Hypomorphic Protein Expression of DNA Polymerase Beta in PolβL301R-V303R/L301R-V303R Knock-In Transgenic Mice Does Not Impact Global DNA Methylation Levels in the Midbrain

DNA polymerase beta (Polβ) is a 39 kDa, single polypeptide enzyme that possesses both gap tailoring and nucleotidyl transferase activity and is the key polymerase involved in base excision repair (BER) and the final steps of active gene demethylation. We demonstrated that residues in the mouse Polβ protein, L301 and V303, are critical for Polβ's interaction with the BER scaffolding protein X-ray repair cross-complementing 1 (XRCC1), and mutation of these residues impairs Polβ's ability to bind to XRCC1, negatively impacting BER complex assembly. We developed Polβ^{L301R-V303R/L301R-V303R} knock-in mice to explore how defects with this essential protein complex impact genome stability in the mouse. We found these mice to be viable and fertile yet exhibited a modest reduction in body weight. Here, we examined the protein and mRNA levels in tissues from wild-type (WT), heterozygous (HET), and homozygous (HOM) Polβ^{L301R-V303R/L301R-V303R} mice and the derived fibroblast cell lines. We show that HOM mice have significantly diminished Polβ protein levels, as compared to WT mice, in several tissues, yet Polβ mRNA levels were not significantly different, suggesting the decreased levels of Polβ protein could not be attributed to lower gene expression. Upon examination of Polβ stability in mouse ear fibroblasts derived from WT and HOM mice, results are consistent with human cell studies that the Polβ^L301R-V303R protein is unstable and undergoes proteasome-mediated degradation. Finally, we evaluated WT, and HOM, liver and brain genomic DNA samples for 5-methylcytosine/5-hydroxymethylcytosine (5mC/5hmC) levels by nanopore sequencing to investigate the impact of suppressed Polβ protein levels on active gene demethylation. As expected, we found tissue-specific trends in methylation, when comparing the brain and liver. However, we were unable to discern substantial differences in methylation levels between WT and HOM mice, suggesting that in the absence of external stressors, low Polβ levels do not impact methylation patterns.