5727

PTCH1

BCNS|HPE7|NBCCS|PTC|PTC1|PTCH|PTCH11;patched 1;PTCH1;patched 1

PTCH protein +12b|PTCH protein +4'|PTCH protein -10|PTCH protein -3,4,5|protein patched homolog 1

9q22.3

protein-coding

Gorlin syndrome is an autosomal dominant disorder that predisposes to basal cell carcinomas of the skin, ovarian fibromas, and medulloblastomas. Unlike other hereditary disorders associated with cancer, it features widespread developmental defects. To investigate the possibility that the syndrome is caused by mutation in a tumor suppressor gene, we searched for loss of heterozygosity in 16 sporadic basal cell carcinomas, 2 hereditary basal cell carcinomas, and 1 hereditary ovarian fibroma and performed genetic linkage studies in five Gorlin syndrome kindreds. Eleven sporadic basal cell carcinomas and all 3 hereditary tumors had allelic loss of chromosome 9q31, and all informative kindreds showed tight linkage between the Gorlin syndrome gene and a genetic marker in this region. Loss of heterozygosity at this chromosomal location, particularly in hereditary tumors, implies that the gene is homozygously inactivated and normally functions as a tumor suppressor. In contrast, hemizygous germline mutations lead to multiple congenital anomalies.

The PTCH1 tumor suppressor gene encodes a receptor for secreted hedgehog (HH) ligands and is important for proper proliferation, differentiation and patterning in almost every tissue and organ during embryogenesis. The PTCH1 protein works as a negative regulator of the HH-signaling pathway by repressing downstream signaling by the coreceptor smoothened (SMOH). mutations in PTCH1 lead to constitutive expression of HH target genes and a relationship between mutated PTCH1 and the most common tumor form in the Western world, Basal Cell Carcinoma (BCC) has been clearly established. We here show that PTCH1 is transcriptionally regulated by three independent promoters generating transcripts with alternative first exons. We demonstrate that only one of two putative Gli-binding sites that were identified in the promoter region of PTCH1 is functional, and that the transactivating Gli proteins, GLI1, Gli2 and GLI3, bind and enhance transcription through this site. Moreover, a strong repression of both basal and induced PTCH1 transcription was observed following expression of a truncated version of GLI3. Most interestingly, the upstream components in the HH-signaling cascade, Sonic HH (SHH) and SMOH, solely operate through the functional Gli-binding site because mutation of the Gli-binding site resulted in the disappearance of the enhanced transcription induced by the Gli proteins, as well as by SHH or SMOH. This finding suggests that transcriptional activation of the PTCH1 gene mediated via the HH-signaling pathway is dependent on the single functional Gli-binding site.

mutations in mouse and human patched (PTCH) genes are associated with birth defects and cancer. PTCH, a 12-pass transmembrane protein, is a receptor for Sonic hedgehog (Shh) signaling proteins. Shh proteins activate transcription of target genes, including PTCH, via GLI transcription factors. Here we identified seven and five isoforms of human and mouse PTCH mRNA, respectively, which are generated by the complex alternative use of five exons as the first exon (exons 1a to 1e in the 5-to-3 order). Although expression profiles of these isoforms were highly variable among human tissues, three of them, PTCHa, PTCHb, and PTCHd, were predominantly expressed in most tissues, PTCHd being most ubiquitous. In contrast, PTCHb was always predominant and reached a maximum at E10.5 during mouse development. These three mRNA isoforms encode three PTCH proteins with distinct N-termini, PTCH(L), PTCH(M), and PTCH(S). The expression of these three isoforms was regulated by GLI transcription factors, and at least two functional GLI-binding sequences were identified, one in exon 1a and the other between exon 1a and exon 1b. PTCH(L) and PTCH(M) were equally active in terms of suppressing GLI-mediated transcription and inducing apoptosis. PTCH(S) protein (encoded by PTCHd), lacking the first transmembrane domain, was more unstable than the other two, resulting in a reduced activity. This study may shed light on the mechanism whereby a single PTCH gene plays a role in both tumor cell growth and embryonic development.

