Academic journal article Genetics

Sources and Structures of Mitotic Crossovers That Arise When BLM Helicase Is Absent in Drosophila

Academic journal article Genetics

Sources and Structures of Mitotic Crossovers That Arise When BLM Helicase Is Absent in Drosophila

Article excerpt

ABSTRACT The Bloom syndrome helicase, BLM, has numerous functions that prevent mitotic crossovers. We used unique features of Drosophila melanogaster to investigate origins and properties of mitotic crossovers that occur when BLM is absent. Induction of lesions that block replication forks increased crossover frequencies, consistent with functions for BLM in responding to fork blockage. In contrast, treatment with hydroxyurea, which stalls forks, did not elevate crossovers, even though mutants lacking BLM are sensitive to killing by this agent. To learn about sources of spontaneous recombination, we mapped mitotic crossovers in mutants lacking BLM. In the male germline, irradiation-induced crossovers were distributed randomly across the euchromatin, but spontaneous crossovers were nonrandom. We suggest that regions of the genome with a high frequency of mitotic crossovers may be analogous to common fragile sites in the human genome. Interestingly, in the male germline there is a paucity of crossovers in the interval that spans the pericentric heterochromatin, but in the female germline this interval is more prone to crossing over. Finally, our system allowed us to recover pairs of reciprocal crossover chromosomes. Sequencing of these revealed the existence of gene conversion tracts and did not provide any evidence for mutations associated with crossovers. These findings provide important new insights into sources and structures of mitotic crossovers and functions of BLM helicase.

MEIOTIC recombination was discovered 100 years ago by T. H. Morgan and his students in classic studies of Drosophila genetics (Morgan 1911). Since that time, a great deal has been learned about the functions, molecular mechanisms, and regulation of meiotic recombination. This process is initiated through the introduction of programmed DNA doublestrand breaks (DSBs), which are then repaired through highly regulated homologous recombination (HR) pathways such that a substantial fraction of repair events produce reciprocal crossovers (reviewed in Kohl and Sekelsky 2013). The chiasmata that form at sites of crossovers help to ensure accurate segregation of homologous chromosomes. In addition, crossovers generate chromosomes with novel combinations of alleles at linked loci, leading to increased genetic diversity.

A quarter century after the discovery of meiotic recombination, Curt Stern, also working with Drosophila,foundthat crossovers can occur in somatic cells (Stern 1936). This phenomenon is usually called "mitotic recombination," although most such events are thought to occur during interphase rather than in mitosis per se. Compared to meiotic recombination, little is known about mitotic recombination. Except in some specialized cases, like antibody gene rearrangement, mitotic recombination occurs in response to DNA damage (spontaneous or exogenously induced). Mitotic recombination, like meiotic recombination, can be initiated by DSBs, but it is unclear whether DSBs constitute a substantial fraction of the events that initiate spontaneous mitotic recombination.

There are crucial differences in how DSB repair proceeds in mitotically proliferating cells compared to meiotic cells (reviewed in Andersen and Sekelsky 2010). First, meiotic DSB repair uses HR exclusively, whereas mitotically proliferating cells use both HR and homology-independent mechanisms. Second, mitotic HR typically involves use of the sister chromatid as a repair template rather than the homologous chromosome, as in meiosis. Third, a substantial fraction of meiotic DSBs are repaired as crossovers, but HR in mitotic cells tends to occur through pathways that do not produce crossovers.

Structure-selective DNA helicases are major contributors to the prevention of crossovers in mitotic cells. Foremost among these is BLM helicase, so-named because mutations in BLM cause the hereditary disorder Bloom syndrome. The predominant clinical features of Bloom syndrome are small size and a high risk for early onset of a broad range of cancers (German and Ellis 1998). …

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