A 14-year-old male presents with a dramatic acceleration in his growth rate over the past 18 months, accompanied by headaches and coarsening facial features. His insulin-like growth factor 1 (IGF-1) level is markedly elevated. An oral glucose tolerance test fails to suppress his growth hormone (GH) level to less than 5 ng/mL, and an MRI confirms a pituitary macroadenoma. Which pathogenic variant is most commonly identified in pediatric patients with pituitary gigantism?