PTCH1 gene (localization 9q22.3) is a tumor suppresor gene which plays a role in the cell cycle regulation. The sequence corresponds to a complex gene locus with a complicated regulation of transcription and alternative splicing. Coding sequence is composed of several different exons 1 and 22 other exons. Gene product is a transmembrane protein of the Patched family. This protein has a function as the receptor of signal hedgehog proteins, especially sonic hedgehog (SHH). The Patched protein inhibits the expresion of the gene's encoding signal proteins belonging to the TGF-beta and Wnt family. Binding of the SHH ligand to the protein disturbs this inhibition and releases the signal cascade.
Mutations in the PTCH1 gene are responsible for an autosomal dominant hereditary disease called Gorlin (Gorlin-Goltz) syndrome or also basal cell nevus syndrome. The syndrome manifests in various developmental anomalies and by a predisposition to various carcinomas, especially skin carcinomas. Evans et al. established the criteria for the diagnosis of Gorlin syndrome. For the evidence of the disease it is necessary to fulfill at least two major criteria or one major and two minor criteria. The major criteria are: 1. manifestation of 1 - 2 cutaneous basaliomas, 2. manifestation of basalioma before the age of 20, 3. odontogenic keratocyst of jaw bone, 4. falx cerebri calcification, 5. anomalies of the shape of ribs, 6. occurence of the same manifestations in 1st degree relatives, 7. presence of two or more palmar pits. The minor criteria represent: 1. macrocephaly, 2. cleft lip/palate, 3. bulging forehead, 4. hypertelorism, 5. skeletal anomalies, 6. ovarian fibromas, 7. medulloblastomas.
Molecular biology of the tumor formation in patients with Gorlin syndrome corresponds to the Knudson's two - hit hypothesis. Germline mutation of the PTCH1 gene on one allele is in 90 % of hereditary basaliomas complemented by the loss of heterozygosity (LOH) in that locus. Developmental and skeletal abnormalities are probably caused due to the PTCH1 haploinsufficiency. The spectrum of mutations finding in the PTCH1 gene is wide. Deletions, insertions, nonsense, missense and splicing mutations were identified over the entire gene, however, most often they are located in two large extracellular and one large intracellular loop.
Examination
We perform the mutation analysis by direct sequencing of the coding section of the PTCH1 gene including exon - intron regions. Examination can be done from blood or tissue (frozen or fixed).
Clinical sensitivity:
Under strict clinical criteria with a precise diagnosis of Gorlingoa syndrome there is a clinical sensitivity for gene PTCH1 at 87% (Lo Muzio, 2013).
Sequence analysis of the coding exons and exon-intron connections detect about 94% of mutations in the gene PTCH1 listed in a publicly available version of the mutation database HGMD (The Human Gene Mutation Database).
MLPA (Multiple-Ligation Probe Amplification) analysis of the mutation in a large-scale (amplification / deletion) detects approximately 5% of mutations in the gene PTCH1 listed in a publicly available version of the mutation database HGMD.
Analytical sensitivity and specificity of the sequencing: 99%.
Limitations:
Mutations deep in the introns and regulatory sequences are not detected. Rare polymorphisms in the location of primers or probes annealing may cause a diagnostic error.
In the case of analysis of somatic mutations sequencing mutations will not be detected, if the altered cell line is not represented by at least 20%.