Progress in molecular biology and translational science, volume 140

Childhood obesity presents an even bigger challenge, due to theincreased chance of developing metabolic syndrome early in adulthood.Obesity significantly decreases the potential of life

P ROGRESS IN

MOLECULAR BIOLOGY AND TRANSLATIONAL

SCIENCE

Genetics of Monogenic and Syndromic Obesity

P ROGRESS IN

MOLECULAR BIOLOGY AND TRANSLATIONAL

Auburn, AL, USA

AMSTERDAM • BOSTON • HEIDELBERG • LONDON

NEW YORK • OXFORD • PARIS • SAN DIEGO

SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO

Academic Press is an imprint of Elsevier

525 B Street, Suite 1800, San Diego, CA 92101-4495, United States

50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, UK

First edition 2016

Copyright © 2016 Elsevier Inc All Rights Reserved.

No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions

This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein).

Notices

Knowledge and best practice in this field are constantly changing As new research and experience broaden our understanding, changes in research methods, profes- sional practices, or medical treatment may become necessary.

Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein In using such information or methods they should

be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility.

To the fullest extent of the law, neither the Publisher nor the authors, contributors,

or editors, assume any liability for any injury and/or damage to persons or property

as a matter of products liability, negligence or otherwise, or from any use or operation

of any methods, products, instructions, or ideas contained in the material herein ISBN: 978-0-12-804615-9

ISSN: 1877-1173

For information on all Academic Press publications

visit our website at https://www.elsevier.com/

Publisher: Zoe Kruze

Acquisition Editor: Mary Ann Zimmerman

Editorial Project Manager: Helene Kabes

Production Project Manager: Magesh Kumar Mahalingam

Designer: Mark Rogers

Typeset by Thomson Digital

E Burgio

European Cancer and Environment Research Institute (ECERI), Bruxelles, Belgium; ISDE International Society of Doctors for Environment (Scientific Office), Arezzo, Italy M.G Butler

Departments of Psychiatry & Behavioral Sciences and Pediatrics, University of Kansas Medical Center, Kansas City, KS, United States of America

D.D Duan

Laboratory of Cardiovascular Phenomics, Center for Cardiovascular Research, Department

of Pharmacology, and Center for Molecular Medicine, University of Nevada School of Medicine, Reno, NV, United States

Obesity is an epidemic in developed countries and becoming ever moreprevalent in developing countries Obesity is strongly associated with severalmetabolic diseases, including type 2 diabetes mellitus, non-alcoholic fattyliver disease, hypertension, arteriosclerosis, and cardiovascular disease It isalso associated with sleep apenea, and some types of cancers such as breastcancer in women and prostate cancer in men It also has significant psycho-logical effects on the obese Therefore obesity has become a major publichealth problem worldwide, with tremendous health, economic and socialcosts Childhood obesity presents an even bigger challenge, due to theincreased chance of developing metabolic syndrome early in adulthood.Obesity significantly decreases the potential of life expectancy increase thatcould be achieved with reduced smoking and improved health care.Obesity is caused by multiple factors, including genetic, environment,and social elements Genetic contribution to obesity was clearly demon-strated with elegant studies in naturally occurring rodent obesity models(such as ob/ob and db/db mice and Zucker Fatty rat) as well as twin studies inhumans Indeed, mutations in several genes, such as leptin, leptin receptor,proopiomelanocortin, prohormone convertase 1/3, melanocortin-4 recep-tor, brain-derived neurotrophic factor, and Sim1, causing monogenic obe-sity have been identified Some of these genes were discussed in previouschapters in this Series.

This volume reviews several aspects of the genetics of monogenic andsyndromic obesity Starting with Butler’s introduction to several examples ofmonogenic and syndromic obesity, Ramos-Molina, Martin, and Lindbergdiscussed mutations in prohormone convertase 1/3 and human obesity Hansummarized the studies on brain-derived neurotrophic factor variants andhuman obesity My students and I reviewed variants in melanocortin-3receptor and ghrelin receptor and obesity Epigenetic aspect of obesity wasreviewed by Migliore and her colleagues Duan and his colleagues provided aguided tour for us “From genome-wide association study to phenome-wideassociation study” as applied to obesity studies Last but not least, Switonskiand his colleagues demonstrated that genetics is also relevant in our bestfriend (dogs) and an important source of our meat (pigs) They also argued

xi

that these species could serve as large animal models for human obesitystudies, complementing rodent studies.

I am immensely grateful to all the authors for their wonderful tions to this volume I also greatly appreciate the expert guidance andenormous patience of the colleagues at Elsevier, Mary Ann Zimmermanand Helene Kabes It has been a great pleasure to work with you on this andother volumes Finally, without the unwavering support of the Series Editor,Professor P Michael Conn, this volume would not possible I thank you allfor the support

contribu-Finally, as always, I thank my family for continuous support and ditional love

uncon-YA-XIONGTAO

Auburn, Alabama

Single Gene and Syndromic

2 Genetic Variant and Genome-Wide Association Studies in Obesity 4

6.1 Genomic Imprinting Defects in Prader –Willi Syndrome 19 6.2 Genetic and Clinical Aspects of Prader –Willi Syndrome 21

6.5 Clinical Findings Associated with Typical 15q11-q13 Deletions

(Type I and Type II) in Prader –Willi Syndrome

32

7 Fragile X Syndrome and the Prader –Willi Phenotype 33

8 Alström Syndrome: A Rare Obesity-Related Single Gene Disorder 35

Abstract

Obesity is a significant health problem in westernized societies, particularly in the United States where it has reached epidemic proportions in both adults and children The prevalence of childhood obesity has doubled in the past 30 years The causation is complex with multiple sources, including an obesity promoting environment with plentiful highly dense food sources and overall decreased physical activity noted for much of the general population, but genetic factors clearly play a role Advances in genetic technology using candidate gene approaches, genome-wide association studies, structural and expression microarrays, and next generation sequencing have

Progress in Molecular Biology andTranslational Science, Volume 140

1

led to the discovery of hundreds of genes recognized as contributing to obesity Polygenic and monogenic causes of obesity are now recognized including dozens of examples of syndromic obesity with Prader –Willi syndrome, as a classical example and recognized as the most common known cause of life-threatening obesity Genetic factors playing a role in the causation of obesity will be discussed along with the growing evidence of single genes and the continuum between monogenic and polygenic obesity The clinical and genetic aspects of four classical but rare obesity- related syndromes (ie, Prader –Willi, Alström, fragile X, and Albright hereditary osteo- dystrophy) will be described and illustrated in this review of single gene and syndro- mic causes of obesity.

Obesityisanincreasingserioushealthproblemrecognizedworldwideandreachingepidemicstatusparticularlyinwesternizedsocieties.Aroleforgenetic factors is now recognized as contributing to obesity and will besummarizedinthis reviewonsinglegene andsyndromic causes ofobesity.TheUnitedStatescurrentlyleadsasthemostobesenation.Thecausationofobesityisclearlycomplex.Therisingobesityprevalenceispartiallyduetoanobesity-promotingenvironmentwithhighlydense,inexpensive,andplentifulfoodsourceswith arelatively sedentarylifestyleduetoadvancesinmoderntechnology leading to decreased energy expenditure for employees in theworkplace.1–3Asurvivaladvantageover timehasledtoathriftyphenotypeduetoamoreefficientuseofcalorieswithfatdepositiontherebycontributing

totheobesityepidemicinthepresenceofincreasedcaloricintake

Majorhealthconcernsrelatedtolongtermobesitystatusincludeinsulinresistance, type 2 diabetes, fatty liver, sleep apnea, cancer, hypertension,cardiovascular disease, stroke, and physical limitations with disabilities.Obesity, as a global public health problem, is on the riseas evidenced in

1997at19.4%;24.5%in2004,33.8%in2008,and35.7%ofadultsin2010.3Childhoodobesityisalsopresentat17%intheUnitedStateswith70%ofobese adolescents becoming obese adults The prevalence of childhoodobesity has doubled in the past 30 years.4 The medical costs for treatingcomplications from an overweight and obesity status are inordinate withestimatesfor treating people for overweight and obesityestimated at $72billionand $198 billion,respectively impacted bythe current obesogenicenvironment.3,5–10

Bodymassindex(BMI)isacommonlyusedmeasureofobesityinadultsandchildren and definedas weight measuredinkilograms dividedbythe

squareofheightinmeters(kg/m2), butthismeasureisnotas accuratefordeterminationofobesityinthepediatricagegroup.ABMIbetween25and

30 is considered overweight while BMIs greater than or equal to 30 areconsideredobeseinadulthood.Sincethe1980s,themeanBMIhasincreased

by0.4–0.5kg/m2perdecadeinadults.InchildhoodaBMIfromthe85thto95thpercentilemaybeconsideredoverweightandequaltoorgreaterthanthe95thpercentileforobesity.3,11–13

