Evidence that haploinsufficiency of Ptch leads to medulloblastoma in mice

Abstract

The PTCH gene encodes a putative tumor suppressor protein; germline alterations in PTCH have been found in patients with the nevoid basal cell carcinoma syndrome (NBCCS). Medulloblastoma, a brain tumor, develops in about 3% of NBCCS patients, and mutations in PTCH have also been described in a subset of sporadic medulloblastomas. The search for the causes of medulloblastoma has been hindered by the lack of an appropriate model system for this tumor type. Recently, a transgenic mouse hemizygous for the Ptch gene was generated by homologous recombination. Medulloblastomas were found in about 19% of these mice within the first 25 weeks after birth. The status of the wild-type PTCH allele in these tumors has not been investigated. For clearer definition of the role of PTCH as a tumor suppressor in medulloblastoma, 13 cerebellar tumors from transgenic Ptch ϩ/Ϫ mice were examined for alterations in the remaining Ptch allele. A single mutation was found in one tumor, a C-to-A substitution changing a tyrosine to a stop codon; all other tumors exhibited a wild-type sequence. Two tumors with normal Ptch cDNA were examined by in situ hybridization. Ptch cDNA was found in tumor cells but not in associated tumor stroma. We also examined the mRNA expression levels for the remaining Ptch allele, as well as for Gli1, a gene known to be transcriptionally activated by Ptch inactivation. Blot analysis of RNA from the 13 tumors shows that Ptch mRNA of appropriate size is expressed in all tumors at varying levels. Expression of Gli1 was increased in tumors compared to normal cerebellum. These results suggest that deletion of one copy of Ptch may be sufficient to promote medulloblastoma development in mice.

Loss ofpatchedand disruption of granule cell development in a pre-neoplastic stage of medulloblastoma

“…Heterozygotes survive to adulthood, but after 3 months of age, 14-20% develop medulloblastoma (Goodrich et al, 1997;Wetmore et al, 2000). The status of the wild-type patched allele in these tumors is controversial: some studies have reported expression of wild-type patched in tumor tissue (Romer et al, 2004;Wetmore et al, 2000;Zurawel et al, 2000), whereas others have suggested that the wild-type allele is epigenetically silenced (Berman et al, 2002). Determining whether patched is lost -and when during tumorigenesis this loss occurs -is crucial for understanding the mechanisms of medulloblastoma formation.…”

Aberrant promoter methylation of previously unidentified target genes is a common abnormality in medulloblastomas–Implications for tumor biology and potential clinical utility

“…While a small subgroup of MBs seems to be characterized by mutations in the human homologue of the drosophila segment polarity gene PTCH1, no clear molecular pathway has been de®ned for the origin of the majority of MBs (Zurawel et al, 2000). Recent studies using Comparative Genome Hybridization (CGH) demonstrated loss of genetic material from chromosomes 8p, 10q, 11, 16q, 17p (in up to 50%) and 22; gains were detected for 17q and 7 and ampli®cations were seen in 5p15.3 and 11q22.3 (Reardon et al, 1997;Russo et al, 1999).…”

Alteration of the PATCHED locus in superficial bladder cancer

“…To our knowledge, such an occurrence has not been reported for the PTC gene. On the other hand, increased PTC expression has been reported in cancers such as BCCs and medulloblastomas even in the presence of PTC LOH (Wicking et al, 1999;Zurawel et al, 2000). Various studies have suggested that any disruption in the equilibrium between PTC, SHH and SMO can lead to tumour formation.…”

“…Rapid induction of BCCs was observed in transgenic mice overexpressing Shh (Oro et al, 1997). Overexpression of SMO was also found in most BCCs (Kallassy et al, 1997) while haploinsufficiency of PTC was shown to induce medulloblastoma (Zurawel et al, 2000) and rhabdomyosarcoma in mice (Calzada-Wack et al, 2002).…”