BRCA1

BRCA1 (breast cancer 1, early onset) is a human gene, some mutations of which are associated with a significant increase in the risk of breast cancer, as well as other cancers. BRCA1 belongs to a class of genes known as tumor suppressors, which maintains genomic integrity to prevent uncontrolled proliferation. The multifactorial BRCA1 protein product is involved in DNA damage repair, ubiquitination, transcriptional regulation as well as other functions.

Gene location

The BRCA1 gene is located on the long (q) arm of chromosome 17 at band 21, from base pair 38,449,843 to base pair 38,530,933

BRCA1 2 pathway prevents leukemias and lymphomas


Function and mechanism
DNA damage repair

The BRCA1 protein is directly involved in the repair of damaged DNA. In the nucleus of many types of normal cells, the BRCA1 protein is thought to interact with RAD51 during repair of DNA double-strand breaks, though the details and significance of this interaction is the subject of debate. These breaks can be caused by natural radiation or other exposures, but also occur when chromosomes exchange genetic material (homologous recombination, e.g. "crossing over" during meiosis). The BRCA2 protein, which has a function similar to that of BRCA1, also interacts with the RAD51 protein. By influencing DNA damage repair, these three proteins play a role in maintaining the stability of the human genome.

BRCA1 directly binds to DNA, with higher affinity for branched DNA structures. This ability to bind to DNA contributes to its ability to inhibit the nuclease activity of the MRN complex as well as the nuclease activity of Mre11 alone. This may explain a role for BRCA1 to promote higher fidelity DNA repair by NHEJ. BRCA1 also colocalizes with γ-H2AX (histone H2AX phosphorylated on serine-139) in DNA double-strand break repair foci, indicating it may play a role in recruiting repair factors.

Transcription

BRCA1 was shown to co-purify with the human RNA Polymerase II holoenzyme in HeLa extracts, implying it is a component of the holoenzyme. Later research, however, contradicted this assumption, instead showing that the predominant complex including BRCA1 in HeLa cells is a 2 megadalton complex containing SWI/SNF. SWI/SNF is a chromatin remodeling complex. Artificial tethering of BRCA1 to chromatin was shown to decondense heterochromatin, though the SWI/SNF interacting domain was not necessary for this role.BRCA1 interacts with the NELF-B (COBRA1) subunit of the NELF complex.

Mutations and cancer risk

Certain variations of the BRCA1 gene lead to an increased risk for breast cancer. Researchers have identified more than 600 mutations in the BRCA1 gene, many of which are associated with an increased risk of cancer.

These mutations can be changes in one or a small number of DNA base pairs (the building blocks of DNA). Those mutations can be identified with PCR and DNA sequencing.

In some cases, large segments of DNA are rearranged. Those large segments, also called large rearrangements, can be a deletion or a duplication of one or several exons in the gene. Classical methods for mutations detection(sequencing) are unable to reveal those mutations. Other methods are proposed: Q-PCR, Multiplex Ligation-dependent Probe Amplification (MLPA), and Quantitative Multiplex PCR of Shorts Fluorescents Fragments (QMPSF). New methods have been recently proposed: heteroduplex analysis (HDA) by multi-capillary electrophoresis or also dedicated oligonucleotides array based on comparative genomic hybridization (array-CGH).

A mutated BRCA1 gene usually makes a protein that does not function properly because it is abnormally short. Researchers believe that the defective BRCA1 protein is unable to help fix mutations that occur in other genes. These defects accumulate and may allow cells to grow and divide uncontrollably to form a tumor.

In addition to breast cancer, mutations in the BRCA1 gene also increase the risk on ovarian, fallopian tube and prostate cancers. Moreover, precancerous lesions (dysplasia) within the Fallopian tube have been linked to BRCA1 gene mutations.