A 28-year-old man presents with progressive facial dysmorphism, multiple lentigines on the lips and eyelids, and recent-onset headaches with visual field defects. Imaging reveals a growth hormone-secreting pituitary adenoma and an incidental cardiac myxoma in the right ventricle. Genetic testing identifies an inactivating mutation in the gene marked **A** on the diagram. Which of the following best explains the multi-organ tumorigenesis in this patient?
A. MEN1 mutations cause loss of menin protein, leading to impaired cell cycle regulation and neuroendocrine tumor predisposition
B. RET proto-oncogene activation results in uncontrolled tyrosine kinase signaling in neural crest-derived tissues
C. Loss of PRKAR1A function leads to constitutive PKA catalytic activity and dysregulated cAMP signaling, driving multi-tissue tumorigenesis
D. VHL inactivation causes HIF-1α stabilization, leading to angiogenic and metabolic dysregulation in renal and adrenal tumors
Explanation
Why Loss of PRKAR1A function leads to constitutive PKA catalytic activity and dysregulated cAMP signaling, driving multi-tissue tumorigenesis is right
The structure marked A is PRKAR1A on chromosome 17q22-q24, which encodes the regulatory subunit type 1-alpha of cAMP-dependent protein kinase A. Inactivating mutations in PRKAR1A are the defining genetic lesion of Carney Complex, an autosomal dominant multiple neoplasia syndrome. Loss of the regulatory subunit removes the brake on PKA catalytic activity, resulting in constitutive (unregulated) PKA signaling. This dysregulated cAMP-PKA pathway drives the characteristic multi-organ tumorigenesis seen in this patient: spotty skin pigmentation (lentigines), cardiac myxomas (often multiple and in any chamber, not just the left atrium), growth hormone-secreting pituitary adenomas (acromegaly/gigantism), and other endocrine tumors. This mechanism is the pathophysiologic cornerstone of Carney Complex as described in Williams Endocrinology.
Why each distractor is wrong
MEN1 mutations cause loss of menin protein, leading to impaired cell cycle regulation and neuroendocrine tumor predisposition: While MEN1 (marked B on the diagram) is another autosomal dominant multiple neoplasia syndrome, it is caused by mutations in MEN1 on chromosome 11q13, not PRKAR1A. MEN1 presents with parathyroid adenomas, pituitary adenomas, and pancreatic neuroendocrine tumors, but does NOT feature the classic triad of spotty skin pigmentation, cardiac myxomas in any chamber, and PPNAD-driven ACTH-independent Cushing syndrome that define Carney Complex.
RET proto-oncogene activation results in uncontrolled tyrosine kinase signaling in neural crest-derived tissues: RET mutations (marked C on the diagram, chromosome 10q11.2) cause Multiple Endocrine Neoplasia type 2 (MEN2) and familial medullary thyroid carcinoma. RET is a proto-oncogene (gain-of-function mutations), not a tumor suppressor like PRKAR1A. MEN2 does not present with cardiac myxomas, spotty skin pigmentation, or PPNAD.
VHL inactivation causes HIF-1α stabilization, leading to angiogenic and metabolic dysregulation in renal and adrenal tumors: VHL mutations (marked D on the diagram, chromosome 3p25) cause von Hippel-Lindau syndrome, characterized by renal cell carcinoma, pheochromocytoma, and hemangioblastomas. VHL is a tumor suppressor involved in hypoxia-inducible factor regulation, not cAMP-PKA signaling. VHL syndrome does not feature cardiac myxomas, spotty skin pigmentation, or the endocrine profile of Carney Complex.
