Grantee Research Project Results
2000 Progress Report: "Patched" Gene Mosacism as a Basis for Differences in Skin Cancer Susceptibility
EPA Grant Number: R827018Title: "Patched" Gene Mosacism as a Basis for Differences in Skin Cancer Susceptibility
Investigators: Burns, Fredric J. , Sun, Tung-Tien , Shore, Roy E. , Roy, Nirmal
Institution: New York University
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1998 through September 30, 2001 (Extended to September 30, 2002)
Project Period Covered by this Report: October 1, 1999 through September 30, 2000
Project Amount: $570,271
RFA: Interindividual Variation in Human Susceptibility to Environmentally-caused Disease (1998) RFA Text | Recipients Lists
Research Category: Human Health
Objective:
This proposal is intended to examine the role of the PTCH (patched) gene as a basis for differences in skin cancer susceptibility of people exposed to ionizing and/or ultraviolet radiation. Of about 2000 people who were exposed to x-irradiation for tinea capitis at 8 years of age, about 10 percent have developed at least 1 skin cancer as of 35 years after exposure. Most interestingly, about 30 subjects have developed 3 or more skin cancers in comparison to the random expectation of only 2 such subjects. An analysis of tumor DNA from donors in the multiple (greater than 8) cancer group revealed no evidence of P53 mutations, even though such mutations are frequently found in such tumors from the general population(1). Instead a high (90 percent) incidence of loss of heterozygosity (LOH) of the PTCH gene was found. Mutations in the PTCH gene lead to the autosomal dominant condition known as, nevoid basal cell carcinoma syndrome (Gorlin's syndrome). The multiple basal cell carcinomas (BCCs) that are characteristic of Gorlin's syndrome tend to occur in association with UV radiation exposure, which supports the idea that inactivation of both PTCH alleles is necessary for carcinogenesis. As none of the donors in our study were afflicted with Gorlin's syndrome, inactivation of both PTCH alleles must have occurred somatically. One way to understand the higher than expected occurrence of multiple basal cell carcinomas is to hypothesize that one PTCH allele was mutated in a skin progenitor cell during embryonic development. If so, the skin of such an individual would be a genetic mosaic containing regions following the lines of Blaschko where the mutated PTCH allele would occur. Multiple cancers might then be the result of the high sensitivity of such a mutated region to exogenous carcinogens as in Gorlin's syndrome. This hypothesis will be tested by examining normal skin epithelial cells in the immediate vicinity of PTCH-altered BCCs to determine if PTCH mutations are present, and if so whether they are identical to mutations seen in the tumor itself.Progress Summary:
A main purpose of this project is to test the idea that multiple basal cell carcinomas are the result of carcinogenic exogenous agents acting within an extensive genetic mosaic of PTCH allele mutations in normal skin epithelial cells. In patients with Gorlin's syndrome, where all skin epithelial cells contain the offending PTCH allele mutations, multiple cancers are frequent in UV exposed regions of skin. If cancers have developed from the hypothesized mosaic, then normal epithelial cells in the vicinity of a cancer should show evidence of a similar or the same alteration in the PTCH gene as found in the cancer itself. By contrast, if cancers have arisen sporadically in single cells, there should be no evidence of PTCH mutations in the normal skin surrounding the tumors.
As an initial group designed to establish proof-of-principle, 14 subjects were selected as likely to have either single or multiple BCCs. These patients, 7 expected to have multiple cancers and 7 expected to have single cancers, were brought into the clinic and examined for presence of BCCs and other skin lesions in the head and neck regions. Three subjects exhibited no skin cancers, but 2 of these had existing lesions removed in the preceding year during visits to their own Dermatologist, and 1 patient refused to give us a blood sample, which leaves a total of 10 informative subjects, i.e., subjects for which both cancer and blood DNA are available. Out of these 10 subjects, a total of 19 BCCs were found and either biopsied or excised. Some individuals had their tumors biopsied at NYU and then preferred further treatment by their own physician. For one such subject, we were able to obtain cancer and normal skin from an excision with the cooperation of the physician and the subject. Surprisingly, none of the subjects opted for the MOHS procedure, which involves microscopic removal of progressive slices of tissue until no cancer cells can be detected, apparently because of the complexity of the procedure in comparison to simple excision.