Medulloblastoma (MB) is the most common malignant pediatric brain tumor which is thought to originate from cerebellar granule cell precursors (CGNPs) that fail to properly exit the cell cycle and differentiate. Although mutations in the Sonic Hedgehog (Shh) signaling pathway occur in 30% of cases, genetic alterations that account for MB formation in most patients have not yet been identified. We recently determined that the cyclin D-dependent kinase inhibitor, p18(Ink4c), is expressed as CGNPs exit the cell cycle, suggesting that this protein might play a central role in arresting the proliferation of these cells and in timing their subsequent migration and differentiation. In mice, disruption of Ink4c collaborates independently with loss of p53 or with inactivation of the gene (Ptc1) encoding the Shh receptor, Patched, to induce MB formation. Whereas loss of both Ink4c alleles is required for MB formation in a p53-null background, Ink4c is haplo-insufficient for tumor suppression in a Ptc(1+/-) background. Moreover, MBs derived from Ptc(1+/-) mice that lack one or two Ink4c alleles retain wild-type p53. Methylation of the INK4C (CDKN2C) promoter and complete loss of p18(INK4C) protein expression were detected in a significant fraction of human MBs again pointing toward a role for INK4C in suppression of MB formation.

Patched1 (PTCH1) is one of the key molecules involved in the Hedgehog (HH) signaling pathway and acts as the receptor of HH ligands. Additionally, PTCH1 inhibits the positive signal transductor Smoothened (SMO). Several PTCH1 splice variants are known but the functional differences among them are not clear. Here, we demonstrate the unique biological properties of the PTCH1 isoforms generated by alternative first exon usage. All isoforms examined worked as functional receptors of both Sonic HH and Desert HH. However, the signaling upregulated isoforms PTCH1-1B and -1C inhibited SMO and the pathway transcription factors glioma 1 (GLI1) and GLI2 to a higher extent than PTCH1-1 and -1Ckid. Moreover, in situ hybridizations allowed the detection of the Ptch1 isoforms in specific structures of the developing mouse embryo. Additionally, the differences in the N-terminal tail had a dramatic influence on the steady states of the proteins, with PTCH1-1B and -1C levels being significantly higher than PTCH1-1 and -1Ckid. This implies that the pronounced signaling inhibitory properties of PTCH1-1B and -1C may be mostly due to this high-protein expression rather than to intrinsic functional differences. Thus, our study supports a role of splicing variation and promoter choice for HH signaling regulation.

BACKGROUND: Cardiac fibroma (CF) is a rare benign tumor that is poorly characterized genetically. CF is more commonly encountered in patients with Gorlin syndrome (3%) than the general population. mutations of the tumor suppressor gene PTCH1 are the underlying cause of Gorlin syndrome. METHODS: Conventional cytogenetic analysis was performed on a peripheral blood and a CF sample from a 2-week-old male. In addition, fluorescence in situ hybridization (FISH) studies were performed to assess the copy number of the PTCH1 gene locus (9q22.3) on metaphase and interphase cells from these same specimens using yeast artificial protein (YAC) probe 891G1 and on representative paraffin-embedded tissue sections of two additional CFs (one arising in a 2-month-old female and the other in a 13-week-old male). None of the patients had Gorlin syndrome. RESULTS: Karyotypically, the following abnormal chromosomal complement was detected in the 2-week-old males CF: 46,XY,del(9)(q22q34)[15]. FISH studies revealed homozygous loss of the PTCH1 locus in the cytogenetically analyzed CF and in the CF arising in the 13-week-old male. Heterozygous loss of this locus was identified in the remaining CF from the 2-month-old female. A mutational mechanism other than deletion may be responsible for PTCH1 inactivation on the other locus in this latter patient. Conventional cytogenetic and FISH studies of the peripheral blood sample from the 2-week-old male were normal. CONCLUSION: These data support a tumor suppressor gene role for PTCH1 in nonsyndromic or sporadic CFs.

Medulloblastoma is an embryonal tumor thought to arise from the granule cell precursors (GCPs) of the cerebellum. PATCHED (PTCH), an inhibitor of Hedgehog signaling, is the best-characterized tumor suppressor in medulloblastoma. However, <20% of medulloblastomas have mutations in PTCH. In the search for other tumor suppressors, interest has focused on the deletion events at the 17p13.3 locus, the most common genetic defect in medulloblastoma. This chromosomal region contains HYPERMETHYLATED IN cancer 1 (HIC1), a transcriptional repressor that is a frequent target of epigenetic gene silencing in medulloblastoma. Here we use a mouse model of Ptch1 heterozygosity to reveal a critical tumor suppressor function for Hic1 in medulloblastoma. When compared with Ptch1 heterozygous mutants, compound Ptch1/Hic1 heterozygotes display a fourfold increased incidence of medulloblastoma. We show that Hic1 is a direct transcriptional repressor of Atonal Homolog 1 (Atoh1), a proneural transcription factor essential for cerebellar development, and show that ATOH1 expression is required for human medulloblastoma cell growth in vitro. Given that Atoh1 is also a putative target of Hh signaling, we conclude that the Hic1 and Ptch1 tumor suppressors cooperate to silence Atoh1 expression during a critical phase in GCP differentiation in which malignant transformation may lead to medulloblastoma.

