A 22-year-old Indian male presents with multiple basal cell carcinomas on his face and trunk, odontogenic keratocysts of the jaw, and palmar pits. Genetic testing confirms a heterozygous germline mutation in the PTCH1 gene at the locus marked **A** (9q22.32). Which of the following best explains the pathogenic mechanism underlying his phenotype?
A. Gain-of-function mutation in TP53 at 17p13.1 causes impaired apoptosis and accumulation of damaged cells
B. Homozygous inactivation of CDKN2A at 9p21.3 results in loss of cell cycle checkpoint control
C. Loss of PTCH1 function leads to constitutive activation of the Hedgehog signaling pathway, driving uncontrolled proliferation in tissues dependent on Hh during development
D. Haploinsufficiency of a tumor suppressor at 3p21.31 impairs DNA mismatch repair mechanisms
Explanation
Why option 1 is correct
The PTCH1 gene at chromosome 9q22.32 encodes a 12-transmembrane receptor that normally inhibits Smoothened (SMO) in the Hedgehog signaling pathway. Loss of PTCH1 function (as occurs with heterozygous germline mutations in Gorlin syndrome) causes constitutive activation of the SHH-PTCH1-SMO-GLI pathway, leading to uncontrolled proliferation in tissues dependent on Hedgehog signaling during development—particularly skin (basal cell carcinomas), jaw (odontogenic keratocysts), and other structures. This is the fundamental pathogenic mechanism of Gorlin syndrome (NBCCS) and directly explains the patient's clinical presentation.
Why each distractor is wrong
Option 2: While TP53 mutations are associated with Li-Fraumeni syndrome and increased cancer risk, the locus at 17p13.1 is not involved in Gorlin syndrome. The anchor diagnosis is caused by PTCH1 mutation, not TP53 gain-of-function.
Option 3: CDKN2A at 9p21.3 is associated with familial melanoma and pancreatic cancer predisposition, not Gorlin syndrome. Additionally, Gorlin syndrome is autosomal dominant with heterozygous mutations, not homozygous inactivation.
Option 4: The 3p21.31 region is not implicated in Gorlin syndrome pathogenesis. PTCH1 at 9q22.32 is the primary gene responsible, and the mechanism involves Hedgehog pathway dysregulation, not mismatch repair defects.
