MSH6

MSH6 MSH6 is a human DNA mismatch repair protein involved in recognition and initiation of repair of base–base mismatches and insertion–deletion loops. It acts within heterodimeric complexes to preserve genomic stability during DNA replication and recombination, contributing to cancer suppression, cellular responses to DNA damage, and somatic hypermutation. MSH6 dysfunction is implicated in hereditary cancer syndromes and somatic mutation signatures observed across diverse tumors.

Function and Mechanism

MSH6 functions primarily as the partner of MutS homolog 2, forming a heterodimer that recognizes mismatches arising during DNA replication. In concert with MutL homolog 1, the complex coordinates excision and resynthesis through recruitment of exonuclease 1, proliferating cell nuclear antigen, and replication protein A, coupling recognition to downstream processing. The heterodimer distinguishes single base mismatches and small insertion–deletion loops; mismatch binding triggers ATP-dependent conformational changes that signal for strand discrimination and cleavage by downstream effectors. MSH6 also participates in recognizing DNA lesions during immunoglobulin somatic hypermutation in collaboration with Activation-induced cytidine deaminase and interfaces with damage signaling pathways involving ATM kinase and ATR kinase to modulate cell cycle checkpoints.

Structure and Domains

MSH6 contains multiple conserved domains common to the MutS family, including an N-terminal region with a mismatch-recognition motif and a C-terminal ATPase domain with Walker A and B motifs. The N-terminal domain mediates interactions with DNA and the MSH2 partner and contains a PWWP-like region implicated in chromatin association, enabling contacts with nucleosomes and histone marks deposited by complexes such as Polycomb repressive complex 2. The central region includes connector motifs that position DNA binding loops and interact with proteins such as Exonuclease 1 and PCNA. The ATPase domain drives nucleotide-dependent conformational cycling that transitions the heterodimer from a scanning state to a sliding clamp state on duplex DNA, facilitating recruitment of MutL homolog 1 and downstream repair machinery. High-resolution structures derived from cryo-electron microscopy and X-ray crystallography informed by studies on bacterial MutS and eukaryotic homologs reveal conserved structural principles shared with proteins studied by groups at institutions like Max Planck Society and Cold Spring Harbor Laboratory.

Genetic Variants and Clinical Significance

Germline pathogenic variants in the MSH6 coding region cause a subset of cases of hereditary cancer predisposition syndromes characterized by early-onset colorectal, endometrial, and other extracolonic tumors associated with microsatellite instability. These syndromes have been delineated through pedigrees studied at centers such as Johns Hopkins Hospital and Mayo Clinic, informing diagnostic criteria adopted by guideline bodies like the National Comprehensive Cancer Network. Somatic inactivation of MSH6 occurs in sporadic tumors via frameshift, nonsense, or missense mutations and contributes to hypermutated phenotypes observed in sequencing studies from consortia including The Cancer Genome Atlas and International Cancer Genome Consortium. Specific missense substitutions can produce dominant-negative effects or reduce ATPase activity, altering mismatch recognition and repair kinetics; functional assays developed at academic laboratories including University of Cambridge and Harvard Medical School are used to classify variants of uncertain significance. Clinical management decisions, including surveillance and surgical strategies, often reference variant status alongside immunohistochemistry and microsatellite instability testing employed in pathology services at institutions like Memorial Sloan Kettering Cancer Center.

Expression and Regulation

MSH6 expression is cell-cycle regulated, peaking in S phase where replication-associated mismatches are most frequent; this regulation is coordinated with expression of proteins such as Cyclin A2 and Proliferating cell nuclear antigen. Transcriptional control involves regulatory inputs from transcription factors characterized in studies at institutions including University of Oxford and Yale University, while post-translational regulation occurs via ubiquitination, phosphorylation, and proteasomal turnover mediated by E3 ligases and kinases such as Casein kinase 2 and CHK1. Chromatin context and histone modifications—studied in laboratories at Broad Institute and European Molecular Biology Laboratory—influence accessibility of MSH6 to replication forks and recombination intermediates. Alternative splicing and promoter variants modulate tissue-specific levels observed in transcriptomic datasets from projects like the Genotype-Tissue Expression Project. Aberrant epigenetic silencing through promoter methylation has been documented in tumor samples analyzed by groups at Institut Curie and Dana-Farber Cancer Institute.

Interactions and Pathways

MSH6 is central to the DNA mismatch repair pathway through stable interaction with MutS homolog 2 and transient interactions with MutL homolog 1, linking recognition to excision by Exonuclease 1 and repair synthesis by polymerases such as DNA polymerase delta. It engages with replication and chromatin factors including Proliferating cell nuclear antigen, Replication protein A, and histone-modifying complexes to coordinate repair during S phase and at stalled replication forks. Crosstalk with checkpoint kinases ATM kinase and ATR kinase integrates repair with cell-cycle arrest, while interactions with BRCA1-associated complexes and recombination factors modulate responses to double-strand breaks and homologous recombination. Dysregulation of these interactions contributes to tumorigenesis, therapeutic resistance to agents studied clinically at centers like MD Anderson Cancer Center, and shapes mutation signatures characterized by sequencing consortia including PCAWG.

Category:DNA repair proteins