SPRTN and TDP1/TDP2 Independently Suppress 5-Aza-2′- deoxycytidine-Induced Genomic Instability in Human TK6 Cell Line

DNA-protein cross-links (DPCs) are generated by internal factors such as cellular aldehydes that are generated during normal metabolism and external factors such as environmental mutagens. A nucleoside analog, 5-aza-2 '-deoxycytidine (5-azadC), is randomly incorpo-rated into the genome during DNA replication and binds DNA methyltransferase 1 (DNMT1) covalently to form DNMT1-DPCs without inducing DNA strand breaks. Despite the recent progress in understanding the mechanisms of DPCs repair, how DNMT1-DPCs are repaired is unclear. The metalloprotease SPRTN has been considered as the primary enzyme to degrade protein components of DPCs to initiate the repair of DPCs. In this study, we showed that SPRTN-deficient (SPRTN-/-) human TK6 cells displayed high sensitivity to 5-azadC, and the removal of 5-azadC-induced DNMT1-DPCs was significantly slower in SPRTN-/- cells than that in wild -type cells. We also showed that the ubiquitination-dependent proteasomal degradation, which was independent of the SPRTN-mediated processing, was also involved in the repair of DNMT1-DPCs. Unexpectedly, we found that cells that are double deficient in tyrosyl DNA phosphodiesterase 1 and 2 (TDP1-/-TDP2-/-) were also sensitive to 5-azadC, although the removal of 5-azadC-induced DNMT1-DPCs was not compromised significantly. Furthermore, the 5-azadC treatment induced a marked accumulation of chromosomal breaks in SPRTN-/- as well as TDP1-/-TDP2-/- cells compared to wild-type cells, strongly suggesting that the 5-azadC-induced cell death was attributed to chromosomal DNMT1-DPCs. We conclude that SPRTN protects cells from 5-azadC-induced DNMT1-DPCs, and SPRTN may play a direct proteolytic role against DNMT1-DPCs and TDP1/TDP2 also contributes to suppress genome instability caused by 5-azadC in TK6 cells.