Mutation of DNA Polymerase β R137Q Results in Retarded Embryo Development Due to Impaired DNA Base Excision Repair in Mice

Abstract

DNA polymerase β (Pol β), a key enzyme in the DNA base excision repair (BER) pathway, is pivotal in maintaining the integrity and stability of genomes. One Pol β mutation that has been identified in tumors, R137Q (arginine to glutamine substitution), has been shown to lower polymerase activity, and impair its DNA repair capacity. However, the exact functional deficiency associated with this polymorphism in living organisms is still unknown. Here, we constructed Pol β R137Q knock-in mice, and found that homozygous knock-in mouse embryos were typically small in size and had a high mortality rate (21%). These embryonic abnormalities were caused by slow cell proliferation and increased apoptosis. In R137Q knock-in mouse embryos, the BER efficiency was severely impaired, which subsequently resulted in double-strand breaks (DSBs) and chromosomal aberrations. Furthermore, R137Q mouse embryo fibroblasts (MEFs) were more sensitive to DNA-damaging reagents, such as methyl methanesulfonate (MMS) and H2O2. They displayed a higher percentage of DSBs, and were more likely to undergo apoptosis. Our results indicate that R137 is a key amino acid site that is essential for proper Pol β functioning in maintaining genomic stability and embryo development.

Loss of DNA polymerase β induces cellular senescence

“…We find increased number of colocalized γH2AX and p53BP1foci in senescing cells, which is amplified by loss of Polβ . We should note that DSB have been shown to accumulate in a knock‐in mouse model of PolB variant R137Q [Pan et al, ], Polβ ‐/‐ cells exposed to MMS [Pascucci et al, ], and an Alzheimer's/Polβ +/ ‐ mouse (3xTgAD/ Polβ ) [Sykora et al, ]. But this is the first report that loss of Polβ alone results in a significant accumulation of colocalized γH2AX and p53BP1, presumed to be DSB.…”

Modulation of trinucleotide repeat instability by DNA polymerase β polymorphic variant R137Q

“…This effect has been further confirmed in a pol βR137Q transgenic mouse model in vivo in a recent study [79]. The results of the studies suggest that the pol β polymorphic variant exhibits significantly reduced DNA synthesis activity in random DNA sequence compared with wild-type pol β, and this may result in genome instability.…”

Trinucleotide Repeat Instability Modulated by DNA Repair Enzymes and Cofactors

“…We demonstrated that this was because the polymorphic variant exhibited similar capability to that of wild-type pol β of skipping over a small template loop structure during BER ( A previous study has shown that pol βR137Q exhibits 30% of wild-type enzymatic activity in the context of random sequence, which in turn reduces BER capacity and increases cellular sensitivity to alkylating DNA damaging agents as well as promotes apoptosis [61]. This effect has been further confirmed in a pol βR137Q transgenic mouse model in vivo in a recent study [62]. The results of the studies suggest that the pol β polymorphic variant exhibits significantly reduced DNA synthesis activity in random DNA sequence compared with wild-type pol β, and this may result in genome instability.…”

“…In addition, the heterozygous pol β variant with deletion of exon 4-6 and 11-13 is associated with an increased risk of ovarian carcinoma [60]. In addition, some functional studies of polymorphic variants of BER enzymes also revealed a linkage between variants of BER enzymes and cancers [61,62].…”

“…Studies of pol β Arg137Gln indicate the impaired DNA synthesis activity, which results in accumulative DNA repair intermediates, contributes to the higher risk in cancer development compared to the wild-type pol β carriers [61,62]. In fact, many non-germline mutants of BER enzymes have also been identified in a variety of cancers: more than 140 pol β mutants with multiple or single amino acids alternation have been identified in different cancers including gastric, colorectal, prostate, lung, breast, bladder, and esophageal cancers [63].…”