A 72-year-old man with Gorlin syndrome presents with multiple asymptomatic, pearly nodules with central ulceration on the face and trunk. Histopathology shows islands of basaloid cells with peripheral palisading and retraction artifact. He has a history of medulloblastoma at age 8 and multiple odontogenic keratocysts. Which molecular alteration is responsible for this syndrome and predisposes to multiple skin tumors?

Explanation

## Gorlin Syndrome (Nevoid Basal Cell Carcinoma Syndrome) ### Molecular Basis **Key Point:** Gorlin syndrome is an autosomal dominant disorder caused by germline loss-of-function mutations in the PTCH1 (patched homolog 1) gene located on chromosome 9q22. This gene encodes a transmembrane receptor that acts as a negative regulator of the Hedgehog signaling pathway. ### Pathophysiology The PTCH1 protein normally suppresses the Smoothened (SMO) receptor. Loss of PTCH1 function leads to: 1. **Constitutive Hedgehog pathway activation** — even in the absence of Hedgehog ligand 2. **Increased cell proliferation** — particularly in basal keratinocytes 3. **Impaired apoptosis** — reduced programmed cell death 4. **Multiple tumor predisposition** — affecting skin, nervous system, and odontogenic tissues ### Clinical Features of Gorlin Syndrome | Feature | Details | |---------|----------| | **Skin manifestations** | Multiple basal cell carcinomas (BCCs) appearing in childhood/adolescence; often >100 lesions by adulthood | | **Odontogenic** | Multiple odontogenic keratocysts (jaw cysts) | | **CNS** | Medulloblastoma (5–10% of patients); increased risk in childhood | | **Skeletal** | Bifid ribs, vertebral anomalies, frontal bossing | | **Ocular** | Cataracts, strabismus | | **Other** | Palmar/plantar pits, cardiac fibromas | **High-Yield:** The combination of **multiple BCCs + odontogenic keratocysts + medulloblastoma history** is pathognomonic for Gorlin syndrome. ### Histopathology of BCC in Gorlin Syndrome The lesions described (pearly nodules with central ulceration, basaloid islands with peripheral palisading, retraction artifact) are classic features of **nodular BCC**, the most common histologic type in Gorlin syndrome. ### Why PTCH1 and Not Other Genes? **Clinical Pearl:** PTCH1 mutations account for ~80% of Gorlin syndrome cases. The Hedgehog pathway is the primary driver of basal cell proliferation in this syndrome, making PTCH1 the key molecular culprit. ```mermaid flowchart TD A[PTCH1 Loss-of-Function Mutation]:::outcome --> B[Loss of Hedgehog Pathway Inhibition]:::outcome B --> C[Constitutive SMO Activation]:::outcome C --> D[Increased GLI Transcription Factor Activity]:::outcome D --> E[Enhanced Cell Proliferation & Survival]:::action E --> F[Multiple BCCs, Odontogenic Keratocysts, Medulloblastoma]:::outcome ``` ### Inheritance and Genetic Counseling - **Autosomal dominant** inheritance with high penetrance (~90%) - **Germline mutation** — present in all cells - **De novo mutations** — ~20% of cases have no family history - Genetic testing is recommended for all suspected cases and their relatives **Mnemonic:** **PTCH1** = **P**atched **T**ransmembrane receptor, **C**arcinoma predisposition, **H**edgehog pathway regulator, **1** = first/primary gene implicated in Gorlin syndrome.