Conservation of transcriptional regulation by BRCA1 and BARD1 in Caenorhabditis elegans.

The tumor-suppressor proteins BRCA1 and BARD1 function as an E3 ubiquitin ligase to facilitate transcriptional repression and DNA damage repair. This is mediated in-part through its ability to mono-ubiquitylate histone H2A in nucleosomes. Studies in Caenorhabditis elegans have been used to elucidate numerous functions of BRCA1 and BARD1; however, it has not been established that the C. elegans orthologs, BRC-1 and BRD-1, retain all the functions of their human counterparts. Here we explore the conservation of enzymatic activity toward nucleosomes which leads to repression of estrogen-metabolizing cytochrome P450 (cyp) genes in humans. Biochemical assays establish that BRC-1 and BRD-1 contribute to ubiquitylation of histone H2A in the nucleosome. Mutational analysis shows that while BRC-1 likely binds the nucleosome using a conserved interface, BRD-1 and BARD1 have evolved different modes of binding, resulting in a difference in the placement of ubiquitin on H2A. Gene expression analysis reveals that in spite of this difference, BRC-1 and BRD-1 also contribute to cyp gene repression in C. elegans. Establishing conservation of these functions in C. elegans allows for use of this powerful model organism to address remaining questions regarding regulation of gene expression by BRCA1 and BARD1.

One of these strains is not like the others: C. elegans DW102 has an altered dependence on brc-1 and brd-1 for regulation of cyp gene transcription.

Several strains of Caenorhabditis elegans with mutations in brc-1 or brd-1 are readily available to aid in elucidating the functions of these two genes in DNA damage repair, meiosis, and gene repression. DW102 is the only C. elegans strain to our knowledge with mutations in both brc-1 and brd-1 . However, several groups have reported the DW102 strain is indistinguishable from wild-type when observing levels of embryonic lethality, sensitivity to radiation, and rates of male progeny, while strains with mutations in either brc-1 or brd-1 display increased occurrence of these phenotypes. Here, RT-qPCR analysis of the cyp-13A gene family, reveals distinctive and aberrant expression patterns in DW102 compared to other brc-1 or brd-1 mutant strains underscoring the need for caution in choosing this strain to draw conclusions about brc-1 and brd-1 functions.