Báo cáo khoa học: BRCA1 16 years later: risk-associated BRCA1 mutations and their functional implications pptx

Although the loss of wild-type BRCA1 function is an important mechanism by which mutations confer increased cancer risk, multiple studies suggest mutant BRCA1 proteins may confer functio

BRCA1 16 years later: risk-associated BRCA1 mutations and their functional implications

Rebecca J Linger1and Patricia A Kruk1,2

1 Department of Pathology and Cell Biology, University of South Florida, Tampa, FL, USA

2 H Lee Moffitt Cancer Center, Tampa, FL, USA

Introduction

Family history is the strongest risk factor for the

development of ovarian cancer and a major risk factor

for the development of breast cancer [1]

Understand-ing how risk-associated mutations contribute to cancer

initiation and progression will provide insight into

molecular mechanisms and aid in better risk

assess-ment, prophylaxis and treatment for carriers The

majority of hereditary ovarian cancers and a significant

proportion of hereditary breast cancers are associated

with mutation of the breast cancer susceptibility gene 1

(BRCA1) [1,2] The objective of this review is to

pro-vide a brief consideration of the normal functions

associated with BRCA1, followed by a discussion of

the types of risk-associated BRCA1 mutation and their

molecular and cellular impact Lastly, we will consider

the clinical implications of these mutations for breast and ovarian cancer patients

BRCA1

The predominantly nuclear BRCA1 protein, which shuttles between the nuclear and cytoplasmic compart-ments, has multiple functions in the cell [3,4] BRCA1 plays an important role in the DNA damage response,

as evidenced by the fact that BRCA1 null mice die early in embryonic development and exhibit chromo-somal aberrations that are exacerbated by a p53 muta-tion [5] (see also [6–8]) BRCA1’s expression and phosphorylation are cyclic, and BRCA1 plays a role

in the cell cycle as well, by regulating key cell cycle

Keywords

BRCA1; breast cancer; mutation;

ovarian cancer; risk

Correspondence

P A Kruk, Department of Pathology and

Cell Biology, MDC 11, University of South

Florida, 12901 Bruce B Downs Blvd,

Tampa, FL 33612, USA

Fax: +813 974 5536

Tel: +813 974 0548

E-mail: pkruk@health.usf.edu

(Received 26 January 2010, revised 27 April

2010, accepted 4 June 2010)

doi:10.1111/j.1742-4658.2010.07735.x

Mutations in the tumor suppressor breast cancer susceptibility gene 1 (BRCA1), an important player in the DNA damage response, apoptosis, cell cycle regulation and transcription, confer a significantly elevated life-time risk for breast and ovarian cancer Although the loss of wild-type BRCA1 function is an important mechanism by which mutations confer increased cancer risk, multiple studies suggest mutant BRCA1 proteins may confer functions independent of the loss of wild-type BRCA1 through dominant negative inhibition of remaining wild-type BRCA1, or through novel interactions and pathways These functions impact various cellular processes and have the potential to significantly influence cancer initiation and progression In this review, we discuss the functional classifications of risk-associated BRCA1 mutations and their molecular, cellular and clinical impact for mutation carriers

Abbreviations

BARD1, BRCA1-associated RING domain protein 1; BRAT, BRCA1 185delAG truncation; BRCA1, breast cancer susceptibility gene 1; BRCT, BRCA1 C-terminus.

controllers, including p21, and by physically

interact-ing with cell cycle regulators (reviewed in [9]) BRCA1

can also recruit chromatin modifying proteins, such

as histone acetyltransferases and histone deacetylases,

and directly interact with other transcription factors

to alter their function (reviewed in [9]) For example,

BRCA1 binds and modulates phosphorylation of

p53 to enhance its transactivation function [10,11]

