The Genetics of Breast Cancer
Currently today, chances are that one in ten women will develop breast cancer at some point in their life (12). Breast cancer is the most common malignancy in women and is responsible for about 18% of all female cancers (7). This is the leading type of cancer in women and the second highest killer of all cancers behind lung cancer. 5-10% of breast cancers is due to genetic predisposition (1). Familial breast cancer is often labeled by several factors including diagnosis at a young age, bilateral breast cancer, affected relatives, and a strong link to ovarian cancer (15). About 7-10% of ovarian cancers occur by germline inheritance.
Inherited breast cancer is characterized by cancer occurring with an alteration in a breast cancer susceptibility gene, which has been passed on. Familial breast cancer refers to a person who has family diagnosed with breast cancer. Characteristics of breast and other cancers that develop in these genetic syndromes have now been studied and been proven to differ from sporadic versions of the same neoplasms.
Two of the genes that are the main causes or breast cancer are BRCA1 and BRCA2. The mutations of these genes account for more than all other inherited breast cancer-causing genes combined. These genes are both autosomal dominant and highly penetrant. Both of these genes contain a high density of repetitive elements. This feature of the genomic region contributes to the chromosomal instability of these genes. Mutations highly arise from the rearrangements of these repetitive regions. BRCA1 and BRCA2 proteins help regulate the homologous recombination and double-strand break repair in response to DNA damage. These jobs are crucial to cellular processes for DNA damage repair, transcriptional regulation, and cell cycle control. Cells lacking the BRCA1 and BRCA2 genes overtime summate chromosomal abnormalities like chromosomal breaks, severe aneuploidy and centrosome amplification. The deficiency of these two genes is the precursor for breast tumor formation. Lifetime risk for second primary breast cancer in BRCA1 or BRCA2 mutation carriers with breast cancer is 40-
60%, with a 5-year risk estimate of about 22-31%. For males, breast cancer in BRCA2 mutation carriers is about 6.3%. Ovarian cancer development in BRCA1 mutation carriers is estimated

around 28-44%, and in BRCA2 carriers, it is about 16-27% (1). Questions that are still unknown today are: Why do the mutation of these two genes that work upon many different pathways specifically lead to breast and ovarian cancer, and also does these genes play a role in the development of breast cancer out of inherited genes (10)?
BRCA1 mutation carriers have an 87% chance of developing breast cancer, and about
50% risk of developing ovarian cancer (9). BRCA1 is located on chromosome 17Q12-q21. This gene consists of 22 exons encoding approximately 5592 nucleotides and will produce a protein of about 1863 amino acids. A little more than half of the coding sequence occurs on exon 11.
This protein spans about 80 kb of genomic DNA. In this highly unstable region, there is a high amount of alu repeats which make this region more susceptible to deletions and alterations.
The exact function of BRCA1 is unknown. There are 126 nucleotides near the beginning of the gene that is similar to the RING finger homology which leads to believe that the BRCA1 protein might be involved in transcriptional regulation. In a small area of the central portion of the protein there is similar homology to the family of granin proteins, which demonstrates that this may be a secreted protein. Another area in the central region of the gene interacts in vitro with RAD51, suggesting that this protein helps with sensing and responding to DNA damage.
There have been over 200 different kinds of BRCA1 mutations identified, most of which that were found in families with breast and/or ovarian cancer. 185delAG and 5382insC account for about a quarter or all mutations found. Frameshift, nonsense, splice site and regulatory mutations, and large genomic deletions that lead to protein truncation or the absence of
BRCA1 protein are responsible for about 90% of all germ-line mutations reported. Frameshift mutations are the easiest type to detect (14). Mechanisms for detecting BRCA1 mutations are denaturing high performance liquid chromatography technique (DHPLC), single-strand conformation polymorphism assay (SSCP), protein truncation test (PTT), direct sequencing, and heteroduplex analysis (HA). The 3’ end of the BRCA1 gene is mostly associated with breast cancer, while the 5’ end is primarily the area of breast cancer and ovarian cancer mutations.
Close to the 5’, BRCA1 has a BRCT domain or “zinc finger” in which a fragment 126 bp similar to this region. This finger has a group of cysteine and histidine residues creating a binding site

