Early onset of pathological polyploidization and cellular senescence in hepatocytes lacking RAD51 creates a pro-fibrotic and pro-tumorigenic inflammatory microenvironment

Background & Aims: RAD51 is a highly conserved DNA repair protein and is indispensable for embryonic viability. As a result, the role of RAD51 in liver development and function is unknown. Our aim was to characterize the function of RAD51 in postnatal liver development. Approach & Results: RAD51 is highly expressed during liver development and during regeneration following hepatectomy and hepatic injury, and is also elevated in chronic liver diseases. We generated hepatocyte-specific Rad51 deletion mouse model using Alb -Cre ( Rad51 -CKO) and AAV8-TBG-Cre to evaluate the function of RAD51 in liver development and regeneration. The phenotype in Rad51 -CKO mice is dependent on Cre recombinase dosage, with Rad51 fl/fl ; Alb -Cre +/+ manifesting a more severe phenotype than the Rad51 fl/fl ; Alb -Cre +/- mice. RAD51 deletion in postnatal hepatocytes results in aborted mitosis and early onset of pathological polyploidization that is associated with oxidative stress and cellular senescence. Remarkable liver fibrosis occurs spontaneously as early as in 3-month-old Rad51 fl/fl ; Alb -Cre +/+ mice. While liver regeneration is compromised in Rad51 -CKO mice, they are more tolerant of carbon tetrachloride (CCl 4 )-induced hepatic injury and resistant to DEN/CCl 4 -induced hepatocellular carcinoma (HCC). A chronic inflammatory microenvironment created by the senescent hepatocytes appears to activate ductular reaction and consequently a transdifferentiation of cholangiocytes to hepatocytes. The newly-derived RAD51 functional immature hepatocytes proliferate vigorously, acquire increased malignancy, and eventually give rise to HCC. Conclusions: Our results demonstrate a novel function of RAD51 in liver development, homeostasis and tumorigenesis. The Rad51 -CKO mice represent a unique genetic model for premature liver senescence, fibrosis and hepatocellular carcinogenesis.