In several of these cases, activation of HH-GLI signaling is mediated by overproduction of HH ligands (e.g., prostate cancer), loss-of-function mutations in <i>PTCH1</i> or gain-of-function mutations in <i>SMO</i>, which occur in the majority of basal cell carcinoma (BCC), SHH-subtype medulloblastoma and rhabdomyosarcoma.
We speculate that PTCH2 modulates tumorigenesis linked to the PTCH1 mutation and is likely associated with the congenital onset of the RMS observed in our patient.
Expression of GLI1 with or without PTCH was detected in substantial subsets of embryonal RMS (ERMS) and US tumors but only rarely in alveolar RMS tumors.
Sporadic rhabdomyomas and rhabdomyosarcomas showed overexpression of PTCH (43/43) and GLI1 (41/43) mRNA, as determined by in situ hybridization, indicating ongoing active hedgehog signalling.
Using mouse genetic models of RMS development, we further show that Fem1a is consistently downregulated in primary RMS from Ptch1+/- mice, from p53-/- mice, from p53+/-; Ptch1+/- mice, and from HGF/SF-Ink4a/Arf-/- mice.
It now appears that constitutive activation of Hedgehog signalling, by inactivating mutations in PTCH1 or activating mutations in the coreceptor SMOH, is required and possibly sufficient for basal cell carcinoma development and also contributes to the formation of a variety of other tumour types, including medulloblastoma and rhabdomyosarcoma.