The roles of Brca1 in DNA damage repair during mouse spermatogenesis

Abstract

Germline mutations of the breast cancer associated gene 1 (BRCA1 ) predispose women to breast and ovarian cancers. In mice, loss of function mutations of Brca1 results in recessive lethality during gestation. Analysis of these mutant embryos revealed that Brca1 plays essential roles in maintaining genomic integrity. However the mechanisms underlying these defects remain to be illustrated. To further define the processes through which BRCA1 maintains genomic integrity, meiosis in mutant mice that are homozygous for a targeted deletion of Brca1 exon 11 (Brca1Delta11/Delta11) was studied. Brca1Delta11/Delta11 embryos died at early gestation, but survived to adulthood if either one or both wild type p53 alleles were also mutated. This study showed that all Brca1Delta11/Delta11 p53+/- and Brca1 Delta11/Delta11p53-/- males were infertile and females exhibited decreased fertility. Mutant males had significantly reduced testicular size and contain no spermatozoa even though mutant testes contained spermatogonia, stem cells, and early stage primary spermatocytes; in their seminiferous tubes. Chromosomal analyses on the spermatocytes indicated that mutant homologous chromosomes paired and advanced to the pachytene stage, however mutant mice were unable to form chiasma, and no spermatocytes developed to the diplotene stage. The block of spermatogenesis occurred right after homologous chromosome pairing at meiosis I. Brca1-Delta11 mutation decreased the frequency of Mlh1 foci formation at crossover sites, affected Rad51 foci formation, and interfered with the relocalization of gamma-H2AX from sites of DNA double strand breaks to the XY body during spermatogenesis. The expression levels of a number of genes that are involved in DNA damage repair were reduced including helicase and silent information regulatory proteins. Termination of spermatogenesis at pachytene stage was accompanied by increased rates of apoptosis. cDNA microarray analyses demonstrated changes in expression of genes involved in both p53-dependent and -independent apoptotic pathways. In conclusion, this study revealed an essential role for Brca1 in meiotic recombination repair and regulation of genetic exchange of homologous chromosomes during spermatogenesis. Loss of Brca1 function led to the accumulation of DNA damage, genomic instability, activation of both p53-dependent and -independent apoptotic pathways and, eventually, failure of spermatogenesis.