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4
Predisposition for
Breast Cancer
Marion Kiechle, MD and Alfons Meindl, PhD
Etiology
High Penetrance Genes, below), risks have
been determined4 (Table 4.1). In the case of
mutation carriers in the BRCA1 or BRCA2
genes (see below), only breast and ovarian
tissues have a relatively high risk to transform into malignant cells. In addition, the
probability to change into an aberrant phenotype is age dependent. Likewise, the probability for a BRCA1 mutation carrier to develop
breast cancer at the age of 50 is approximately
29%. However, women with a carrier status
and born after 1940 may have higher risks.4,5
Only slightly increased risks for transformation are described for such tissues as prostate,
colon, and pancreas.8,9
The known forms of hereditary breast cancer are inherited in an autosomal dominant
mode. It should be noted, however, that inactivation of one allele is only a prerequisite for
tumor generation and is not sufficient to initiate cancer in normal epithelial breast cells. Indeed, for tumor initiation, the second allele has
to be inactivated. This can happen either by
loss of heterozygosity in the according regions10,11 or by epigenetic effects such as
methylation (see Chapters 26 and 27).12 Tumor
progression requires the accumulation of additional aberrations in the cellular genome, including the disturbance of TP53 expression
that allows cell escape from apoptosis.13 Based
on histologic parameters and gene expression
profiling, BRCA1 tumors can be distinguished
from BRCA2 and sporadic breast cancer samples. Tumor samples from BRCA1 mutation
carriers are normally high grade, infiltrating
ductal breast cancers14 and more often estrogen and HER-2/neu receptor negative than
sporadic samples.14,15 In contrast, BRCA2 tumors are more difficult to separate from sporadic cases by histologic classification only, but
it should be feasible through more validated
expression profiling data; for example, tumors
from BRCA2 mutation carriers show an
Breast cancer is the most frequent carcinoma
in women. In more developed countries, the
probability of contracting breast cancer between the ages of 20 and 80 is approximately
7.8%, that is, 1 in 13 women.1 A comparison of
familial and sporadic cases shows that familial cancer diagnosed at a younger age is frequently bilateral and frequent in men.2 In
recent years it has been estimated that 5–15%
of breast cancer cases involve genetic events.
However, careful epidemiologic investigations
now indicate that only one third (5%) might be
caused by monogenic factors. Most inherited
breast cancer cases are now assumed to be
polygenic, and most diagnosed breast cancer
cases are sporadic without familial clustering
(FIG. 4.1).1,3 In contrast, it can no longer be excluded that at least some of the sporadic cases
may be caused through a combination of polygenic variants and exogenous factors.3
The penetrance of inherited mutated alleles is modulated by additional low penetrant
genetic variants (modifier genes), environmental factors, or both. Factors such as nulliparity,
physical exercise, diet, obesity, and birth age
influence the age at onset in gene mutation
carriers in both sporadic and familial cancer.4
Indeed, it can be observed in pedigrees from
families with multiple occurrences of breast
and/or ovarian cancer that the age at onset of
the disease is decreasing.4,5 It is still controversial whether the use of oral contraceptives
increases the risk of developing breast cancer
in BRCA mutation carriers. Some studies calculate a small but statistically significant
risk,6 whereas other studies have demonstrated no greater risk.7
The penetrance of predisposing genes for
breast cancer is also dependent on age and
tissue type. For the two known breast cancer
predisposition genes, BRCA1 and BRCA2 (see
48
Department of Obstetrics and Gynecology,
Klinikum rechts der Isar, Technical University of Munich, Munich, Germany
Chapter 4
Predisposition for Breast Cancer
15% hereditary
breast carcinoma
2% BRCA1
85% sporadic
breast carcinoma
1% BRCA2
2% BRCAX
5-10% Familial
Clustering
FIGURE 4.1 Origins of breast carcinoma.
