Double-Strand Breaks Induce Nuclear-Cytosolic Shuttling of Polymorphic DNA Mismatch Repair Protein MutS Homolog 3 and Binding to NEMO/IKKγ in Colon Cancer Cells.

Background and Aims: Human MutS homolog 3 (MSH3) is a DNA mismatch repair protein that recognizes postsynthetic slippage mistakes at microsatellite sequences consisting of dinucleotide or longer repeats in concert with its heterodimer partner MSH2. MSH3 has also been implicated in DNA double-strand break (DSB) repair as part of Homologous Recombination. Loss of MSH3 function is an acquired somatic defect seen in 50% of colorectal cancers and triggered by proinflammatory interleukin-6 signaling, causing a reversible nuclear-to-cytoplasmic shift of the protein. With MSH3's shift to the cytosol, microsatellite frameshift mutations accumulate. Here we examined MSH3 and Δ27bpMSH3, an MSH3 polymorph that alters function of MSH3's nuclear localization signal with enhancement for cytosolic retention with interleukin-6 and oxidative stress, for evidence of dysfunction with induced DSBs. Methods: We employed immunofluorescent microscopy, nuclear-cytosolic protein fractionation, immunoprecipitation and quantitative reverse transcriptase polymerase chain reaction to track the location and amount of MSH3 and Δ27bpMSH3 after inducing DSBs with etoposide. Results: Cells containing polymorphic Δ27bpMSH3 were more susceptible to induced DSBs, and induced DSBs triggered nuclear-to-cytosolic shift of Δ27bpMSH3 that involved binding to the NFκB protein NEMO/IKKγ. Conclusion: These findings suggest that inflammatory proteins are important for Δ27bpMSH3 translocation to the cytosol but also for stabilizing MSH3 when separated from nuclear MSH2.