Repressing the hedgehog signalling pathway

Abstract

The Hedgehog (Hh) signalling pathway is essential for metazoan development and aberrant activation of the pathway is found in tumours. Mutations in Hh pathway components are found in several tumour types, including basal cell carcinoma (BCC) and medulloblastoma (MB). The outline of the Hh pathway is evolutionary conserved, with Patched (Ptch) as the Hh receptor and Smoothened (Smo) as the signal transducer that relays the signal into the cytoplasm to activate the transcription factors Ci/Gli, which regulate gene transcription and cellular responses leading to cell proliferation and/or differentiation.

These studies focus on the function of Ptch1 and Suppressor of fused (Sufu), both negative regulators of Hh signalling. In paper I, we showed that genetic ablation of Sufu in mice results in embryonic lethality around embryonic day 9 with failure to close the neural tube. Sufu-/- embryos showed high Gli activity that was not repressable at the level of Smo. Sufu+/- mice develop jaw keratocysts and a skin phenotype with BCC-like lesions, alopecia, increased skin pigmenation, and abberant sebaceous gland morphology. Our data show that in contrast to the situation in Drosophila, Sufu has a central role in mammalian Hh signalling, and its loss-offunction leads to ligand-independent activation of the Hh pathway.

In paper II, we have investigated the possibility that Sufu would modulate the phenotype of Ptch1+/- mice or vice-versa. We found that the frequency of MB was not altered in Sufu+/-Ptch+/- mice compared to Ptch1+/- mice. All MB in Ptch1+/- mice and all but one MB in Sufu+/-Ptch1+/- mice had lost expression of the Ptch1 wt allele, indicating that this is the critical genetic change leading to MB formation in these mice. Skin from Sufu+/-, Ptch1+/- and Sufu+/-Ptch1+/- mice developed BCClike epidermal lesions. The number of such lesions was increased in Sufu+/-Ptch1+/- mice compared to Sufu+/- and Ptch1+/- mice. Our data indicate a differential importance of Sufu and Ptch1 as tumour suppressors in MB versus the BCC-like lesions.

In paper III, we investigated the unique properties of PTCH1 isoforms generated by alternative first exon usage. All isoforms functioned as Hh receptors. However, the 4 isoforms induced by Hh signalling inhibited pathway activity to a higher extent than those not regulated by Hh signalling. In situ hybridization allowed the detection of the Ptch1 isoforms in specific structures of mouse embryos. This study supports a role of splicing variation and/or promoter choice for Hh signalling regulation.

In paper IV, we created a conditional Ptch1 allele in mice and deleted Ptch1 by using the ARR2PBi-Cre transgenic line. Ptch1 was deleted in prostate and seminal vesicles but we found no abberant phenotype in these tissues up to the age of 12 months. In contrast, BCC-like epidermal lesions were initiated by Cre-mediated Ptch1 recombination in solitary cells in interfollicular epidermis and hair follicles. Skin with BCC-like lesions showed upregulation of Gli1, an indicator of Hh pathway activity. The skin proliferations arose both from interfollicular epidermis and hair follicles. Our results indicate that loss of Ptch1 in keratinocytes drives them into a hair follicle fate.