Eukaryotic DNA mismatch repair (MMR) involves both exonuclease 1 (Exo1)-dependent and Exo1-independent pathways. We found that the unstructured C-terminal domain of Saccharomyces cerevisiae Exo1 contains two MutS homolog 2 (Msh2)-interacting peptide (SHIP) boxes downstream from the MutL homolog 1 (Mlh1)-interacting peptide (MIP) box. These three sites were redundant in Exo1-dependent MMR in vivo and could be replaced by a fusion protein between an N-terminal fragment of Exo1 and Msh6. The SHIP-Msh2 interactions were eliminated by the msh2 mutation, and wild-type but not mutant SHIP peptides eliminated Exo1-dependent MMR in vitro. We identified two S. cerevisiae SHIP-box-containing proteins and three candidate human SHIP-box-containing proteins. One of these, Fun30, had a small role in Exo1-dependent MMR in vivo. The Remodeling of the Structure of Chromatin (Rsc) complex also functioned in both Exo1-dependent and Exo1-independent MMR in vivo. Our results identified two modes of Exo1 recruitment and a peptide module that mediates interactions between Msh2 and other proteins, and they support a model in which Exo1 functions in MMR by being tethered to the Msh2-Msh6 complex.

In the version of this article initially published, some of the information in Supplementary Table 3 was incorrect. The correct Supplementary Table 3 is now provided online. The error has been corrected in the HTML version of the article.

Colorectal cancer (CRC) is one of the leading causes of cancer deaths, with an increasing rate of CRC diagnosis in younger individuals. MMR is the DNA repair mechanism that repairs DNA mispairs and small insertions or deletions remaining after replication. MMR is also required for apoptosis after certain types of exogenous DNA damage that result in damage-associated mispairs. MMR defects are the underlying cause of Lynch syndrome, a familial cancer predisposition syndrome that increases susceptibility to multiple cancers, specifically colorectal cancer. MMR defects are also commonly found in sporadic colorectal cancers. Model systems such as Saccharomyces cerevisiae, Escherichia coli, and human cell lines have been used to study the MMR proteins and pathways. The basics of the MMR mechanism are fairly well understood; however, proteins associated with MMR are still being identified, and the roles of these proteins are largely unknown. We have identified the yeast protein Rad5 as an interactor with the yeast MMR proteins Msh2 and Mlh1. Rad5 is a helicase and an E3 ubiquitin ligase which is involved in post-replicative repair and damage tolerance. However, to date, Rad5 has no known role in MMR. We have determined that these interactions are conserved through evolution to human Rad5 homologs, HLTF and SHPRH. The Rad5 interactions with Mlh1 and Msh2 appear to be split between the human homologs with human Msh2 interacting with HLTF and human Mlh1 interacting with SHPRH. We have found that loss of SHPRH induces resistance to MMR-mediated apoptosis. Current experiments are in progress to determine the binding sites between these proteins. We are also investigating what functional impact the Rad5 homologs have on mutation rate and MMR-induced apoptosis and how the interactions affect the roles of MMR. Together this will allow for a deeper understanding of how accessory proteins may influence canonical and non-canonical MMR. Since MMR status is currently used to determine patient treatment, understanding how commonly mutated accessory factors interact is critical. Citation Format: Anna Kristin Miller, Guogen Mao, Christine Rahal, Christopher Putnam, Eva Goellner. HLTF and SHPRH in mismatch repair and cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 799.

DNA mismatch repair (MMR) repairs replication errors, and MMR defects play a role in both inherited cancer predisposition syndromes and in sporadic cancers. MMR also recognizes mispairs caused by environmental and chemotherapeutic agents; however, in these cases mispair recognition leads to apoptosis and not repair. Although mutation avoidance by MMR is fairly well understood, MMR-associated proteins are still being identified. We performed a bioinformatic analysis that implicated Saccharomyces cerevisiae Rad5 as a candidate for interacting with the MMR proteins Msh2 and Mlh1. Rad5 is a DNA helicase and E3 ubiquitin ligase involved in post-replicative repair and damage tolerance. We confirmed both interactions and found that the Mlh1 interaction is mediated by a conserved Mlh1-interacting motif (MIP box). Despite this, we did not find a clear role for Rad5 in the canonical MMR mutation avoidance pathway. The interaction of Rad5 with Msh2 and Mlh1 is conserved in humans, although each of the Rad5 human homologs, HLTF and SHPRH, shared only one of the interactions: HLTF interacts with MSH2, and SHPRH interacts with MLH1. Moreover, depletion of SHPRH, but not HLTF, results in a mild increase in resistance to alkylating agents although not as strong as loss of MMR, suggesting gene duplication led to specialization of the MMR-protein associated roles of the human Rad5 homologs. These results provide insights into how MMR accessory factors involved in the MMR-dependent apoptotic response interact with the core MMR machinery and have important health implications into how human cells respond to environmental toxins, tumor development, and treatment choices of tumors with defects in Rad5 homologs.