Twinandfamily studiesstrongly implicategeneticfactorsas playinganimportant role in the development of obesity Genetic predisposition forobesitycanalsobeinfluencedbyethnicity.Monozygotictwinstudiesshowconcordanceforobesityatarangefrom70to90%,whileindizygotictwinsrangefrom35to45%.Heritabilityestimatesderivedfromtwinstudiesreport

arepresentativenumberof77%forBMI.14,15IncreasedphysicalactivityandexercisecanlowertheeffectsofgenesimpactingBMIandthereforeobesitystatus.Hence,genesplayanimportantcentral roleinthedeterminationofBMIand pathogenesisofobesity.Inaddition,stronggeneticfactorsinflu-encepercentagebodyfat, waistcircumference,energyexpenditure,eatingbehavior,andlevelofphysicalactivity.Specificgenemutationsarefoundin5–10%ofobesityinchildhoodsupportedbyarecentreviewof370recog-nized genesidentified inthe literature playing a role in obesity.16 Severalgeneshavebeenstudiedinrelationshiptoobesity,particularlyinmongeniccausation;forexample,MC4R,whichisthesinglemostcommonlyrecog-nized gene causingchildhood obesityand found in about4% of cases.6,7Structural chromosomal anomalies involving genomic regions containingcausativegenesforobesityhavebeenreportedsuchas3p25duplicationsand16p11.2deletions.17

With continually improved and varied complex genetic techniquesrequiring smaller quantities of DNA(and RNA)such as next generationsequencingfurthersupportedbyprometaphasechromosomeanalysis,fluo-rescenceinsituhybridization,linkageandgenome-wideassociationstudies(GWAS),copynumbervariant(CNV),andsinglenucleotidepolymorphism(SNP)probesutilizedinhighresolutionmicroarrayshavebeeninstrumental

inidentifyingstructural,chromosomal,andDNAabnormalitiesingenomicregionsandcandidategenescausingbothrareandcommonformsofobesity.Specifically,chromosomeabnormalities(deletions,duplications)havebeenrecorded in humans with dysmorphic and syndromic obesity includingchromosome 1p36 deletion; chromosome 2q37.3 deletion; chromosome3p23duplication;3p25.3duplication(containstheGHRLgene);chromo-some 4q32.1 duplication; 4q35.1 duplication; chromosome 5p13.1

duplication; chromosome 6q16.2 deletion (SIM1gene); 6q22.2 deletion;6q24.3 duplication; 6q15-q21 deletion (SIM1 gene); 6q16-q21 deletion(SIM1gene);6q16.1-q16.3deletion(SIM1gene);chromosome7q36dele-tion; chromosome 9p23 deletion; 9q34 deletion; 9q34.3 deletion;9q33.3q34 duplication; chromosome10q22.3q23.2 duplication; chromo-some 11p12-p14 deletion; 11p13-p14.2 deletion (BDNF gene); 11p11.2deletion;chromosome12p13.1duplication;12qterdeletion;chromosome14q32.2hypomethylationstatus(maternaldisomy14);chromosome16q13duplication; 16p11.2 deletion (SH2B1 gene); 16q11.2-q13 duplication(FTO gene); chromosome 18q12.2-q21.1 deletion; chromosome 19q12-q13.2 duplication; 19q13.2 deletion; chromosome 20q13.13-q13.32deletion;chromosome22q11.2deletion;chromosomeXq26.3-q27.3dele-tion (FMR1 gene); Xq23q25 duplication; Xp11.3p21.1 duplication;Xp11.4q11.2 inversion; and Xq27.1-q28 deletion (FMR1 gene)17(Table 1) Small deletionsinvolving the chromosome 16p11.2 band havebeenreportedin0.5–0.7%ofindividualswithsevereobesityandnotfound

innonobesehealthy individuals.It includesthe SH2B1gene,an related gene that impacts leptin sensitivity and hence caloric intake.18ChromosomemicroarraystudieshaveshownrareCNVswhicharegreaterthan2Mbinsizeandfoundin1.3%ofindividualswithobesityandevidence

obesity-of disrupting multiple candidate genes for obesity (eg, POMC, UCP1,GHRL).Hence,theresearch ofCNVsinthestudyofobesityhasmerits.Recently,Butleretal.16summarized theliteratureand authoritativecom-puterwebsitesandfound370clinicallyrelevantandknowngenesreportedforobesityandplottedthegenesonchromosomeideogramstorepresentavisualdisplayofgenedistribution(Fig.1AandB)

Identificationofgenelociormarkersfordiseasestatesbecameableinlate2005andisusedtodiscoveranumberoflociforobesity.ThefirststudyofitstypetouseDNAmarkersinchildhoodobesitywaspublishedin

avail-2010andtwolociwerefound(ie,SDCCAG8andTNKS/MSRA).19Nowlarger,moreadvancedgeneticlocistudieshave utilized14existingGWASdata sets to identify additional loci for obesity.3 The GWAS method isconsideredhigh-throughputwiththeadvantage toassay millions ofDNAmarkersasSNPsthatspantheentirehumangenome.Whencombinedwith

CommonvariantsinanumberofgenesencodingproteinsinvolvedwithobesityincludeleptinandleptinreceptorswhichregulatecaloricintakeandhavebeenfoundtobeassociatedwithBMIandobesitystatusidentifiedin

Table 1 List of Chromosome Locations and Obesity Genes in Relationship to Obesity Phenotypes.

Genetic Factors and Relationship

(AGRP); 17q21 (HOXB5); 17q21.31 (PYY); 18q21.32 (MC4R)

Genetic loci associated with child

and/or adult BMI levels

including extreme obesity

obesity—novel loci

EPHA6-UNQ6114 Associated with childhood and/

or adult waist to hip ratio

LYPLAL1; C12orf51; LY86; HOXC13;

Figure 1 (A) Obesity gene ideogram part A High resolution human chromosome ideograms (850 band level) with symbols representing recognized genes for obesity positioned at the chromosome band location The upper “p” and lower “q” arms for each chromosome are separated by the centromere area 16 (B) Obesity gene ideogram part B High resolution human chromosome ideograms (850 band level) with symbols representing recognized genes for obesity positioned at the chromosome band location The upper “p” and lower “q” arms for each chromosome are separated by the centromere area.16

severalhumanpopulations.Anotherhormonethatplaysaroleinglucoseandfatty acid regulation and production is adiponectin which is coded by aspecificgeneandknowntobelowerinobeseindividualsandinthosewithtype2diabetes.Othervariantsincludethecannabinoidreceptor1(CNR1),dopamine receptor 2 (DRD2), Fat mass and obesity associated (FTO),insulin induced gene 2 (INSIG2), serotonin receptor2C (HTR2C), andSLC6A4.Another important obesitygeneis peroxisomeproliferator-acti-vatedreceptorgamma(PPARγ)foundtobeassociatedwithbothobesityandtype2diabetesalongwiththeFTOgenewhichisconsideredthemostrobust

(B)

Figure 1 (Continued)

common obesity–susceptibility locus found to date The frequency for aminorallele(ie,rs9939609)oftheFTOgenerangesfrom38to44%intheCaucasianpopulationandclearlycorrelateswithweightgainbyproducinghigherBMIlevelsandobesity.3,6,7,20–25

UsingtheGWASapproach,largermetaanalysisstudieshavebeencarriedoutbytheGenetic InvestigationofAnthropometricTraits(GIANT)con-sortium in Caucasians and continues to uncover obesity-related genesincludingtransmembraneprotein18(TMEM18),potassiumchanneltetra-merization domain containing 15 (KCTD15), glucosamine-6-phosphatedeaminase2(GNPDA2),SH2Badaptorprotein1(SH2B1),mitochondrialcarrier 2 (MTCH2), and neuronal growth regulator 1 (NEGR1).Furthermore, novel loci reported involve chromosome regions 1q25,3q27, and 12q13 A more recent, expanded GIANT metaanalysis studyinvolving249,796individualshasuncovered32BMI-associatedgeneticlociincluding 10 loci previously reported and associated with BMI and 4associated with weight gain and/or increased waist–hip ratio (SEC16B,TFAP2B, FAIM2, NRXN3) Eighteen loci were reported to benovel (RBJ-ADCY3-POMC, GPRC5B-IQCK, MAP2K5- LBXCOR1,QPCTL-GIPR, TNNI3K, SLC39A8, FLJ35779-HMGCR, LRRN6C,TMEM160-ZC3H4, FANCL, CADM2, PRKD1, LRP1B, PTBP2,MTIF3-GTF3A, ZNF608, RPL27A-TUB, and NUDT3- HMGA1).Common CNVs were also found including a 21kb deletion which was

50kb upstream to the GPRC5Bgene Continuationof analytical studies

bythisconsortiumgrouphaveconfirmedpreviouslyidentifiedobesitylocisuch as FTO, SEC16B, MC4R, GIPR-QPCTL, ADCY3-DNAJC27,BDNF,and MAP2K5 and newadditional gene loci (CDKAL1, PCSK1,GP2,GNPDA2, TFAP2B, PAX6, CDKAL1, and KLF9) associated withBMI.3Hence,candidategene,DNAlinkage,andGWASapproacheshaveledtotheidentificationoflargesetsofgenesandinvolvedgenomicregionsfound to be associated with obesityand obesity-related endophenotypes.Evidencenowexistsfor thepresenceof370genesplaying a roleand 153genesassociatedwithknownreproductionand infertilitywith21ofthesegenesincommoninbothobesityandinfertility.16

GeneticformsofobesitycanbegroupedintoMendelianor singlegene and syndromic or multifactorial These include recessive gene

inheritance,partialgenedeficienciesorduplications,genomicstructuralvariations or CNVs, andpolygenic causes Monogenic forms or singlegeneconditionscausingobesityhavebeenreportedforatleasteightgenesincluding leptin (LEP), leptin receptor (LEPR), proopiomelanocortin(POMC),prohormoneconvertase 1(PCSK1),melanocortin4receptor(MC4R),single-mindedhomolog1(SIM1),brain-derivedneurotrophicfactor(BDNF),andtheneurotrophictyrosinekinasereceptortype2gene(NTRK2).Thehypothalamicleptin–melanocortinsystemiscriticalforregulating energy balance with disturbances leading to severe obesitydisorders.3,6,7Theseobesitygenesareknowntoprimarilyaffectcommonpathwaysinvolvinglipidmetabolism,depositionortransport,foodseek-ingbehaviorandcalorieselectiontypes(ie, fat,protein,carbohydrates),level of physical activity, and forms of energy expenditure related toemploymentor recreation.

