Liver failure caused by herpes simplex virus thymidine kinase plus ganciclovir therapy is associated with mitochondrial dysfunction and mitochondrial DNA depletion.

Herpes simplex virus thymidine kinase (HSV-tk) converts ganciclovir (GCV) into an active compound, which can be incorporated into DNA molecules and terminate DNA synthesis. Gene transfer of HSV-tk followed by GCV administration has been used with success to treat experimental cancer and this strategy has entered into clinical trials. Although it is thought that the cytotoxic effect occurs mainly in tumoral dividing cells, where mitotic activity favors integration of the genotoxic compound into nuclear DNA, there are concerns of potential damage to normal nondividing cells. In the present work we have explored the mechanisms of HSV-tk/GCV toxicity and in particular whether this therapy may cause lesions of mitochondrial DNA (mtDNA) and mitochondrial dysfunction. We found that the administration of GCV to rats injected with adenovirus encoding HSV-tk induced hepatocellular damage characterized by the presence of apoptotic bodies, ballooning of hepatocytes, and severe hepatic steatosis with mitochondria enlargement and cristae dissolution at the ultrastructural level. Remarkably, Southern blot analysis showed substantial reduction in the amount of mtDNA in the liver. Using radiolabeled GCV we could demonstrate incorporation of this compound into both nuclear and mtDNA in HSV-tk-transduced rat hepatocytic cell line MCA-RH7777 and subsequent alteration of mitochondrial function. Our observations confirm that GCV can damage both nuclear and mtDNA in cells transduced with HSV-tk and that this effect could be responsible for severe mitochondrial dysfunction and toxicity in normal nondividing cells. These data are relevant for the design of clinical trials using adenoviral vectors encoding HSV-tk.