A 72-year-old man with Gorlin syndrome presents with multiple asymptomatic, pearly nodules with central ulceration on the face and trunk. Histopathology shows islands of basaloid cells with peripheral palisading and retraction artifact. He has a history of medulloblastoma at age 8 and multiple odontogenic keratocysts. Which molecular alteration is responsible for this syndrome and predisposes to multiple skin tumors?