Several obesity-related syndromic genetic disorders are identified inhumans, both common and rare and will be discussed later Monogeniccauses ofmorbidobesityareuncommonbut markedobesityand extremeeating behavior(hyperphagia) are keyfeaturesof severalrare genetic syn-dromesincludingPrader–Willi,Alstro¨m,Bardet–Biedl,Albrighthereditaryosteodystrophy(AHO),Cohen,andfragileXsyndromes.Recognizedgenesplaying aroleinthesedisorders areknown[eg, SNRPNforPrader–Willisyndrome(PWS),GNASforAHO,FMR1forfragileXsyndrome(FXS)],26whichwillbediscussedlater.Understandingthemolecularbasisoftheseraredisordersandtheirgeneticmechanismsinvolvingthecontroloffoodintakeandenergybalanceinthegeneralpopulationwillbeimportanttoaddresstheobesityepidemic.CodingandnoncodingRNAexpressionpatterns,specif-icallymicroRNAsandsmallnucleolarRNAs(snoRNAs)havingimportantregulatory roles in biological processes, need to be better characterizedincluding their impacton appetite regulation,gene–environmentinterac-tion, adipocyte differentiation, and biochemicalpathways.16 Genetic var-iants havebeen reportednear theMC4Rand FTO geneswhichincreasebodyweightinthosecarryingthesevariants,withmutationsoftheMC4Rgenepresentinabout2%ofallobeseindividuals.Maleandfemalehetero-zygous carriers weigh 15–30kg more, respectively compared with theirrelativeswithouttheMC4Rgenechangesormutations.3,6,7Moreresearch

isneededto examinegeneticdifferencesamongobeseandnonobeseviduals,particularlyrareCNVstogivenovelinsightintothegeneticcausa-tion and architecture of obesity and associated infertility in the generalpopulation

indi-The consequences of increased weight and obesity can shorten lifeexpectancyaswellasaffectingreproductionwithdysfunctioninovulation,spontaneousabortions,andoverallinfertility.Adversepregnancyoutcomesarealsonotedincludingpreeclampsia,fetalgrowthfailurewithprematuredelivery,and gestationaldiabetes.About15%ofallwomenintheUnitedStatesarealsoinfertilewithadvancedageplayingarole.Theprevalenceofmaternal obesity in the US population is increased with more womenhaving obesity-related reproductive problems Gradual and sustainedmaternalweightlossis neededtoimprovemenstrualcycles andovulationand thus reproductive rates and outcomes.27,28Weight lossis consideredthefirst line of treatment in those women with reproductivefailure andobesity-relatedinfertility.Oneofthemostcommoncausesofsubfertilityinwomen with obesity is polycystic ovarian syndrome which is associatedwith the androgen receptor (AR) gene.29,30 The AR gene produces asteroid hormone-activated transcription factor important in regulatingandrogenactivityandsensitivitytosexhormonesinbothmalesandfemalesinvolvedinweightand bodycomposition.ThisX-linkedgenecontainsapolymorphicCAGtrinucleotide repeatlengththatis inverselycorrelatedwithgeneexpression.31

ButlerandManzardo29examinedthepolymorphicARgenewithCAGrepeats and measures of weight and BMI in a cohort of nonsyndromicobese and lean controls, compared with PWS subjects PWS is a rareobesity-relatedgenetic disorder with growth hormone (GH)deficiency,hypogonadism, andnatural sexhormonedeficits.The effectsof theARgeneCAGrepeatlengthwasexaminedinrelationshiptoandrogen-medi-atedresponseandobesity-relatedfactorsrelevanttohumaninfertilityandreproduction.The average CAG repeat length in base pair size did notsignificantlydifferbysubjectgroupbutwassignificantlypositivelycorre-lated with height amonglean andobese males,but not present in PWSmales.Anegativecorrelationwasalsoobservedforweightamongfemaleswhen grouped together The summary results support the role of sexhormones and the AR gene interaction in obesity and infertility, bothcardinalfeaturesseeninPWS.TheARgeneCAGrepeatlengthisamarkerfor increased androgen sensitivityandshorter CAG repeatlength in thisstudypredictedsmallerstatureinnon-PWSadultmales.Thisprocessmayacceleratefusionofbonegrowthplatesandreducethelengthofthegrowthphase.Hence,increasedandrogeneffectsfromshorterCAGrepeatlengths

in non-PWS females could impact pregnancy-related weight gain andpossiblypregnancyoutcomes

4 OBESITYGENESANDTHEIRENCODED PROTEINS

One ofthe keyobesity-related proteinsis leptin whichisa 16-kDasecreted protein and encoded by the leptin (LEP) gene This gene isexpressedandsecretedbyadipocytes,butitsreceptorisprimarilyexpressed

in thehypothalamus.Leptin plays a major role infood intake regulation,energy balance, and body weight Mutations and polymorphisms of theleptin and leptin receptorgenesare associated withobesity in Caucasians

as wellas POMC withthegenelocated on chromosome2 Its proteinisproduced by the hypothalamus and thus POMC plays a role in feedingbehavior.ThePOMCgeneexpressionlevelispositivelyregulatedbyleptin.AlterationsinthePOMCgeneincludingaframeshiftmutationhavebeenreportedtocauselossoffunction.Thisresultsinearly-onsetobesity,adrenalinsufficiency,andredhairpigmentation.POMCalsoproducesβ-MSHandβ-endorphinpeptidesviaaproteolyticprocessinvolvingtheenzyme,pro-hormone convertase (PC), and when altered produces an aberrant fusionproteinwithlowerbindingaffinitytotheMC4R.ThiscanhavedevastatingeffectsbecauseMC4Rplaysanimportantroleinbothfoodintakeandbodyweightbycontrollingleptineffectsandshowntocausesevereobesitywhendisruptedinmice.6,7

Multiplenonsenseandmissensemutationsingeneshavebeenidentified

inhumansandstronglyassociatedwithmanyobesity-relatedtraitsincludingthe BDNF gene located on chromosome 11 Its receptor is a tyrosinereceptorkinaseBandencodedbytheNTRK2genewhichregulateseatingbehaviorandenergybalancewithgrowingevidenceinhumansasplayingaroleinobesity.Inaddition,theconditionalknockoutofBDNFgeneinmicewilldevelopobesityandhyperactivity.TheSIM1geneislocatedonchro-mosome 6 and also important in the differentiation and function of thecentral nervous system When this gene is disturbed, early-onset obesityoccursalongwithincreasedlineargrowth,hyperinsulinemia,andhyperlep-tinemiainhumans.TheFTOgeneinhumansconsistsofnineexonsandislocatedonchromosome16.FTOgeneexpressionismostabundantinthebrainbased onmouse studies,particularlyinhypothalamic nuclei,anareainvolvedinenergybalance.ThemRNAlevelsfortheFTOgenearefoundspecificallyinthearcuatenucleuswhichregulatesfeedingpatterns.3Currentresearch and supporting evidence directly identifies human disorders ofenergy balance and obesity such as Prader–Willi, Alstro¨m, and Albrighthereditaryosteodystrophysyndromes as targets for study inunderstanding

thegeneticsofobesity.PWSandAHOarebothdueto errorsingenomicimprintingorepigenetics.Epigeneticsreferstochangesingeneexpressionwithout altering the DNA code and influenced by theparent of origin.Epigeneticsisanemergingfieldofstudywhichimpactsthedevelopmentofmultiplediseasesanddisordersincludingobesityandwillrequireadditionaltestingtogainabetterunderstandingofpathogenesis.26

Genetic factors which predispose weight gain and development ofobesityinhumans canalso impact theresponsetointervention interms

of weight loss For example, those individuals with MC4R or POMCgenemutations appear to respond to a reducedcaloricdiet andexerciseprogrambutthosewithMC4Rmutationsfailtomaintainweightlostafterintervention.Hence,thegene–environmentinteractionneedstobecon-sideredwhentreating thoseaffectedwith obesity.Accessto amultidisci-plinary team may be helpful in achieving weight loss including geneticevaluations and screening for obesity-related disorders or monogeniccauses, dietary consultation, and behavior/pharmaceutical therapy.Thereisa growinglistof pharmaceuticalagentsanddrugs underclinicaltrialdevelopmenttotargetknowngeneticcausesofobesity,forexample,PWS.Informationlearnedthrough thisprocessmay havedirectapplica-tionon obesity inthe generalpopulation Bariatricsurgerymay also beconsidered.However,thisistypicallyusedasafinaloptionfortreatingandmanagingexogenousobesitybutwithonlylimitedexperienceinindivi-dualswith syndromicobesity

