Breast Cancer Susceptibility Genes

In 1994 and 1995 two geneswere identified that
confer susceptibility to breast and ovarian
cancer when mutated, the breast cancer genes
BRCA1 and BRCA2. Both genes encode multifunctional
proteins with important cellular
functions in genomic stability, homologous recombination,
and double-stranded and transcription-
coupled DNA repair (see p. 80). The
BRCA1 and BRCA2 proteins interact and play a
role in cell cycle control (see p. 112) and in
development. An autosomal dominant susceptibility
mutant allele in one of these genes is
considered the main cause of the cancer in
about 5–10% of patients. Mutations in other
genes are involved in some cases. The direct
causative role of BRCA1 and BRCA2 mutations is
difficult to assess in individual patients. Different
mutations as well as polymorphic variants
occur throughout the genes.

The breast cancer susceptibility gene BRCA1

The BRCA1 gene on chromosome 17 at q21.1
consists of 24 exons spanning 80 kb of genomic
DNA that encode a 7.8 kb mRNA transcript. The
protein has 1863 amino acids. Exon 11 is quite
large (3.4 kb). About 55% of all mutations occur
in exon 11. Although some mutations occur
relatively frequently in other exons, they tend to
be evenly distributed throughout the gene (only
some mutations are shown). The deletion of an
adenine (A) and a guanine (G) in nucleotide
position 185 (185delAG) and the insertion of a
cytosine in position 5382 (5382insC) account
for about 10% of mutations each. These mutations
are particularly frequent in the Ashkenazi
Jewish population.
The protein has five main functional domains.
The RING finger region near the N-terminus at
amino acids 1–112 defines a zinc-binding
domain of conserved cysteine and histidine residues
that mediate protein—protein or protein-
DNA interactions. This region is also the site of
heterodimerization of BRCA1 and BARD1
(BRCA1-associated RING domain 1). Other
functional domains define the central part of
the BRCA1 protein. These are two nuclear localization
signals (NLS) and two protein-binding
domains, one for p53 protein, retinoblastoma
(RB) protein, and RAD50 and RAD51. RAD50 and
51 are proteins involved in recombination
during mitosis and meiosis, and in recombinational
repair of double-stranded DNA breaks.
The C-terminus contains a region involved in
transcriptional activation and DNA repair.

The breast cancer susceptibility gene BRCA2

The BRCA2 gene on 13q12 comprises 27 exons
spanning 80 kb of genomic DNA that encode a
10.4 kb mRNA transcript. Its protein has 3418
amino acids. Exon 11 is large (11.5 kb), as in
BRCA1. Mutations occur throughout the gene
(only some are shown). A deletion of thymine at
nucleotide position 6174 (6174delT) is relatively
(1%) frequent in the Ashkenazi Jewish
population.
The BRCA2 protein has a transcriptional activation
domain near the N-terminus and a nuclear
location signal (NLS) near the C-terminus. A
large central domain consists of eight copies of
a 30–80-amino-acid repeat, which are conserved
in all mammalian BRCA2 proteins (BRC
repeats).

BRCA1 and BRCA2 genes

The BRCA1 and BRCA2 genes are expressed
ubiquitously with the highest levels of expression
in thymus and testis. The spatial and temporal
expression patterns of Brca1 and Brca2 in
the mouse fetal and adult tissues are essentially
identical, with highest expression of both in
rapidly dividing tissues during differentiation,
especially in mammary epithelium. In the
mammary gland both genes are expressed
during puberty and pregnancy, and their expression
is reduced during lacation.

Retinoblastoma

Retinoblastoma (McKusick 180200) is the most
frequent tumor of the eye in infancy and early
childhood. It occurs in 1 of 15000–18000 live
births. This tumor results from loss of function
of both alleles of the retinoblastoma gene RB1.
Tumor initiation is preceded by two steps as A.
Knudson predicted in 1971 in his “two-hit” hypothesis
(tumor suppressor gene, p. 318). The
first predisposing mutation in one allele may
occur either in a retinoblast, an undifferentiated
retinal cell in the developing embryo, or
in the germline. The other allele is inactivated
by a second mutation.

Phenotype

Retinoblastoma occurs in one eye or both eyes.
An important early sign is the so-called “cat’s
eye,” awhite shimmer out of the affected eye (1)
or the development of strabismus. One or
several tumors originate from the retina (2).
The tumor progresses rapidly (3). The relative
proportions of the genetic types of retinoblastoma
are about 60% somatic mutations (nonhereditary
form) and 40% germline mutations,
transmitted as an autosomal dominant trait
(hereditary form, in about 10–15%, due to
transmission from a parent; the remainder due
to a new mutation). New mutations usually affect
a paternal allele (about 10: 1). In about 10%
of carriers of a germline mutation no tumor
develops (nonpenetrance).

Retinoblastoma locus on chromosome 13

The RB1 locus at 13q14.2 was first defined with
cytogenetically visible interstitial deletions.