Mutations in the human DNA mismatch repair gene MSH2 are associated with hereditary nonpolyposis colorectal cancer as well as a significant proportion of sporadic colorectal cancer. The inactivation of MSH2 results in the accumulation of somatic mutations in the genome of tumor cells and resistance to the genotoxic effects of a variety of chemotherapeutic agents. Here we show that the DNA repair and DNA damage-induced apoptosis functions of Msh2 can be uncoupled using mice that carry the G674A missense mutation in the conserved ATPase domain. As a consequence, although Msh2 G674A homozygous mutant mice are highly tumor prone, the onset of tumorigenesis is delayed as compared with Msh2-null mice. In addition, tumors that carry the mutant allele remain responsive to treatment with a chemotherapeutic agent. Our results indicate that Msh2-mediated apoptosis is an important component of tumor suppression and that certain MSH2 missense mutations can cause mismatch repair deficiency while retaining the signaling functions that confer sensitivity to chemotherapeutic agents.
Mammalian Exonuclease 1 (EXO1) is an evolutionarily conserved, multifunctional exonuclease involved in DNA damage repair, replication, immunoglobulin diversity, meiosis, and telomere maintenance. It has been assumed that EXO1 participates in these processes primarily through its exonuclease activity, but recent studies also suggest that EXO1 has a structural function in the assembly of higher-order protein complexes. To dissect the enzymatic and nonenzymatic roles of EXO1 in the different biological processes in vivo, we generated an EXO1-E109K knockin (Exo1 EK ) mouse expressing a stable exonuclease-deficient protein and, for comparison, a fully EXO1-deficient (Exo1 null ) mouse. In contrast to Exo1 null/null mice, Exo1 EK/EK mice retained mismatch repair activity and displayed normal class switch recombination and meiosis. However, both Exo1-mutant lines showed defects in DNA damage response including DNA double-strand break repair (DSBR) through DNA end resection, chromosomal stability, and tumor suppression, indicating that the enzymatic function is required for those processes. On a transformation-related protein 53 (Trp53)-null background, the DSBR defect caused by the E109K mutation altered the tumor spectrum but did not affect the overall survival as compared with p53-Exo1 null mice, whose defects in both DSBR and mismatch repair also compromised survival. The separation of these functions demonstrates the differential requirement for the structural function and nuclease activity of mammalian EXO1 in distinct DNA repair processes and tumorigenesis in vivo.somatic hypermuation | scaffold function | ssDNA E xonuclease 1 (EXO1) belongs to the XPG/Rad2 family of metallonucleases and was first described as a 5′-3′ exonuclease associated with meiosis in Schizosaccharomyces pombe (1). Since then EXO1 has been implicated in a multitude of eukaryotic DNA metabolic pathways and in maintaining genomic integrity. It is involved in DNA mismatch repair (MMR) by hydrolyzing DNA mismatches (2-4), in DNA double-strand break repair (DSBR) through DNA end resection (5-7), in B-cell development through the generation of antibody diversity (8), and in telomere maintenance by promotion of telomeric recombination (9). Biochemical analysis had shown that the N-terminal half of EXO1 possesses 5′-3′ exonuclease and 5′ flap-endonuclease activities (10). However, these apparently distinct functions now are thought to be mechanistically unified (11).MMR is essential for maintaining the integrity of eukaryotic genomes by removing misincorporated nucleotides that result from erroneous replication. During MMR, the repair of distinct types of mismatches is initiated by two partially redundant MutS homolog (MSH) complexes: the MSH2-MSH6 (MutSα) heterodimer, that recognizes and binds to single-base mispairs and single-base insertion/deletions, and the MSH2-MSH3 (MutSβ) complex that primarily interacts with single-base and larger insertions/deletions. Subsequent to mismatch recognition by the MSH complexes, a MutL homolog (MLH) complex con...
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