Progressinclinicalevaluations,awareness,andgenetictestingwillhelptoidentify DNA factors contributing to obesity specific for each individualbasedontheirfamilyhistory,lifestyle,andobesogenicenvironmentcommon

inmodernsociety Othercontributing factorsmayincludeenvironmentalchemicals,heavymetals,andadditivesupplements.NextgenerationDNAsequencingincludingexomeandwhole-genomeapproacheswillgenerateextensivegeneticdataandcomprehensivegeneticmapsofpotentially pre-disposingcausativefactorsforobesity.Thisdetailedpictureofbiologicalandmolecularmechanismsandpathwayswillbekeyinunderstandingthedevel-opmentofchildhoodobesityandeatingbehavior

PediatricobesityintheUnitedStatesisontherisewithaboutone-fifthofchildrenbeing affected.4 Most or greater than 50% of casesof childhoodobesityareexogenousinnatureand resultfromexcessiveenergyintakeinrelationshiptoenergyexpenditureforextendedperiodsoftime.Somecases

ofobesityare associatedwith hormonalor geneticfactors,or syndromes.Hormone imbalances such as hypothyroidism, Cushing disease, GH

orPOMCmutations.ExamplesofsyndromicobesityincludePrader–Willi,Alstro¨m,andBardet–Biedlsyndromes

Criticalfeaturescanbedistinguishedbetweenthemorerareendogenousobesitygeneticdisorderswhen comparedwiththemore commonexoge-nousformsofobesity.Thosewithendogenousobesitymayhaveanearlieronsetofobesity,lackofsatiety,inadequatelineargrowthpatterns,dysmor-phic features, and cognitive/behavior problems To identify differencesbetweenthetwoobesityformsaclinicalgeneticsevaluationmayberequired

to rule outdysmorphic/syndromic causes, obtainfamily historydata, andrequestdietitianconsultationsforreviewingfoodrecordsandcalorieintake

Inaddition,appropriatehormonalandgeneticlaboratorytestingwouldbeorderedandresultsinterpretedapplicableforindividualmedicalcare

Syndromic obesity may result from a single gene condition (eg,Cohen,Alstro¨m,Bardet–Biedlsyndromes)orerrorsinimprinting(ie,epi-genetics) as seen in PWS involving chromosome 15 or AHO involvingchromosome20.Othersyndromesmayhavemorethanonecause,leading

tothecollectionoffindingssuchasobesityfoundinDownsyndromeorthePrader–Williphenotype(PWP)ofFXSmales.Wewillreviewthegeneticsofsyndromic obesityand clinical presentationof classical and underreportedrare causes of obesity There are several syndromic causes of obesity inhumans that are currently recognized; some are common such as Downsyndromewhileothersarerare[eg,Alstro¨msyndrome(ALMS)](Table2).PWSisthefirstexampleofsyndromicobesitytobediscussed.Itwasfirstdescribedin1956withninecasesandsincethattimewellover1000caseshavebeenreported.32PWSisaneurodevelopmentaldisorderresultingfromerrors in genomic imprinting (epigenetics) with loss of only paternallyexpressedgeneslocatedonchromosome15.Itresultsusuallyfromadenovodeletionofthe15q11-q13region.33–36Adeletionofthe15q11-q13regionfrom the mothercauses a different clinical disorder known as Angelmansyndrome.About70%ofthosewithPWSwillhavethe15q11-q13deletionwhiletheremaining(25–30%ofcases)willhaveboth15sfromthemotheror

a defectof theimprintingcentercontrolling theactivityof theimprinted

PWSshowsarangeofmildlearningdeficitsandcharacteristicbehaviorproblemsincludingself-injury(skinpicking),outbursts,obsessivecompul-sions,temper tantrums, and food seeking withhyperphagia beginning inchildhood.Additionalclinicalfindingsincludegrowthandotherhormonedeficienciesleading to short stature, small hands and feet,hypogonadism,hypogenitalism,hypotonia,apoorsuck,feedingdifficultiesduringinfancy,andaparticularfacies(Fig.2).InfertilityispresentinbothPWSmalesandfemales.Obesitycanresultifnotcontrolledinearlychildhoodand belifethreatening.37PWSisfoundinabout1in10,000–30,000babieswithalowerprevalenceof1in30,000–50,000individuals;PWSaffectsbetween350,000and400,000peopleworldwide.38MostcasesaresporadicbutPWSiscon-sideredoneofthemostcommonknowngeneticcausesoflifethreateningobesityaffectingallracesandethnicgroups34(Table3)

FXS is the most common cause of intellectual disability that runs infamilies.ItisduetoatripletrepeatmutationoftheFMR1genelocatedatchromosomeXq27.3whileacarrierstatusorpremutationformofthisgenecanbeseeninfemalesatriskofdevelopingafullmutationinoffspringinthenextgeneration.FXSisassociatedwithobesitybutasubsethasfeaturesincommonwithPWSreferredtoasthePrader–Williphenotype(PWP).FXSgenerallyaffectsmalesandwasfirstreportedin1969.39Itisfoundinabout1

in 4000 males in the general population.40 Prominent clinical features

Table 2 Syndromic Causes of Obesity in Humans.

elon-ofmaleswiththisgenemutationhavefeaturessimilarlyseeninPWSinghypotonia,developmentaldelay,shortstature,asmall penis,behavioralproblems,excessiveeating,andmarkedobesity41–43(Table3)

includ-A third rare separate genetic obesity-related disorder is ALMS Thissyndrome isdueto mutationsof a singlegene(ALMS1) locatedon chro-mosome 2p13.ALMSoccursinabout1in 1,000,000individuals44 andischaracterizedbymultiorgan involvement with fibrosis,progressive vision,and hearingloss,accompaniedbyobesityinchildhood.Insulin resistance,type 2 diabetes mellitus, and high lipid levels are noted, along with

Figure 2 Frontal and profile view of an 8-year-old male with PWS showing the typical facial features, central obesity, and gastrostomy-tube site located on the abdomen.

Syndrome (PWS)

Narrow forehead, almond-shaped eyes, strabismus, short nose with thin upper lip, downturned corners of mouth, dry sticky saliva, enamel hypoplasia

Severe hypotonia, short stature, obesity, osteoporosis, small hands and feet, scoliosis, hypopigmentation, head tilt forward, hypogenitalism

Mild learning impairment, hyperphagia, skin and rectal picking, difficulty with transitions, stubbornness, temper tantrums, perseverative speech, autism, obsessive compulsions, unusual skill with jigsaw puzzles, high pain tolerance

Paternally derived 15q11-q13 deletion (70% of cases), maternal disomy 15 (about 25% of cases), imprinting defects (1–3% of cases)

almond-Obesity, delayed puberty, small penis, hypotonia

Developmental delay, food seeding behavior and hyperphagia, difficulty with transitions, perseverative speech, hand flapping, poor eye contact, autism, obsessive compulsions

FMR1 gene triplet repeat mutations (at chromosome Xq27.3)

Alstro¨m Syndrome

(ALMS)

Round face, deep-set eyes, thickened skull, thick ears, frontal hair loss

Wide, flat feet with brachydactyly, scoliosis, dental anomalies, truncal obesity, short suture, hypogonadism,

cardiomyopathy, vision rod dystrophy) and hearing loss, type 2 diabetes, progressive pulmonary, renal and hepatic problems with fibrosis

(cone-Developmental delay, balance disturbances and neurosensory deficits, depression, autism, obsessive compulsions

ALMS1 gene mutations (at chromosome 2p13)

(AHO) neck, delayed dental

eruption or enamel hypoplasia

metatarsals, short distal phalanx

of thumb, small stature, osteoporosis, subcutaneous mineral deposits and basal ganglia, thickened calvarium, variable hypocalcaemia and hyperphosphatemia, occasionally hypothyroidism, hypogonadism and infertility, lens opacity or cataracts, optic atrophy, scoliosis and vertebral anomalies

center domain associated with different forms of pseudohypoparath- yroidism (PHP) and pseudopseudohy- poparathyroidism (PPHP) depending on parent of origin Maternal inheritance leads to PHP-Ia (AHO plus hormone resistance) while paternal inheritance leads to PHPP or AHO without evidence of resistance

to parathyroid hormone (PTH)

hypogonadism;shortstature;andcardiac,liver,lung,andkidneyproblems.The organ systems are often complicated by fibrotic changes notedthroughoutlife.ObesityisobservedinmostchildrenwithALMSandtheyarenotedtohavesignificantandrapidweightgainbeginningwiththefirstyearoflife.45,46 Theobesityis predominantlydistributed inbothsubcuta-neousandvisceralcompartments.Severalendocrinedisturbances,includinghypogonadism and hypothyroidism, are known to occur in this raresyndrome,as wellas decreased growthvelocitywithadvancing age Mostadolescents and adults are reported with short stature and low levels ofinsulin-like growth factors and GH Chronic respiratory tract infectionsare common beginning in early childhood with some becoming severeandleadingto chronicbronchitis,asthma,and obstructivepulmonarydis-ease.Pulmonaryhypertensionisalsocommonwithsevereinterstitialoblit-eratingfibrosisreported, consistentwiththefibroticchanges occurringinothermajororgans.44,45

