Mechanisms of genetic instability in a single S-phase following whole genome doubling

Doubling of the full chromosome content -whole genome duplications (WGDs)- is frequently found in human cancers and is responsible for the rapid evolution of genetically unstable karyotypes 1–3. It has previously been established that WGDs fuel chromosome instability due to abnormal mitosis owing to the presence of extra centrosomes and extra chromosomes 4–8. Tolerance to ploidy changes has been identified in different model organisms and cell types 5,6,9–12, revealing long term cellular adaptations that accommodate ploidy increase. Importantly, however, the immediate consequences of WGDs as cells become tetraploid are not known. It also remains unknown whether WGD triggers other events leading to genetic instability (GIN), independently of mitosis. In this study, we induced tetraploidy in diploid genetically stable RPE-1 cells and monitored the first interphase. We found that newly born tetraploids undergo high rates of DNA damage during DNA replication. Using DNA combing and single cell sequencing, we show that replication forks are unstable, perturbing DNA replication dynamics and generating under- and over-replicated regions at the end of S-phase. Mechanistically, we found that these defects result from lack of protein mass scaling up at the G1/S transition, which impairs the fidelity of DNA replication. This work shows that within a single interphase, unscheduled tetraploid cells can accumulate highly abnormal karyotypes. These findings provide an explanation for the GIN landscape that favors tumorigenesis after tetraploidization.