Structural, molecular and cellular functions of MSH2 and MSH6 during DNA mismatch repair, damage signaling and other noncanonical activities

Abstract

The field of DNA mismatch repair (MMR) has rapidly expanded after the discovery of the MutHLS repair system in bacteria. By the mid 1990s yeast and human homologues to bacterial MutL and MutS had been identified and their contribution to hereditary non-polyposis colorectal cancer (HNPCC; Lynch Syndrome) was under intense investigation. The human MutS homologue 6 protein (hMSH6), was first reported in 1995 as a G:T binding partner (GTBP) of hMSH2, forming the hMutSα mismatch-binding complex. Signal transduction from each DNA-bound hMutSα complex is accomplished by the hMutLα heterodimer (hMLH1 and hPMS2). Molecular mechanisms and cellular regulation of individual MMR proteins are now areas of intensive research. This review will focus on molecular mechanisms associated with mismatch binding, as well as emerging evidence that MutSα and in particular, MSH6, is a key protein in MMR-dependent DNA damage response and communication with other DNA repair pathways within the cell. MSH6 is unstable in the absence of MSH2, however it is the DNA lesion-binding partner of this heterodimer. MSH6, but not MSH2, has a conserved Phe-X-Glu motif that recognizes and binds several different DNA structural distortions, initiating different cellular responses. hMSH6 also contains the nuclear localization sequences required to shuttle hMutSα into the nucleus. For example, upon binding to O6meG:T, MSH6 triggers a DNA damage response that involves altered phosphorylation within the N-terminal disordered domain of this unique protein. While many investigations have focused on MMR as a post-replication DNA repair mechanism, MMR proteins are expressed and active in all phases of the cell cycle. There is much more to be discovered about regulatory cellular roles that require the presence of MutSα and, in particular, MSH6.

Mismatch repair single nucleotide polymorphisms and thyroid cancer susceptibility

“…Firstly, because it is biologically plausible: MSH6 integrates the MutSα complex, a sensor of genetic damage that, besides its role in the repair of replication errors, cooperates with other DNA repair and damage-response signalling pathways to allow for cell cycle arrest, DNA repair and/or apoptosis of genetically damaged cells. Of importance for DTC susceptibility, MutSα ensures accurate homologous recombination repair of double strand breaks and cooperates with MUTYH in the repair of 8-oxoGuanine [reviewed in (27)(28)(29)], lesions that commonly arise from IR exposure, the most well-known DTC risk factor. Secondly, because of the functional impact of MSH6 mutations: Experimental studies in MSH6-deficient yeast, mice or human cells demonstrate that MSH6 mutations results in partial MMR deficiency (mild mutator phenotype, characterized by weak microsatellite instability, MSI-L) and increased cancer susceptibility in animal models (27).…”

“…Interestingly, the MSH6 NTR is absent from prokaryotic MutS which, coincidentally, does not share some of the functions of eukaryotic MutSα (e.g., activation of apoptosis) (63), suggesting a critical role for this region in such processes. As extensively reviewed in Edelbrock et al (28), several sequence motifs in the NTR may be of relevance to the multitude of actions performed by MSH6, including: 1) a short, conserved PCNA interacting protein (PIP) motif, located near the N-terminal extreme, that allows PCNA binding; 2) a PWWP sequence motif, distal to the PIP box, that mediates interactions with chromatin and chromatin-associating proteins; 3) a conserved motif near the NTR C-terminus, rich in positively charged amino acids that (through electrostatic attraction) contributes to nonspecific DNA binding and stabilizes the MutSα-DNA interaction (possibly modulating the residence time of MutSα at the lesion site); 4) nuclear localization sequences (NLSs, e.g., a conserved Ser-Pro-Ser sequence-amino acids 41-43-containing phosphorylated serines), that may contribute to the nuclear import of MutSα; and 5) multiple phosphorylation sites (19 out of the 23 identified in MSH6, according to the updated list at http://www.uniprot.org/uniprot/P52701), that may be involved in the post-translational regulation of MutSα stability, nuclear import and differential downstream signalling for MMR and DNA damage response. The NTR may also be responsible for other protein interactions.…”

“…However, given its location-in the MSH6 NTR, near a NLS containing two phosphorylation sites (Ser41 and Ser43)-it is possible that rs1042821 interferes with phosphorylation of these residues (both MAPK recognition motifs) and hence with the post-translational regulation of MutSα stability, nuclear import or activity (28). The rs1042821 SNP may also interfere with non-specific DNA binding since glutamate, contrasting with glycine, is negatively charged at physiological pH.…”

“…MMR thus prevents base substitutions or repeat sequence instability, greatly increasing DNA replication fidelity and safeguarding genomic integrity (27). MMR also participates, among other cellular processes (e.g., mitotic and meiotic recombination, immunoglobulin class switching), in the recognition of DNA damage induced by genotoxic chemicals, UV light, IR or oxidative stress (e.g., oxidative lesions, double strand breaks, pyrimidine dimers and inter-strand crosslinks) and subsequent repair (in cooperation with other repair pathways) or damage-induced cytotoxicity (downstream signalling for cell cycle arrest and apoptosis) (28)(29)(30). MMR's role is therefore critical to carcinogenesis: loss of MMR (e.g., inactivating mutation) greatly increases the rate of spontaneous mutation, leading to a mutator phenotype, and results in microsatellite instability (MSI), a hallmark of MMR defects (27,29,31).…”

Clinicopathologic Features and Germline Sequence Variants in Young Patients (≤40 Years Old) With Pancreatic Ductal Adenocarcinoma

“…However, these occur in the immediate vicinity of the stop codon, which indicates that the transcript is translated normally. 25 It has also been reported that critical domains are concentrated within the N-terminal side of MSH6, [26][27][28] whereas mutations in the ATP binding domain within the C-terminal side of MSH6 are unlikely to affect the mismatch binding activity biochemically. 29 Therefore, the clinical significance and functional effects of these 2 variants in MSH6 were not clear, so they were classified as VUS.…”