Table 4.1
Risks for BRCA1 or BRCA2 Mutation Carriers to
Develop Breast and/or Ovarian Cancer
BRCA1
BRCA2
Age (yr)
BCa
OvCa
BCa
OvCa
40
21%
3%
17%
2%
50
39%
21%
34%
4%
60
58%
40%
48%
6%
80
80–90%
60%
80%
30%
expression profile that is different from BRCA1
but also from sporadic tumors.16
Predisposition Markers
It is estimated that 10–15% of all breast cancer
cases may be caused by genetic predisposition.
The inheritance of a single defective allele is
most likely in a family exhibiting clustering of
breast cancer cases, especially if it includes
cases diagnosed before age 50. Consequently,
a dominant monogenic trait can be demonstrated in families with multiple occurrences of
breast cancer (high penetrance genes). In contrast, the segregation of multiple deficient alleles is most likely in families with fewer cases
of late onset breast cancer (low penetrance
genes). The genes associated with hereditary
forms of breast cancer are listed in Table 4.2 and
are discussed in the following paragraphs.
High Penetrance Genes
To date, two genes that feature frequent mutations associated with breast cancer predisposition in high risk families have been isolated and
validated.17–19 Other genes also involved in the
DNA repair or apoptosis pathways have been
characterized but only rarely were shown to be
mutated. Some of these genes are restricted to
site-specific breast cancer, whereas others are
associated with other cancer entities.
BRCA1 The first gene found to be mutated in
families with multiple occurrences of breast
and/or ovarian cancer was the BRCA1 gene.17,19
It has been classified as a classic tumor suppressor gene, and consequently inactivation of
the second allele in the resulting tumor could be
demonstrated (see above). It is located at 17q21;
comprises 24 exons, 22 of them coding; and
encodes for a protein consisting of 1863 amino
acids. A typical pedigree of a family with a
BRCA1 mutation is given in FIGURE 4.2. Currently,
more than 2000 different deleterious mutations
in the gene are listed in the international
Breast Cancer Information Core (BIC) database (http://research.nhgri.nih.gov/bic/). Most
of the putative pathogenic mutations found
in the gene are either frame-shift or stop mutations that similarly cause the production of
truncated proteins.5,17 Only a few missense
mutations, causing single amino acid substitutions, could be demonstrated to be disease causing.20,21 Recently, a high prevalence of large
gene rearrangements in the BRCA1 gene has
been reported for some populations.22,23 Mutation profiles for different countries have been
published,5,24–26 and it has become apparent
that a mosaic of founder- and populationspecific mutations exists. Founder mutations
are often of Ashkenazi Jewish origin and
were shown to be present in different white
populations.5,24–26
Only a few missense mutations in the
BRCA1 gene can be clearly associated with
familial breast cancer. These include amino
acid residues that are part of the zinc finger
49
50
MOLECULAR ONCOLOGY OF BREAST CANCER
Table 4.2
List of Genes Associated with Hereditary Breast Cancer
Syndrome
Gene/Region
Primary Carcinoma
Secondary Carcinoma
Familial breast/
ovarian carcinoma
BRCA1
17q21
Breast and
ovaries
Prostate,colon
leukemia
Familial breast/
ovarian carcinoma
BRCA2
13q13
Breast and
ovaries
Prostate, pancreas
male breast
Familial breast/
bilateral b.c.
BACH1
17q22
Breast
None
Familial breast/
bilateral b.c.
RAD51
15q15
Breast
None
Li-Fraumeni
syndrome
TP53
17p13
Sarcomas,
breast
Brain, leukemia
Cowden
syndrome
PTEN
10q23
Hamartomatous
lesions
Polyps
Peutz-Jeghers
syndrome
STK11
19q
Gastrointestinal
polyps
Melanin spots
on lips, b.c.
Ataxia telangiectasia
ATM
11q22
Lymphomas
gliomas
Heterozygous:
breast cancer
Li-Fraumeni/
breast cancer
CHEK2
13q21
Breast
sarcomas
Brain, leukemia
High Penetrance Genes
Low Penetrance Genes
b.c., breast cancer.
