Last Updated: September 15, 2022
BRCA1 Gene and Protein Functions
The BReast CAncer 1, early onset gene (BRCA1) which encodes the breast cancer type 1 susceptibility protein was originally identified in 1994 as a causative gene in hereditary breast and ovarian cancers. The current designation of the BRCA1 gene is BRCA1, DNA repair associated. Another gene shown to be causally associated with inherited breast and ovarian cancers is identified as BRCA2 (BRCA2, DNA repair associated). The BRCA2 gene was also characterized in 1994.
Both the BRCA1 and the BRCA2 genes encode tumor suppressor proteins, defined by the characteristic that loss of their normal function results in an increased risk for the development of cancer. Breast and ovarian cancer risks vary by type and location of BRCA1 or BRCA2 mutations. Although the BRCA1 and BRCA2 encoded proteins are both involved in processes related to the repair of damaged DNA, their functions are distinct.
The cancers that result from mutations in the BRCA1 gene can be defined, in part, by their histopathology. Breast tumors resulting from BRCA1 mutations show an excess of medullary histopathology and are of higher histologic grade than sporadic forms of breast cancer. Histologic grading is a pathology related term that defines the level of abnormality of cancer cells. These BRCA1 mutant cancers are also more likely, than sporadic breast tumors, to be estrogen receptor-negative and progesterone receptor-negative, and are less likely to demonstrate overexpression of the HER2 protein. The HER2 (human EGF receptor 2) protein is encoded by the ERBB2 gene. These later three gene expression related characteristics of BRCA1 mutation involved tumors places them within the category of what is referred to as “triple-negative” breast cancers. Pathogenic mutations in the BRCA2 gene have also been associated with triple-negative breast cancer.
The BRCA1 gene is located on chromosome 17q21.31 and is composed of 24 exons that generate 368 alternatively spliced mRNAs. However, as yet only five have been characterized to encode functional protein. The mRNA that encodes what is considered the full-length BRCA1 protein (also designated as p220) generates a protein of 1863 amino acids. One effect of the alternative splicing of BRCA1 derived mRNAs is that the encoded proteins exhibit different subcellular locations and functions. Some alternative splicing events have been associated with disease causing mutations.
The BRCA1 protein contains a cysteine-rich RING domain in the N-terminus of the protein. The RING designation is derived from Really Interesting New Gene. The RING domain is a zinc-binding domain that is found in numerous other regulatory proteins. In the C-terminal region of the BRCA1 protein there are tandem domains referred to as BRCT (BRCA1 C-terminal) domains. The BRCT domains are found in several other proteins and in particular these domains are integral to the activity of several cell cycle check point proteins that function in response to DNA damage. In the BRCA1 protein the tandem BRCT domains constitute a phosphopeptide recognition domain that binds peptides containing a phosphorylated motif: SXXF. In the SXXF motif the S is Ser which, when phosphorylated, constitutes the form of the motif recognized by the BRCT domain in BRCA1. The X and F in the motif represent any amino acid and Phe, respectively.
BRCA1 as a Ubiquitin Ligase
BRCA1 functions as a ubiquitin ligase when in a heterodimeric complex with another protein identified as BRCA1-associated RING domain protein which is encoded by the BARD1 gene. The BARD1 protein contains an N-terminal RING domain and a C-terminal motif that has significant homology to the BRCT domains that exist near the C-terminus of BRCA1. When BRCA1 forms the heterodimer with BARD1 the complex functions as an E3 ubiquitin ligase. Mutations in the N-terminal RING domain of BRCA1 causes lack of interaction with BARD1 which results in loss of the E3 ubiquitin ligase activity. These BRCA1 mutations are often associated with cancer predisposition.
BRCA1 Functions in DNA Double-Strand Break (DSB) Repair
In addition to the interactions between BRCA1 and BARD1, BRCA1 has been shown to interact with several other proteins through its BRCT domain. These other BRCA1-interacting proteins are involved in numerous aspects of DNA damage repair, especially homologous recombination-mediated DNA double-strand break (HR-DSB) repair. Interaction of the protein ABRAXAS1 with the C-terminal BRCT domain of BRCA1 occurs through the BRCT domain-mediated interaction of the phospho-SXXF motif in the ABRAXAS1 protein. The interaction between BRCA1 and ABRAXAS1 facilitates the recruitment of the complex to sites of DNA damage. BRCA1/ABRAXAS1 complexes are also involved in the cell cycle checkpoint that control transition from the G2 phase of the cell cycle to M-phase.
Another important protein that interacts with the C-terminal BRCT domains of BRCA1, and as such is important to the role of BRCA1 in DSB repair, is BRCA1-interacting protein C-terminal helicase 1 (encoded by the BRIP1 gene). The BRIP1 encoded protein is a member of the RecQ DEAH helicase family. DEAH represents the single letter amino acid codes (Asp, Glu, Ala, His) of the conserved domains in RNA helicases that contain it (referred to as a DEAH-box). The DEAH-box helicases are related to the DEAD-box helicases. The DEAD/DEAH helicases are involved in a variety of processes related to RNA metabolism such as transcription, splicing, transport from the nucleus to the cytoplasm, and RNA decay. The BRIP1 gene has also been called the BRCA1-associated carboxyl-terminal helicase, BACH1 and is also known as the Fanconi anemia J (FANCJ) gene.
