Mitotic DNA Synthesis in Untransformed Human Cells Preserves Common Fragile Site Stability via a FANCD2-Driven Mechanism That Requires HELQ

FANCD2-dependent mitotic DNA synthesis relies on PCNA K164 ubiquitination

“…However, MiDAS in cancerous and non-transformed cell models is not identical. 16 , 17 A recent study found that RAD18-mediated PCNA K164 ubiquitination, and not sumoylation, was critical for MiDAS in human cancer cell lines. 18 We extend these findings to 293T cancer cells and, importantly, non-transformed RPE-1 cells, as PCNA K164R mutants in both backgrounds were defective in MiDAS ( Figure 4 ).…”

“…FANCD2 is a canonical marker of under-replicated loci, including CFSs, and is the main regulator of MiDAS in non-transformed cells. 16 , 17 FANCD2 exists as a homodimer and in response to replication stress one subunit is exchanged for FANCI, forming a FANCD2-FANCI heterodimer. Ubiquitination of this DNA-bound complex by the FA core complex locks it onto DNA.…”

“…Human cancer cells can utilize RAD52- or FANCD2-dependent MiDAS 15 ; however, we demonstrated that in non-transformed (i.e., non-cancerous) human cells, the RAD52-dependent mechanism is absent, and instead, MiDAS relies on FANCD2. 16 , 17 Interestingly, a recent study found that PCNA K164 ubiquitination by RAD18 is required for MiDAS in cancer cell lines but did not investigate its necessity in non-transformed human cells. 18 …”