The Hedgehog signalling pathway and its role in cancer

Mutations in Hedgehog (Hh) pathway genes such as Patched1 are of established importance in Basal cell carcinoma (BCC). However, BCC is unusual because it has indolent behaviour and rarely metastasises. Hh pathway gene mutations are rare in cancers with more typical malignant behaviour, leading to the hypothesis that the pathway plays an important role in BCC but has limited importance in cancers with typical malignant behaviour. This has been addressed in three ways. First, evidence that superficial BCC is a clonal proliferation driven by alterations at the Patched1 locus was sought. Second, evidence of Wnt pathway activation in BCC was sought by looking for nuclear accumulation of -catenin, because Wnt is a putative Hedgehog pathway target gene. Third, to determine whether the Hh pathway was important in a cancer with typical malignant behaviour, breast cancer was studied. In superficial BCC, microdissection of individual tumour nests and analysis for LOH in six cases using three microsatellite markers at the Patched1 locus was performed. Only one of the microsatellites was successfully employed. In four of the cases, all nests showed no LOH and in two cases, all nests lost the same allele. Concordant patterns of allelic loss/retention within cases represented strong evidence of clonality. In addition, the two cases with LOH suggested that clonal proliferation in superficial BCCs might be driven by Patched1 alternations. Accumulation of nuclear -catenin was found in 20 out of 86 BCCs and had a significant relationship with proliferation, indicating that the Hh pathway may act through its target genes. Alternative factors influencing -catenin distribution were sought: no relationship between nuclear -catenin and expression of E-cadherin was seen and no -catenin mutations in eight cases were found. Four breast cancer cell lines and HBL100, derived from normal mammary epithelium, showed substantially lower expression of Hh pathway target genes than BCC and were unresponsive to the pathway ligand, Sonic Hh, suggesting that the pathway is not important in this tumour. However, a relationship was found between the expression of Patched1 and in vitro invasion, the importance of which is uncertain. In conclusion, these results support the hypothesis that the Hh pathway is important in BCC but not in cancers with typical malignant behaviour.