A 52-year-old man from Mumbai presents with progressive vision loss and a fundoscopic examination reveals multiple retinal hemangioblastomas and a renal mass on imaging. His 16-year-old son has recently been diagnosed with a hemangioblastoma of the cerebellum. Genetic testing confirms a heterozygous germline mutation in the VHL gene. The patient is counseled about his increased cancer risk. Which molecular mechanism best explains why individuals with inherited VHL mutations have a predisposition to multiple tumors?