Replication across O⁶-methylguanine activates futile cycling of DNA mismatch repair attempts assisted by the chromatin-remodelling enzyme Smarcad1.

This article focuses on the mechanistic investigation of futile mismatch repair (MMR) cycling induced by O6-methylguanine (meG) lesions during DNA replication in a chromatin context. Using replication-competent Xenopus egg extracts, the study demonstrates that meG–T mispairs, but not meG–C pairs, efficiently activate MMR, leading to iterative cycles of strand excision and resynthesis on the nascent strand, consistent with the futile cycling model. The research further shows that replication across meG-containing DNA generates persistent single-strand breaks on the daughter strand and that the chromatin remodeler Smarcad1 facilitates the retention of these breaks, thereby assisting futile MMR cycling. These findings provide experimental evidence that chromatin replication across meG lesions induces damage-promoting futile MMR cycling, with Smarcad1 playing a supportive role in this process.