I:1
II:2
III:2
I:2
OvarialCa
54J
II:1
MammaCa bilateral
Dx: 42/42J
OvarialCa bilateral
Dx: 60/60J
verst: 62J
III:1
MammaCa
bilateral
Dx: 34/42J
IV:1
9J
IV:2
14J
FIGURE 4.2 A mutation in the BRCA1 gene, causing a truncated protein (1364X).
II:4
II:3
OvarialCa
51J
III:3
III:4
27J
For individual: III:1
Calculated risk of cancer (34)
Heteozygote risk: 98.1%
Chapter 4
1
ATM
ATR
CHK2
Predisposition for Breast Cancer
Sensing/signaling DNA damage
Kinases
Sensors
P
2
MSH2
MSH6
BLM
BRCA1
MRE11, RAD50, Nbs1
BARD1
6
MRE11
RAD50
Nbs1
DSB resection
ZBRK1
CtlP
Ubiquitin ligation
P
4
3
SWI/SNF
BACH1
HDAC
P300/CBP
Transcriptional regulation
of GADD45
5
Altering DNA topology
at damage sites
MSH2
MSH6
RNA pol. II
P300/CBP
Transcription, transcription-coupled repair
FIGURE 4.3 Role of the BRCA1 protein in the DNA repair process. (From Ref. 28.)
(Cys61Gly, Cys64Y) and amino acids located in
the BCRT domain that have been shown to interact with different proteins (e.g., W1837R).
Several strategies have been established to
evaluate the character of such unclassified
variants. These include segregation analysis,
evolutionary methods to identify functionally
important amino acid sites, loss of heterozygosity analysis in the corresponding tumor
samples, and functional assays.20,21
A variety of assay approaches has now become available, enabled by the advanced characterization of the BRCA1 protein. Based on
the initial observation that the BRCA1 protein
is a caretaker of chromosomal stability, several studies demonstrated that it is involved
in the repair of double-stranded DNA breaks
(for details, see Chapter 25). Such genomic
lesions can be repaired by nonhomologous end
joining, recombination between homologous
DNA sequences, or single-strand annealing.27,28 In addition, other than its local activities at sites of DNA damage, BRCA1 seems
to be involved in processes that are located
upstream and downstream of DNA damage
responses. On the one hand, BRCA1 is part
of protein complexes that apparently have
functions intrinsic to the sensing and signaling of different types of DNA lesions; on the
other hand, it operates as a transcriptional
regulator of genes, whose expression affects
downstream biologic responses (FIG. 4.3). The
local activities of BRCA1 at sites of DNA damage have been elucidated by several experiments. Sites of DNA damage are rapidly
marked by the phosphorylation of the histone
species H2A-X, and BRCA1 interacts with the
MRE11/RAD50/Nbs1 protein complex to migrate to such sites and resect double-stranded
DNA breaks.28,29 It may be necessary to alter
the DNA topology at the damage sites via
recruiting the protein complexes SWI/SNF,
BACH1, HDAC, and P300/CBP for effective
DNA repair as well as the regulation of downstream processes. Included in these downstream processes are the transcriptional
regulation of GADD45 and transcription coupled repair (FIG. 4.3). Surprisingly, to date, only
the BACH1 gene has been associated with
hereditary breast cancer (see Infrequently
51
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MOLECULAR ONCOLOGY OF BREAST CANCER
Mutated Genes, below). Upstream processes of
these local activities require the phosphorylation of the BRCA1 protein via the kinases
ATR, ATM, and CHK2 proteins (FIG. 4.3).30–32 Interestingly, these two latter kinases could be
associated with hereditary breast cancer (see
The ATM Gene and The CHK2 Gene, below).
Apart from the breast and ovary, compared with many organs in which there is no
cancer predisposition risk, several other organs such as the pancreas, prostate, and colon
have a slightly increased risk of cancer development in BRCA1 mutation carriers.8,9 This
observation indicates that BRCA1 disruption
may have tissue-specific effects that favor malignant transformation. It has been suggested
that BRCA1 functions as an inhibitor of estrogen receptor signaling.33 Studies have substantiated this observation.34,35 Evidence that
BRCA1 mediates repression of the estrogen
receptor has been reported, and this process
is correlated with the down-regulation of the
transcriptional coactivator p300.34 Further
characterization of this pathway may help to
explain the tissue specificity of cancer development in BRCA1-deficient cancers.