MostindividualswithALMSarereportedto havenormalintelligence,althoughmildtomoderatedelayinreachingmajormilestonesmaybenoted,such as speech problems Major depression, obsessive compulsions, andpsychoticdisturbancesmayoccurparticularlyduringadulthoodinALMS.TheproteinencodedbytheALMS1geneisknowntocauseALMSwhendisturbedand is relatedto ciliary function as similarly seen inan anothergeneticobesitydisorderknownasBardet–Biedlsyndromewhichconsistsofmore than one type.47 The ALMS1 genecontains 23 exons withseveralalternatively spliced transcripts (isoforms).48 ALMS occurs as a result ofmutationsofthisproteincodinggeneandinheritedasanautosomalrecessivedisorder(Table3).45,46,49

A second obesity-related genetic disorder due to epigenetics is AHO.AHO was first reported in 194250 and is caused by errors in genomicimprintingfound on chromosome 20.26AHO resultsfrom an end-organresistancetoPTH and otherassociated hormones.Obesityisalsoa majormanifestationalongwithsmallstature,mild mentaldeficiency,a particularfacialappearance,shortfourthandfifthmetacarpals,skeletalanomalies,anddelayed dental eruption Osteoporosis, subcutaneous calcium deposits,hypothyroidism, hypogonadism, ocular problems, and cataracts are occa-sionallyseen.42,51Twomajorclinical variants areknownfor thiscomplexdisorderandreferredtoasPHP(PHP-IaorPHP-Ib)andPPHPdepending

onthepresenceorabsenceofadditionalhormoneresistance,alongwiththepresence of the AHO phenotype and the specific pattern of inheritance(Table3)

6 PRADER–WILLISYNDROME

Mostgenesandtranscriptsonthechromosome15q11-q13regionarejecttogenomicimprintingwithdisturbancesleadingtoPWS,arareobesity-relateddisorder.35Lossordeletionofgeneallelesthatareonlyactiveonthechromosome 15receivedfromthefatherand controlledbyan imprintingcenterwithinthe15q11-q13regionleadstoPWS.Thesesamealleleswhenpresentonthematernalchromosome 15aresilencedbyepigeneticfactorsusually through methylation Loss of activity of the maternally expressedgene(ie,UBE3A)inthe15q11-q13region,usuallybythesamedeletionbutfromthemotherleadstoAngelmansyndrome,anentirelydifferentclinicaldisorder.PWSandAngelmansyndromewerethefirstexamplesoferrorsingenomic imprinting causingdisease inhumans It is now recognized thatmultiplesyndromesaffectgrowth,forexample,Beckwith–Wiedemannsyn-dromewithovergrowthasamanifestationandSilver–Russellsyndromewithgrowthretardationas afeatureare duetoerrors ingenomicimprinting.26About1%ofhumangenesarethoughttobeimprinted(estimatedatabout

sub-150innumber);manycodeforgrowthfactors,relatedproteins,andtorsandareexpressedononlyonechromosomedependingontheparentoforigin Paternallyexpressed imprinted genes are thought to enhance thegrowthof thefetus whilematernallyexpressed imprinted genesaremorelikelytoinhibitgrowth.Genomicimprintingisnowthoughttoplayaroleinotherdiseasessuchasmalignancies,diabetes,andtheagingprocess.26,52,53GenomicimprintingisrelatedtothemethylationofcytosinebasesintheCpGdinucleotidesoftheDNAmoleculewhicharekeyregulatoryelements

recep-ofgenes.Almostallimprintedgeneshave aCpG-richdifferentiallyylated region (DMR) which usually relates to allele repression Manyimprintedgenesare arrangedinclusters (imprinteddomains)on differentchromosomes under control of an imprinting center affecting animalgrowth, development, and viability Imprintedgenes may alsocontribute

meth-tobehaviorandlanguagedevelopment,alcoholdependency,schizophrenia,and possibly bipolar affective disorders In addition, the phenomena ofgenomic imprintingwith abnormal imprintingand loss of heterozygosity(LOH)contributestoawiderangeofmalignancies.52–54

Theexpressionofimprintedgenesmaybetissue-andstagespecificwithone of the parental alleles being differentiallyexpressed only at a certain

of an imprinted gene is not absolute Thus, a potential role of genomicimprinting in thedifferentiation oftissue types may be to determinethetranscription rate of genes that influence growth through a fine balancebetweentheexpressionofthetwoparentalalleles.55

Experimentalevidencesuggeststhatgenomicimprintingevolvedabout

150 million years ago in a common live-born mammalian ancestor afterdivergencefromegg-layinganimals.56Imprintinggenesprovidethepaternaland maternal genomes the ability to exert counteracting growth effectsduring embryonic development.57 Approximately 1% of all mammaliangenesare thought to be imprintedwith thefirst gene(H19) reported to

beimprintedinhumansin1992.58Sincethen, manyimprintedgenesarenowcandidatesforhumandiseaseincludingcancer,obesity,anddiabetes.56Imprintedgenesaretargetsforenvironmentalfactorstoinfluenceexpres-sion through epigenetics whereby the expression level is altered withoutchangingtheDNAnucleotidecodingstructure.Imprintingdisturbanceshavebeen reported in classical genetic disorders such as Beckwith–Wiedemann,Angelman,andPrader–Willisyndromeswhiletheincidenceofthesedisordersareincreased in those individuals conceived with the use of assisted repro-ductivetechnology(ART).Hence,ARTmayincreaseimprintingdefectsbychangingtheregulationofimprintedgenes.59

Epigenetics involve various processes altering gene activity withoutchangingtheprimarynucleotidesequenceoftheDNAmolecule.Acom-mon process for controlling gene activity is methylation A gene that ismethylated(inactivated)canbereactivatedinmaleorfemalegametogenesisforthenextgeneration.Forexample,amaternallyimprintedgene(inacti-vated by methylation) may be unmethylated by male gametogenesis andtransmittedasanactivegeneinthesperm

Agenome-wide searchfor imprintedgenesinthehuman genomehasidentifiedover150candidateimprintedgenesinvolving115chromosomebands.60 The number of human diseases or disorders due to genomicimprinting maybe greater than 100 conditions as a consequence of aninappropriategenetic alteration,such as a deletion or uniparental disomyinvolving a gene or chromosome region Humans are predicted to havefewer imprintedgenesthan mice, butthetypes ofhuman genesinvolvedare markedly different from mice Therefore, questions have been raisedabout the use of mice as models for human diseases, particularly thoseinvolved with imprinted genes, and assessing environmental factors thatmayimpactongenesandtheiractivity.Examplesofclassicalhumandisorders

related to alterations of genomic imprinting, besides Prader–Willi andAngelman syndromes, include Silver–Russell syndrome, Beckwith–Wiedemann syndrome, AHO, and more recently, uniparental disomy 14(bothpaternalandmaternalforms).26,61,62

Genes clustered together under the regulation of a singlecontrollingelementsuggestpossibleinvolvementofhigherorderregulatoryelementsshowingallelicspecificDNAreplication.Genescontributedbythemothergenerallyreplicateorexpressatdifferentratesthangenescontributed

imprinting-bythefather.However,inappropriatemethylationmaycontributetotumorformation bysilencing tumor-suppressing genesor byactivating growth-stimulatinggenes.In mammals,DNAmethylation patternsareestablishedand maintained during development by three distinct DNA cytosinemethyltransferases(Dnmt1,Dnmt3a,andDnmt3b).Inmammaliansomaticcells,cytosinemethylationoccursin60–80%ofallCpGdinucleotidesthatare not randomly distributed in thegenome Heavily methylated hetero-chromatinandrepetitivesequencescontributetogenesilencing.MostCpGislandslocatedatthepromoterregionsofmanyactivegenesaremethylationfree.UnderstandingthefunctionsofDNAmethylationanditsregulationinmammaliandevelopmentwillhelptoelucidatehowepigeneticmechanismsplay a role in human diseases such as neurobehavioral problems andcancer.26,63

