Revisiting the concept of mitotic DNA synthesis: is it a mitosis-driven reaction?

More than a decade ago, a novel DNA repair pathway termed mitotic DNA synthesis (MiDAS) was proposed to complete DNA replication during early mitosis and minimise chromosome missegregation. This proposal was based on unscheduled incorporation of 5-ethynyl-2′-deoxyuridine (EdU) at common fragile sites (CFSs) in mitotic cells that experienced mild replication stress, together with the involvement of proteins implicated in break-induced replication (BIR). Since then, increasing attention has focused on the mechanisms underlying MiDAS. However, the widely adopted MiDAS assay has several fundamental limitations and cannot conclusively demonstrate de novo DNA synthesis initiated during mitosis. In this Comment, I discuss these limitations and clarify the conceptual distinction between the phenomenon of DNA synthesis in mitosis and MiDAS DNA repair. I propose alternative explanations and urge caution in attributing MiDAS solely to mitotic BIR without rigorously considering other possibilities, including the continuation of DNA replication across the G2-M transition and BIR initiated in G2 that persists into mitosis. These considerations highlight the need for experimental approaches capable of distinguishing newly initiated mitotic DNA synthesis from pre-existing DNA synthesis/repair. This Comment urges caution in attributing MiDAS solely to mitotic BIR, encouraging the use of approaches capable of distinguishing newly initiated mitotic DNA synthesis from pre-existing replication.