Chromatin Structure and DNA Repair

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Previously, we discovered mutations in the mu2 gene in the fruit fly, Drosophila, that allow the recovery of chromosome aberrations that have lost a natural telomere and regained a structure that protects the chromosome end. These neotelomeres have lost the DNA motifs normally associated with telomeres, but retain the proteins that protect the chromosome ends and distinguish them from chromosome breaks. Recently, we found that the MU2 protein is distributed along chromosome arms in the absence of DNA damage, but upon radiation induced damage to DNA redistributes to the radiation-induced repair foci. The protein also accumulates at sites of meiotic recombination, which are induced by DNA double strand breaks. When present in these foci, MU2 acts as a scaffold, with one end of the protein binding to a complex of the repair proteins MRE11, RAD50 and NBS and the other end to a phosphorylated form of variant histone H2Av known as gammaH2Av. (Drosophila H2Av is an ortholog of human H2AX.) Based on sequence alignments, domain structure and protein function, MU2 appears to be an ortholog of the human MDC1 protein. Mutations in the mu2 gene cause a decrease in the number and size of radiation induced repair foci and meiotic recombination foci. The rate of DNA repair appears to be reduced, although repair is not blocked. Similarly, cell cycle regulation in response to DNA damage is decreased in these mutants, but not blocked entirely. In an attempt to understand the interaction of factors that control telomere stability and chromatin structure, we are looking for chromatin proteins that interact with MU2. One of these is heterochromatin protein 1 (HP1a), which binds to MU2 in the absence of DNA damage. After radiation treatment, MU2 is drawn to repair foci throughout the nucleus. Repair foci form in heterochromatin, which is HP1a rich, condensed chromatin, within a few seconds of treatment, then they are removed from heterochromatic to nuclear domains with more relaxed euchromatin. RNAi knockdown of HP1a prevents the removal of foci from heterochromatin, and increases G2/M arrest and apoptosis. These observations suggest that DNA repair requires a relaxed chromatin structure to proceed. The scaffold protein that holds the complex together in Drosophila also interacts with proteins that regulate the timing of oogenesis. When we reduce the amount of MU2 protein using a mutation, the level of one regulatory protein ORB, a Drosophila homolog of mammalian cytoplasmic polyadenylation element binding protein (CPEB), is also decreased, as are down stream effector proteins. The progression of chromosomal events thus temporarily becomes delayed relative to the cellular events of oogenesis, although the mutant oocytes eventually recover and mutant females are fertile. These results are consistent with the idea that there is communication between chromosomal events during meiosis and developmental progression of oogenesis in order to keep them in synch.