Figure 1.

Shh Pathway Showing Some of the Proteins Implicated in HH

(A) In the presence of SHH, PTCH no longer inhibits SMO from translocating to the membrane, where it inhibits phosphorylation of GLI2 and GLI3 by PKA (PRKACA). Full-length GLI2 and GLI3 translocate to the nucleus and, together with GLI1 and the transcriptional coactivator CREBBP, promote transcription of downstream target genes⁹

(B) In the absence of SHH, SMO remains localized to endosomes in the cytoplasm. GLI2 and GLI3 are bound by the SUFU complex and phosphorylated by PKA,¹⁵ leading to their cleavage. The cleaved protein translocates to the nucleus, where it acts to repress target gene transcription.

14 out of 38 individuals with HH had confirmed somatic mutations in the Shh pathway; an additional five individuals had unconfirmed findings in this pathway. Four individuals had confirmed somatic point mutations in GLI3, and an additional individual had a CNV that encompassed this gene. Three individuals had confirmed somatic mutations in PRKACA. Other proteins in the Shh pathway implicated in HH because of their occurrence in CNVs are SHH, SMO, CREBBP, GLI2, and STK36 (or, FU). Not shown on this simplified diagram are IHH, LRP2, LRP5, AKT1, and specific WNT and BMP pathway members that encode proteins in the Shh pathway, or that closely interact with the pathway, and are located within CNVs found in individuals with HH.¹⁶, ¹⁷, ¹⁸, ¹⁹