Exo1 independent DNA mismatch repair involves multiple compensatory nucleases

Abstract

Functional DNA mismatch repair (MMR) is essential for maintaining the fidelity of DNA replication and genetic stability. In hematopoiesis, loss of MMR results in methylating agent resistance and a hematopoietic stem cell (HSC) repopulation defect. Additionally MMR failure is associated with a variety of human malignancies, notably Lynch syndrome. We focus on the 5′ → 3′ exonuclease Exo1, the primary enzyme excising the nicked strand during MMR, preceding polymerase synthesis. We found that nuclease dead Exo1 mutant cells are sensitive to the O6-methylguanine alkylating agent temozolomide when given with the MGMT inactivator, O6benzylguanine (BG). Additionally we used an MMR reporter plasmid to verify that Exo1mut MEFs were able to repair G:T base mismatches in vitro. We showed that unlike other MMR deficient mouse models, Exo1mut mouse HSC did not gain a competitive survival advantage post temozolomide/BG treatment in vivo. To determine potential nucleases implicated in MMR in the absence of Exo1 nuclease activity, but in the presence of the inactive protein, we performed gene expression analyses of several mammalian nucleases in WT and Exo1mut MEFs before and after temozolomide treatment and identified upregulation of Artemis, Fan1, and Mre11. Partial shRNA mediated silencing of each of these in Exo1mut cells resulted in decreased MMR capacity and increased resistance to temozolomide/BG. We propose that nuclease function is required for fully functional MMR, but a portfolio of nucleases is able to compensate for loss of Exo1 nuclease activity to maintain proficiency.

MutLγ promotes repeat expansion in a Fragile X mouse model while EXO1 is protective

“…Since EXO1 is protective, it may be that expansions arise via the Exo1 -independent sub-pathway of MMR [ 46 , 47 ]. This pathway has been suggested to involve strand displacement or excision by one or more other nucleases [ 46 ].…”

Impact of Charged Particle Exposure on Homologous DNA Double-Strand Break Repair in Human Blood-Derived Cells

“…In addition, we recently reported that the more pronounced formation of 53BP1 foci after X-ray-induced DSB in HSPC was a consequence of reduced NF-κB activity ( 20 ). Compromised NF-κB-mediated BRCA1-CtIP activation ( 73 ) can explain the observed relative shift to error-prone repair pathways in HSPC, possibly under participation of EXO1 nuclease as a resection factor ( 74 , 75 ). This might also be relevant for particle radiation-induced DSB because we similarly found accumulation of 53BP1 foci after X-ray and carbon ion exposure (intermediate LET) of HSPC.…”

Germline Mutations in FAN1 Cause Hereditary Colorectal Cancer by Impairing DNA Repair

“…7,8,12 However, being MMR proficiency an inclusion criteria in our study, tumors developed by FAN1 mutation carriers showed microsatellite stability and/or normal expression of MMR proteins (Supplementary Figure 9). The fact that a plethora of nucleases, including FAN1, EXO1 and MRE11, can carry out the required nuclease activity for the MMR function, 12 might explain the absence of MMR deficiency in FAN1 mutation carriers' tumors.…”