Sarcopenia Age-Related Muscle Wasting and Weakness: Mechanisms and Treatments P26 ppsx

While several studies have examined genetic variation in the ESR1 gene in relation to muscle strength measures, none have confirmed any association.. Vandevyver and colleagues 1999 exa

analysis revealed a significant association with handgrip strength that was completely explained by the rs1800169 A-allele, such that A/A individuals exhibited lower handgrip strength compared to G-allele carriers In a follow-up study, Roth et al (2008) examined multiple polymorphisms in the CNTFR gene in association with strength variables in 465 men and women (20–90 year) For the C174T polymor-phism, T-allele carriers exhibited significantly higher quadriceps and hamstrings concentric and eccentric isokinetic strength at both 30 and 180 deg/s compared

to C/C carriers, but these differences were not significant after adjustment for lower limb lean mass No differences were observed for polymorphisms in the promoter region or elsewhere in the gene De Mars and coworkers (2007)

exam-ined polymorphisms in both the CNTF and the CNTFR genes in 493 middle-aged

and older men and women with measures of knee flexor and extensor strength

T-allele carriers of the C-1703T polymorphism in CNTFR exhibited higher

strength levels for multiple measures compared to C/C homozygotes, including all knee flexor torque values In middle-aged women, A-allele carriers at the T1069A

locus in CNTFR exhibited lower concentric knee flexor isokinetic and isometric torque compared to T/T homozygotes The CNTF null allele was not associated with any strength measures, nor were any CNTF*CNTFR interactions observed These findings indicate the potential for significant influences of CNTF and

CNTFR gene variants on skeletal muscle strength, though inconsistencies have

been noted for CNTFR The frequency of the rare A/A genotype in CNTF is so low

that, despite some consistent findings of lower muscle strength, public health sig-nificance is uncertain, though clinical importance may be had for those particular individuals

Estrogen Receptor (ESR1) The estrogen receptor alpha is expressed in skeletal muscle, indicating a potential sensitivity to estrogen signaling (Wiik et al 2009)

While several studies have examined genetic variation in the ESR1 gene in relation

to muscle strength measures, none have confirmed any association Salmen et al (2002) examined 331 early postmenopausal women during a 5-year hormone

replacement therapy trial for associations with the ESR1 gene Neither baseline nor 5-year grip strength values were associated with ESR1 genotype Vandevyver and

colleagues (1999) examined 313 postmenopausal Caucasian women with measures

of grip and quadriceps strength and reported no associations with ESR1 genotype

Grundberg et al (2005) reported no association between a TA-repeat

polymor-phism in the ESR1 gene and several muscle strength measures in 175 Swedish

women (20–39 year) Ronkainen and co-workers (2008) examined ESR1 genotype

in 434 older women (63–76 year) and found no significant association with hand grip or knee extension strength or leg extension power

Insulin-like Growth Factor 2 (IGF2) Two studies have examined the IGF2

gene in relation to strength phenotypes Sayer et al (2002) performed grip strength

analysis in 693 older men and women and examined association with the IGF2 ApaI polymorphism IGF2 genotype was associated with grip strength in men but

not women, with G/G genotype having lower strength compared to A/A genotype carriers Interestingly, an independent but additive effect of birth weight on grip strength values was also noted in men Schrager and colleagues (2004) examined

the same ApaI polymorphism in relation to muscle strength and power phenotypes

in 485 men and women They reported significantly lower arm and leg isokinetic strength measures in A/A women compared to G/G women, differences that were

not observed in men IGF2 is imprinted in mammals such that only the paternal

allele is transcribed (Zemel et al 1992), thus analyses in these studies focused on comparing homozygote groups rather than heterozygotes The results of these

stud-ies stand in direct contrast to each other, and indicate that any influence of IGF2

genotype on strength-related traits is going to be minor or the result of interaction with other yet-to-be identified factors

