p53-independent abrogation of a postmitotic checkpoint contributes to human papillomavirus E6-induced polyploidy

Polyploidy is often an early event during cervical carcinogenesis, and it predisposes cells to aneuploidy, which is thought to play a causal role in tumorigenesis. Cervical and anogenital cancers are induced by the high-risk types of human papillomavirus (HPV). The HPV E6 oncoprotein induces polyploidy in human keratinocytes, yet the mechanism is not known. It was believed that E6 induces polyploidy by abrogating the spindle checkpoint after mitotic stress. We have tested this hypothesis using human keratinocytes in which E6 expression induces a significant amount of polyploidy. We found that E6 expression does not affect the spindle checkpoint. Instead, we provide direct evidence that E6 is capable of abrogating the subsequent G(1) arrest after adaptation of the mitotic stress. E6 targets p53 for degradation, and previous studies have shown an important role for p53 in modulation of the G(1) arrest after mitotic stress. Importantly, we have discovered that E6 mutants defective in p53 degradation also induce polyploidy, although with lower efficiency. These results suggest that E6 is able to induce polyploidy via both p53-dependent and p53-independent mechanisms. Therefore, our studies highlight a novel function of HPV E6 that may contribute to HPV-induced carcinogenesis and improve our understanding of the onset of the disease.