Lastly, BRCA1 is capable of ubiquitin ligase activity

when heterodimerized with BRCA1-associated RING

domain protein 1 (BARD1) [12] The loss of these

cellular functions of BRCA1 may contribute to cancer

by promoting genomic instability and accumulation

of cancer-causing mutations [6], a process further

accelerated by p53 mutation, a common characteristic

of BRCA1 mutant ovarian cancers [13] BRCA1 mutation carriers have a 30% risk of developing ovar-ian cancer during their lifetime [14] and a 50–80% risk of developing breast cancer before the age of

70 years [6]

Types of BRCA1 mutation

All types of BRCA1 mutation have been reported, including frameshift, nonsense, missense, in-frame insertions and deletions, splice altering mutations, mutations in the untranslated regions, as well as silent mutations The majority of risk-associated mutations are frameshift or nonsense mutations that result in a premature stop codon and truncated protein product

BRCA1

DNA damage

response

Chemosensitivity Apoptosis Proliferation Transcription/gene Tumorigenesis

regulation

Transactivation

B B

domain

185delAG 5382InsC N-terminal 602aa*

N-term 302 aa*

N-term 771 aa*

185delAG

5382InsC

5677InsA

ΔN

aa303-1863*

185delAG

185delAG M1775K P1749R Y1853STOP Q1756InsC Δ500-1863*

Δ1314-1863*

ΔNLS*

ΔNLS/C+NLS*

Δ515-1091*

Δ BamH1 N-terminal 1313aa*

Δ Kpn1 N-terminal 771aa*

Δ EcoR1 N-terminal 302aa*

Δ500-1863*

5083del19 Δ1808-5556*

Ser1841Asn

5382InsC

M1775K

P1749R

C64G

T826K

M1775R

ΔN aa303-1863*

* Denotes synthetic mutation

1835STOP 340STOP Δ343-1081*

Δ 515-1092*

5677InsA ΔEcoR1 N-term 302aa*

CT-BRCA1 aa1293-1863*

N-terminal 602aa*

Δ11 splice variant

ΔN aa303-1863*

1835STOP 340STOP Δ343-1081*

Δ 515-1092*

Δ 542*

BRCA1 tr/tr aa1-900*

N-terminal 602aa*

ΔRING splice variant*

trBRCA1 (N-term 300aa)* Δ11 splice variant W1777Stop*

ΔRING splice variant*

Development

Q1756InsC

Y1853STOP

M1775K/R

P1749R

C64G

T826K

1835STOP 340STOP

Ser1841Asn

5083del19

B

A

Fig 1 BRCA1 mutations and their cellular and physiological impact (A) Domain structure of BRCA1 protein and the location of risk-associ-ated mutations discussed NES, nuclear export signal; NLS, nuclear localization signal (B) BRCA1 mutations categorized by cellular pro-cesses in which each has been found to lack function or exhibit function different from the wild-type The nomenclature used for each mutation was that used in the original research article, or a structural description if designation was not descriptive of the mutation or mutant structure.

(NIH Breast Cancer Information Core Database,

http://research.nhgri.nih.gov/bic/) Risk-associated

trun-cation mutations are found throughout the entire

BRCA1 coding sequence (Fig 1) and result in mutant

proteins that vary in length and structural impairment

For example, the nonsense mutation Y1853X, which

lacks the last 11 amino acids, is only missing a small

portion of the second BRCT (BRCA1 C-terminus)

