Synthetic Lethality of Drosophila in the Absence of the MUS81 Endonuclease and the DmBlm Helicase Is Associated With Elevated Apoptosis

Abstract

Mus81-Mms4 (Mus81-Eme1 in some species) is a heterodimeric DNA structure-specific endonuclease that has been implicated in meiotic recombination and processing of damaged replication forks in fungi. We generated and characterized mutations in Drosophila melanogaster mus81 and mms4. Unlike the case in fungi, we did not find any role for MUS81-MMS4 in meiotic crossing over. A possible role for this endonuclease in repairing double-strand breaks that arise during DNA replication is suggested by the finding that mus81 and mms4 mutants are hypersensitive to camptothecin; however, these mutants are not hypersensitive to other agents that generate lesions that slow or block DNA replication. In fungi, mus81, mms4, and eme1 mutations are synthetically lethal with mutations in genes encoding RecQ helicase homologs. Similarly, we found that mutations in Drosophila mus81 and mms4 are synthetically lethal with null mutations in mus309, which encodes the ortholog of the Bloom Syndrome helicase. Synthetic lethality is associated with high levels of apoptosis in proliferating tissues. Lethality and elevated apoptosis were partially suppressed by a mutation in spn-A, which encodes the ortholog of the strand invasion protein Rad51. These findings provide insights into the causes of synthetic lethality.

Drosophila FANCM Helicase Prevents Spontaneous Mitotic Crossovers Generated by the MUS81 and SLX1 Nucleases

“…In Drosophila, each of the three putative mitotic HJ resolvases (MUS81-MMS4, GEN, and MUS312-SLX1) are essential when BLM is absent (Trowbridge et al 2007;Andersen et al 2009Andersen et al , 2011. The experiments described above show that none of the resolvases are essential when FANCM is absent and that at least males are fertile (we did not assay female fertility since several of these resolvases are required .…”

“…The mus81; Gen Fancm triple mutants survive to the pharate adult stage, which is the same stage at which mus81; Blm double mutants die but much later than lethality of Gen Blm double mutants, which only survive until the second instar larval stage. Preventing recombination partially suppresses mus81; Blm and Gen Blm lethality: mus81; Blm spn-A mutants are semiviable (70% live to adulthood), and Gen Blm spn-A mutants survive to the pupal stage (Trowbridge et al 2007;Andersen et al 2011). To ask whether mus81; Gen Fancm inviability is similarly due to recombination defects, we made mus81; Gen Fancm spn-A quadruple mutants.…”

“…Life cycle stages and Blm mutant results are modified from Andersen et al (2011). meiotic crossovers are generated by a complex whose catalytic subunit is MEI-9, which is orthologous to XPF/Rad1 (Sekelsky et al 1995). No role in generating meiotic crossovers has been detected for MUS81 (Trowbridge et al 2007), so it will be interesting to determine whether the extra meiotic crossovers in Fancm mutants are dependent on MEI-9, MUS81, or another resolvase or combination of resolvases.…”

“…Mitotic crossovers are highly elevated in Drosophila Blm mutants, and these mutants have defects in SDSA and meiotic recombination (Adams et al 2003;McVey et al 2004a,b;Kohl et al 2012). Blm mutations are synthetically lethal with mutations in mus81, mus312 (encodes the ortholog of Slx4), Slx1, or Gen (Trowbridge et al 2007;Andersen et al 2009Andersen et al , 2011. As in yeast, different double mutants have different phenotypes that reveal different functional overlaps.…”

The <i>mus</i>309 mutation, defective in DNA double-strand break repair, increases the frequency of X-ray-induced somatic crossing over in <i>Drosophila melanogaster</i>, but the effect is not dose-rate dependent

“…They are the "dissolution model" [22] and the "disruptase model" [23][24][25]. These models are based on studies of the effect of the mus309 gene on gap repair and on mitotic exchange in the germ line of the males, or on the interaction of mus309 with other genes being involved in DSB repair.…”