DNA polymerase epsilon and delta proofreading suppress discrete mutator and cancer phenotypes in mice

Abstract

Organisms require faithful DNA replication to avoid deleterious mutations. In yeast, replicative leading- and lagging-strand DNA polymerases (Pols epsilon and delta, respectively) have intrinsic proofreading exonucleases that cooperate with each other and mismatch repair to limit spontaneous mutation to less than 1 per genome per cell division. The relationship of these pathways in mammals and their functions in vivo are unknown. Here we show that mouse Pol epsilon and delta proofreading suppress discrete mutator and cancer phenotypes. We found that inactivation of Pol epsilon proofreading elevates base-substitution mutations and accelerates a unique spectrum of spontaneous cancers; the types of tumors are entirely different from those triggered by loss of Pol delta proofreading. Intercrosses of Pol epsilon-, Pol delta-, and mismatch repair-mutant mice show that Pol epsilon and delta proofreading act in parallel pathways to prevent spontaneous mutation and cancer. These findings distinguish Pol epsilon and delta functions in vivo and reveal tissue-specific requirements for DNA replication fidelity.

Fig. 1.

Survival and cancer phenotypes of Pol ε proofreading-deficient mice. (A) Kaplan-Meier survival estimates. Mice were followed for long-term survival and observed daily until moribund or unexpected natural death. Dark red indicates Pole^e/e (n = 36) and Pole^+/e (n = 35) in C57BL/6 genetic background after removal of the neomycin selection cassette (Neo⁻); light red indicates Pole^e/e (n = 35) and Pole^+/e (n = 45) in a mixed C57BL/6:129/Sv genetic background with the neomycin selection cassette still present (Neo⁺); blue indicates Pold1^e/e (n = 40) in C57BL/6 (Neo⁻); purple indicates Pole^e/ePold1^e/e (n = 35) in C57BL/6 (Neo⁻); black indicates wild-type (WT; n = 37) C57BL/6; green indicates Mlh1^Δ/Δ (n = 27) in C57BL/6. One month = 30.4 days. (B) Spontaneous tumor incidences. Moribund mice were euthanized and necropsied, and tumors were diagnosed by histology. *, Incidences among 32 wild-type (WT), 33 Pole^e/e, 36 Pold1^e/e, 26 Mlh1^Δ/Δ, and 34 Pole^e/ePold1^e/e mice. ^†, Tumors with 15% or greater incidence in 1 or more groups. See Table S1 for details and rare tumors.

Fig. 2.

Mutator phenotypes conferred by defective Pol ε and Pol δ proofreading. (A) Mutant frequencies in vivo. Wild-type, Pole^e, and Pold1^e mice harboring the cII transgene were euthanized at 6–8 weeks of age, and cII mutant frequencies were determined in DNA isolated from whole thymus (purple inverted triangles), whole lung (green diamonds), unfractionated bone marrow (red triangles), and small intestine epithelium (blue squares). Each datum point is the average of 5–7 mice. Bar graphs are averages of 4 tissues with standard deviations. (B) Mutation rates and spectra in cultured fibroblasts. Cell lines were derived from 14- to 16-day embryos, and spontaneous mutant rates were determined by fluctuation analyses using maximum likelihood estimates. Data are from multiple experiments with 2–6 independently derived cell lines of each genotype (except for Pole^+/e, in which 1 cell line was analyzed in a single experiment). Parentheses indicate 95% confidence intervals. *, Ouabain-resistant (Oua^R) or 6-thioguanine–resistant (6-TG^R) mutants per cell division. ^†, Number of 6-TG^R clones with Hprt base substitutions (BS) or ±1 frameshifts (FS). ^‡, Microsatellite instability (MIN) expressed as number of clones with variant microsatellites/total number of clones screened. ^§, ND, not determined. ^‖, Base-substitution and frameshift values from 6-TG^R Msh2^Δ/Δ clones. See Fig. S3 and Table S2 for details of mutation spectra.