Similar Effects of Brca2 Truncation and Rad51 Paralog Deficiency on Immunoglobulin V Gene Diversification in DT40 Cells Support an Early Role for Rad51 Paralogs in Homologous Recombination

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Abstract
BRCA2 is a tumor suppressor gene that is linked to hereditary breast and ovarian cancer. Although the Brca2 protein participates in homologous DNA recombination (HR), its precise role remains unclear. From chicken DT40 cells, we generated BRCA2 gene-deficient cells which harbor a truncation at the 3′ end of the BRC3 repeat (brca2tr). Comparison of the characteristics of brca2tr cells with those of other HR-deficient DT40 clones revealed marked similarities with rad51 paralog mutants (rad51b, rad51c, rad51d, xrcc2, or xrcc3 cells). The phenotypic similarities include a shift from HR-mediated diversification to single-nucleotide substitutions in the immunoglobulin variable gene segment and the partial reversion of this shift by overexpression of Rad51. Although recent evidence supports at least Xrcc3 and Rad51C playing a role late in HR, our data suggest that Brca2 and the Rad51 paralogs may also contribute to HR at the same early step, with their loss resulting in the stimulation of an alternative, error-prone repair pathway.