Insights into DNA Damage in Regenerating Young and Aged Mouse Livers

Abstract Background The maintenance of genome stability is a key process to slow aging. One of the mechanisms ensuring this stability is the correct coordination of origins of replication (ORI), resulting in the successful transmission of DNA. Previously, we mapped and compared ORI firing between young and aged mice in vivo, using a regenerating liver mouse model. We confirmed a decreased hepatocyte ORI efficiency in aged mice, known to have impaired liver regeneration. ORI firing proved to be fully rescued when blocking the aged mice's ATR serine/threonine-protein kinase, suggesting that the DNA replication checkpoint actively mediates ORI firing impairment upon DNA damage detection. Aims To explore DNA damage differences between young and aged mice regenerating livers. Methods To induce proliferation, mice were subjected to partial hepatectomy (PH) and liver sections were harvested at different timepoints. Immunohistochemistry staining (IHC) were used to address proliferation and DNA damage, using Ki67 and serine 139 phosphorylated histone H2AX (g-H2AX) as markers, respectively. Results We confirmed a lack of Ki67 signal in young and aged mice prior to PH. The signal takes off at 24-28h post-PH and reaches its peak at 36-48h, which is significantly lower in aged mice. After 48h post-PH, young hepatocytes’ Ki67 reaches its basal level 120h post-PH. However, aged hepatocytes’ Ki67 is maintained at low levels overtime. Next, we compared DNA damage kinetics between young and aged mice livers. Both mouse groups present an increase of g-H2AX upon PH, higher in young mice. The g-H2AX signal decreases in young hepatocytes after 48h post-PH until disappearing 120h post PH. Aged mice hepatocytes maintain the g-H2AX rates overtime after 48h post-PH. Conclusions Our data suggests that hepatocytes develop DNA damage upon proliferation, which is able to be resolved in young mice hepatocytes but remains present in aged livers, ultimately leading to impaired liver regeneration.