for the ion zinc, which elicits the fold of the amino acid chain to build the loop. Only 1% of human genes include this sequence. Normally, this enables the protein to work with other molecules, sometimes proteins, and at times RNA or DNA. The zinc finger region close to the N terminal of the BARD1 protein has been discovered to interact with the BRCA zinc finger, creating a heterodimer where both are found close to each other. Since many transcription factors bind to DNA only after creating heterodimers, BARD1 and BRCA1 are believed to cooperate in DNA recognition of certain sequences (12). Studies have shown that in normal cells, the BRCA1 protein in located intranuclearly. In cancer cells however, the protein may be locating inside the nucleus and in the cytoplasm. This is believed to be a mechanism of developing breast tumor pathogenesis (9). Testing has shown that the BRCA1 protein is tumor suppressive. This was proven in an experiment where wild-type gene was inoculated to breast and ovarian tumor cells using retroviral vectors. MCF-7 tumors were inhibited in tested nude mice when wild-type BRCA1 were present, even when mutated BRCA1 proteins were there also, supporting that BRCA1 acts as a tumor suppressant (14).
The BRCA1 gene is a coactivator of the androgen receptor. BRCA1 enhances androgen receptor signaling. This works through androgen receptor activation function. BRCA1 and the p160 coactivators synergistically potentiate androgen receptor signaling in breast and prostate cancer cell lines. The p160 coactivators bind to NRs to strengthen ligand-dependent receptor signaling by recruiting to the target promoter a large, multisubunit coactivator complex that has histone acetyltransferase activity. The coactivators work with the androgen receptor through both of its activation functions. BRCA1 interacts with the androgen receptor NTD and the GRIP1 COOH-terminus. Overexpression of this gene relieves the inhibitory polyglutamine effect on androgen receptor transactivation activity. This activity shows to decrease with increasing polyglutamine length and may help with inhibition of p160-mediated coactivation
(8). BRCA1 also has a BRCT domain in the C end region which is like those in proteins of mammals that binds to p53. This suggests that BRCA1 may work with this protein for transcriptional activation of other genes (3). BRCA1 protein also has three signal sequences of nuclear localization, which allows protein to enter to the nucleus from the cytoplasm. This

allows protein to bind to a cytosolic receptor, with an importin-alpha and an importin-beta.
This is able to bind to RAD51, a protein involved in meiotic recombination and repair, which shows that is has nuclear function.
Carriers of the BRCA2 mutation gene usually have a later onset of disease than BRCA1 mutation carriers. With this mutation, carriers are at a higher increased risk of breast cancer in males and females, and of ovarian, prostatic, pancreatic, gall bladder, bile duct and stomach cancers, and melanoma (10). BRCA2 mutation carriers have an 85% risk of breast cancer and a 10-20% chance of developing ovarian cancer. Mutations of this gene are responsible for 6% of all male breast cancers. The ovarian cancer cluster region is an area where increased mutations raise the risk of ovarian cancer in a 3.3 kb region inside exon 11.
This region is important for ovarian specific tissue regulation (9). BRCA2 is similar, but not identical to the BRCA1 gene. This gene is found on chromosome 13a12-q13, specifically between regions D13S289 and D13S267, comprising of 10.254 nucleotides with approximately 26 exons encoding a protein of 3,418 amino acids. There are two regions identified on the BRCA2 gene that works with RAD51. Known as the BRC repeats, this area is a repetition of eight amino acids located centrally of the gene that interacts with RAD51 to bind in vitro separated by unrepeatable fragments of varying sizes. It is believed that this gene aids in function for recombination and in double strand DNA repair (9). The N terminal of the gene is similar to the activation domain of c-Jun, a transcription factor that includes exon 3, has been shown to activate transcription in vitro. Mutations of this gene are found along the entire length of the gene. Two of the mutations account for 33% found of all germline alterations. Similar to
BRCA1, BRCA2 tends to result from frameshift mutations due to deletion or insertion of one or more nucleotides (14). The specific function of this gene has not been identified. BRCA2 expression is high in mammary epithelial cells and in the testicles where rapid proliferation occurs. This expression is decreased by growth factors deprivation and increased by glycorticoids (15).
Li-Fraumeni syndrome (LFS) is caused by another highly penetrant gene that could onset early development of breast cancer. Caused by germline mutations in the p53 gene

located on chromosome 17p13, this syndrome produces a variety of tumors including sarcomas, lymphomas, and breast cancers (14). It is estimated that carriers of p53 mutations have a 50% chance of developing breast cancer by the age of 50 years. Rarely do post-menopausal women with familial LFS will develop breast cancer (15). The p53 gene is believed to be involved in the regulation of cellular proliferation. Levels of the nuclear phosphoprotein are maintained by the cell cycle through two mechanisms. One method is that p53 RNA transcription increases by 10- to 20- fold late in the G1 phase, and the other mechanism is that normal p53 protein is rapidly eliminated by its short half-life, which is about 6-30 minutes. This works to quickly provide protein to be quickly broken down. This keeps levels of protein low. Mutant forms of p53 differ in the wild type by failure to bind to the simian virus 40 tumor antigen, but do form stable complexes with hsc70. This may increase the half-life, raising already levels of protein in cells transformed with a mutant p53 gene. This gene mutation is believed to be an oncogenic mutation in the way that they alter amino acid residues that have been highly maintained throughout evolution. Mutations of this gene throughout the region have been showed to alter normal functions of the protein. The mutated form of this gene is able of forming oligomeric complexes with wild type p53. This gene in humans is found on the short arm of chromosome 17 in an area where deletion occurs in many different human tumors. This deletion is followed by the mutation of remaining copies of the p53 gene, killing off any wild type p53 in the cell. This eventually leads to the inability to control growth. These suggest that p53 may perform as a tumor suppressor or a recessive oncogene. More than 60% of breast cancers have deletions of alleles on 17p, and it has been found that 45% of breast cancers over express the p53 protein. Allelic deletion and over expression does not correlate to each other however (5).
Cowden disease (CD) is a rare, autosomal dominant inherited disorder characterized by multiple hamartomatous, papillomatous, and mucosal lesions of the skin, mucous membranes, intestinal polyps, facial trichelloma, acral keratoses, and an increased risk of the development of breast and thyroid cancer. Skin is involved in 90-100% of cases, and thyroid is involved in
66% of cases. An estimated 30% of women with this disease will have bilateral breast cancer by