The clinical outcome of patients with
germline mutations in BRCA1 has been evaluated in several studies. Most informative is
the comprehensive study of Robson et al.15 in
which two retrospective cohorts of Ashkenazi
Jewish women undergoing breast-conserving
treatment for invasive cancer were genotyped
and the clinical characteristics of the women
with and without BRCA founder mutations
were compared. In brief, BRCA1 mutation
carriers demonstrated a significantly shorter
overall survival. This adverse prognosis could
not be linked to tumor size or axillary nodal
status. This study also demonstrated that
women with BRCA1 mutations are at substantially increased risk for developing metachronous contralateral breast cancer, whereas no
increased risk could be seen for the development of ipsilateral breast cancer when compared with noncarriers.
BRCA2 A second high penetrance gene
found to be mutated in families with multiple
occurrences of breast cancer is the BRCA2
gene. It is located at 13q12.3 and consists of 27
exons, 26 of them coding for a protein of 3418
amino acids.18,19 A typical pedigree with a mutation in the BRCA2 gene is given in FIGURE 4.4.
Similar to the BRCA1 gene, most of the defined cancer-causing mutations are either
frameshift or stop mutations that produce
truncated proteins. More than 1000 deleterious mutations in the BRCA2 gene are listed in
the international BIC database. However,
compared with BRCA1, more missense mutations can be seen in the BRCA2 gene. Because
the BRCA2 protein is not as well characterized as the BRCA1 protein, it is necessary to
evaluate the character of the observed unclassified variants carefully. Overall, it appears
that most of the listed unclassified variants in
the BRCA2 gene are rare polymorphic variants (unpublished data) rather than pathogenic. Analogous to the BRCA1 gene, founder
mutations are presented frequently in most
populations.36
I:1
II:1
III:5
III:3
I:2
II:2
Breast cancer
diagn: 50ys
III:6
III:4
Prostate cancer
Breast cancer
diagn: 70ys
III:7
III:1
III:2
Breast cancer
diagn: 63ys
Breast cancer
diagn: 52ys
IV:4
IV:6
IV:7
IV:5
IV:10
IV:8
IV:9
Breast cancer
diagn: 46ys
V:1
V:2
II:5
II:4
II:3
unclassified
carcinoma
V:3
FIGURE 4.4 A mutation in the BRCA2 gene, causing a truncated protein (S1882X).
IV:11
IV:1
Chapter 4
Predisposition for Breast Cancer
Nucleus
DNA damage/
replication
DNA-repair
BRCA1
FA complex
A
G
C
F
E
BRCA2
RA
RAD5
D5
D2
K561
1
1
D2
RAD51
K561
Ub
DNA-repair
foci
FIGURE 4.5 BRCA2 and FA proteins in DNA repair. (From Ref. 44.)
Mutations in the BRCA2 gene are normally
found in site-specific breast cancer families, including male breast cancer. However, families
with a clustering of pancreatic cancer were also
shown to harbor BRCA2 mutations.37,38 In one
study,37 26 European families in which at least
two first-degree relatives had a histologically
confirmed diagnosis of pancreatic ductal adenocarcinoma were screened for mutations in the
BRCA2 gene, and three deleterious and missense aberrations were found. In addition, it
could be shown that 2% of men with early onset
prostate cancer harbor germline mutations in
the BRCA2 gene. Male BRCA2 mutation carriers under the age of 60 have a 26-fold risk increase of developing prostate cancer compared
with control subjects.39 However, further studies are required to confirm that BRCA2 mutations are associated with a higher penetrance
in familial cancers of the pancreas and prostate
than are BRCA1 mutations.