repeat, whereas the 39 amino acid 185delAG mutant

lacks all of BRCA1’s known functional domains

A smaller percentage of risk-associated BRCA1

mutations are point mutations classified as missense

mutations Like truncation mutations, missense

muta-tions occur throughout the entire BRCA1 coding

sequence (Fig 1) [15], although it is difficult to

deter-mine the clinical importance of these mutations

because of their rarity and because they do not often

result in gross structural or functional loss Therefore,

many missense mutations remain ‘variants of unknown

significance’ [16] The functional significance of the

RING and BRCT domains, as well as the substantial

conservation of their sequences, fuel speculation that

many missense mutations in these areas are probably

linked to cancer predisposition Nonetheless, several

missense mutations have already been linked to breast

and⁄ or ovarian cancer predisposition, including C61G,

M1775K and P1749R

BRCA1 is thought to act as a classical tumor

sup-pressor and the loss of BRCA1’s cellular functions is

thought to occur through bi-allelic inactivation

Carri-ers of mutations have one germline hit (the inherited

mutated copy of BRCA1) and, in the tumor, a second

somatic hit usually through the loss of heterozygosity

[6] The observed phenotype of enhanced breast and

ovarian cancer risk is generally thought to result from

the loss of some or all wild-type functions of the

BRCA1 gene product

However, countless studies have revealed the

com-plexities of signaling molecule and transcription factor

interactions, as well as cellular adaptations in response

to the unique selective pressures of tumor initiation

and progression Therefore, it is important to

investi-gate all possible molecular mechanisms by which a

mutation may contribute to the disease phenotype

Mutant proteins may antagonize wild-type proteins in

a dominant negative manner, resulting in the loss of

remaining wild-type function [17], or they may engage

in unique molecular interactions and manifest novel

functions independent of the loss of wild-type protein

function [18] Likewise, BRCA1 mutations may

con-tribute to cancer risk through the loss of wild-type

BRCA1 function or through the gain of function

asso-ciated with mutant BRCA1 proteins

Loss of function mutations

As mentioned previously, several lines of evidence sug-gest the loss of wild-type BRCA1 function as a com-mon mechanism for enhanced breast and ovarian cancer risk (Table 1) Similar to BRCA1 knockout mice and cell lines, elevated levels of aneuploidy and loss of heterozygosity indicative of an impaired DNA damage response have been noted in breast cancer tis-sue from mutation carriers compared with control breast cancers, as well as in the human BRCA1 trun-cated breast cancer cell line, HCC1937 (reviewed in [6]) In structural protein studies, Tischkowitz et al [19] suggested that structural alterations in the BRCT phosphopeptide-binding pocket caused by the BRCA1 M1775K missense mutation contributed to enhanced breast and ovarian cancer risk through diminished transactivation and binding to other DNA damage response proteins Likewise, Williams et al [20] found that decreased stability of BRCA1 missense and trun-cation mutants resulting from aberrant protein folding contributed to the loss of BRCA1 function and enhanced cancer risk

Expression of mutant BRCA1 constructs in the absence of wild-type BRCA1 frequently fails to restore wild-type BRCA1 function Scully et al [21] utilized the c radiation-sensitive HCC1937 breast cancer cell line, which lacks wild-type BRCA1 and carries two 5382InsC BRCA1 alleles that code for a frameshift and premature stop signal at codon 1829, and were able to decrease c radiation sensitivity with restoration

of wild-type BRCA1 However, transfection of several BRCA1 mutants into these cells failed to alter radia-tion sensitivity In agreement, the addiradia-tion of wild-type BRCA1 expression into breast cancer cell lines that exhibit low wild-type BRCA1 expression due to the presence of a single wild-type BRCA1 allele inhibited growth However, expression of the risk-associated truncation mutants 1835STOP and 340STOP, as well

as the synthetic internal deletion mutants D343-1081 and D 515-1092, failed to alter cell growth, tumor for-mation and tumor progression in nude mice [22] Lastly, introduction of wild-type BRCA1 into HCC1937 breast cancer cells and IGROV 1 ovarian cancer cells inhibited tumor initiation and growth, whereas a synthetic BRCA1 mutant lacking the last

542 amino acids did not [23] Interestingly, Cousineau

& Belmaaza [24] hypothesized that reduced gene dos-age of wild-type BRCA1 in mutation carriers is solely responsible for altered DNA damage repair, subse-quent mutation accumulation and increased cancer risk Using MCF7 breast cancer cells that harbor a single copy of wild-type BRCA1 and exhibit enhanced