Myostatin-Related Genes After myostatin’s discovery in the late 1990s, it emerged as a potential target of gene association studies and multiple

investigations reported associations with skeletal muscle strength, but the sample sizes were very small owing in part to low allele frequencies of the common poly-morphisms Seibert et al (2001) reported lower strength in older African American

women (70–79 year) with the R-allele compared to K/K genotype at the MSTN K153R polymorphism, but the sample size was quite low (n = 55) Corsi et al

(2002) reported lower isometric muscle strength (averaged across eight muscle groups) in R-allele carriers of the K153R polymorphism in 450 older men and women Though consistent with the findings of Seibert (2001), the sample size of R-allele carriers was only seven making the findings inconclusive Because the common polymorphisms have rare allele frequencies, the clinical significance of

within the myostatin signaling pathway, including the myostatin receptor

(activin-type II receptor B; ACVR2B) and follistatin (FST), a myostatin inhibitor Walsh

et al (2007) examined the genetic association of ACVR2B and FST with muscle strength in 593 men and women across the adult age span In women but not men,

ACVR2B haplotype was significantly associated with knee extensor concentric peak

torque FST haplotype was not associated with muscle strength Kostek et al (2005) reported significant associations with the MSTN gene in 23 African Americans for biceps isometric strength The FST gene was also associated with baseline

one-repetition maximum strength levels Again, the sample sizes of the genotype groups with significant findings were small making the clinical relevance of these findings uncertain but generally not compelling

Vitamin D Receptor (VDR) Vitamin D deficiency has been consistently associated with lower muscle strength (Ceglia 2008) and has been discussed as

a potential mechanism of sarcopenia (Montero-Odasso and Duque 2005) In one

of the first gene associations for skeletal muscle traits, Geusens et al (1997)

demonstrated a significant relationship between the VDR BsmI polymorphism

and both isometric quadriceps and hand grip strength in 501 elderly, healthy women, with 23% higher quadriceps strength and 7% higher grip strength in the b/b compared to B/B genotype carriers These findings were subsequently sup-ported in a subgroup of these same women (Vandevyver et al 1999) In contrast, Grundberg et al (2005) examined two polymorphisms (poly A repeat and BsmI)

within VDR in relation to muscle strength in 175 women aged 20–39 year

They found greater hamstrings isokinetic muscle strength in women homozygous for the shorter poly A repeat (ss) compared to women homozygous for the long poly A repeat (LL) No associations were reported with quadriceps or grip strength Similar findings were reported for the BsmI variant (b and B alleles) given the significant linkage disequilibrium between the s and B alleles Thus, the B/B genotype group exhibited higher hamstrings strength in contrast to the Geusens et al findings Roth and colleagues (2008) reported significant

associa-tions with the VDR FokI polymorphism (f and F alleles) and knee extensor

iso-metric strength in 302 older Caucasian men (f/f higher than F/F), but these associations were no longer significant once leg FFM was accounted for in the models, suggesting that the genotype-strength associations were explained by differences in muscle mass Wang et al (2006) examined the ApaI, BsmI, and

TaqI VDR polymorphisms in 109 young Chinese women in relation to knee and

elbow torque measures At the ApaI locus, A/A women exhibited lower elbow flexor concentric peak torque and lower knee extensor eccentric peak torque compared to either A/a or a/a carriers For the BsmI locus, the b/b carriers dem-onstrated lower knee flexor concentric peak torque than the B-allele carriers No associations were observed for the TaqI locus Windelinckx and colleagues (2007) examined the BsmI, TaqI, and FokI VDR polymorphisms in 493 middle-aged and older men and women for association with various muscle strength phenotypes, with BsmI and TaqI combined in a haplotype analysis In women, the FokI polymorphism was associated with quadriceps isometric and concentric strength, with higher levels in f/f homozygotes compared to F-allele carriers In men, the BsmI/TaqI haplotype was associated with quadriceps isometric strength with Bt/Bt homozygotes exhibiting greater strength than bT haplotype carriers

