Mitotic slippage causes nuclear instability in polyploid cells

Whole-genome duplication (WGD), leading to polyploidy, can arise in physiological and pathological contexts ^1–5 . WGD can occur via non-canonical cell cycles such as mitotic slippage, cytokinesis failure or endoreplication ^1,3 . However, whether the route of WGD shapes the behaviour of the resulting polyploid cells remains unclear. Here, we compared these routes under both physiological and non-physiological conditions. Remarkably, only mitotic slippage led to widespread nuclear abnormalities defined by highly variable nuclear deformations that we termed nuclear instability. These nuclei were softer and more vulnerable to microtubule-driven deformations leading to invaginations. Mechanistically, we found defects in chromatin compaction and H3K9me2 chromatin-lamina interactions. Moreover, transcriptional stability was disrupted as early as G1, with effects intensifying by G2. Importantly, we observed similar nuclear instability in megakaryocytes, which are physiological polyploid cells that we demonstrate here to be generated by mitotic slippage, providing a potential explanation for their atypical nuclear architecture ^6,7 . In striking contrast, nuclear shape was stable in different physiological polyploid cells generated by cytokinesis failure and endoreplication. Overall, our findings highlight that the path to polyploidy matters and that mitotic slippage uniquely destabilizes nuclear architecture and gene expression, with implications for both physiology and disease.