Alternative first exons of PTCH1 are differentially regulated in vivo and may confer different functions to the PTCH1 protein

Abstract

The PTCH1 gene is a human tumour suppressor gene frequently mutated in basal cell carcinoma (BCC) and several other tumour types. It encodes a receptor for soluble factors of the hedgehog family. Binding of hedgehog to the receptor relieves its inhibitory action on the transmembrane co-receptor Smoh. In this study we describe alternative first exons of the PTCH1 tumour suppressor gene and show that they are differentially regulated in normal tissues, exon 1B being expressed at very low levels and the major mRNA species containing exon 1 or 1A. Exon 1B transcripts were found to be specifically upregulated in nodular BCCs. The different PTCH1 transcripts all encode proteins that interact with Smoh in doubly transfected cells. Furthermore, functional assays demonstrated that whereas all PTCH1 isoforms can inhibit the activity of SHH, only the PTCH1B isoform is capable of fully inhibiting Smoh activity. The results indicate that in tumour cells the PTCH1B promoter is specifically activated and importantly, that the N-terminal part of PTCH1 including exon 1B is required for full inhibition of Smoh signaling but not for physical interaction with Smoh.

Distinct roles of first exon variants of the tumor-suppressor Patched1 in Hedgehog signaling

“…In previous reports, we and others showed that PTCH1 has several first exons, with the expression of exons 1B and 1C being upregulated by HH signaling activation (Kogerman et al, 2002;Shimokawa et al, 2004;Å gren et al, 2004;Nagao et al, 2005). However, the functional differences among the PTCH1 isoforms were not fully clarified.…”

“…We also found a third variant, which joined the alternative 5 0 -splice site within PTCH1 splice variants specific function T Shimokawa et al exon 1C directly to exon 3 (PTCH1-1CkidDE2, data not shown). The open reading frames (ORFs) of PTCH1-1CDE2 and PTCH1-1CkidDE2 were the same as the ORFs of the previously characterized variants PTCH1-1 and PTCH1-1A (Kogerman et al, 2002). On the other hand, PTCH1-1Ckid had a fusion ORF, resulting from the joining of an upstream ORF to the PTCH1-1C ORF and consequently coded for a PTCH1 variant with a unique N-terminal tail.…”

“…PTCH1 is reported to have at least four first exons, 1, 1A, 1B and 1C (Johnson et al, 1996;Hahn et al, 1996a, b;Kogerman et al, 2002;Shimokawa et al, 2004;Nagao et al, 2005), and moreover undergoes exon skipping (Smyth et al, 1998;Nagao et al, 2005) or alternative exon insertion events (Uchikawa et al, 2006). The alternative promoters for exons 1B and 1C have GLI1 binding sites, consequently HH signaling activation results in upregulation of both PTCH1-1B and -1C (Shimokawa et al, 2004).…”

Selective haploinsufficiency of longer isoforms of PTCH1 protein can cause nevoid basal cell carcinoma syndrome

“…[8][9][10] As the 5 0 exonic structure of PTCH1 is conserved in humans and mice, alternative splicing between the first and second Reverse-transcription-PCR products were subcloned into the pGEM-T Easy vector and Escherichia coli was transformed. DNA was extracted from the transformed E. coli and sequenced.…”

“…7 Focusing on the 5 0 region of the PTCH1 gene, we found that alternative splicing takes place between five alternatively used first exons and one commonly used second exon (Figure 1a). [8][9][10] This alternative splicing leads to the production of at least three isoforms of the PTCH1 protein, PTCHL, PTCHM and PTCHS (Figure 1b, wild-type allele). This alternative splicing is conserved in humans and mice suggesting a mechanism whereby a single PTCH1 gene has a role in both tumor cell growth and embryonic development.…”