The chromosome 15q11-q13 region contains several million DNA basepairswithdozensofimprintedgenesand/ortranscriptsclusteredandunderthecontroloftwoimprinting-controllingcenters(oneforPWSandoneforAngelman syndrome) in this cytogenetic region There is also a nonim-printeddomainorclusterofgeneswhichareequallyexpressedfromeither

oftheparentalchromosome15s.33,35LowcopyDNArepeatscalledconsarelocatedat theendofthischromosomeregionand arenovelwithdesignationat chromosome15q11-q13breakpointsites.64,65Theserepeti-tiveDNAareascontainafunctionalHERC2genewhichislocateddistallyatthechromosome15q11-q13regionfoundatbreakpointBP3.TwoHERC2pseudogeneswhicharelocated attwoproximal breakpointsites(BP1andBP2)areseparatedbyabout500kbsizeDNAsegment66(Fig.3).Becausethesechromosome15breakpointsitescontainsimilarDNAsequences,theycan contribute to nonhomologous or mispairing of thechromosome 15sduringmeiosisIleadingtoaberrantrecombinationorcrossing-overevents

dupli-Figure 3 Chromosome 15 ideogram showing the location of genes in the 15q11-q13 region BP1, BP2, and BP3 are the three common chromosome 15 breakpoints in the region at the site of breakage leading to the larger typical Type I deletion between BP1 and BP3 and the smaller Type II deletion between BP2 and BP3 The dark gray (blue in the web version) colored rectangle-shaped symbols represent paternally expressed genes (eg, MAGEL2) which when disturbed leads to PWS The gray (red in the web version) colored square-shaped symbols represent maternally expressed genes and the UBE3A gene when disturbed causes Angelman syndrome The light gray (green in the web version) colored rectangle-shaped symbols represent genes (eg, CYFIP1) that are expressed on both the maternal and paternal chromosome 15s.

in the offspring and hence PWS when the deletion is from the father’schromosome 15 or Angelmansyndrome when the deletionis present onthechromosome15donatedbythemother

About 70% of individuals with PWS will show the typical paternaldeletion of the15q11-q13 region and will be de novoin originand notinherited.Therearetwotypesofthetypical15q11-q13deletion,TypeIandType II withtheType I deletion being larger (about6.5Mb insize) andinvolvingchromosomebreakpointsBP1andBP3.33,35Thisisdetectablebyhigh resolution chromosomal microarray analysis (Fig 4A) The smallerType II deletion (about 5.3Mb in size) involves breakpoints BP2 andBP3.67Fourgenes(TUBGCP5,CYFIP1,NIPA1,andNIPA2)arelocatedbetweenBP1andBP2(Fig.3)andarenotimprinted.Theyshowbiallelicornormal expression from either thematernal or paternal chromosome15.Individualsrecentlyreported withspeechdelay andautistic characteristicsare shown to have deletions and/or duplications of only the four genesbetweenBP1andBP2 Inabout5%ofindividualswithPWS, anunusual

oratypicaldeletioncanbepresentthatiseitherlargerorsmallerinsizethanthetypicalTypeIorTypeIIdeletion.68–70

Several studies have shown that individuals with the larger typical15q11–q13Type I deletion have more severeneurodevelopmental symp-toms,as compared to those with PWS or Angelman syndrome with thesmallertypical TypeIIdeletions.71,72 Bittel et al.73 later reported findingsfrommRNAisolatedfromlymphoblastoidcelllinesestablishedfrommaleswithPWSforfourgenes(ie,NIPA1,NIPA2,CYFIP1,andTUBGCP5)inthegenomicareabetweenBP1andBP2inthe15q11.2chromosomeband.Theyreportedthat24–99%ofthephenotypicvariabilityinbehavioralandacademic measures obtained in their subjects could be explained by theindividualgeneexpression levels.Dykensand Roof74 laterexamined andreportedbehaviorfindingsinamixedcohortofyoungandoldsubjectswithPWS and showed a relationship between their genetic subtypes and age.They found negative associations between age and behavior in the15q11–q13Type I deletionsubtypeonlywhich implicated nonimprintedgenesbetween breakpoints BP1 and BP2, specifically the CYFIP1 gene.DisturbedexpressionofCYFIP1isseeninotherdevelopmentaldisabilitiesincludingthosewith15qdisorderswithoutPWS.

Chai et al.75 showed that these four genes are highly conserved andbiallelicallyexpressed.NIPA1ornonimprintedinPrader–Willi/Angelmansyndrome 1 geneis the best studied gene and associated with autosomaldominanthereditaryspasticparaplegia.76NIPA1alsomediatesMg2+trans-portandis highlyexpressedinneuronaltissue.Jiangetal.77later reportedchildhoodabsenceepilepsywhenmutationswerefoundintheNIPA2gene.TheTUBGCP5geneortubulingammacomplexassociatedprotein5gene

isinvolvedinneurobehavioraldisordersincludingADHDandOCD[24]

Entire length of chromosome from PWS individual with loss of heterozygosity due

to uniparental maternal disomy 15 (meiosis II error)

Chromosome regions (DNA segments) with loss of heterozygosity (B)

60000kb 80000kb 100

Figure 4 (Continued)

CYFIP1orcytoplasmicfragileXmentalretardation1(FMR1)interactingprotein 1 gene interacts with FMRP in a ribonucleoprotein complex.FMRP is theproduct of the FMR1 gene whichis associated with FXS,the most common cause of familial intellectual disability that primarilyaffects males.40Hence,theimportanceofthesefourgenesincausationofneurologicaldevelopmentandfunctionsbutnotallindividualswithdefectswithinthe15q11.2band (ie, microdeletionsor microduplications)shareaclinicalphenotypeorareclinicallyaffected.Therefore,thisregioncontainsgeneticmaterialthatshowsincompleteorlowpenetrancealong withvar-iableexpressivity.Populationsurveysshowthatabout0.25%ofcontrolswillhave the 15q11.2 BP1–BP2 microdeletion with penetrance estimated at10.4%representingatwofoldincreaseoverthegeneralpopulation.78,79

In summary, studies of the 15q11.2 BP1–BP2 microdeletion or theBurnsideButlersyndromefoundthataffectedindividualswillshowdevel-opmentalandlanguagedelay,neurobehavioraldisturbances,andpsychiatricproblems that can vary from person to person However, this emergingsyndrome is nowrecognized with a prevalence ranging from 0.57–1.27%

ofpatientspresentingforhighresolutionmicroarrayanalysisaccountingforatwo- tofourfoldincreasecomparedwithcontrols.79Autismisreported inthisdisorderandthischromosomeanomalyisconsidered oneofthemorecommonfindingsinthosepresentfor microarrayanalysis.Seizures,schizo-phrenia,andmilddysmorphicfeaturesarelesscommonlyseeninthiscon-ditionbutareatrisk

ThesecondmostcommongeneticcauseofPWSismaternaldisomy15wherebothchromosome15scomefromthemotherandisfoundinabout25%ofaffectedindividuals.35Therearethreerecognizedformsofmaternaldisomy 15 These include maternal heterodisomy 15 with two differentchromosome15sfromthemotherduetoerrorsinthefirststageofmeiosis(meiosisI)fromnondisjunctionandwithoutcross-overeventsorshufflingofgenesfromthetwomaternal chromosome15s.Asecondformismaternalisodisomy15withtwoidenticalchromosome15sfromthemotherduetoerrorsintheseconddivisionofmeiosis(meiosisII)ortheequationalphaseduetonondisjunction.Thethirdformissegmentalmaternalisodisomy15withtwopartiallydifferentchromosome15sreceivedintheoffspringfromthemotherduetoerrors inmeiosisIfromnondisjunction withcross-overevents leading to segments of isodisomy or DNA sequence regions withidenticalgenealleles.HighresolutionCNV/SNPmicroarraystudieshaveshownthatifDNAsegmentsof10MbsizeareaorlargerareseenwithLOH

on a specific chromosome with a normal copy number,then uniparental

disomyispresent.Forexample,ifthisLOHoccursonchromosome15inthepresenceofDNAmethylationtestingshowingaPWSgeneticpattern,thenthisrepresentsmaternaldisomy15(Fig.4B).AnindividualwithPWSandmaternal disomy 15 can be at risk for a second genetic condition, if themotherisacarrierofanautosomalrecessivegenemutationonchromosome

15 for a disorder ifthe geneis located in theLOH or isodisomic regionwithidentical DNA sequencesrepresenting two copies of the samegeneallele.33,35

Maternaldisomy15isthoughttoarisefromanerroringametogenesisinthefemalewiththeeggcontainingtwochromosome15sfromthemotherand if fertilized bya normal sperm witha single chromosome 15 then atrisomy15zygoteresults.26,80Trisomy15islethalandarelativelycommoncauseofspontaneousabortions.Inaneventoftrisomy15rescue,theextrachromosome 15 is not passed in the nextcell division in the developingembryo.Thus,anormal46chromosomenumberisnowestablishedintheembryofromanabnormal47chromosomecountandleadstoviabilityofthefetus.Ifthefather’schromosome15islostthenthetworemainingchromo-some15sfromthemotherwillleadtomaternaldisomy15andthefetusisbornwithPWSduetohisgeneticsubtype

Another geneticphenomenoncanoccur infemales withPWSduetomaternaldisomy15whichinvolvestheXchromosome.FemaleshavetwoXchromosomes(onefromthefatherand onefromthemother)whilemaleshave only one X chromosome; however, the numberof active X-linkedgenesremainsconstantinbothsexesdueto genedosage compensationinfemaleswithorwithoutPWS.Femalesgenerallyinactivateoneoftheir XchromosomesatrandomwhichthenequalsthenumberofX-linkedgenesfoundinthemale.ThisprocessofXchromosomeinactivationoccursveryearlyinpregnancy Occasionally,this processis notrandom and skewnessoccurswhichcan allowfor expression ofX-linked conditionsinfemales.Hence,thetrisomy15rescueeventintheearlypregnancyofadevelopingfemalewithPWSandmaternaldisomy15mayallowforasmallnumberofcellstosurviveandtopopulateembryodevelopment.ThesesmallnumberofcellsrescuedbythetrisomyeventmayhavethesameXchromosomeactiveleadingtoX chromosomeinactivation skewness.Thisallowsfor thepres-enceofanX-linkedconditionifthemotherisacarrierofanX-linkedgeneandPWSduetomaternaldisomy15.35

