Human developmental disorders and the Sonic hedgehog pathway

Sonic hedgehog (Shh) is a morphogen that is crucial for normal development of a variety of organ systems, including the brain and spinal cord, the eye, craniofacial structures, and the limbs. Mutations in the human SHH gene and genes that encode its downstream intracellular signaling pathway cause several clinical disorders. These include holoprosencephaly (HPE, the most common anomaly of the developing forebrain), nevoid basal cell carcinoma syndrome, sporadic tumors, including basal cell carcinomas, and three distinct congenital disorders: Greig syndrome Pallister-Hall syndrome, and isolated postaxial polydactyly. These conditions caused by abnormalities in the SHH pathway demonstrate the crucial role of SHH in complex developmental processes, and molecular analyses of these disorders provide insight into the normal function of the SHH pathway in human development.

Developmental genes and cancer: role of patched in basal cell carcinoma of the skin

Many genes originally identified because of their role in embryonic development are also important in postnatal control of cell growth and differentiation. Mutations in some of these genes have been shown to cause cancer. Basal cell carcinoma (BCC) of the skin is the most common cancer in humans. More than 750000 new cases are diagnosed annually, and the incidence is rising. BCCs are slow-growing, locally invasive tumors that rarely metastasize but can result in extensive morbidity through local recurrence and tissue destruction. Epidemiologic studies suggest that sunlight (particularly UVB radiation) is a strong risk factor for BCC formation, although other factors are also involved. The nevoid basal cell carcinoma syndrome (NBCCS), a rare genetic disorder, is characterized by predisposition to BCCs and other tumors as well as to a wide range of developmental defects. NBCCS maps to chromosome 9q22.3, and loss of heterozygosity at this site in both sporadic and hereditary BCCs suggests that it functions as a tumor suppressor. The gene for NBCCS was recently cloned and is the human homologue of the Drosophila gene "patched." Genetic studies in Drosophila show that patched is part of the hedgehog signaling pathway, which is important in determining embryonic patterning and cell fate in multiple structures of the developing embryo. Human patched is mutated in both hereditary and sporadic BCCs, and inactivation of this gene is probably a necessary, if not sufficient, step for BCC formation. Delineation of the biochemical pathway in which patched functions may lead to rational medical therapy for BCCs and possibly for other tumors associated with NBCCS.

Basal cell nevus syndrome

PURPOSE OF REVIEW

Basal cell nevus syndrome (BCNS), is a hereditary condition transmitted as an autosomal dominant trait exhibiting high penetrance and variable expressivity. Inherited or spontaneous mutations in the human homologue of the Drosophila patched gene underlie the disorder and in addition to tumor predisposition, are associated with a range of 'patterning' defects. Recent advances, with glimpses of possible therapies are emerging, but because of the wide-ranging nature of phenotypic expression and overlap with other syndromes, there is difficulty. Finally, because of the importance of PTCH and paralogous genes in many species other than humans, reports appear in a correspondingly wide range of journals, which makes 'keeping abreast' difficult.

RECENT FINDINGS

Progress has been achieved in understanding the role of Gli-1, 2, & 3 in development of 'sporadic' BCCs and BCNS. Expression of PTCH1 is now known to be regulated by alternative promoters and a single functional Gli-binding site. Expression of FOXE1 as a new transcriptional target of Gli2 has been demonstrated in human epidermis and BCCs. Finally, the discovery of Shh pathway inhibitors such as cyclopamine, a naturally occurring alkaloid and ornithine decarboxylase inhibition suggest possible interventional therapies.

SUMMARY

In BCNS, phenotype does not correlate with position of mutations within Patched, suggesting genetic makeup and environment modulate effects of premature protein truncation induced by PTCH mutation. These developmental abnormalities occur as a result of haplo-insufficiency in heterozygotes for the mutated gene, whereas neoplastic complications arise from a classical two-hit tumor suppressor gene model. Attention is therefore turning toward TP53 and PTCH associations.

Multiple neoplasms following craniospinal irradiation for medulloblastoma in a patient with nevoid basal cell carcinoma syndrome. Case report

A 28-year-old man presented to the authors' hospital with multiple intracranial tumors. At 2 years of age, he had undergone resection of a medulloblastoma and received adjunctive craniospinal irradiation. Subsequently, he was diagnosed with nevoid basal cell carcinoma syndrome, Gorlin's syndrome. Since his first presentation, he has required surgery for multiple basal cell carcinomas, an osteochondroma of the rib, two meningiomas, a trigeminal schwannoma, and a pleomorphic liposarcoma, all of which arose within the radiation field. Despite this impressive list of benign and malignant neoplasms, the patient is relatively well and leads a normal life. The authors examine the relationships between Gorlin's syndrome and radiation therapy and the subsequent development of tumors.