In a study involving 107 COPD patients and 104 healthy controls, Hopkinson

et al (2006) reported Fok1 F/F carriers had lower quadriceps isometric strength than f-allele carriers The b-allele of the Bsm1 polymorphism was associated with greater strength compared to B-allele carriers in COPD patients but not in controls In summary, VDR genetic variation has been associated with muscle strength variables in numerous studies, though inconsistencies have been noted Studies having examined the BsmI locus are mixed with regard to their findings and future studies need to incorporate the haplotype of BsmI and TaqI rather

than looking at either site independently The VDR FokI site is considered

strength in f/f compared to F/F carriers, so this site should be investigated more thoroughly for possible clinical significance

In summary, several genes have been associated with skeletal muscle strength phenotypes in multiple studies While none of these genes can yet be tagged as conclusively contributing to inter-individual variation in strength phenotypes, their consistency across multiple studies is encouraging These genes will require additional validation and clarification as to their specific roles in modifying strength-related traits, with the eventual goal to determine their clinical importance

to sarcopenia

4.2 Genetic Variation and Skeletal Muscle Mass

Table 3 summarizes the genes that have been studied in relation to skeletal muscle mass measurements, focusing on genes associated with baseline muscle mass values; genes related to muscle mass adaptation to exercise training are discussed

in a later section Genes that have been studied in only one paper or that have not

been replicated in some way and are not discussed here include: MTHFR (Liu et al

2008b); CNTF and CNTFR (Roth et al 2000, 2008); COL1A1 (Van Pottelbergh

et al 2001); TNF (Liu et al 2008a); IL15 and IL15RA (Pistilli et al 2008); COMT

2008); and IGF2 (Schrager et al 2004)

Angiotensin Converting Enzyme (ACE) The majority of papers examining the

mass phenotypes, though some studies have examined both Most have shown no

Charbonneau et al (2008) reported higher quadriceps muscle volume in D/D com-pared to I/I carriers in a study of 225 older men and women (50–85 year) Thus, it

appears unlikely that ACE genotype contributes significantly to muscle mass

phe-notypes, which is similar to the conclusion for muscle strength traits

Alpha Actinin 3 (ACTN3) As discussed above, several studies have examined

the potential for the ACTN3 R577X polymorphism to explain variability in muscle

strength measures Many of those same papers have also examined muscle mass variables, though the results are less consistent Vincent and colleagues (2007) did not observe any genotype difference in FFM determined by bioelectrical imped-ance in their study of 90 young men Norman et al (2009) reported no significant genotype associations with FFM determined by skinfold measurements in 120 young men and women Delmonico et al (2008) reported no significant genotype associations with DXA-measured FFM in their study of 1,367 older adults (70–79 year) Walsh et al (2008) examined 848 adult men and women (22–90 year) and found that X/X women displayed lower levels of both total body FFM and lower limb FFM compared with R/X + R/R women Concomitant differences were noted for muscle strength that were explained by the FFM differences, as discussed in the previous section No genotype-related differences were observed in men Thus,

mass and the ACTN3 null allele, indicating at best a minor role for this

polymor-phism in explaining inter-individual variability in this trait

Androgen Receptor (AR) Walsh and colleagues (2005) examined the

associa-tion between the AR CAG-repeat polymorphism with muscle strength and mass

variables in two cohorts of older men and women Though they found no

associa-tion between muscle strength and AR genotype, significant genotype associaassocia-tions

with FFM were observed in the men of both cohorts The androgen receptor is a nuclear transcription factor, for which testosterone is an important ligand The

CAG-repeat sequence in exon 1 of the AR gene appears to modulate receptor

tran-scriptional activity (Chamberlain et al 1994) Subjects were grouped according to

Table

the length of the CAG repeat, with subjects grouped for short and long fragments Men in both cohorts with the long fragment lengths demonstrated significantly greater appendicular skeletal muscle mass and higher relative total lean mass The results could not be explained by genotype-based differences in either bioavailable

or total testosterone Additional work is required to determine the extent to which

the AR CAG-repeat polymorphism contributes to muscle mass variation, though

these consistent findings in two cohorts is encouraging

Myostatin-Related Genes Despite the strong physiological evidence behind

myostatin as a candidate gene for muscle mass traits, genetic variation in the MSTN

