BRCA2 GENE; BRCA2

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BRCA2 GENE; BRCA2

CLONING

Wooster et al. (1995) identified the BRCA2 gene by positional cloning of a region on chromosome 13q12-q13 implicated in Icelandic families with breast cancer (612555). The candidate disease gene was likely to be located in a 600-kb interval centered around D13S171. Using yeast artificial chromosome and P1 artificial chromosome contigs to identify trapped exons within that region, Wooster et al. (1995) screened human fetal brain, placental, monocyte, and breast cancer cDNA libraries. They identified a cDNA encoding a 2,329-amino acid protein, but suggested that it may not represent the entire gene. Northern blot analysis demonstrated expression in normal breast epithelial cells, placenta, and a breast cancer cell line (MCF7).

Tavtigian et al. (1996) determined the complete coding sequence and exonic structure of BRCA2 and examined its pattern of expression. The composite BRCA2 cDNA sequence assembled consisted of 11,385 bp, but did not include the polyadenylation signal or poly(A) tail. Conceptual translation of the cDNA revealed an ORF beginning at nucleotide 229 and encoding a protein of 3,418 amino acids. There was no signal sequence at the end of terminus, and there were no obvious membrane-spanning regions. The highest levels of expression were observed in breast and thymus, with slightly lower levels in lung, ovary, and spleen. Tavtigian et al. (1996) noted that the BRCA2 protein, like the BRCA1 protein (113705), is highly charged; roughly one-quarter of the residues are acidic or basic.

Connor et al. (1997) described the mouse Brca2 gene. They sequenced cDNA for the entire 3,329-amino acid Brca2 protein and found that, like Brca1, Brca2 is relatively poorly conserved between humans and mice (approximately 60%). Brca2 was transcribed in a diverse range of mouse tissues, especially the testis, ovary, and midgestation embryo. Brca2 was also expressed in the mammary gland and was apparently induced upon pregnancy. The pattern of expression was strikingly similar to that of Brca1.

Warren et al. (2002) cloned and characterized the chicken Brca2 gene. The gene is organized similarly to the human BRCA2 gene, but is more compact. The chicken gene encodes a protein of 3,399 amino acids, which is poorly conserved with mammalian BRCA2 proteins, having only 37% overall amino acid sequence identity with human BRCA2. However, certain domains are much more highly conserved, indicating functional significance. The authors speculated that knowledge of the evolutionarily divergent chicken Brca2 sequence may be useful in distinguishing sequence variants from mutations in the human BRCA2 gene.


GENE STRUCTURE

Tavtigian et al. (1996) determined that the human BRCA2 gene contains 27 exons. They noted that both the BRCA1 and BRCA2 genes have a large exon 11, translational start sites in exon 2, and coding sequences that are AT-rich; both span approximately 70 kb of genomic DNA and are expressed at high levels in testis.


MAPPING

Wooster et al. (1994) mapped the BRCA2 gene to chromosome 13q12-q13.

Couch et al. (1996) generated a detailed transcription map of the 1.0-Mb region on 13q12-q13 containing the BRCA2 gene. Evidence for 7 genes, 2 putative pseudogenes, and 9 additional putative transcription units was obtained.

Connor et al. (1997) found that the mouse Brca2 gene maps to mouse chromosome 5, consistent with its localization on human 13q12.


GENE FUNCTION

Jensen et al. (1996) noted that BRCA2 includes a motif similar to the granin consensus at the C terminus of the protein. BRCA1 also has sequence homology and biochemical analogy to the granin protein family.

Studying the expression of Brca2 in murine mammary epithelial cells as a function of proliferation and differentiation, Rajan et al. (1996) demonstrated that Brca2 mRNA expression is tightly regulated during mammary epithelial proliferation and differentiation, and appears to be coordinately regulated with Brca1 expression. Both genes showed mRNA expression that was upregulated in rapidly proliferating cells; was downregulated in response to serum deprivation; was expressed in a cell cycle-dependent manner, peaking at the G1/S boundary; and was upregulated in the differentiating mammary epithelial cells in response to glucocorticoids. The results suggested that these genes are induced by, and may function in, overlapping regulatory pathways involved in the control of cell proliferation and differentiation.

Milner et al. (1997) showed that the portion of human BRCA2 encoded by its third exon shares homology with a known transcription factor and is capable of activating transcription, thus indicating a potential function of BRCA2. The exon 3 sequence at the N terminus of BRCA2 (within a region highly conserved between human and mouse) showed sequence similarity to the activation domain of JUN (165160). They found that the activation potential within exon 3 is under negative control of inhibitory regions (IR1 and IR2) present immediately on either side of exon 3. The finding that BRCA2, like BRCA1, has transcriptional activation potential provides functional evidence of a relationship between the 2 proteins. Indeed, the fact that mutations found naturally in breast cancers disrupt the activation potential of both BRCA1 and BRCA2, indicates that compromising this activity may be an important step in the generation of a subset of familial breast cancers. Mutations found outside these activation domains may affect other functions.

Daniels et al. (2004) showed that BRCA2 deficiency impairs the completion of cell division by cytokinesis. Brca2 inactivation in mouse embryo fibroblasts (MEFs) and HeLa cells by targeted gene disruption or RNA interference delayed and prevented cell cleavage. Impeded cell separation was accompanied by abnormalities in myosin II organization during the late stages in cytokinesis. Daniels et al. (2004) suggested that BRCA2 may have a role in regulating these events, as it localizes to the cytokinetic midbody. The authors concluded that their findings linked cytokinetic abnormalities to a hereditary cancer syndrome characterized by chromosomal instability and may help to explain why BRCA2-deficient tumors are frequently aneuploid.

The complete scientific article may be viewed online.