Academic journal article Genetics

Multiple Functions of Drosophila BLM Helicase in Maintenance of Genome Stability

Academic journal article Genetics

Multiple Functions of Drosophila BLM Helicase in Maintenance of Genome Stability

Article excerpt

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.

BLM is an ATP-dependent helicase that belongs to the RecQ family (ELLIS et al. 1995). Mutations in BLM cause Bloom Syndrome (BS), a rare, autosomal recessive disorder characterized by proportional dwarfism, sterility, and immunodeficiency. BS patients have an increased incidence of many types of cancers, including leukemias, lymphomas, and carcinomas. BS cell lines are genomically unstable, showing a high rate of chromosome breaks and rearrangements and increased exchange between sister chromatids and homologous chromosomes (CHAGANTI et al. 1974; GERMAN et al. 1977).

In vitro, the human BLM protein acts on structures mimicking those formed during DNA replication and recombination. It promotes branch migration of Holliday junctions (HJs) and unwinds HJs and D-loops (KAROW et al. 2000; VAN BRABANT et al. 2000; BACHRATI et al. 2006). Biochemical assays have also revealed a strand-annealing activity that may act in conjunction with its helicase activity (CHEOK et al. 2005; MACHWE et al. 2005). Together, these activities suggest that BLM may function during DNA replication, DNA repair, and/or meiotic recombination. The exact roles that BLM plays in these multiple contexts are currently the subject of intense investigation.

Accumulating evidence suggests that BLM plays an important role in the recovery of damaged and/or stalled replication forks. BLM accumulates at sites of stalled replication forks, where it interacts with repair and checkpoint proteins, including p53, 53BP1, and Chk1 (SENGUPTA et al. 2003, 2004). In addition, in vitro studies have shown that BLM can regress a stalled or collapsed replication fork in such a way that the damage or blockage can be bypassed (RALF et al. 2006).

Other studies suggest that BLM also acts during the repair of DNA double-strand breaks (DSBs). BLM interacts with the homologous recombination repair proteins Rad51, Mlh1, and replication protein A via its N and C termini (BROSH et al. 2000; PEDRAZZI et al. 2001; WU et al. 2001). These interactions, viewed in light of the increased crossover phenotype seen both in BS cells and in embryonic stem cells of BLM knockout mice, are consistent with BLM acting within one or more repair pathways that do not result in crossovers (CHESTER et al. 1998; HU et al. 2005).

BLM may also function in meiotic recombination, but its role in this process is not well understood. In mouse spermatocytes, BLM foci associate with the synaptonemal complex and often colocalize with the recombination proteins RPA, Rad51, and Dmc1 (WALPITA et al. 1999; MOENS et al. 2000). Mutations in SGS1, which encodes the sole RecQ helicase in Saccharomyces cerevisiae, have variable effects on meiotic recombination. …

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