Proliferation, chemosensitivity, tumorigenesis

acids Synthetic

Proliferation, chemosensitivity

acids Truncated:

spontaneous recombination or ‘hyper-recombination’,

they showed that transfection of MCF7 cells with

wild-type BRCA1 diminished hyper-recombination and

chemosensitivity, whereas addition of the 5382InsC

BRCA1 mutation affected neither endpoint These

studies further support a role for the loss of wild-type

BRCA1 function as a contributing factor to enhanced

breast and ovarian cancer risk

It is important to note that many of the

aforemen-tioned studies attempted to delineate BRCA1 mutant

function in model systems lacking normal levels of

wild-type BRCA1, which makes it difficult to

discrimi-nate between the contribution of BRCA1 mutants and

the loss of wild-type BRCA1 to disease risk However,

several studies utilizing a wild-type BRCA1

back-ground clearly support the loss of BRCA1 wild-type

function for cancer risk For example, although the

overexpression of type BRCA1 in several

wild-type BRCA1 cancer cell lines and COS cells

upregulat-ed p21 expression, several synthetic deletion and

trun-cation mutants and risk-associated BRCA1 mutants,

including P1749R, Q1756InsC (aka 5382InsC) and

Y1853STOP (aka 5677InsA), a frameshift mutation

resulting in a premature stop codon that lacks the last

11 amino acids [25], failed to alter p21 expression [26]

Gain of function mutations

Although mutations resulting in a premature stop

codon are typically susceptible to nonsense-mediated

mRNA decay, mounting evidence suggests that mutant

mRNA and proteins are not uniformly degraded

Per-rin-Vidoz et al [27] found that several BRCA1

muta-tions were unaffected by mRNA decay, including

185delAG and 5382InsC, two of the most common

risk-associated BRCA1 mutations [28] Truncation

mutant mRNAs may avoid decay by translation

re-ini-tiation at a methionine codon downstream of the

pre-mature stop codon [29], and consequently, may

contribute aberrant gene products coding for

trunca-tion proteins exhibiting varying degrees of protein

sta-bility that may impart novel cellular functions [30] It

is important to consider that detection of some mutant

BRCA1 proteins in clinical samples has proven

unsuc-cessful due to technical challenges such as

cross-reac-tivity of antibodies with wild-type BRCA1 However,

validation studies of mutant proteins in tissue samples

are ongoing and will provide a framework within

which to view experimental studies of mutant function

BRCA1 mutant proteins may participate in novel

protein–protein interactions as a result of aberrant

cel-lular localization Rodriguez et al [31] found that

exogenous missense and truncation mutants lacking a

small portion of the BRCA1 C-terminal, including 5382InsC, exhibited aberrant cytoplasmic localization

in breast cancer cells, whereas larger truncations resulted in enhanced nuclear localization of mutants Aberrant localization may result from mutation or loss

of the nuclear localization or export signals, impaired recognition of these signals as a result of improper protein folding, or altered interaction with binding partners that impact BRCA1 localization, such as BARD1 [31]

Mutant BRCA1 proteins may convey unique pheno-types by inhibiting the normal function of wild-type BRCA1 in a dominant negative manner by binding BRCA1 and inhibiting its interaction with other pro-teins, or by sequestering BRCA1 binding partners Likewise, mutant proteins may also convey unique functions by interacting with novel proteins and⁄ or regulating alternative genes Indeed, a significant pro-portion of BRCA1-associated breast cancer tissue sam-ples [32], as well as primary cells from mutation carrier-derived ovarian cancer cell xenograft tumors [33], exhibit loss of the wild-type BRCA1 allele con-comitant with increased mutant allele copy number Consequently, mutant BRCA1 proteins have been shown to impact a range of cellular functions, includ-ing development, proliferation, chemosensitivity, apop-tosis and gene regulation (Fig 1, Table 1)

Role of gain of function mutations for development, cellular proliferation, chemosensitivity, apoptosis and gene regulation