When DNA damage occurs, BRCA1/ABRAXAS1 complexes have been shown to bind to regions of DSB through association with the ABRAXAS1-binding complex originally call abraxas receptor-associated protein-80 (RAP80). The human RAP80 encoding gene is identified as UIMC1 (ubiquitin interaction motif containing 1). The UIMC1 protein associates with ubiquitylated histones at sites of DNA damage sites through its tandem ubiquitin-interacting motif domain. Another protein that is one of the first to interact with histones at sites of DNA damage is mediator of DNA damage checkpoint 1 (encoded by the MDC1 gene). Following MDC1 binding, UIMC1 binds, all of which facilitates the BRCA1-mediated repair of DSB. Another important protein in the process of BRCA1-mediated DSB repair was originally identified through its interactions with the BRCA2 protein. This protein is called partner and localizer of BRCA2 and is encoded by the PALB2 gene. BRCA1 is also interacts with RBBP8 encoded protein (retinoblastoma protein-binding protein 8, endonuclease) which was originally identified C-terminal-binding protein 1 (CTBP) interacting protein (CtIP). BRCA1/RBBP8 complexes interact with the MRE11/RAD50 complex that is found directly bound to the ends of DSB. The role of BRCA1/RBBP8 is to promote 5′-end resection of DSB in the early steps of the synthesis-dependent strand annealing pathway of homologous recombination.
BRCA1 in Cell Cycle Control
Cell cycle checkpoint regulators are protein complexes that can induce cell cycle arrest at the G1 to S phase or G2 to M phase transitions and can also induce cell cycle arrest during S phase. The protein complexes that involve BRCA1 in the processes of DNA DSB repair are also referred to as checkpoint regulators.
The BRCA1/UIMC1 complex has been shown to activate G2 to M phase checkpoint signaling. The recruitment of BRCA1/UIMC1 complexes to sites of DNA damage also lead to chromatin retention of BRCA1. The retention of BRCA1 to sites in chromatin promotes phosphorylation of the serine/threonine kinase identified as checkpoint kinase 1 (encoded by the CHEK1 gene) following activation of the G2 to M phase checkpoint activation. The BRCA1/UIMC1 complex requires interaction with RBBP8 to activate DNA damage-induced phosphorylation of CHEK1.
Double-strand breaks in DNA during replication result in stalling of replication forks. BRCA1 is involved in transient inhibition of DNA replication which allows for DNA repair to take place before replication restarts. The interactions between BRCA1 and BRIP1 are required for these replication checkpoints during S phase which suggests that this complex probably works in the normal processes of DNA replication. The changing protein interactions that occur with BRCA1, resulting in control of multiple cell cycle transitions, is the function of the BRCT domain in the BRCA1 protein. Dysfunction in cell cycle checkpoint regulation as a result of impaired formation of the various BRCA1 complexes, due to mutations in the BRCA1 gene, are likely to contribute to cancer predisposition.
Clinical Association of BRCA1 Mutations
Mutations in the BRCA1 gene that are associated with inherited forms of breast and ovarian cancer have been localized to four distinct regions of the BRCA1 encoded protein. There are three regions referred to as breast cancer cluster regions (BCCR) and one referred to as an ovarian cancer cluster region (OCCR). The following designations for these various mutations are using nucleotide numbering of the coding region of the full-length BRCA1 protein. The first BCCR (identified as BCRR1) is located between nucleotides 179 and 505. The BCRR2 is located between nucleotides 4328 and 4945, while the BCCR3 is located between nucleotides 5261 and 5563. The OCCR is located between nucleotides 1380 and 4062. These designation resulted from analysis of DNA from nearly 20,000 female carriers of BRCA1 mutations. In this population of females 46% had breast cancer, 12% had ovarian cancer, 5% had breast and ovarian cancer, and 37% had no cancer.
To date over 1766 different mutations have been identified in the BRCA1 gene. These mutations include missense mutations, nonsense mutations, insertions, deletions, insertions and deletions, and splice site mutations. Very nearly all of the 1766 characterized mutations are pathogenic with the greatest concentration of pathogenic mutations occurring in exon 11. Inherited (germline) mutations in BRCA1 confer a lifetime risk of developing breast cancer of 85% and for developing ovarian cancer of 40%. In addition to breast and ovarian cancers, mutations in BRCA1 have also been associated with lymphoma, leukemia, melanoma, prostate, pancreatic, stomach, and colorectal cancer. However, unlike the clear risks for developing breast and ovarian cancers, the risks for these other cancers in patients with BRCA1 mutations is not clear.