Abstract DNA polymerase eta (Polη), a product of the Xeroderma pigmentosum variant (XPV) gene, plays a unique role in translesion DNA synthesis (TLS). It is a short-lived proteasomally degraded protein and its steady-state level is tightly controlled by multiple pathways. In this study, we have identified the deubiquitylating enzyme ubiquitin-specific protease 7 (USP7) as a novel regulator of Polη stability. Polη and USP7 interact in vitro and in vivo. Overexpression of wild-type USP7, but not its interaction-defective and catalytic mutants, deubiquitylate Polη and increase its steady-state level. The data demonstrate that both physical interaction and catalytic activity of USP7 are necessary for Polη deubiquitylation and stability regulation. Interestingly, knockout of USP7 also increased the steady-state levels and slowed the turnover of both Polη and p53, which is likely caused by destabilizing MDM2, a targeting E3 ligase for ubiquitylation of both Polη and p53. Furthermore, ectopic expression of USP7 upregulated the UV-induced PCNA monoubiquitylation. In contrast, UV-induced PCNA monoubiquitylation was not observed in XPV cells ectopically expressing USP7. Taken together, our results demonstrated that USP7 facilitates UV-induced PCNA monoubiquitylation by directly and indirectly regulating Polη stability. (The work was supported by grants from NIH.) Note: This abstract was not presented at the meeting. Citation Format: Jiang Qian, Qianzhen Zhu, Kyle Pentz, Jinshan He, Qien Wang, Altaf A. Wani. USP7 modulates UV-induced PCNA monoubiquitylation by regulating DNA polymerase eta stability. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4893. doi:10.1158/1538-7445.AM2014-4893