DNA has been extracted from 19 adequate part 1 tumor DNA samples and the corresponding normal bloods. LOH analysis has been performed on this DNA for 6 microsatellite markers in the region of 9q22.3. The results show LOH near the patched locus in 12/19 BCCs, which is significantly lower than the ratio found in preliminary work with patients exhibiting multiple cancers, and the distribution of the positive LOH locations varied somewhat from that seen in the initial survey, i.e., there a greater incidence at D9S 180 than seen previously. By using SSCP followed by direct sequencing, a DNA sample from normal blood indicated the existence of a 14 base deletion in exon 6 of the XPA gene, strongly suggesting XPA heterozygosity. Based on a biopsy sample from each of the 5 BCCs discovered in the clinic, a similar analysis indicated the existence of the same 14 base deletion in all 5 BCCs. Further analysis based on allelic identification verified in 2/2 BCCs that the normal XPA allele had been deleted. These data imply that the genetic loss of 1 XPA allele greatly increases the susceptibility to BCC induction by ionizing radiation probably through the somatic deletion or recombinational loss of the remaining normal XPA allele. If the somatic XPA event is linked to the PTCH event, as seems likely based on LOH patterns, the defective XPA allele should have been lost 50 percent of the time, and yet cancers with this configuration were not seen. Apparently, abrogation of the XPA repair capability was a significant selective factor for development of BCCs, presumably because of the lack of protection from subsequent exposures to UV radiation.
A second major goal is to establish the DNA sequence changes that inactivate the PTCH gene in people developing skin cancers as a result of exposure to ionizing radiation. A possibly related finding to this second goal is the existence in 2 of 10 patients of a third microsatellite band at D9S 280 in blood DNA, suggesting these people may be mosaics at this chromosome location. If true this mosaicism must have been established early in embryonic development, because it affects approximately 50 percent of adult blood cells. One individual has 2 BCCs that show LOH patterns that differ from that seen in the blood indicating that in the BCCs the allele pattern has reverted to 2 alleles. These are extremely preliminary findings but suggest that the mosaicism we are interested in may be present and detectable in the blood as well as in the skin. Further studies are being conducted to confirm and extend these very interesting early findings relating possible mosaicism in blood to similar patterns in the BCCs.
Future Activities:
Project will be continued as planned.Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 4 publications | 3 publications in selected types | All 1 journal articles |
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Type | Citation | ||
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Zhao P, Zhu X, Liu Y, Wang B, Wang C, Burns FJ. Solar ultraviolet radiation and skin damage: An epidemiological study among a Chinese population. Archives of Environmental Health 1998;53:405-409. |
R827018 (1999) R827018 (2000) |
not available |
Supplemental Keywords:
ultraviolet radiation, risk assessment, sensitive populations, carcinogen, children, genetic predisposition, susceptibility., RFA, Health, PHYSICAL ASPECTS, ENVIRONMENTAL MANAGEMENT, Ecosystem Protection/Environmental Exposure & Risk, Ecosystem/Assessment/Indicators, Ecosystem Protection, Disease, Risk Assessments, Ecological Effects - Environmental Exposure & Risk, Ecological Effects - Human Health, Physical Processes, Ecological Indicators, Risk Assessment, health effects, deterministic linkages, ecological effects, ecological risk assessment, biomarkers, assessment models, gene-environment interaction, environmental stressor, exposure, patched gene mosaic, adverse human health affects, environmental consequences, environmental mutagens, patched gene, human exposure, susceptibility, ecosystem indicators, environmental stressors, assessment methods, biological markers, skin cancer, biomedical research, exposure assessment, cancer risk, skin cancer susceptibilityProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.