Li-Fraumeni Syndrome

A 28-year-old woman presents with a painless right breast lump of 6 weeks duration. Her family history is striking: mother died at age 34 from bilateral premenopausal breast cancer, maternal grandmother from osteosarcoma at age 41, elder brother from glioblastoma multiforme at age 25, and a 4-year-old niece from adrenocortical carcinoma. Core-needle biopsy confirms grade 3 invasive ductal carcinoma (ER+, PR+, HER2−). Germline multigene panel testing identifies a pathogenic missense mutation in the TP53 gene. The karyotype shown in the diagram marked **A** demonstrates the chromosomal location of this mutation. Which of the following best describes the primary consequence of TP53 loss of function in this patient's cancer predisposition?