MEN1 Syndrome

A 38-year-old woman presents with recurrent peptic ulcers refractory to high-dose proton pump inhibitors, nephrolithiasis, and recent visual field defects. Laboratory findings show serum calcium 11.6 mg/dL with elevated PTH, fasting gastrin 1200 pg/mL with positive secretin stimulation test, and prolactin 280 ng/mL. Pituitary MRI confirms a macroadenoma. Her father and paternal uncle have hypercalcemia with "stomach ulcers," and her sister has amenorrhea. The pedigree pattern marked **A** in the diagram shows autosomal dominant inheritance with high penetrance across three generations involving parathyroid, pancreatic, and pituitary tumors. Which of the following BEST describes the underlying genetic mechanism in this family?