Retinoblastoma - RB1 Gene Mutation at 13q14

An 18-month-old female infant presents with leukocoria noted in family photographs and new-onset divergent squint of the right eye. Examination under anaesthesia reveals the structure marked **C** in the diagram—a chalky-white intraretinal tumor mass with overlying vitreous seeds filling more than 50% of the vitreous cavity. MRI confirms an intraocular lesion without optic nerve or pineal involvement. RB1 gene testing at 13q14 is arranged, and the diagnosis is unilateral sporadic retinoblastoma (Group D, International Classification). Which of the following best explains the pathogenesis of this tumor in the context of the RB1 mutation?