Cowden Syndrome PTEN 10q Autosomal Dominant

A 38-year-old woman presents with macrocephaly, trichilemmomas, cobblestone oral papillomatosis, and acral keratoses. She has been diagnosed with invasive ductal carcinoma of the breast and follicular variant papillary thyroid carcinoma. Her mother had endometrial cancer at age 45 with macrocephaly, and her brother has Lhermitte-Duclos disease. Genetic testing identifies a pathogenic mutation at the locus marked **A** in the diagram. Which of the following best explains the molecular mechanism underlying the multiple hamartomas and increased cancer susceptibility in this patient?