Studies of the BRCA2 gene in the Icelandic population have determined that a founder effect also exists in that population; the majority of high-risk breast and ovarian cancer patients share a common haplotype flanking BRCA2, which is associated with 999del5 mutation.55 Research also has shown that 8.5% of breast cancer patients, 7.9% of ovarian cancer patients, and 2.7% of prostate cancer patients under 65 years of age in Iceland carry the 999del5 mutation. Finally, founder mutations have been identified in Finland. These include the 9346-2A>G mutation and the 999del5 mutation, which suggests that individuals from Finland might have introduced this mutation into Iceland. Analysis of haplotypes associated with nine recurrent mutations in BRCA2 suggested that all are ancestral, as no evidence was found of multiple origins of common BRCA2 mutations.
Biological Function of BRCA2
The biological function of the BRCA2 gene is not yet well understood, but like BRCA1, it is thought to play a role in DNA damage response, possibly within some of the same pathways. Importantly, BRCA1 and BRCA2 are now known to be associated with one another, a finding that confirms their function in the same cellular pathways. The regions of the gene that appear to be very similar in human BRCA2 and murine Brca2 have been studied by several groups. Interestingly, both Brca1 and Brca2 are expressed in the mouse mammary gland concurrently, with the steady-state levels of their respective messenger RNAs both up-regulated during pregnancy. Like BRCA1, BRCA2 has been localized to the cell nucleus.
BRCA2 and Transcription
An N-terminal transactivation domain, consisting of repression and activation components, has been identified in BRCA2. Unlike BRCA1, however, BRCA2 has not been shown to interact with the RNA polymerase II holoenzyme, although studies have demonstrated that BRCA2 interacts with both BRCA1 and RAD51. BRCA2 does bind directly to RAD51, which in turn has been shown to bind to both p53 and the RNA polymerase II holoenzyme. These observations suggest a possible role for BRCA2 in regulation of transcription, although no direct BRCA2-DNA interactions have yet been reported.
BRCA2 and Development
BRCA2 hybridization using mouse embryonic tissue showed widespread expression in many developing tissues, suggesting that BRCA2 may play a role in tissue development. Sharan et al. produced the first Brca2 nullizygous mice.
The null allele resulted in embryonic lethality at day 8.5 of embryogenesis, the same time that Brca2 expression levels became detectable in the normal animals. Heterozygous mice were healthy and fertile. Suzuki et al. also demonstrated that embryos lacking functional Brca2 die before day 9.5 of development. The death of the Brca2 nullizygous mice during early postimplantation suggests a role for Brca2 in cellular proliferation and development by Sharan and Suzuki.
The mouse models described carried Brca2 mutations that truncated the protein in the region encoded by exon 10 or early exon 11, eliminating the BRC repeats. However, viable mice with two copies of truncated Brca2 have been generated by two groups. In these animals, truncation of Brca2 occurred in regions more toward the 3’ portion of the gene than in previous models. The allele described by Connor et al. contained seven of the eight BRC repeats. The Friedman et al. mice retained three of the four BRC repeats, were sickly and growth retarded, but survived to adulthood. Some developed thymic lymphomas. Interestingly, these mice also had a marked defect in chromosomal segregation;cultured fibroblasts demonstrated generation of bizarre chromosomal forms in only three to four passages. These data provide support for the hypothesis that BRCA2 is important in recombination, as suggested by high expression levels in the thymus and testes.