mus309 encodes DmBLM

mus309 encodes a RecQ helicase that is the Drosophila ortholog of BLM, the helicase that is defective in Bloom syndrome. Although we should call this protein MUS309, we have been calling it DmBLM to make the orthology clear to non-Drosophilists. For gene and protein sequence, please refer to the Flybase page for mus309.
 

DmBLM in DSB Repair

Drosophila mus309 mutants are hypersensitive to ionizing radiation, suggesting a role in DSB repair. A hallmark of Bloom syndrome cells is the occurrence of high rates of crossovers between sister chromatids and homologous chromosomes. We have also detected a high frequency of mitotic crossing over in mus309 mutants, both in the male germline and in somatic tissues. Two models have been proposed to explain the ability of BLM to prevent crossing over:

DHJ dissolution. In vitro, BLM and the associated protein TOP3α can carry out the dissolution of double-Holliday junction structures, as shown at right. The DHJ is thought to be a key intermediate in generating meiotic crossovers, at least in some organisms. Thus, many researches believe the dissolvase activity of BLM explains the ability of this protein to prevent crossovers, because in the absence of BLM, DHJs undergo resolution, which can lead to crossovers.

D-loop dissociation. Our studies with mus309 mutants led us to propose an alternative explanation for the increased crossovers in the absence of BLM: the disruptase model. In this model, BLM promotes dissociation of the nascent strand from a D-loop, as shown at right, thereby promoting SDSA. This activity has also been well documented in vitro, and has been suggested to account for the anti-recombination activity of E. coli RecQ and the ability of RecQ to prevent illegitimate recombination.

We believe that both the dissolvase and disruptase activities are important to BLM function, but that the disruptase activity is likely to predominate during mitotic DSB repair, and the dissolvase activity during some types of replication fork repair and when resolution is blocked during meiotic recombination. For details, see

M.D. Adams, M. McVey, and J. Sekelsky (2003) Drosophila BLM in double-strand break repair by synthesis-dependent strand annealing. Science 299: 265-267.

M. McVey, J. LaRocque, M.D. Adams, and J. Sekelsky (2004) Formation of deletions during double-strand break repair in Drosophila DmBlm mutants occurs after strand invasion. Proc. Natl. Acad. Sci. USA 101: 15694-15699.

M. McVey, S. Andersen, Y Broze, and J. Sekelsky (2007) Multiple functions of the Drosophila Blm helicase in maintaining genome stability. Genetics 176: 1979-1992.

DmBLM in Replication Fork Repair

Eukaryotic RecQ helicases have long been thought to play important roles in responding to blocked or broken replication forks. It is likely that the high frequency of mitotic crossing over in mus309 mutants is a consequence of the inability of these mutants to properly respond to spontaneous DSBs that arise from replication forks problems. Another manifestation of this function is that mus309 mutations are synthetically lethal with mutations in mus81 or mms4. For our interpretation of the causes of this lethality, please refer to our mus81 page.
 

DmBLM in Embryonic Development

Females carrying null mutations in mus309 are almost completely sterile. They lay fertilized eggs with normal morphology, but only a few percent of these hatch into larvae; those that hatch make it to adulthood, indicating DmBLM is only essential during early embryogenesis. By staining DNA in fixed embryos, we found frequent anaphase bridges in syncytial blastoderm embryos. In these stages, replication is extremely rapid - only 6-8 minutes. This is possible because a large number of replication origins are fired. A consequence of this is that there are a large number of converging replication forks. We hypothesize that DmBLM facilitates resolution of converging forks. This function appears to be separate from the function of DmBLM in DSB repair, because we isolated an allele that permits female fertility while preventing normal DSB repair. For details, see

M. McVey, S. Andersen, Y Broze, and J. Sekelsky (2007) Multiple functions of the Drosophila Blm helicase in maintaining genome stability. Genetics 176: 1979-1992.

DmBLM in Meiotic Recombination

The allele mentioned above has allowed us to assess meiotic recombination defects in mus309 mutants. We measured crossovers and found that they are decreased by 50% overall (in contrast to the increase in mitotic crossovers). Furthermore, the distribution of COs was essentially randomized. We have not tried to intrepet this finding, because some of these crossovers are presumably pre-meiotic, and this is only a separation-of-function allele. For details, see

M. McVey, S. Andersen, Y Broze, and J. Sekelsky (2007) Multiple functions of the Drosophila Blm helicase in maintaining genome stability. Genetics 176: 1979-1992.