Basal cell carcinoma of the skin is the most common type of cancer in humans. The majority of these tumors displays aberrant activation of the SONIC HEDGEHOG (SHH)/PATCHED pathway, triggered by mutations in the PATCHED tumor suppressor gene, which encodes a transmembrane receptor of SHH. In this study, we took advantage of the natural genotype (PATCHED(+/-)) of healthy keratinocytes expanded from patients with the nevoid basal cell carcinoma or Gorlin syndrome to mimic heterozygous somatic mutations thought to occur in the PATCHED gene early upon basal cell carcinoma development in the general population. PATCHED(+/-) epidermis developed on a dermal equivalent containing wild-type (WT) PATCHED(+/+) fibroblasts exhibited striking invasiveness and hyperproliferation, as well as marked differentiation impairment. Deciphering the phenotype of PATCHED(+/-) keratinocytes revealed slight increases of the transcriptional activators GLI1 and GLI2-the latter known to provoke basal cell carcinoma-like tumors when overexpressed in transgenic mice. PATCHED(+/-) keratinocytes also showed a substantial increase of the cell cycle regulator cyclin D1. These data show for the first time the physiological impact of constitutive heterozygous PATCHED mutations in primary human keratinocytes and strongly argue for a yet elusive mechanism of haploinsufficiency leading to cancer proneness.

Dysregulation of the Hedgehog signaling pathway is central to the development of certain tumor types, including medulloblastoma and basal cell carcinoma (BCC). Patched1 (Ptch1) and suppressor of fused (Sufu) are two essential negative regulators of the pathway with tumor suppressor activity. Ptch1(+/-) mice are predisposed to developing medulloblastoma and rhabdomyosarcoma, while Sufu(+/-) mice develop a skin phenotype characterized by basaloid epidermal proliferations. Here, we have studied tumor development in Sufu(+/-)Ptch1(+/-) mice to determine the effect of compound heterozygosity on the onset, incidence, and spectrum of tumors. We found significantly more (2.3-fold) basaloid proliferations in Sufu(+/-)Ptch1(+/-) compared to Sufu(+/-) female, but not male, mice. For medulloblastoma, the cumulative 1-yr incidence was 1.5-fold higher in Sufu(+/-)Ptch1(+/-) compared to Ptch1(+/-) female mice but this strong trend was not statistically significant. Together this suggests a weak genetic interaction of the two tumor suppressor genes. We noted a few rhabdomyosarcomas and pancreatic cysts in the Sufu(+/-)Ptch1(+/-) mice, but the numbers were not significantly different from the single heterozygous mice. Hydrocephalus developed in approximately 20% of the Ptch1(+/-) and Sufu(+/-)Ptch1(+/-) but not in Sufu(+/-) mice. Interestingly, most of the medulloblastomas from the Sufu(+/-)Ptch1(+/-) mice had lost expression of the remaining Ptch1 wild-type allele but not the Sufu wild-type allele. On the contrary, Sufu as well as Gli1 and Gli2 expression was upregulated in the medulloblastomas compared to adult cerebellum in Ptch1(+/-) and Sufu(+/-)Ptch1(+/-) mice. This suggests that Sufu expression may be regulated by Hedgehog pathway activity and could constitute another negative feedback loop in the pathway.

Inactivation of homologous recombination (HR) or nonhomologous end-joining (NHEJ) predisposes to a spectrum of tumor types. Here, we inactivated DNA double-strand break repair (DSBR) proteins, DNA Ligase IV (Lig4), Xrcc2, and Brca2, or combined Lig4/Xrcc2 during neural development using Nestin-cre. In all cases, inactivation of these repair factors, together with p53 loss, led to rapid medulloblastoma formation. Genomic analysis of these tumors showed recurring chromosome 13 alterations via chromosomal loss or translocations involving regions containing Ptch1. Sequence analysis of the remaining Ptch1 allele showed a variety of inactivating mutations in all tumors analyzed, highlighting the critical tumor suppressor function of this hedgehog-signaling regulator. We also observed genomic amplification or up-regulation of either N-Myc or cyclin D2 in all medulloblastomas. Additionally, chromosome 19, which contains Pten, was also selectively deleted in medulloblastoma arising after disruption of HR. Thus, our data highlight the preeminence of Ptch1 as a tumor suppressor in cerebellar granule cells and reveal other genomic events central to the genesis of medulloblastoma.