Essentially all BRCA1 knockouts are embryonic lethal

in mice (reviewed in [34]) However, mice homozygous for a specific synthetic mutation truncating the BRCA1 protein by half are viable, although highly susceptible

to multiple tumor types, including lymphomas, sarco-mas, and carcinomas⁄ adenocarcinomas of the colon, endometrium, lung, liver and mammary gland [35] Interestingly, introduction of a synthetic BRCA1 trun-cation mutant encoding the first 300 BRCA1 amino acids inhibits mammary gland differentiation and structural formation during murine development, despite the presence of wild-type BRCA1 [36] Like-wise, when injected into the cleared murine mammary fat pad, primary human breast epithelial cells trans-fected with the BRCA1 D11 splice variant or murine BRCA1-W1777Stop (which mimics the human 1835STOP mutation), undergo limited differentiation and branching and develop extensive hyperplasia [37] The 5677InsA insertion mutation, resulting in a frameshift and premature stop signal at codon 1853,

inhibits proliferation of DU145 human prostate cancer

cells expressing a low level of wild-type BRCA1 more

efficiently than exogenous wild-type BRCA1 [38],

whereas a synthetic N-terminal mutant was found to

inhibit physical interaction of wild-type BRCA1 and

cyclin D1 [39] In contrast, an exogenous C-terminal

fragment of BRCA1 can enhance normal breast

epithe-lial cell growth, possibly by acting in a dominant

nega-tive manner to inhibit wild-type BRCA1’s growth

suppressive function [40] Similarly, whereas

over-expression of wild-type BRCA1 in the ID8 mouse

ovarian epithelial cell line diminished proliferation,

chemosensitivity and tumorigenicity of

intraperitone-ally injected cells, expression of a synthetic truncation

mutant encoding the first 602 amino acids of BRCA1

yielded enhanced proliferation and chemosensitivity

Furthermore, when injected intraperitoneally, cells

expressing the mutant were significantly more

tumori-genic [41] It should be noted, however, that BRCA1

mutants have also been shown to exhibit some residual

wild-type growth function as a result of remaining

intact domains For example, mouse embryonic

fibro-blasts homozygous for D11 BRCA1 exhibited a failed

G2-M checkpoint [42], whereas breast cancer cells

expressing only the 5382InsC mutant maintained an

intact G2-M checkpoint [21]

Fan et al [39] reported that in DU145 prostate cancer

cells expressing low levels of wild-type BRCA1,

overex-pression of wild-type BRCA1 or 5677InsA increased

to-poisomerase inhibitor cytotoxicity, which could be

reversed by transfection of synthetic mutants DEcoRI

(amino acids 1-302) and DKpnI (amino acids 1-771),

yielding chemoresistant cells Likewise, in the HCC1937

breast cancer cell model system lacking endogenous

wild-type BRCA1, the addition of exogenous wild-type

BRCA1enhanced chemoresistance, which was reversed

by cotransfection of DEcoRI and DKpnI [39] This

suggests that mutants can, at least in part, overturn

wild-type BRCA1 function, thereby supporting a role

for gain of function BRCA1 mutations

The 185delAG (BRAT) mutation, which imparts

upon carriers a 66% lifetime risk of developing

ovar-ian cancer [43], arises from the deletion of two

nucleo-tides (AG) in the second exon of the BRCA1 gene

This deletion results in a reading frame shift that

pro-duces a premature stop signal at codon 39 and a

trun-cated protein product Using SV-40 transfected

ovarian surface epithelial cells from women with the

BRAT mutation, we found that mutant cells exhibited

enhanced apoptosis and caspase 3 activation in

response to staurosporine [44], possibly related to

diminished levels of phospho-Akt, XIAP and cIAP1

[45] To rule out the possible contribution of wild-type

BRCA1 haploinsufficiency to altered apoptosis in 185delAG cells, BRAT was expressed in wild-type BRCA1 ovarian surface epithelial cells In agreement with our earlier studies, BRAT enhanced caspase 3-mediated apoptosis and diminished levels of phospho-Akt, cIAP1 and XIAP [46] In more recent studies, we found that BRAT upregulated the expres-sion of maspin [47], a tumor suppressor important in apoptosis, invasion and metastasis that is uniquely overexpressed in several tumor types, including ovarian cancer [48] Maspin expression has been correlated with cisplatin sensitivity in ovarian cancer cell lines and longer progression-free and overall survival times

