Polyploidy in Liver Regeneration and Adaptation to Chronic Injury

The liver contains diploid and polyploid hepatocytes (tetraploid, octaploid, etc.), with polyploids comprising ≥90% of the hepatocyte population in adult mice. Moreover, polyploid hepatocytes form multipolar spindles in mitosis, leading to individual chromosome gains and/or losses and random aneuploidy. The molecular mechanisms that regulate polyploidization have been well-characterized; however, it is unclear if diploid and polyploid hepatocytes function similarly in multiple contexts. The effect of aneuploidy on liver function is also unknown and the degree of liver aneuploidy is debated, with reports showing aneuploidy affects 5-60% of hepatocytes. We used mice lacking E2f7 and E2f8 in the liver (LKO), which have a polyploidization defect. We found diploid hepatocytes were enriched 20-fold in LKO livers and nearly all LKO hepatocytes were euploid compared to control hepatocytes, which were mostly aneuploid, suggesting polyploids are required for production of aneuploid progeny. Livers from LKO mice maintained normal function but became highly tumorigenic when challenged with tumor-promoting stimuli, suggesting that tumors in LKO mice were driven, at least in part, by diploid hepatocytes capable of rapid proliferation. Indeed, LKO hepatocytes were more proliferative and out-competed control hepatocytes in competitive repopulation studies. To eliminate potentially confounding effects associated with E2f7/E2f8 deficiency, diploid and polyploid hepatocytes from wild-type mice were examined. The wild-type diploid cells also showed a proliferative advantage, entering and progressing through the cell cycle faster than polyploid cells, both in vitro and during liver regeneration. Finally, to investigate the role of diploids and polyploids in chronic injury we bred LKO mice onto a tyrosinemia background, a disease model where the liver can develop disease-resistant, regenerative nodules. Survival was significantly reduced in tyrosinemic LKO mice, compared to controls, but they maintained the ability to form regenerative nodules. Molecular analyses revealed that the nodules in the tyrosinemic livers were generated via aneuploidy and inactivating mutations. In summary we identified diverse, context-specific roles for diploids and polyploids in liver function, demonstrating diploid hepatocytes are the most proliferative and have the highest capacity for liver regeneration, and polyploid hepatocytes are required for the formation of aneuploid progeny and facilitate adaptation to chronic liver disease.