Nevoid basal cell carcinoma syndrome. Profile of genetic and environmental factors in oncogenesis

Nevoid basal cell carcinomas (NBCCs) are a prototype of a genetic form of basal cell carcinoma. These basal cell cancers, rather than being caused by genetic factors alone, are most likely the product of genetic and environmental factors. The NBCC syndrome provides a model for studying tumors induced by ionizing radiation and for viewing carcinogenesis as a multistage process explainable by a minimum of two steps. The interaction of genetic and environmental factors in producing tumors to which an individual is predisposed can be studied in patients with the NBCC syndrome and childhood medulloblastoma that was treated by radiation therapy. Individuals with the NBCC syndrome represent a special subgroup with a hereditary predisposition to basal cell carcinoma in whom ionizing radiation may supply the subsequent mutation necessary for tumor development. The genetically altered epidermis underlying the palm and sole pits found in patients with the syndrome represents basal cell carcinoma in situ from which basal cell carcinomas develop, albeit infrequently. The restrained biologic behavior of most of these tumors contrasts with the usual destructive behavior of the NBCCs of the head and neck in the same patient.

Several PATCHED1 missense mutations display activity in patched1-deficient fibroblasts

Mutations in mouse and human patched1 (ptc1) genes are associated with birth defects and cancer. Ptc1 is a receptor for Hedgehog (Hh) signaling proteins. Hh proteins activate transcription of target genes, including ptc1, and Ptc1 represses those genes, both by regulating the activity of Gli transcription factors. We have established mammalian cell lines with reduced Ptc1 function and a lacZ reporter to investigate Hh signal transduction. Embryonic fibroblasts were derived from mice, heterozygous or homozygous for a ptc1 mutation that inserts lacZ under the control of the ptc1 promoter (ptc1-lacZ). In heterozygous ptc1 cells, ptc1-lacZ was expressed at low levels but could be induced by Sonic Hedgehog (Shh) and Gli-1. Homozygous ptc1 cells expressed high levels of ptc1-lacZ without Hh stimulation. ptc1-lacZ expression was dependent on cell density in ptc1 homozygotes and Hh-stimulated heterozygotes but was independent of density when Gli1 was used to activate ptc1-lacZ. A wild-type ptc1 transgene introduced into homozygous ptc1 cells greatly reduced ptc1-lacZ expression. Expression of either half of Ptc1 alone resulted in improper maturation of the protein and a failure to complement the ptc1(-/-) cells. When co-expressed, both Ptc1 halves matured and had an activity similar to that of the intact protein. Three missense PTCH1 mutations exhibited significant functions in homozygous ptc1 cells. The missense mutants retained activity when expressed at about 10-fold lower levels and appeared as stable as wild-type Ptc1. These studies suggest that some tumors and disease phenotypes may arise from small reductions in PTCH1 activity.

Nevoid basal cell carcinoma syndrome. Multiple basal cell carcinomas of the palms after radiation therapy

A patient with the nevoid basal cell carcinoma syndrome had been treated with radiation therapy to the hands at 5 years of age. Multiple basal cell carcinomas of the palms and dorsa of the hands developed when the patient was 28 years of age. It is proposed that the radiation therapy induced tumors in palmar pits that seem to represent a forme fruste of basal cell carcinoma. Radiation therapy should be avoided in individuals with the nevoid basal cell carcinoma syndrome.

Multiple synchronous pigmented basal cell carcinomas following radiotherapy for Hodgkin's disease

BACKGROUND

Multiple basal cell carcinomas (BCCs) are infrequently seen in patients under 30 years of age. Their occurrence at a young age is often linked to some genodermatosis, including Nevoid Basal Cell Carcinoma Syndrome (NBCCS). The exposure to ionizing radiation is also considered to be a predisposing factor in the development of BCCs.

METHODS

We report the case of a 35-year-old patient who presented with seven synchronous, nodular, brownish-pigmented BCCs, confined within the radiation-treated cutaneous areas, 15 years after receiving Cobalt-60 (60Co) irradiation for Hodgkin's disease.

RESULTS

On the basis of clinical, radiological, and anamnestic data we excluded a NBCCS, thus proposing irradiation as the cause of the multiple synchronous pigmented BCCs.

CONCLUSIONS

Previous therapeutic ionizing radiation leads to an increased risk of BCCs confined to the region of the body to which radiotherapy was delivered. We consider our patient's BCCs represents a late adverse effect of the treatment with Cobalt-60.

Rearrangement of the same chromosome regions in different SV40 transformed human skin keratinocyte lines is associated with tumourigenicity

Twelve different human keratinocyte strains were transformed with recombinant plasmid pSV6-1 which contained an origin defective SV40 genome. When injected into athymic nude mice lines produced either squamous cell carcinomas (SCC) in all animals, SCC in some animals and epidermal cysts in others, or epidermal cysts only in all the animals. The tumourigenic capacity of the lines could be correlated with the chromosomal changes present initially in the transformed cells. Lines which produced SCC in all the animals within a short period of time all showed simultaneous loss of part of chromosomes 3p, 8p and 11p in one homologue. Lines which were not tumourigenic did not show these simultaneously appearing rearrangements. These specific rearrangements are acquired in vitro and the time taken for a recognisable tumour to appear is related to the proportion of such cells in the line. The rearrangement of the same chromosome regions in different tumourigenic cell lines suggests that genes in these regions are important in the development of squamous cell carcinoma, possibly by loss of heterozygosity, at particular loci.