Of further interest is the recent finding that
biallelic mutations in the BRCA2 gene have
been found in two Fanconi complementation
groups.40 Fanconi anemia (FA) is an autosomal
recessive disorder affecting all bone marrow
elements and is associated with cardiac, renal,
and limb malformations as well as dermal
pigmentation changes. Fanconi patients also
have an increased cellular sensitivity to DNA
cross-linking agents. FA has eight identified
complementation groups, and for the complementation groups B and D1, mutations in the
BRCA2 gene can be identified. This finding has
now linked the FA pathway to the BRCA1/
BRCA2 mediated DNA repair process (FIG. 4.5).
Several FA proteins, including FANCA,
FANCC, FANCE, FANCF, and FANCG, form a
constitutive complex in the nucleus of human
cells that in response to DNA damage mediates
the monoubiquitination of FANCD2. The activated FANCD2 protein is then translocated to
chromatin and the DNA-repair foci that contain
BRCA1 and BRCA2 (FANCD1). BRCA2 is
known to bind directly to RAD5141 and, similar
to BRCA1, is involved in homology-directed
DNA repair. Also, similar to the BRCA1 protein,
the BRCA2FA pathway can be linked to ATM
and to another upstream activator, the MRE11
complex. A study indicates that in response to
cellular exposure to ionizing radiation, the ATM
protein directly phosphorylates FANCD2.42
Thus, the FANCD2 protein functions in mediating two signaling pathways by undergoing two different types of posttranslational
modifications. A link to the MRE11 complex
(NBS-MRE11-RAD50) can be established
through the demonstration that the activated
form of FANCD2 and NBS1 are colocalized
in nuclear foci. Either the disruption of the
MRE11-binding carboxy terminal end of NBS1
or the disruption of the FANCD2 monoubiquitination site results in mitomycin hypersensitivity.43 In summary, both BRCA proteins are
involved in DNA repair, a cellular mechanism
activated in response to irradiation or chemically induced lesions. As well as the ATM and
NBS pathways, respectively, the FA pathway
has now linked to this cellular network system
that maintains genomic integrity and stability.44
53
54
MOLECULAR ONCOLOGY OF BREAST CANCER
As in the case of BRCA1 mutation carriers,
the clinical outcome for BRCA2 mutation carriers has been evaluated.15 In contrast to females
with predisposing mutations in the BRCA1
gene, females with mutations in the BRCA2
gene have no reduced survival after breast cancer therapy. However, similar to BRCA1 carriers, these patients carry an approximately 30%
risk of developing contralateral breast cancer
after 10 years of posttreatment follow-up has
elapsed. Further follow-up studies are required
to confirm these initial data obtained in Ashkenazi Jewish women.
BRCAX Although most of the high risk families with multiple occurrences of site-specific
breast cancer or breast and ovarian cancer (six
or more cases) map either to 13q13 or 17q21,
no further predisposing loci have been determined for most families exhibiting fewer cases
of breast cancer. Different models have been
developed to determine the genetic transmission modes for the relatively large number of
BRCA1/BRCA2 negative families. Based on
such models, in most of the families showing
familial clustering, polygenic traits may segregate. For only a limited number of families,
further high penetrance genes acting in an
autosomal dominant form are expected.45
However, one published study has predicted
the involvement of an as yet unknown autosomal recessive gene in a significant portion of
such families.46
Infrequently Mutated Genes Based on the
observation that BRCA1 and BRCA2 are involved in double-stranded DNA break-repair
mechanisms, several groups have searched for
aberrations in other genes located in this pathway. The BACH1 gene (BRCA1-associated
C-terminal helicase-1) codes for a nuclear protein that acts directly with the highly conserved
C-terminal BRCT repeats of the BRCA1 protein. The predicted 1249 amino acid long protein, encoded by 20 exons, contains the seven
helicase-specific motifs that are conserved
among members of the DEAH helicase family.
Cantor et al.47 identified a germline mutation
affecting the helicase domain in 2 of 65 patients
with either early onset or familial breast cancer. In subsequent experiments the same group
demonstrated that these two and a novel third
mutation impair the function of the BACH1
protein.48 This novel protein is both a DNAdependent ATPase and a 5 to 3-DNA helicase.