in ovarian cancer patients [49], and may be involved in BRAT-mediated enhanced chemosensitivity [47] Lastly, Thangaraju and colleagues [50] found that co-expression of 5382InsC and 5677InsA with wild-type BRCA1 inhibited the wild-wild-type protein’s ability to enhance apoptosis in breast and ovarian cancer cells Several studies support a role for BRCA1 mutants

in gene regulation For example, wild-type BRCA1 and 5677InsA inhibited exogenous estrogen receptor alpha transactivation, but co-transfection of DBamHI, DKpnI and DEcoRI reversed this phenomenon [39] Similarly, the synthetic BRCA1 mutant (D500-1863), which encodes a protein less than a third the length

of the wild-type, inhibited wild-type BRCA1-mediated activation of a p53 reporter [10] Likewise, using the mouse mammary gland-specific expression of wild-type BRCA1, a risk-associated mutation that truncates the protein at amino acid 340, or a BRCA1 splice variant that omits the N-terminal 72 amino acids, Hoshino

et al [51] showed that the splice variant mediated hyperproliferation and enhanced lobule formation in the mammary gland In addition, tumorigenesis and death were accelerated in mice expressing the splice variant In separate studies, Quaresima and colleagues [52] performed microarray analysis on HeLa cells stably expressing vector, wild-type BRCA1 or the founder mutation 5083del19, which encodes a BRCA1 protein missing the last 193 amino acids, and, conse-quently both BRCT domains, and found differential regulation of multiple genes, including upregulation of periostin Furthermore, periostin levels were also increased in serum and breast cancer tissue from a small number of patients carrying this mutation In other studies, expression of a synthetic truncation mutant maintaining the first third of the BRCA1 pro-tein enhanced p53 expression in 1D8 mouse epithelial ovarian cancer cells and downregulated constituents of the SAPK⁄ JNK and MAPK ⁄ ERK1 ⁄ 2 pathways [53] Finally, the missense mutation Ser1841Asn, which

is associated with enhanced breast cancer risk,

upregu-lates D52 (TD52) and the folate receptor alpha

(FOL1) in HeLa cells [54] This regulation is clinically

relevant, as expression of these genes correlates with

tumor progression in breast [55,56] and ovarian

cancers [57,58]

