Cell cycle regulation as a mechanism for functional separation of the apparently redundant uracil DNA glycosylases TDG and UNG2

Abstract

Human Thymine-DNA Glycosylase (TDG) is a member of the uracil DNA glycosylase (UDG) superfamily. It excises uracil, thymine and a number of chemical base lesions when mispaired with guanine in double-stranded DNA. These activities are not unique to TDG; at least three additional proteins with similar enzymatic properties are present in mammalian cells. The successful co-evolution of these enzymes implies the existence of non-redundant biological functions that must be coordinated. Here, we report cell cycle regulation as a mechanism for the functional separation of apparently redundant DNA glycosylases. We show that cells entering S-phase eliminate TDG through the ubiquitin–proteasome system and then maintain a TDG-free condition until G2. Incomplete degradation of ectopically expressed TDG impedes S-phase progression and cell proliferation. The mode of cell cycle regulation of TDG is strictly inverse to that of UNG2, which peaks in and throughout S-phase and then declines to undetectable levels until it appears again just before the next S-phase. Thus, TDG- and UNG2-dependent base excision repair alternates throughout the cell cycle, and the ubiquitin–proteasome pathway constitutes the underlying regulatory system.

Thymine DNA Glycosylase Is a CRL4Cdt2 Substrate

“…Several forms of TDG regulation have been discovered: 1) SUMOylation facilitates TDG dissociation from abasic sites on DNA (59); 2) both acetylation and phosphorylation regulate the access of TDG to DNA (60,61); and 3) S phasespecific proteolysis eliminates TDG during DNA replication (28). Here, we show that TDG is also destroyed after DNA damage, and our results indicate that replication and damage-dependent destruction of TDG both involve the E3 ubiquitin ligase CRL4…”

“…Interestingly, TDG is eliminated from S phase cells by ubiquitin-mediated proteolysis (28). The S phase destruction of TDG might prevent the collision of DNA replication forks with TDG-AP site complexes that have not yet been disrupted by TDG SUMOylation (28).…”

“…We conclude that, like recombinant TDG, endogenous TDG is targeted by CRL4 Cdt2 during DNA repair. TDG Destruction during S Phase-TDG is destroyed during S phase in human cells (28). We therefore sought to recapitulate replication-coupled destruction in egg extract.…”

Uracil in DNA and its processing by different DNA glycosylases

“…Uracil-DNA glycosylases T. Visnes et al 565 cell-cycle regulated. However, it is regulated opposite to UNG2 by displaying the highest expression in the G1-phase and the lowest in the S-phase (Hardeland et al 2007). While TDG has not been assumed to have an important function in uracil repair compared with the 'leading' enzymes UNG2 and SMUG1, this issue is far from settled and not based on good experimental evidence.…”

DNA damage response by single-strand breaks in terminally differentiated muscle cells and the control of muscle integrity

“…2 This occurs during differentiation-associated gene reprogramming at transcriptional level as in the case of genes coding for proteins shared by DNA repair and replication (e.g., replicative DNA polymerases, Flap structure-specific endonuclease 1, proliferating cell nuclear antigen and DNA ligase 1) 3 or repair proteins that are cell-cycle related (e.g., XRCC1 (X-ray repair complementing defective repair in Chinese hamster cells 1), uracil-DNA glycosylase). 4,5 Alternatively, post-translational modifications may modify the efficiency of specific DNA repair components as in the case of transcription factor II H that, because of reduced ubiquitination, may lead to decreased global genomic nucleotide excision repair typical of differentiated cells. 6 Exposure of single-stranded (ss) DNA and/or the generation of double-strand breaks (DSB) are powerful activators of DDR by recruiting and activating two protein kinases, ataxia telangiectasia and Rad3-related (ATR) or ataxia telangiectasia mutated (ATM), respectively, at the site of DNA lesion.…”