The helicase activity is strictly ATP dependent,
and when a Pro47Ala missense mutation is present, this activity is abrogated. Due to another
missense mutation (Lys52Arg), ATPase activity
is disrupted as the unwinding of DNA and RNA
hybrids, an additional function associated with
the wild-type. Finally, one missense mutation
maintains the ATPase activity but disturbs the
unwinding process. Further studies will show
whether other variants in the gene will have at
least a minor effect on hereditary breast cancer,
for example, in combination with CHEK2 (see
The CHK2 Gene, below).
Mouse experiments demonstrated that
BRCA2 interacts with RAD51 (see BRCA2,
above). Recently, Pellegrini et al.49 showed, on
the basis of crystal structures, that the direct
binding is mediated by the BRC repeat
(BRCA2) and the RecA-homology domain
(RAD51). The BRC repeat mimics a motif in
RAD51 that serves as an interface for
oligomerization between individual RAD51
monomers, thus enabling BRCA2 to control
the assembly of the RAD51 nucleoprotein filament that is essential for strand-pairing reactions during DNA recombination. In parallel
to the still ongoing functional works on
BRCA2 and RAD51, several groups looked for
aberrations in the RAD51 gene. In a study
with 20 patients from breast cancer families,
Kato et al.50 identified a missense mutation
(Arg150Gln) in two unrelated females, both
presenting with bilateral breast cancer. In addition, a single nucleotide polymorphism in
the 5-untranslated region of the RAD51 gene
(135g → c) has been associated with an increase for BRCA2 mutation carriers.51 However, further studies, including quantitative
reverse transcriptase-polymerase chain reaction, are required to substantiate this finding.
Genes Associated with Cancer Syndromes Breast cancer is also a component
of several other cancer syndromes, including Li-Fraumeni syndrome (OMIM 151623),
Cowden syndrome (OMIM 158350), and PeutzJeghers syndrome (OMIM 175200). All these
syndromes are rare, and mutations in the
genes causing them contribute only a small
fraction of hereditary breast cancer cases.
The TP53 gene is mutated in Li-Fraumeni
syndrome, which is characterized by the
occurrence of multiple cancers, including
childhood sarcoma and breast cancer (see
Chapter 23).52 Mutations in the TP53 gene
Chapter 4
are found in approximately 70% of the LiFraumeni families. A higher cancer risk in
females than in males was observed by using
follow-up studies.53 Quite recently, mutations
in the CHEK2 gene have also been associated
with Li-Fraumeni syndrome (see The CHK2
Gene, below). Although only a few germline
mutations in TP53 contribute to hereditary
breast cancer, somatic inactivation of TP53 in
BRCA mutation related tumors is frequently
identified (see above).54 It is now accepted that
inactivation of TP53 prevents the initiation of
apoptosis, prevents the depletion of cells that
have accumulated genomic lesions, and thus
contributes to the establishment and progression of malignancy.
Multiple hamartomatous lesions, especially of the skin, mucous membranes, breast,
and thyroid, and craniomegaly are characteristic lesions of the Cowden syndrome. In 1997,
mutations in the PTEN (phosphatase and
tensin homolog) gene were identified in families with the Cowden syndrome phenotype.55
Although germline mutations in PTEN are
rare, PTEN is frequently inactivated in sporadic cancers, including glioblastomas and
prostate cancer (see Chapter 23).56 Recent
data indicate that PTEN is involved in the regulation of apoptosis56 and angiogenesis.57 During both processes, the PTEN protein acts
through the well-recognized tumor progression associated phosphoinositol-3-kinase and
Akt-dependent pathways.
A serine-threonine kinase, the STK11/
LKB1 gene, was found to be mutated in several families with Peutz-Jeghers syndrome.58
This is an autosomal dominant disorder characterized by melanocytic macules of the lips,
multiple gastrointestinal hamartomatous
polyps, and an increased risk for various neoplasms, including gastrointestinal and breast
cancer. Carrier females with a mutation in the
STK11 gene also have an elevated risk for
breast cancer, as has been shown in a British
study in which 33 Peutz-Jeghers families were
evaluated by follow-up data.59 Whether the
STK11 gene also plays a role in sporadic
breast cancer, as suggested in a recent study,60
requires validation with larger patient groups.