2005) Kostek et al (2009) did report strength differences for MSTN in a small number of African American subjects, as noted above Two studies have examined myostatin-related genes in relation to muscle mass phenotypes In 593 men and women across the adult age span, Walsh et al (2007) reported significant

associa-tions between follistatin (FST) haplotype and leg FFM in men but not women, but

no association with FFM and haplotype structure in the myostatin receptor,

ACVR2B Strength differences were discussed in the previous section Kostek et al (2005) also examined the FST gene and found that African Americans carriers of

the FST T-allele had greater biceps CSA than A/A genotype carriers for the

A-5003T polymorphism, but sample sizes were small There is little compelling

evidence that MSTN or myostatin-related genes are major contributors to skeletal

muscle mass, though minor contributions are indicated

Thyrotropin-Releasing Hormone Receptor (TRHR) As described above, Liu and colleagues (2008a) identified TRHR as a potential candidate gene for skel-etal muscle mass from the first genome-wide association study for this trait After the initial genome-wide analysis that identified two polymorphisms in the

consisting of over 6,000 total white and Chinese subjects and consistent signifi-cant associations with LBM were observed in those analyses Importantly,

inter-actions between TRHR and genes in the growth hormone/insulin-like growth

factor (GH/IGF1) pathway were explored and tentative connections were

indi-cated Though only a single paper, the multiple replications pointing to TRHR

provide strength for this as a potentially important candidate gene for muscle mass variation

Vitamin D Receptor (VDR) VDR genetic variation has been studied fairly

extensively for muscle strength phenotypes, as described above, but fewer studies have focused on skeletal muscle mass Van Pottelbergh and colleagues (2001) reported associations between the TaqI (T and t alleles)/ApaI (A and a alleles) haplotypes and lean mass in 271 older men (>70 year) The highest lean mass was observed in the At-At haplotype group, which differed most from haplotypes con-taining T-allele homozygosity (e.g., aT-aT, AT-aT, and AT-AT haplotypes) This relationship was not observed, however, in a group of younger men from the same study Roth et al (2008) reported significant associations with the VDR FokI poly-morphism (f and F alleles) and leg FFM in 302 older Caucasian men, with con-comitant differences in muscle strength as noted above No significant differences

were associated with the VDR BsmI site This study is described in more detail in

the section on genes specifically associated with sarcopenia Thus, only two studies

have examined VDR genotype in relation to skeletal muscle mass phenotypes, but

the results provide some evidence for positive association

In summary, remarkably few studies have provided evidence of genetic associa-tion of specific candidate genes with muscle mass phenotypes despite the strong heritability of the trait The strongest findings are perhaps those with the least

evi-dence, as TRHR and AR have at least been replicated, but only one research group

has contributed to each of those studies Presumably the advent of genome-wide association studies will provide a greater push for identifying potential candidate genes with relevance to skeletal muscle mass

4.3 Genetic Variation and Sarcopenia

While a number of studies have addressed specific genes and genetic variants in relation to skeletal muscle strength and mass phenotypes, only one study to date has specifically targeted a measure of sarcopenia per se Roth and colleagues (2004)

analyzed the influence of the VDR BsmI and FokI variants on muscle strength and

mass in a cohort of 302 older (58–93 year) Caucasian men with measures of FFM

by DXA VDR FokI genotype was significantly associated with total lean mass, appendicular lean mass, and normalized appendicular lean mass (all P < 0.05), with

the F/F group demonstrating significantly lower mass than the F/f and f/f groups

In addition, the group categorized the men as normal or sarcopenic based on the definition of Baumgartner et al (1998), which relies on a cutoff value based on

significant 2-fold higher risk for sarcopenia in VDR Fok I F/F homozygotes than carriers of the f-allele (OR = 2.17; 95%CI = 1.19–3.85; P = 0.03) Quadriceps

