Identification and characterization of two novel PTCH1 splice variants

All isoforms examined worked as functional receptors of SHH. However, the isoforms PTCH1-1B and PTCH1-Δ10 inhibited SMO and the pathway transcription factor glioma 1 (GLI1) to a greater extent than did PTCH1-Δ15. In addition, PTCH1-1B and PTCH1-Δ10 (but not PTCH1-Δ15) can be negative regulators of the HH pathway. These results indicate that the SSD domain and the C-terminal region are essential for maximal repressor function of PTCH1. Additionally, SMO inhibition by PTCH1 occurred through a nonstoichiometric, catalytic mechanism, indicating that this inhibition was less dependent on the dose of the PTCH1 protein. Finally, all these isoforms have been revealed to inhibit GLI1 activation by either the classical HH signaling pathway or a new pathway not reliant on both SMO and apoptosis. Thus, our study clearly demonstrated the unique involvement of the two novel PTCH1 splice variants in HH signal transduction.