Low Penetrance Genes
Currently, three genes are considered to act as
low penetrance genes for hereditary breast
cancer. In contrast to high penetrance genes,
Predisposition for Breast Cancer
such genes act in a concerted way, either in
connection with variants in other predisposing
genes or in combination with epigenetic or
environmental factors. In addition, several
modifier genes in breast cancer have been
described and are summarized elsewhere.61
The ATM gene A first gene in which variants have been associated with breast cancer is
the ATM gene (OMIM 208900). The ATM gene
is mutated on both alleles in ataxia telangiectasia and codes for a member of the phosphatidylinositol-3 kinase family of proteins.
Similar to the two BRCA proteins, ATM plays a
key role in monitoring genomic integrity and
regulating cell cycle checkpoints, DNA repair,
and apoptotic pathways induced by doublestranded DNA breaks. After the occurrence of
double-stranded breaks in the cell, ATM interacts with and phosphorylates a number of different targets, including p53, Nibrin, CtIP,
BRCA1, and CHEK2 (FIGS. 4.3 and 4.6).62 Given
that there are several confirmed direct functional links between the ATM and BRCA1 proteins, the ATM gene also has been implicated
in breast carcinoma susceptibility.
In two larger mutation screening reports,
variants in the ATM gene have been linked to
familial breast cancer.62,63 In both studies, missense mutations in the ATM gene were shown
to be specific for familial cases but were absent
from normal control subjects (Table 4.2). However, additional studies are required to confirm that ATM mutations confer increased
susceptibility to breast cancer.
The CHK2 gene Cell cycle checkpoint
kinase 2 (CHEK2 [OMIM 604373]) plays an
important role in the maintenance of genome
integrity and in the regulation of the G2/M cell
cycle checkpoint. It has been shown to interact
with other proteins involved in DNA repair
processes, such as BRCA1 and TP53.32,64
These findings suggest that CHK2 could be
an attractive candidate gene associated with
susceptibility for a variety of cancers (FIG. 4.6).
Patients with Li-Fraumeni syndrome were
analyzed, and in a small proportion of families
a single mutation in exon 10 of CHK2 was
identified.65 This del1100C variant causes a
stop codon after amino acid 380 abrogating the
kinase activity. The CHK2 Breast Cancer Consortium analyzed 718 families with breast
and/or ovarian cancer that harbor neither a
BRCA1 nor a BRCA2 mutation. The 1861delC
mutation was identified in 5.1% of the families
55
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MOLECULAR ONCOLOGY OF BREAST CANCER
DNA damage
or
ATM
P
P
P
p53
P
CHEK2
P
ATR
P P P
P
BRCA1
P
p53
Cell-cycle arrest
Apoptosis
DNA repair
FIGURE 4.6 Possible role of CHK2 in breast cancer development. (From Ref. 64.)
compared with a prevalence of 1.2% in a
healthy control group, indicating a low penetrance of this mutation.66
To establish the relevance of CHK2 mutations in the German population, all 14 coding
exons were analyzed in 516 families negative
for mutations in both the BRCA1 and BRCA2
genes. As in other studies, a statistically significant association for the exon 10 mutations
was observed (P 0.006), and carriers had an
almost fourfold increased risk for familial
breast cancer (relative risk, 3.9). However, the
prevalence of the 1100delC mutation in both
familial breast cancer patients (1.6%) and control subjects (0.5%) in the German population
is significantly lower than reported for other
white populations. It shows independent segregation for breast cancer in two of four families analyzed. Furthermore, no increased
breast cancer risk was found for the most common missense mutation I157T or the splice
site mutation IVS2 1G → A, which added
together for more than half of the variants
(12 of 21) observed in patients.67
The low prevalence of the exon 10 deletion
mutations together with no or an uncertain
elevation in risk for other CHK2 mutations
suggests only a limited relevance of mutations
in the CHK2 gene for familial breast cancer.