Taken together, these studies support a gain of

func-tion role for some mutafunc-tions The presence or absence

of a mutant function, as well as its impact on the cell,

is probably very specific to each mutation and factors

impacting mutant function, including mutant protein

size, loss⁄ maintenance of various domains, or

struc-tural changes resulting in novel domains These studies

must also be viewed in a cautionary manner Gain and

loss of function experiments provide valuable insight

into the mechanism of BRCA1 mutant functions

However, until the presence of stable mutant proteins

is validated clinically, it is necessary to remain mindful

of the limitations, as well as the promise, of this type

of experimental study

Clinical impact of gain of function

mutations

Studies investigating the effect of BRCA1 mutant

pro-teins in the context of wild-type BRCA1 are clinically

important They represent the genotypic and

pheno-typic state of disease-free mutation carriers before the

loss of both wild-type BRCA1 alleles Novel functions

mediated by mutant proteins have been shown in

vari-ous model systems to significantly impact proliferation

and apoptosis and, therefore, have the potential to

influence cancer initiation, progression and, ultimately,

prognosis for patients carrying mutations Although

some mutants may retain specific wild-type BRCA1

functions, others may enhance the risk of cancer

devel-opment by antagonizing BRCA1’s tumor suppressive

functions Further investigation of mutant protein

function is warranted, as a better understanding of the

function of specific mutations could greatly improve

risk assessment and prognostic value for mutation

carriers

A better understanding of BRCA1 mutant functions

may also help to identify novel drug targets for

treat-ment and prophylaxis of mutation carriers Novel

interacting proteins and signaling pathways, as well as

downstream target genes, may reveal as yet

unidenti-fied players in BRCA1 mutation-associated breast and

ovarian cancer Data from our laboratory suggest that

genes important for cancer initiation and progression,

such as maspin, are differentially regulated in normal

human ovarian epithelial cells expressing the BRAT

mutation [47] Furthermore, compared with sporadic

breast cancer tissue, BRCA1 mutation-associated

breast cancer samples reveal more chromosomal aberrations in specific regions, potentially containing additional tumor suppressors important in BRCA1-dependent tumor initiation and progression [59] An understanding of specific interacting proteins, signaling pathways and target genes involved in the mechanism

of enhanced breast and ovarian cancer risk conveyed

by each mutation provides the opportunity for muta-tion-specific personalized therapy for mutation carriers Similar mutations may also share common functions and respond to similar therapeutic strategies Further-more, targeting functions of BRCA1 mutants that probably contribute to premalignancy, cancer initiation and the early stages of tumor growth holds great promise for effective prophylactic measures that are less invasive than oophorectomy and mastectomy

It is interesting to speculate that cells heterogeneous for risk-associated mutations, although nontumorigenic

in their current state, may represent an initial step towards cellular transformation, although additional changes may be necessary for these cells to become malignant Likewise, early changes that may promote malignant transformation, including enhanced telo-meric instability, have been observed in cell lines gener-ated from normal ovarian surface epithelial cells of women with a strong family history of ovarian cancer [60] (reviewed in [61]) Furthermore, several studies have found more frequent occurrence of deep invagin-ations in the ovary surface, dysplasia, hyperplasia and⁄

or surface papillae in high-risk prophylactically removed ovaries versus normal ovaries [62–64], suggest-ing that early ‘premalignant’ changes may already exist

in those carriers The possibility of independent mutant BRCA1 functions does not exclude the contribution

of other oncogenes, tumor suppressors or invasion⁄ metastasis-promoting proteins Conversely, these early changes probably facilitate further cellular changes that manifest in the aggressive phenotype seen clinically in hereditary breast and ovarian cancer

Lastly, there are salient differences between the mechanisms of tumor initiation and progression of breast and ovarian cancer in BRCA1 mutation carriers The lifetime risk for development of breast cancer is higher than that for ovarian cancer [14], and carriers

do not always develop both types of disease Further-more, the importance of differential expression and stoichiometry of transcription factors and signaling molecules in different tissues is also well established The impact of specific mutants is, therefore, probably context specific Holt and colleagues [22] observed a series of BRCA1 mutants to be largely ineffective in inhibiting the growth of breast cancer cells However, one mutant was shown to inhibit the growth of three

ovarian cancer cell lines You et al [65] also found cell

type-specific BRCA1 mutant functions Although

expression of the 185delAG mutation in immortalized

ovarian surface epithelial cells and ovarian cancer cells

revealed multiple downstream effectors and physiologic

impacts [46,47], primary and immortalized cells derived

from normal breast tissue of a 185delAG mutation

carrier did not show a significant difference in growth,

stress response, growth in soft agar or tumorigenicity

when compared with normal breast epithelial cells

homozygous for wild-type BRCA1 [66] Several

epide-miological studies have observed differential ovarian

and breast cancer risk based on the location of the

truncation mutation within the BRCA1 gene [67,68]

Disparate risk levels may represent tissue-specific

degrees of importance for the specific functions lost or

gained as a result of each mutation, and the interplay

of these factors

In conclusion, it is clear from a wide range of model

systems and endpoints that BRCA1 mutations are

capable of significant physiological impacts

Further-more, molecular and phenotypic changes are evident in

mutation carriers These changes may result from loss

of wild-type BRCA1 function, gain of function

muta-tions or both Consequently, further experimental and

clinical studies of mutant BRCA1 proteins are

war-ranted, and will provide a better understanding of

mutation-associated breast and ovarian cancer and

improve the strength of prognosis and efficacy of

pro-phylaxis and treatment for mutation carriers

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