DNA mismatch repair is a primary regulator of mutation production. The pathway is well known for its multiple mutation-avoidance functions, including the correction of DNA biosynthetic errors, inhibition of recombination between quasi-homologous sequences, and participation in the cellular response to DNA damage. Inactivation of human mismatch repair is the cause of Lynch syndrome, the most common hereditary cancer. Mismatch repair defects also have implications for cancer treatment because inactivation of the pathway renders cells resistant to the cytotoxic effects of certain anti-tumor drugs, a consequence of its function in the DNA damage response. Surprisingly, components of the mismatch repair system are also required for the generation of certain mutations including expanded triplet repeat sequences which are the cause of a number of neurodegenerative diseases. We are interested in the molecular mechanisms responsible for mismatch repair function in the control of mutation production. To this end we propose continuation of four ongoing lines of work: (1) MutL? interaction with MutS? is believed to play an important role in the initiation of mismatch repair, while interaction of the PCNA replication clamp with MutL? is required for activation and strand direction of the MutL? endonuclease. We will establish the molecular stoichiometries of these complexes and pursue further clarification of their nature. In the case of the PCNA*MutL? complex, we will also attempt to identify MutL? motif(s) required for the interaction. This aspect of the project will include evaluation of the role of zinc in Mut? endonuclease function. (2) The somatic expansion stage of (CAG)n/(CTG)n neurodegenerative diseases, which depends on the mismatch repair activities MutS?, MutL? and MutL?, can occur in post-mitotic cells. We have found that small CAG or CTG extrusions in covalently continuous DNA can serve as noncanonical PCNA loading sites, thus resulting in MutS?-dependent activation of MutL? endonuclease, which provides a simple mechanism for repair activation on non-replicating DNA. We are extending these studies to the analysis of (CAG)n/(CTG)n repeat processing by the human mismatch repair system in both extract and purified systems. Parallel studies will address the instability of expanded (GAA)n/(TTC)n Friedreich's ataxia disease alleles, which are subject to double strand cleavage in a MutS?? and MutL? endonuclease-dependent manner. (3) Mismatch repair function is required for checkpoint and apoptotic responses to O6-methylguanine, the cytotoxic lesion produced by SN1 DNA methylators. Heteroduplex DNA that contains O6-methylguanine on the template strand supports iterative cycles of abortive excision and repair that continues for several hours in cell extracts. We will attempt to clarify the nature of this reaction. (4) Ongoing collaborative studies with the laboratoy of Lorena Beese will address structural features of DNA processing by the human mismatch repair system with emphasis on the PCNA*MutL? complex.

Public Health Relevance

DNA mismatch repair provides multiple mutation avoidance functions, and its inactivation is the cause of the most common hereditary cancer syndrome. Surprisingly, action of the pathway is also required for the production of certain mutations, including expanded triplet repeat sequences, the primary cause of a number of neurodegenerative disorders. By clarifying the molecular nature of mismatch repair, we hope to understand its functions in controlling the occurrence of mutation.

National Institute of Health (NIH)
National Institute of General Medical Sciences (NIGMS)
Research Project (R01)
Project #
Application #
Study Section
Molecular Genetics B Study Section (MGB)
Program Officer
Willis, Kristine Amalee
Project Start
Project End
Budget Start
Budget End
Support Year
Fiscal Year
Total Cost
Indirect Cost
Duke University
Schools of Medicine
United States
Zip Code
Sherrer, Shanen M; Penland, Elisabeth; Modrich, Paul (2018) The mutagen and carcinogen cadmium is a high-affinity inhibitor of the zinc-dependent MutL? endonuclease. Proc Natl Acad Sci U S A 115:7314-7319
Genschel, Jochen; Kadyrova, Lyudmila Y; Iyer, Ravi R et al. (2017) Interaction of proliferating cell nuclear antigen with PMS2 is required for MutL? activation and function in mismatch repair. Proc Natl Acad Sci U S A 114:4930-4935
Chen, Yu-Tsung Shane; Wu, Jianhong; Modrich, Paul et al. (2016) The C-terminal 20 Amino Acids of Drosophila Topoisomerase 2 Are Required for Binding to a BRCA1 C Terminus (BRCT) Domain-containing Protein, Mus101, and Fidelity of DNA Segregation. J Biol Chem 291:13216-28
Modrich, Paul (2016) Mechanisms in E. coli and Human Mismatch Repair (Nobel Lecture). Angew Chem Int Ed Engl 55:8490-501
Qiu, Ruoyi; Sakato, Miho; Sacho, Elizabeth J et al. (2015) MutL traps MutS at a DNA mismatch. Proc Natl Acad Sci U S A 112:10914-9
Lindsey-Boltz, Laura A; Kemp, Michael G; Reardon, Joyce T et al. (2014) Coupling of human DNA excision repair and the DNA damage checkpoint in a defined in vitro system. J Biol Chem 289:5074-82
Shao, Hongbing; Baitinger, Celia; Soderblom, Erik J et al. (2014) Hydrolytic function of Exo1 in mammalian mismatch repair. Nucleic Acids Res 42:7104-12
Pluciennik, Anna; Burdett, Vickers; Baitinger, Celia et al. (2013) Extrahelical (CAG)/(CTG) triplet repeat elements support proliferating cell nuclear antigen loading and MutL? endonuclease activation. Proc Natl Acad Sci U S A 110:12277-82
Tseng, Quincy; Orans, Jillian; Hast, Michael A et al. (2011) Purification, crystallization and preliminary X-ray diffraction analysis of the human mismatch repair protein MutS?. Acta Crystallogr Sect F Struct Biol Cryst Commun 67:947-52
Liu, Yiyong; Kadyrov, Farid A; Modrich, Paul (2011) PARP-1 enhances the mismatch-dependence of 5'-directed excision in human mismatch repair in vitro. DNA Repair (Amst) 10:1145-53

Showing the most recent 10 out of 56 publications