Therefore, at present it appears premature to
include CHK2 in genetic counseling for patients with an unexplained high familial incidence of breast cancer.
Androgen Receptor Gene Mutations in
the androgen receptor (AR) gene have been
linked to different disease entities. Expansions
of CAG repeats within the coding region can
cause spinal and bulbar muscular dystrophy
(Kennedy disease) and different forms of androgen insensitivity.68 The first link to breast
cancer for AR was confirmed by the identification of germline mutations in two brothers with
breast cancer and Reifenstein syndrome.69
More recently, a polymorphic CAG repeat in the
AR gene has been associated with a poorer prognosis for BRCA1 mutation carriers. Comparison of 165 women with and 139 women without
breast cancer revealed that women carrying at
least one allele with 29 or 30 CAG repeats manifested the disease earlier.70
In summary, an evaluation of genes localized in either apoptosis, DNA repair, or
endocrine signaling will provide further low
penetrance or modifier genes. Ongoing mapping and cloning studies will show whether a
gene segregates recessively or dominantly in
high risk families.
Chapter 4
Predisposition Testing and Risk Management
Because of the complexity of genetic counseling in hereditary breast cancer, referral to a
subspecialized multidisciplinary clinic staffed
by practitioners with expertise in this field is
generally recommended. In this setting, a new
client can be educated by an experienced
genetic counselor about the a priori risks associated with breast cancer predisposing
germline mutations. At the same visit, the
client can also receive information from medical and surgical/gynecologic oncologists concerning screening, diagnosis, and treatment
options. Finally, this approach also allows the
client to meet with a psychologist/psychiatrist
to consider the important psychologic and
emotional issues associated with breast cancer
predisposition testing and the implications for
the client and her family. In such a multidisciplinary counseling setting, the sensitivity
and specificity of the applied molecular techniques, the indications for testing, and possible risk managements in BRCA1/2 mutation
carriers and/or BRCA1/2 negative females
from putative BRCAX families can be thoroughly considered.
Methods
A variety of laboratory techniques has been
used for screening for genetic mutations in
hereditary breast cancer. In addition, protein
truncation tests have been developed as prescreening methods71 and can be applied for the
rapid identification of the likely presence of underlying pathogenic mutations. However, it is
now accepted that screening of index patients
in families with either multiple cases or early
onset manifestation of breast cancer should be
applied by investigating the entire gene to also
detect missense mutations and putative splice
sites. Two independent methods that are applied currently in routine diagnosis include the
direct double-stranded sequencing approach,
as performed by the company Myriad Genetics,19,24 and denaturing high-performance liquid chromatography (DHPLC) analysis of all
coding exons with the adjacent intronic
sites.5,36 In families with a significant prior
probability of a BRCA mutation, testing for
large gene rearrangements22,23 also has to be
included. For this approach, a kit based on multiplex ligation-dependent probe amplification
is available now.72 Overall, the sensitivity of
this technique is greater than 90%, and when
Predisposition for Breast Cancer
either complete direct double-stranded sequencing or comprehensive DHPLC analysis is
combined with multiplex ligation-dependent
probe amplification, the specificity is greater
than 90%.
Indications for Testing
Genetic testing is recommended only after
multidisciplinary genetic counseling. The criterion for testing is a 20% probability of the
presence of a mutation.73–75 Candidates for
BRCA1/2 genetic testing are as follows:
• Two breast cancer cases under age 40 or
three under age 50 years;
• Four cases of breast cancer under age
60 years;
• More than four cases of breast cancer at
any age;
• Ovarian and breast cancer in a family
(breast cancer under age 50 years if only
one ovarian and one breast cancer case);
• Early onset female breast cancer at under
age 60 years and male breast cancer at any
age;
• Single case of breast cancer at under age
40 years if the patient is of Ashkenazi
Jewish decent.
Risk Management
Three categories of options for management of
women with BRCA mutations are available:
screening and surveillance, prophylactic surgeries, and chemoprevention. These options
should be discussed with women in detail to
allow them to make informed decisions regarding their health care.
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