Deregulated DNA polymerase beta induces chromosome tumorigenesis and instability. how key proteinCprotein interactions regulate cellular responses to stress. INTRODUCTION The vital importance of genome maintenance is underscored by the evolution of multiple DNA repair mechanisms, each of which functions on a specific type or class of damaged DNA. Of these, the base excision repair (BER) pathway plays a critical role in repairing base damage and DNA single-strand breaks that emerge from both endogenous and exogenous sources. Failure to repair such DNA lesions can lead to accumulation of DNA mutations and chromosome alterations. As such, defects in DNA repair pathways or proteins can predispose to cancer and disease onset (1). Such defects in DNA repair can arise from mutations in essential active site amino acid residues (2), as well as those critical for post-translational modifications (3), proteinCprotein interactions (4) or protein complex assembly or dis-assembly (5). This study focuses on somatic mutations found in the gene for MCH6 DNA polymerase (Pol) and its impact on the BER pathway. The BER pathway plays a major role in the repair of endogenous and exogenous DNA damage that induces alkylated bases, oxidatively modified bases, base deamination and DNA hydrolysis (6). Pol is the primary DNA polymerase involved in BER and both its 5deoxyribose phosphate (5dRP) lyase and nucleotidyl transferase activities are important for BER (7,8). Mutations in Pol are found in many human cancers and recently, as many as 75% of the tumors analyzed in a colon cancer cohort were found to bear mutations in the coding region or the UTR region of the gene (9C11). Modification of key amino acid residues impacting the 5dRP lyase and nucleotidyl transferase functions of Pol impairs BER efficiency and results in increased sensitivity to many DNA damaging agents (7,8). In addition, mutations that alter the structure of Pol can affect its activity (12,13), such as the R137Q variant that confers cell sensitivity to the alkylating agent methyl methanesulfonate (14) or the P242R mutant that predisposes the cell to genomic instability and transformation (15). Pol is critical for both the gap-tailoring and gap-filling functions of BER (7,8,16). Pol is a bi-functional, two-domain, 39 kDa enzyme (17). The N-terminal 8-kDa domain of Pol possesses 5dRP A939572 lyase activity that removes the sugar-phosphate lesion (5dRP) during BER. The 31-kDa polymerase domain of Pol is responsible for gap-filling DNA synthesis during BER and resides within the C-terminus (17). As we and others have described, these repair functions of Pol are promoted or enhanced via essential proteinCprotein interactions (18,19) as part of the suggested hand-off or baton mechanism of BER (20). Of these protein partners, Pol interacts with X-ray repair cross complementing 1 (XRCC1) (21,22), flap endonuclease 1 (FEN1) (23,24), apurinic/apyrimidinic (AP) endonuclease 1 (APE1) (25), proliferating cell nuclear antigen (PCNA) (26) and p53 (27), among others. Many somatic mutations of Pol have been identified A939572 (9), including those that may prevent critical proteinCprotein interactions, such as the R137Q mutation that disrupts the interaction of Pol with PCNA (14). Numerous studies have suggested that cellular A939572 homeostasis of Pol protein levels is important for proper cellular function and genome maintenance. Low levels of Pol increase cancer susceptibility (28,29), while overexpression of Pol is associated with increased carcinogenesis (30C32). As such, protein degradation plays a central role in regulating many processes of DNA repair and the cellular response to DNA damage (33,34). As we have shown, part of the homeostatic regulation of the Pol protein is mediated by its interaction with XRCC1, since free Pol (not bound to XRCC1) can be targeted for.
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