Novel Mechanisms of Genetic Instability in Cancer  

Abstract:  Repetitive sequences have the capacity to adopt 
alternative DNA structures (e.g. H-DNA and Z-DNA) and stimulate 
genetic instability.  **However, the biological significance and 
the mechanisms of this DNA-structure-induced genetic instability 
are unknown.**   Here, we report that H-DNA- and Z-DNA-forming 
sequences are enriched at mutation hotspots in human cancer 
genomes, implicating them in cancer etiology.  We found that 
H-DNA-induced mutations were suppressed in human cells deficient 
in the nucleotide excision repair (NER) nucleases, ERCC1-XPF and 
XPG.  Further, we found that these nucleases cleaved H-DNA.  
Notably, the absence of XPA reduced XPF association with H-DNA, 
consistent with a requirement for functional NER, where XPA 
recruits ERCC1-XPF to H-DNA.  Interestingly, Z-DNA was not 
processed in the same fashion as H-DNA, but instead we identified 
a unique pathway for its mutagenic repair that included the 
mismatch repair protein complex, MSH2-MSH3 and the NER complex, 
ERCC1-XPF.  Surprisingly, the depletion of MSH2 resulted in the 
loss of the enrichment of XPF at the Z-DNA region.  These results 
support a role for a unique functional interaction between 
ERCC1-XPF and MSH2-MSH3 in Z-DNA-induced mutagenesis in a 
mechanism distinct from that of their roles in canonical NER 
and MMR, whereby MSH2-MSH3 binds the Z-DNA region and recruits 
ERCC1-XPF to the site.  These results suggest novel mechanisms 
of genetic instability triggered by H-DNA and Z-DNA through 
distinct structure-specific, cleavage-based pathways, and provide 
critical information on the role that DNA structure may play in 
the etiology of cancer and other human diseases.