mus-cle strength was also significantly lower in the F/F group compared to the F/f and f/f groups, but this association was eliminated when the analysis controlled for dif-ferences in total body lean mass No significant difdif-ferences were associated with

the VDR BsmI site Thus, VDR FokI genotype was significantly associated with

lean mass and sarcopenia in this cohort of older Caucasian men, with concomitant differences in muscle strength Vitamin D deficiency has been consistently associ-ated with lower muscle strength (Ceglia 2008), and appears to be relassoci-ated to type II fiber atrophy (Pfeifer et al 2002), thus making it an important potential mechanism

in the etiology of sarcopenia in some individuals (Montero-Odasso and Duque 2005) The FokI polymorphism in the VDR gene affects the translational start site

of the gene (Arai et al 1997; Jurutka et al 2000) thus making it a potentially

func-tional polymorphism, though other variants in the VDR gene may interact in a more

remains to be done to take the many genes outlined above and address the clinical relevance of sarcopenia in particular

4.4 Genetic Variation and Skeletal Muscle Adaptation

to Training

Though not an emphasis of this chapter, several studies have examined the role of genetic variation in the adaptation of skeletal muscle to exercise training, especially strength or resistance training The adaptation of skeletal muscle to strength train-ing is a heritable trait in itself (Thomis et al 1998b) and linkage studies have been successfully performed using such traits as outcome variables, as described above (Chagnon et al 2000; Sun et al 1999) Moreover, specific genes have been studied and specific gene variants identified as being potentially important to skeletal muscle adaptation The bulk of these studies have been described most recently in the updated Human Gene Map for Performance and Health-Related Fitness Phenotypes (Bray et al 2009)

Genetic variation important to skeletal muscle adaptation has relevance for sar-copenia in multiple contexts First, the identification of particular genes that con-tribute to inter-individual variation in skeletal muscle adaptation provide insights into the basic biology of skeletal muscle, which could be exploited in multiple ways

to facilitate new or improved intervention techniques for muscle disorders and sar-copenia in particular Second, the possibility exists that the same gene variants important to skeletal muscle adaptation could also be important to skeletal muscle development and thus baseline phenotypes, though the case can equally be made that different genetic contributions can be expected for these two different traits Finally, because exercise training in general and strength training in particular are considered some of the most important interventions for the prevention and treat-ment of sarcopenia (Roth et al 2000), understanding the genetic contributions to muscle adaptation, especially in older men and women, will allow improved appli-cation of such interventions via genetic screening

A number of genes have been identified as potentially important for skeletal muscle adaptation, though arguably none have emerged as clinically meaningful as

of this writing Similar to the situation with baseline skeletal muscle phenotypes, the bulk of these genes remain unreplicated or replicated across different training stimuli or measurement methods, making traditional genetic replication analysis challenging In fact, the variations on exercise training interventions are arguably more numerous than those related to measurement of skeletal muscle strength, and variations on both of these are often seen across different gene association studies related to muscle strength adaptation Genes studied in relation to skeletal muscle

adaptation include: PPARD with muscle volume response to lifestyle intervention

strength and volume responses to strength training (Kostek et al 2005, Hand et al 2007); RST with upper arm muscle strength and muscle CSA responses to strength

ACTN3 , and MYLK in different studies with muscle damage in response to a

muscle strength and mass responses to various exercise training protocols (Folland

Pescatello et al 2006; Frederiksen et al 2003); IL15 and IL15RA (IL-15 receptor) with muscle strength and size responses to strength training (Riechman et al 2004, Pistilli et al 2008); MSTN and FST polymorphisms with muscle strength and size traits in response to strength training (Thomis et al 1998b; Kostek et al 2009; Ivey

et al 2000); ACTN3 with muscle strength and size responses to strength training

response to strength training (Devaney et al 2009)

