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Focus on DNA damage and repair
Figure 1: Male fly in which P-element induced
double-strand breaks in eye progenitor cells are
being repaired by homologous recombination
(red patches) and non-homologous end-joining
In order to accurately replicate and pass on their genetic material, cells must
repair DNA damage as it arises. Two of the most dangerous types of DNA damage
are double-strand breaks and interstrand
crosslinks. Failure to repair these lesions can result in cell death by
apoptosis, while inaccurate repair can be mutagenic. Many human diseases,
including Fanconi Anemia, Bloom Syndrome, and other cancer-prone disorders, are caused by defects
in repair mechanisms that deal with DNA breaks and crosslinks.
Homologous recombination repair
Double-strand breaks can be repaired by two main classes of pathways: non-homologous end-joining and homologous recombination. Homologous recombination (HR) utilizes a homologous template for repair and is usually considered to be error-free. However, experiments done using budding yeast suggest that HR can be mutagenic in certain contexts. We have demonstrated that error-prone, translesion polymerases, such as polymerase eta, polymerase zeta, and Rev1, have important roles during HR repair in flies (Kane et al., 2012). We are now testing the hypothesis that HR is also mutagenic in metazoans and that translesion DNA polymerases are responsible for at least a portion of this mutagenesis.
Non-homologous end joining
End joining represents a flexible set of mechanisms that can repair
double-strand breaks when a homologous template is unavailable. Classical end
joining (C-NHEJ) involves the protection, processing, and subsequent ligation of
broken ends, and depends on the Ku70/80 heterodimer and the DNA ligase IV/XRCC4
complex. Cells lacking C-NHEJ proteins can utilize one or more alternative
end-joining (alt-EJ) mechanisms. Currently, alt-EJ is poorly defined.
Modeling genome instability-induced tumorigenesis
Recently, we have shown that flies lacking the Bloom Syndrome DNA helicase, BLM, develop gut and germline tumors early in their adult life (Garcia et al., 2011). Blm mutant flies also have a higher spontaneous mutation frequency and are prone to genome rearrangements and chromosome translocations. Because gut and germline cells continue to proliferate in adult flies, we hypothesize that genome rearrangements that occur in blm mutant adults activate tumor-promoting pathways. We are currently screening other candidate DNA repair mutants for tumor phenotypes.
Figure 2: Two potential pathways of alternative end joining. (i) microhomology-mediated end joining, which involves annealing at pre-existing microhomologous sequences. (ii) synthesis-dependent microhomology-mediated end joining, which involves DNA synthesis and production of nascent microhomologies.
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