ThethirdmajorcategoryofgeneticsubtypesinPWSis animprintingdefect.Thesedefectsmaybeduetomicrodeletionsoftheimprintingcenter

orduetoepimutationsthroughDNAmethylationerrorsingametogenesis

If the father carries an imprinting defect (microdeletion) that he inheritsfromhismother’schromosome15,heisunaffectedduetothepresenceofhisfather’s normalchromosome15.However, whenhepassestheimprintingdefectinhischromosome15ontohisoffspring,thatoffspringwillthenhavePWS.Theriskforhimtopassthedefecttohisoffspringwouldbe50%.37

Therearedozensofgenesandtranscriptslocatedinthe15q11-q13regionwithmostunderanimprintingcentercontrol.About10genesareimprintedandallbut2(UBE3AandATP10C)arepaternallyexpressedandregulated

byDNAmethylation(methylationequatestogeneinactivityandlation equates to gene activity) The maternally expressed UBE3A genecausesAngelmansyndrome.35,36

unmethy-TheSNRPN(smallnuclearribonucleoproteinN)andasecondproteincodingsequence(SNURF,orSNRPNupstreamreadingframe)arelocated

in the 15q11-q13 region Exons 4–10 of the complex bicistronicSNURF–SNRPNgeneencodeacorespliceosomalprotein(SmN)involved

inmRNAsplicinginthebrain,whereasexons1–3encodea71-amino-acidprotein enriched inarginine residues Adisruption of this complexlocuswillcauselossoffunctionofpaternallyexpressedgenesinthisregion,leading

to PWS.35,66 Multiple copies of noncoding C/D box snoRNAs orSNORDs involved in RNA processing are embedded within the longSNURF-SNRPN transcript These include SNORD64, SNORD107,SNORD108, SNORD109A, and SNORD109B (previously referred to

as HBII-13), HBII-436, HBII-437, HBII-438A,and HBII-438B, tively.DeletionsofothersnoRNAsalsolocatedinthesameregionhavebeenimplicatedincausingaPWSphenotype,81specificallySNORD115(HBII-52)andSNORD116(HBII-85).Otherimprintedgenesthatarenotcom-ponents oftheSNURF-SNRPN genecomplex locusand located proxi-mallyareMKRN3,MAGEL2,NDN,and C15orf2.Theyare involvedinbraindevelopmentandfunction.35,36

respec-Necdin(NDN)isapaternallyexpressedgenefromtheciatedprotein(MAGE) familyandrequiredforcellcycleproliferationanddifferentiation It is expressed in the hypothalamus, thalamus, and ponssuggestingaroleinbraindevelopmentandaxongrowth.Micelackingthisgeneshowdelayedmigrationofthesympatheticneurons,neonatallethality,and respiratory problems A second gene in this region is the MAGEL2gene whichis paternally expressed invarious brain regions including the

melanoma-asso-hypothalamus.Itappearstoplayaroleincircadianrhythm,brainstructure,behavior,andmaintenanceoffertilityrecentlyreportedtobeassociatedwithautism.82TheMKRN3geneisamemberofthemakorin(MKRN)RINGfingerproteingenefamilythatencodesaspecificgroupofproteins(makor-ins)andpresentinawidevarietyofeukaryotes.TheMKRN3isabundantlyexpressed (paternal only) in the developing brain and nervous system.Mutationsofthisgenehaverecentlybeenfoundinindividualswithpreco-ciouspuberty.83

Anotherclassofgeneslocatedinthedistalareaofthe15q11-q13regionare not imprinted and include three gamma aminobutyric acid (GABA)receptorsubunits(GABRB3,GABRA5,andGABRG3)withevidenceofunequal expression(paternalbias).66 Thedisturbancesof receptorsubunitgenesforGABA,amajorinhibitoryneurotransmitter,havebeenimplicated

in a number of symptoms associated with PWS including hunger,obsessive–compulsivedisorder, andaltered visualperception andmemory.ThePgeneencodesaproteinrequiredforpigmentproductionandislocated

inthedistalendofthechromosome15q11-q13regionandnotedto haveequalexpressionfromeachparentalallele.Mutationsofthisgeneareknown

tocauseoculocutaneousalbinismII.33,35,36

The UBE3A and ATP10C genes are located distal to the paternallyexpressedSNURF–SNRPNcomplexgenelocusandimprintedwithmater-nalexpressiononly.A maternal 15q11-q13deletionincludingthe UBE3AgeneormutationsofthisgenecausesAngelmansyndrome.Severaltranscripts

inthe15q11-q13regionarethoughttoreadinanantisensedirection,which

iscomplementarytoDNAsequencesofothergenesintheregion,butinareverse direction thereby impacting on the regulation or control of geneactivityincludingtheUBE3Aantisensetranscript

Butler et al.68 reported a submicroscopic deletion of the 15q11-q13regionapproximately100–200kbinsizeina5-year-oldfemalewithPWS

in 1996 as one of the first PWS subjects with an atypical small deletiondetectable only with molecular genetic or cytogenetic techniques andincludedtheimprintingcentercontrollingelementwithadjoininggeneticregionsnowrecognized asthesnoRNAs(SNORDs).Later,Sahooetal.81describedamalechildwithfeaturesofPWSandapaternaldeletioninvolvingtheSNORDs,particularly SNORD109A,the SNORD116genecluster,andpartofthesnoRNASNORD115cluster.DeSmithetal.84alsoreported

a19-year-oldmalewitha PWSphenotypeanda 187kbmicrodeletionofchromosome 15q11-q13encompassing SNORD116 Therefore,growingevidence including genetic data in mice support that paternal loss of

SNORD116 maycontribute to energy homeostasis,growth pattern, andreproduction;alldisturbedinPWS.Largerdeletionshavealsobeenreportedwhichincludethe15q14bandandinindividualswithfeaturesofPWSbutwithadditionalfindings.Theseincludehearingdeficits,cutaneouseartags,and congenital heart defects The expanded 15q11-q14 deletioncontainsabout60protein-coding genesand makingitlarger (abouttwice thesize)thanthetypical15q11-q13deletionseeninPWS.69

PWS is a complexgenomic imprinting disorder characterized bymental,behavior, and physicalfindingswith obesityas themost significant healthproblem Key neuroendocrine peptides produced by the gastrointestinalsystem are knownto play a role in feeding and eating behaviors thataredisturbedinPWS.85,86 Forexample, ghrelinstimulateseating andpeptide

YYinhibitseating,butwhendisturbedcanleadtoabnormaleatingpatternsand obesity Plasma ghrelin levels are elevated in PWS as noted duringinfancyandearlychildhood.71Thiselevationpotentiallycontributessignif-icantlytohyperphagiaseenasacardinalfeatureinPWSandusuallybeginsinearly childhood, if not controlled In addition, the noncoding snoRNASNORD115isthoughttoregulatealternativesplicingofthehumansero-tonin 5-HT2C receptor gene, an important receptor that contributes tonormaleatingbehaviorinhumans.87WhenthissnoRNAexpressionpattern

is disturbedthroughloss offunction(eg,deletedin PWS)then analteredreceptorresultsleadingtoexcessiveeatingbehaviorandobesity

Bittel et al.88 also reported on gene expression patterns inmales withPWSandfoundseveraldisturbedgeneswhencomparedwithcontrolmales.Several of these genes were involved in eating behavior and obesity(ADIPOR2,MC2R,SAG,HCRT,STAR,OXTR)andserotoninrecep-tors(eg, HTR2B).Hence,evidenceto datesupportsboth disturbedneu-ropeptideandgene/transcriptproductsplayingaroleinthePWSphenotyperequiringfurtherstudies

PWSisgenerallydividedintotwomajorstagesofclinicalcourseopment.Thefirststageischaracterizedbyinfantilehypotonia,temperatureinstability, a weak cry and poor suck, feeding difficulties, developmentaldelay,andhypogonadism/hypogenitalism.Thesecondstageoccursinearlychildhood(2–5 years of age) andis characterizedbydevelopmental delay,behavioralproblems(skinpicking,tantrums,obsessivecompulsions),speechdelay, an insatiable appetite, food seeking with rapid weight gain, and

devel-subsequentobesity, ifdietis notcontrolled There arenowseveral nizednutritionalphasesandsubphasesreferredtoas0,1a,1b,2a,3,and4.Theseoccurfromthetimeofpregnancywithdecreasedfetalmovements(ie,phase0)thenfollowedbyseverehypotoniaandfeedingdifficultiesininfancy.Transitions then occur to an increased weight phase with or withoutincreasedcaloricintakeandfollowedbyfoodseekingwithhyperphagiainearlychildhoodandprogressedintoadulthood(ie,phases3and4).89Shortstatureandsmallhands/feetalsooccurinPWSduetoGHdefi-ciency, along with rumination, sleep problems, physical inactivity, anddecreasedpainsensitivity.Hypopigmentation,hypogenitalism/hypogonad-ism,scoliosis,sleepapnea,enamelhypoplasia,anddecreasedsalivaaremoreapparentlaterinlifeandaccompaniedbyalmond-shapedpalpebralfissures,down-turnedcorners of themouth, a narrow bifrontal diameter, a shortnose,andasmallchin.34Severalofthesefeaturesmaybepresentininfancybut become more evident in childhood into adolescence and adulthood.Hypothalamic dysfunction is implicated in many manifestations of thisgeneticobesity-relatedsyndromewithdecreasedgrowthvelocity,endocrinedisturbancesaccompaniedbybehavioralandintellectualdisabilities(anaver-ageIQof65).

