117 Non-B DNA structure-induced genetic instability in mammalian cells

Abstract

Repetitive regions of genomic DNA can adopt a number of different structures, e.g. Z-DNA and they are at risk for increased genetic instability and disease development [As reviewed in (Zhao, Bacolla, Wang, & Vasquez, 2010)]. We found Z-DNA structure is intrinsically mutagenic in cultured mammalian cells and in chromosomes of transgenic mice (Wang Carbajal, Vijg, DiGiovanni, & Vasquez, 2008). Our recent data showed CG14 repeats, a model Z-DNA-forming sequence, could stall DNA polymerase progress in mammalian COS-7 cells, while AT14 repeats that does not form Z-DNA conformation, did not affect replication. These data suggested a replication-dependent model of Z-DNA-induced mutagenesis in mammalian cells. In addition, we also found that Z-DNA caused mutation and DNA breakage in the absence of DNA replication, and the spectrum of mutation from unreplicated system tended to have more large deletions than replicated DNA, suggested a replicated-independent pathway of Z-DNA-induced genetic instability (Wang, Christensen, & Vasquez, 2004). Several DNA repair proteins were found to be involved in Z-DNA induced mutagenesis, but in different ways: adding XPA protein back in, otherwise, deficient XP12RO cells increased Z-DNA-induced mutation frequency by −50%, while expressing MSH2 protein in Hec59 cells reduced Z-DNA-induced mutation frequency by −25%. Interestingly, cytosine methylation in CpG repeats induced higher frequencies of deletions and rearrangements with a broader mutant size distribution than that found with unmethylated CpG repeats. We discovered that unusual structural features of the methylated CpG sequences precluded local nucleosome assembly leading to genetic instability. Thus, non-B DNA-induced mutation appears to be a very complex process where DNA replication, repair and epigenetic modification are all involved.