Deficiency of a novel mismatch repair activity in a bladder tumor cell line

DNA mismatch repair in cancer immunotherapy

Tumors defective in DNA mismatch repair (dMMR) exhibit microsatellite instability (MSI). Currently, patients with dMMR tumors are benefitted from anti-PD-1/PDL1-based immune checkpoint inhibitor (ICI) therapy. Over the past several years, great progress has been made in understanding the mechanisms by which dMMR tumors respond to ICI, including the identification of mutator phenotype-generated neoantigens, cytosolic DNA-mediated activation of the cGAS-STING pathway, type-I interferon signaling and high tumor-infiltration of lymphocytes in dMMR tumors. Although ICI therapy shows great clinical benefits, ∼50% of dMMR tumors are eventually not responsive. Here we review the discovery, development and molecular basis of dMMR-mediated immunotherapy, as well as tumor resistant problems and potential therapeutic interventions to overcome the resistance.

The hMSH2(M688R) Lynch syndrome mutation may function as a dominant negative

The hMSH2(M688R) mismatch repair (MMR) gene mutation has been found in five large families from Tenerife, Spain, suggesting it is a Lynch syndrome or hereditary non-polyposis colorectal cancer (LS/HNPCC) founder mutation. In addition to classical LS/HNPCC tumors, these families present with a high incidence of central nervous system (CNS) tumors normally associated with Turcot or constitutional mismatch repair deficiency (CMMR-D) syndromes. Turcot and CMMR-D mutations may be biallelic, knocking out both copies of the MMR gene. The hMSH2(M688R) mutation is located in the ATP hydrolysis (ATPase) domain. We show that the hMSH2(M688R)-hMSH6 heterodimer binds to mismatched nucleotides but lacks normal ATP functions and inhibits MMR in vitro when mixed with the wild-type (WT) heterodimer. Another alteration that has been associated with LS/HNPCC, hMSH2(M688I)-hMSH6, displays no identifiable differences with the WT heterodimer. Interestingly, some extracolonic tumors from hMSH2(M688R) carriers may express hMSH2-hMSH6, yet display microsatellite instability (MSI). The functional analysis along with variability in tumor expression and the high incidence of CNS tumors suggests that hMSH2(M688R) may act as a dominant negative in some tissues, while the hMSH2(M688I) is most likely a benign polymorphism.

Identification of regulatory factor X as a novel mismatch repair stimulatory factor

This report describes the identification and purification of a novel mismatch repair stimulatory factor from HeLa cell extracts. This activity copurifies with a proliferating cell nuclear antigen-dependent 5 ' --> 3 ' DNA excision activity during several purification steps but is resolved from the excision activity during gel filtration chromatography using Sephacryl S-300. After purification to near homogeneity, the stimulatory factor is associated with three polypeptides with apparent molecular masses of 68, 36, and 30 kDa. Peptide sequencing analysis by tandem mass spectrometry identified the stimulatory factor as the heterotrimeric regulatory factor X (RFX) complex, which regulates transcription of the class II major histocompatibility complex by facilitating histone acetylation and is defective in the human hereditary immunodeficiency syndrome called bare lymphocyte syndrome. This conclusion was confirmed by the facts that purified recombinant RFX stimulates mismatch repair in an in vitro reconstituted mismatch repair system and that depletion of RFX from nuclear extracts or RFX knockdown in cells reduces mismatch repair activity. As expected, RFX knockdown cells display instability in microsatellite sequences. The possible role of RFX in human MMR repair is discussed.

Mechanisms and functions of DNA mismatch repair

DNA mismatch repair (MMR) is a highly conserved biological pathway that plays a key role in maintaining genomic stability. The specificity of MMR is primarily for base-base mismatches and insertion/deletion mispairs generated during DNA replication and recombination. MMR also suppresses homeologous recombination and was recently shown to play a role in DNA damage signaling in eukaryotic cells. Escherichia coli MutS and MutL and their eukaryotic homologs, MutSalpha and MutLalpha, respectively, are key players in MMR-associated genome maintenance. Many other protein components that participate in various DNA metabolic pathways, such as PCNA and RPA, are also essential for MMR. Defects in MMR are associated with genome-wide instability, predisposition to certain types of cancer including hereditary non-polyposis colorectal cancer, resistance to certain chemotherapeutic agents, and abnormalities in meiosis and sterility in mammalian systems.