the age of 50 (16). Cowden disease is due to a germline mutation in BMPR1A, or bone morphogenetic protein. The inherited gene is the mutation of tumor suppressant gene PTEN
(or MMAC1 or TEP1). It is located on chromosome 10q22-q23 between interval D10S1761 and
D10S541. So far there has been no correlation between genotype and phenotype (17). The protein product of this gene a phosphatase that regulates the function of other different proteins by removing phosphate groups from those molecules. The PTEN protein is believed to promote cell death by negatively controlling the phosphoinositide 3-kinase. This is the signally pathway for regulating cell growth and survival by dephosphorylating the 3 position phosphoinositide. These mutations may lead to overproliferation of cells, creating hamartomatous growth.
Ataxia-telangiectasia (AT) is a rare, low penetrant, autosomal recessive disorder in which patients experience progressive ongoing neurologic degeneration, immune dysfunction, premature ageing and an increased risk of lymphoproliferative malignant disease. Studies have shown that relatives of AT patients that are heterozygous for this gene are at an increased risk of breast cancer. Although this gene is less penetrant, it is fairly common. This disease is characterized by neurological problems, specifically abnormalities of balance, recurrent severe sinus and respiratory infections, and dilated blood vessels on the surface of the skin and in the eyes. Patients usually show immune system abnormalities and are very sensitive to the effects of radiation treatments. It is estimated that approximately 1% of the entire population is heterozygous for this mutation gene. Heterozygotes do not show major symptoms of this disease, but it is proven that they have an underlying cellular radiosensitivity, but not as severe as homozygotes (4). This gene could account for about 8% of all breast cancer patients under 40 years of age. This means that this could be responsible for the larger development of breast cancer than by BRCA1 and BRCA2 mutations combined. This gene is located on chromosome 11q22-23, encoding a 12-kb protein similar to TEL1 and MEC1. TEL1 is a gene monitoring telomere length, and MEC1 is a yeast cell cycle checkpoint gene (15).
Another low-penetrance breast cancer susceptibility gene is the HRAS1 allele. This proto-oncogene is located on chromosome 11p15.5, with a repeat unit 28 base pairs long. Four

“common” alleles accounts for about 90% of the alleles at this locus, differing in size from
1,000 to 2,500 base pairs. This allele is estimated to be in about 6% of the general population.
This could account for 9% of all breast cancers. Recently studied is the correlation between the HRAS1 VNTR locus and BRCA1 that produces a twofold increase in ovarian cancer risk among BRCA1 mutation carriers. In this study, HRAS1 mutation did not affect breast cancer risk, but suggests that it may alter the expression of a breast cancer susceptibility gene indirectly influencing breast cancer susceptibility (15).
Additional risk factors for breast cancer are age, geographical variation, exposure to radiation, and lifestyle. The susceptibility of breast cancer increases with age, doubling about every 10 years until menopause, when the rates decrease dramatically. Different countries have variable rates showing that environmental factors may have influence over increased susceptibility to breast cancer. With increased fast-food intake and consumption of hormonetreated foods, these habits are questionable to the inducing of breast tumor formation.
Women who have never borne children are at greater risk, while women who have become pregnant and breast feed are at lower risk. Later age of pregnancy may increase susceptibility also, especially after 30 years of age. Atypical changes in ductular epithelium also increases risk, known as atypical epithelial hyperplasia. Obesity is associated with a twofold increase in the risk of breast cancer in postmenopausal women (7). Regular exercise, limiting fat intake, and avoiding long term hormone replacement therapy are actions to take to further decrease risk. Alcohol is even a risk factor. Women who drink half a glass of wine everyday increase their risk of developing breast cancer by 6%. Early and regular screening for breast cancer is crucial to the detection of formation of tumors, such as self-examinations or mammographies.
Early treatment upon finding breast cancer is also important before additional growth occurs.
There has been at least eight different susceptibility genes identified in hereditary breast cancer. BRCA1 and BRCA2 are two of the most important of these, but studies still continue to explore the possibility of other affecting genes. Functions of these genes are continually still being researched to identify the exact mechanisms. This is some of the basis for the development of future potential treatments and medications for breast cancer. The

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