recog-Duetodevelopmentaldelay, PWSinfantssit independentlyby1year

ofage,crawlat16months,walkatabout2years,andtalk(10words)at39months but generallyhave a weak or absent cry with little spontaneousactivity.34,90This isduetodecreasedmuscletoneandstrength.Excessivesleepinesswithdiminished swallowingandsucking reflexes arecommonandoftennecessitategavagefeedingswithuseofspecialnipplesorgastro-stomy tube placement to address the feeding problems Growth para-metersshouldbeassessedregularlyusingrecentlyestablishedstandardizedgrowthchartsdevelopedforPWSinfants.91Caloriesareadjustedaccord-ingly,butfatsrequiredforbraingrowthshouldnot berestricted asPWSinfants usually require fewer calories than recommended to avoid rapidweightgain.Vitaminandmineralintakesuchascalciumshouldbeadded,

aswell ascaloricintakemonitored closelybya dietitian.Developmentalassessments, early stimulation programs, and occupational and physicaltherapy services are recommended.37 Endocrine disturbances includingthyroid, growth, and sexhormones and cortisol levels should bemoni-tored from infancy Myopia and impaired vision may be recognized inearlychildhood

ManychildrenwithPWSwillbemainstreamedintheschoolsetting,butspecialeducationandsupportservicesareoftenrequired.PWSchildrenwill

generally havestrengthsrelated toreading,and withvisualand long-termmemoryskills,butmayhaveweakermath,sequentialprocessing,andshort-term memory skills Verbalskills may be relative strengths particularly inthosewithPWShavingmaternaldisomy15.Unusualskillofworkingwithjigsawpuzzlesarealsonotedandmorecommoninthosewiththe15q11-q13deletion.37,92

Duringadolescence,hypogonadismandhypogenitalismbecomesmorepronouncedinthevastmajorityofindividualswithPWSduetohypotha-lamichypogonadismleadingtolowtestosteroneandestrogenlevels.About90% of males will have cryptorchidism along with a micropenis and anunderdevelopedscrotum.34A hypoplastic labia majoraand minora withasmallclitorisareoftenseeninPWSfemalesandpubertyisgenerallyabsentordelayed in both males and females Menarche in females may occur butdelayed untilabout30 years of age but inrare cases,pregnancies in PWSfemaleshavebeenreported.37PWSmalesareinfertile

About90%ofindividualswithPWSwithoutGHtreatmentwilldevelopshort stature byadulthood The averageadult male will have a height of

155cmandtheadultfemalewillbe147cmtall.34,37Smallhandsandfeetarecommon during adolescence and adulthood and often accompanied byscoliosisandkyphosis.Bracingorsurgerymayberequired.PWSadolescentsmayweighmorethan 300poundsifcaloric intakeisnotcontrolledand if

GHwasnotadministeredatayoungerage.Eatingrelateddeathsdooccurincluding choking on gorged food and gastric necrosis with rupture.93Therefore,toavoidthesecomplications,lockingtherefrigeratorand foodcabinetstopreventaccesstofoodandexcessiveeatingareoftenrequiredandaccompaniedbyclosesupervisionbyallcaregiversinsideandoutsideofthehome setting.94 Althoughpsychotropicdrugs areoftenprescribedtohelpcontrol behavior problems, no specific medication has been consistentlyeffective Rigorous control of the food environment which should besecuredisrecommendedtoavoidhyperphagiaandincluderegularexerciseprograms (about 30min/day) to manage weight and theexcessive eatingbehaviorandassociatedhealthcomplications.37,95Mildmentaldeficiencyiscommonfor thefamily backgroundandobsessivecompulsions,outbursts,andtempertantrumsmaybetriggeredbywithholdingfood.Self-injury(eg,skinpicking)isalsocommonbeginninginchildhoodwhichcorrelateswiththePWSgeneticsubtype,particularlythosewiththe15q11-q13deletion.71Behavioralproblemsmaybeginby3yearsofageandmanifestaspoorpeerrelationships,immaturity,andinappropriatesocialbehavior,whicharerem-iniscentofautism

As shortstatureis amain featureof PWSindividuals dueto GHciency,recombinanthumanGHwhichisFDAapprovedisnowprescribedforPWSbeginning ininfancyandthroughoutchildhoodtoachievemax-imumadultheightultimatelywithinthenormalrange.33,37Beneficialeffectsarenotedonstature,weight,andbodycomposition(increasedmuscle anddecreased fat mass) with GH treatment in PWS.96–98 Positive effects onmotordevelopment,strength,andcognitiveeffectshavealsobeenreported

defi-inbothchildrenandadultsduringGHtreatment.99Thequalityoflifeisalsoimproved in adults.100 The prevalence of scoliosis in PWS is high(30–80%),33,37 andthusa concerninthosetreatedwithGH andneedsto

be closely monitored Recommended starting GH dose in children is0.18–0.3mg/kgperweekgivenasdailysubcutaneousinjectionswithcarefulevaluationofclinicalstatus,boneagemeasures,surveillanceforscoliosis,andserum IGF-1 levels at regular intervals should be performed as well asmonitoringregularcaloricintakerecordsregularly.101Plottingtheindivid-ualgrowthparameters(height,weight,headcircumference)onsyndrome-specific standardized PWS growth charts is recommended for standard

ofcare.91,102

Weight control requires a strict dietary plan and coordination with adietitian Increased physical activity is also encouraged and recorded.Caloricintakeisrestrictedto6–8cal/cmofheightforweightlossbeginning

inearlychildhood andto 10–12 cal/cm ofheight to maintain weightfornongrowth hormone treated individuals with PWS particularly duringchildhood.Ageneralrecommendationfor dietaryintake forPWSadoles-centsoradultsmayinclude800cal/daytoloseweightor1000–1200cal/day

to maintain weight if foodseeking is controlled and exercise program inplace.95 Adiet planshouldbe adjustedtoincludecaloriesfrom proteinat30–35%;45%forcarbohydrates;andabout30%forfat.37Calcium,vitaminsupplements,andfishoilarerecommendedthroughoutlife

Deletions(TypeIandTypeII)inPrader–WilliSyndrome

Differences in cognitive, psychological, and behavioral findings in youngadultswithPWSwasfirstreportedbyButleretal.71in2004withthelonger15q11-q13TypeIdeletioninvolvingbreakpointsBP1andBP3versusthosewiththeshorterTypeIIdeletioninvolvingbreakpointsBP2andBP3.PWSindividualswiththelongerTypeIdeletionscoresignificantlyworseinself-injurious (skin picking) assessments and in maladaptive behavior findings

when compared to those with the smaller Type II deletion.Obsessive–compulsivebehaviorwasalsomorefrequentlyseeninPWSsub-jects with the Type I deletion Academic achievement scores did differbetweenPWSsubjectswiththeshorterTypeIIversuslongerTypeItypical15q11-q13 deletion Thisstudy supported thattheloss of genes betweenBP1andBP2whendeletedwouldincreasetheseverityofthebehavioralandpsychological problemsseen inPWS Forexample, theadaptivebehaviorscoresweregenerallyworseinPWSindividualshavingtheTypeIdeletion,suchasobsessive–compulsivebehaviors.ThosewithTypeIdeletiongener-allyhadpoorermathandreadingskills.Additionalproblemsincludedpoorervisual-motorintegration,worseadaptivebehavior,andmorecompulsionsinthosesubjectswithTypeIIdeletion,particularlyrelatingtogroomingandbathingskillsandcompulsionsthatdisrupteddailyliving.Intellectualabilityand academic achievement were also poorer in those PWS subjects withTypeIdeletionsaswellasvisualprocessing,comparedwiththosewithType

toaCGGtripletrepeatexpansiongreaterthan200insizeinthe5’lated region of the fragile X mental retardation 1 (FMR1) gene.40,43,104Thosewitha smaller repeatsize(50–200) wouldbeconsidered as carriers

untrans-orhavingapremutationformwhilethenormalnumberofCGGrepeatsislessthan50andusuallylessthan30.Theexpandedfullmutationusuallyleads

tomethylationandlittletonomRNAistranscribed;thus,resultinginalack

ofproductionofthefragileXmentalretardationprotein(FMRP)encoded

bytheFMR1gene.40,104LackofthisproteiniscorrelatedwithanincreasedchildhoodgrowthrateinFXSpatientsandsupportedbyevidencefromtheFXSknockout mousemodelshowing enhancedgrowthratefor themiceandobesity.Interestingly,about10%ofindividualswithFXSwillhavesevereobesity,hyperphagia,hypogonadism,ordelayedpubertyasseeninPWS.43