Multiple Functions of Drosophila BLM Helicase in Maintenance of Genome Stability

Abstract

Bloom Syndrome, a rare human disorder characterized by genomic instability and predisposition to cancer, is caused by mutation of BLM, which encodes a RecQ-family DNA helicase. The Drosophila melanogaster ortholog of BLM, DmBlm, is encoded by mus309. Mutations in mus309 cause hypersensitivity to DNA-damaging agents, female sterility, and defects in repairing double-strand breaks (DSBs). To better understand these phenotypes, we isolated novel mus309 alleles. Mutations that delete the N terminus of DmBlm, but not the helicase domain, have DSB repair defects as severe as those caused by null mutations. We found that female sterility is due to a requirement for DmBlm in early embryonic cell cycles; embryos lacking maternally derived DmBlm have anaphase bridges and other mitotic defects. These defects were less severe for the N-terminal deletion alleles, so we used one of these mutations to assay meiotic recombination. Crossovers were decreased to about half the normal rate, and the remaining crossovers were evenly distributed along the chromosome. We also found that spontaneous mitotic crossovers are increased by several orders of magnitude in mus309 mutants. These results demonstrate that DmBlm functions in multiple cellular contexts to promote genome stability.

Bloom Syndrome Helicase Stimulates RAD51 DNA Strand Exchange Activity through a Novel Mechanism

“…In humans, males homozygous for BLM mutations are infertile, and heterozygous individuals display increased frequencies of chromosome abnormalities in their spermatozoa (1). In Drosophila melanogaster, mutations in the BLM gene reduce the frequency of meiotic recombination and alter cross-over distribution (54). Stimulation of the DNA strand exchange activity of DMC1 may be related to a meiotic function of BLM.…”

Annealing of Complementary DNA Sequences During Double-Strand Break Repair inDrosophilaIs Mediated by the Ortholog of SMARCAL1

“…We used the well-characterized P{w a } assay in the germline of male Drosophila (Figure 3) (Kurkulos et al 1994;McVey et al 2007). P{w a } is a 14-kb P element that is a nonlethal insertion into the first intron of the essential gene scalloped (sd) on the X chromosome (Figure 3, inset).…”

“…In Blm mutants, which are believed to be defective in D-loop disassembly, the synthesis length in repair products is significantly shorter than in wild-type males (McVey et al 2007). In addition, many repair events have deletions from the break site into the flanking sd gene; these are hypothesized to arise from nucleolytic cleavage of D-loops that cannot be disassembled (Adams et al 2003;McVey et al 2004a).…”

mus309 mutation, defective in DNA double-strand break repair, affects intergenic but not intragenic meiotic recombination in Drosophila melanogaster: Corrigendum

“…The error does not affect the main conclusion of the paper, namely that mus309 has an effect on intergenic but not on intragenic meiotic recombination. On the other hand, after the correction the results are now in correspondence with the results of other authors, like McVey et al (2007). The mus309 mutation has a stronger effect on crossing over frequency in the chromosome regions where the frequency of crossovers in the wild type females is lowest in relation to the physical map length of the chromosome than in the regions where crossovers respectively are most common.…”