A germline polymorphism of DNA polymerase beta induces genomic instability and cellular transformation.

Several germline single nucleotide polymorphisms (SNPs) have been identified in the POLB gene, but little is known about their cellular and biochemical impact. DNA Polymerase ß (Pol ß), encoded by the POLB gene, is the main gap-filling polymerase involved in base excision repair (BER), a pathway that protects the genome from the consequences of oxidative DNA damage. In this study we tested the hypothesis that expression of the POLB germline coding SNP (rs3136797) in mammalian cells could induce a cancerous phenotype. Expression of this SNP in both human and mouse cells induced double-strand breaks, chromosomal aberrations, and cellular transformation. Following treatment with an alkylating agent, cells expressing this coding SNP accumulated BER intermediate substrates, including single-strand and double-strand breaks. The rs3136797 SNP encodes the P242R variant Pol ß protein and biochemical analysis showed that P242R protein had a slower catalytic rate than WT, although P242R binds DNA similarly to WT. Our results suggest that people who carry the rs3136797 germline SNP may be at an increased risk for cancer susceptibility.

DNA polymerase beta is critical for mouse meiotic synapsis.

We have shown earlier that DNA polymerase beta (Pol beta) localizes to the synaptonemal complex (SC) during Prophase I of meiosis in mice. Pol beta localizes to synapsed axes during zygonema and pachynema, and it associates with the ends of bivalents during late pachynema and diplonema. To test whether these localization patterns reflect a function for Pol beta in recombination and/or synapsis, we used conditional gene targeting to delete the PolB gene from germ cells. We find that Pol beta-deficient spermatocytes are defective in meiotic chromosome synapsis and undergo apoptosis during Prophase I. We also find that SPO11-dependent gammaH2AX persists on meiotic chromatin, indicating that Pol beta is critical for the repair of SPO11-induced double-strand breaks (DSBs). Pol beta-deficient spermatocytes yielded reduced steady-state levels of the SPO11-oligonucleotide complexes that are formed when SPO11 is removed from the ends of DSBs, and cytological experiments revealed that chromosome-associated foci of replication protein A (RPA), RAD51 and DMC1 are less abundant in Pol beta-deficient spermatocyte nuclei. Localization of Pol beta to meiotic chromosomes requires the formation of SPO11-dependent DSBs. Taken together, these findings strongly indicate that Pol beta is required at a very early step in the processing of meiotic DSBs, at or before the removal of SPO11 from DSB ends and the generation of the 3' single-stranded tails necessary for subsequent strand exchange. The chromosome synapsis defects and Prophase I apoptosis of Pol beta-deficient spermatocytes are likely a direct consequence of these recombination defects.

The R280H X-ray cross-complementing 1 germline variant induces genomic instability and cellular transformation.

X-ray repair cross complementing protein 1 (XRCC1) plays an important role in base excision DNA repair (BER) as a scaffolding protein for BER enzymes. BER is one of the basic DNA repair pathways repairing greater than 20,000 endogenous lesions per cell per day. Proper functioning of XRCC1, one of the most important players in BER, was suggested to be indispensable for effective DNA repair. Despite accumulating evidence of an important role that XRCC1 plays in maintaining genomic stability, the relationship between one of its most predominant variants, R280H (rs25489), and cancer prevalence remains ambiguous. In the current study we functionally characterized the effect of the R280H variant expression on immortal non-transformed mouse mammary epithelial C127 and human breast epithelial MCF10A cells. We found that expression of R280H results in increased focus formation in mouse C127 cells and induces cellular transformation in human MCF10A cells. Cells expressing R280H showed significantly increased levels of chromosomal aberrations and accumulate double strand breaks in the G1 cell cycle phase. Our results confirm a possible link between R280H and genomic instability and suggest that individuals carrying this mutation may be at increased risk of cancer development.

DNA polymerase beta is critical for genomic stability of sperm cells.

Maintaining genome integrity in germ cells is important, given that the germ cells produce the next generation of offspring. Base excision repair is a DNA repair pathway that is responsible for the repair of most endogenous DNA damage. A key enzyme that functions in this repair pathway is DNA polymerase beta (Pol ß). We previously used conditional gene targeting to engineer mice with sperm deleted of the Pol B gene, which encodes Pol ß. We characterized mutagenesis in the sperm of these mice and compared it to wild-type and mice heterozygous for the Pol B gene. We found that sperm obtained that were heterozygously or homozygously deleted of the Pol B gene exhibited increased mutation frequencies compared to wild-type sperm. We identified an increase in transition mutations in both heterozygously and homozygously deleted sperm, and the types of mutations induced suggest that a polymerase other than Pol ß functions in its absence. Interestingly, most of the transversions we observed were induced only in heterozygous, compared with wild-type sperm. Our results suggest that haploinsufficiency of Pol ß leads to increased frequencies and varieties of mutations. Our study also shows that Pol ß is critical for genome stability in the germline.