5 Conclusions and Future Directions

Despite remarkably high heritability values, only modest progress has been made

in identifying the specific genetic contributors to skeletal muscle strength and mass phenotypes Only seven genes have been positively associated with strength-related

within the replication analyses Similarly, only four such genes have been identified for muscle mass and two of those genes were internally replicated rather than being confirmed in a second paper (Table 3) No genes have been replicated for

associa-tion with sarcopenia per se, though VDR has been associated with sarcopenia in one

study and associated with muscle mass and strength phenotypes in multiple studies

Not only have few genes been identified, but their contribution to genetic variation is also generally quite small None of the genes identified in the present chapter have been shown to conclusively contribute more than 5% of the inter-individual variation to their respective traits, and most are on the order of 1–3% These results mirror what has recently been found for other highly heritable traits: genome-wide association studies are finding genes with relatively small influence that in no way explain the overall genetic influence predicted by

genome-wide association studies and most genetic association studies to date in that these have focused almost exclusively on single nucleotide polymorphisms, which though important are not the only DNA-related components that contribute

to genetic influence In addition to typical polymorphisms, copy number variation (CNV; multiple copies of the same gene), epistasis (multiple genes coordinated

in a pathway), complex gene*environment interactions, and epigenetic factors are also contributing to the genetic component of inter-individual variability (Altshuler et al 2008) and these more complex phenomena are just beginning to

be studied in large-scale investigations

An important contributor to inter-individual variation in age-related muscle traits will likely be epigenetic factors, which have already been shown to be impor-tant to aging tissues in general (Kahn and Fraga 2009) Epigenetics generally refers

to chemical modifiers to DNA and histone proteins that alter DNA regulation without a direct change to the DNA sequence itself with consequences for normal development and disease risk (Hirst and Marra 2009) DNA methylation has been shown to decline with aging in several species including humans (Bollati et al 2009) and DNA methylation has important consequences for gene expression Importantly, modification of epigenetic factors appears to be related to environmen-tal conditions (Foley et al 2009; Baccarelli et al 2009) So, both age and environ-ment are likely to contribute to epigenetic changes in skeletal muscle tissue that will alter gene regulation and contribute to age-related losses in strength and mass, thus affecting physical function How environmental conditions will alter epigenetic factors in a way meaningful for skeletal muscle traits and sarcopenia risk is as yet unclear, but certainly this represents another avenue of exploration for future studies

An underlying theme when considering the genetic aspects of skeletal muscle traits generally and sarcopenia in particular is that of a “threshold” level for these traits below which physical function (e.g., activities of daily living) is impaired Once a person’s strength falls below a certain threshold, physical function becomes impaired Such a threshold would surely be defined differently for each individual, but within reason we can expect clinically meaningful thresholds to be established across various physical characteristics, especially sex, age, height, weight, and body composition This threshold concept has been discussed by a number of groups (Ferrucci et al 1997; Walston and Fried 1999; Visser et al 2005; McNeil

et al 2005)

Because genetic variation (including epigenetics) will tend to have subtle influences on skeletal muscle and sarcopenia-related traits, the general hypoth-esis is that genetic variation will tend to push trait values closer to or farther away from this threshold, thus altering an individual’s risk for impaired physical function Thus, identifying individuals with genetic susceptibility to lower levels

of skeletal muscle strength or mass who are closer to their likely threshold for physical limitation will allow for early, targeted interventions to help prevent early losses This is the concept behind personalized or genetic medicine Early identification for individuals genetically susceptible to sarcopenia could result

in a dramatic improvement in health care costs, by introducing interventions prior to the onset of associated infirmities Of course, finding these genes and developing the individualized interventions will take many years if the last decade provides any clue to future progress One potential approach to speed discovery will be to examine genes related to bone structure and mass, which may have a pleiotropic influence on skeletal muscle traits (Karasik and Kiel 2008) The development of more sophisticated genome-wide association studies that include copy number variants may also aid in this search Even if genes of only minor effect are identified that don’t lend themselves to genetic screening and personalized medicine, those genes will point to potential physiological pathways that can be manipulated through more typical means and lend insight into the underlying etiology of sarcopenia in different individuals (Khoury et al 2007; Burke 2003)