Use of intercross outbred mice and single nucleotide polymorphisms to map skin cancer modifier loci

Variants at the OCA2/HERC2 locus affect time to first cutaneous squamous cell carcinoma in solid organ transplant recipients collected using two different study designs

BACKGROUND

Variants at the oculocutaneous albinism 2 (OCA2)/HECT and RLD domain containing E3 ubiquitin protein ligase 2 (HERC2) locus have been associated with pigmentation phenotypes and risk of developing several types of skin cancer.

OBJECTIVES

To evaluate OCA2/HERC2 locus variants for their impact on time to develop cutaneous squamous cell carcinoma (cSCC) in organ transplant recipients (OTRs) who are at elevated risk of developing cSCC.

METHODS

Participants were solid OTRs ascertained from two centres (n = 125 and 261) with an average of 13·1 years of follow-up post-transplant. DNA was available for genotyping for all participants, in addition to medical records and questionnaire data. The Ohio State University study had a case-control design with prospective follow-up, and the University of California San Francisco study was a national cross-sectional survey with retrospective chart review.

RESULTS

OCA2 variants rs12913832 and rs916977 were significantly associated with time to first cSCC post-transplant. OTRs homozygous for the brown-eye alleles of rs916977 (GG) and rs12913832 (AA) had significant delays of time to first cSCC post-transplant compared with individuals homozygous for the blue-eye alleles (hazard ratio 0·34, P < 0·001 and hazard ratio 0·54, P = 0·012, respectively). Both variants were highly associated with eye colour in the combined studies (P < 0·001).

CONCLUSIONS

This study is the first to show an association between OCA2/HERC2 variants and time to first cSCC post-transplant. This may impact dermatological screening recommendations for high-risk populations.

Fine mapping reveals that promotion susceptibility locus 1 (Psl1) is a compound locus with multiple genes that modify susceptibility to skin tumor development

Although it is well known that the majority of human cancers occur as the result of exposure to environmental carcinogens, it is clear that not all individuals exposed to a specific environmental carcinogen have the same risk of developing cancer. Considerable evidence indicates that common allelic variants of low-penetrance, tumor susceptibility genes are responsible for this interindividual variation in risk. We previously reported a skin tumor promotion susceptibility locus, Psl1, which maps to the distal portion of chromosome 9, that modified skin tumor promotion susceptibility in the mouse. Furthermore, Psl1 was shown to consist of at least two subloci (i.e., Psl1.1 and Psl1.2) and that glutathione S-transferase alpha 4 (Gsta4), which maps to Psl1.2, is a skin tumor promotion susceptibility gene. Finally, variants of human GSTA4 were found to be associated with risk of nonmelanoma skin cancer. In the current study, a combination of nested and contiguous C57BL/6 congenic mouse strains, each inheriting a different portion of the Psl1 locus from DBA/2, were tested for susceptibility to skin tumor promotion with 12-O-tetradecanoylphorbol-13-acetate. These analyses indicate that Psl1 is a compound locus with at least six genes, including Gsta4, that modify skin tumor promotion susceptibility. More than 550 protein-coding genes map within the Psl1 locus. Fine mapping of the Psl1 locus, along with two-strain haplotype analysis, gene expression analysis, and the identification of genes with amino acid variants, has produced a list of fewer than 25 candidate skin tumor promotion susceptibility genes.

New outbred colony derived from Mus musculus castaneus to identify skin tumor susceptibility loci

Susceptibility to tumor development varies among mice strains. Using inbred NIH and wild-derived outbred Mus spretus backcrosses, skin cancer-susceptibility loci were mapped [Nagase et al. 1995. Nat Genet 10: 424-429; Nagase et al. 1999. Proc Natl Acad Sci USA 96: 15032-15037], and Skts13 was identified as the Aurka gene using a conventional linkage in conjunction with haplotype analysis [Ewart-Toland et al. 2003. Nat Genet 34: 403-412]. In the present study, we examined another wild-derived outbred Mus musculus castaneus in which 10.3% of the analyzed SNPs showed heterogeneity among the colony. All mice examined were completely resistant to the two-stage skin carcinogenesis protocol using 7.12-dimethylbenz(a)anthracene (DMBA)/12-O-tetradecanoylphorbol-13-acetate (TPA), and this resistant phenotype was dominant when we crossed them with the highly susceptible strain FVB. By scanning F1 backcross progeny between M. m. castaneus and FVB, we found a highly significant linkage for tumor multiplicity on Chromosome 4, which was overlapped with the Skts-fp1 locus, found in the previous study using FVB and PWK cross [Fujiwara et al. 2007. BMC Genet 8: 39]. The linkage was observed in all pedigrees from the five F1 founders, while the linkage for papilloma size on Chromosome 4 was mapped only in pedigrees from founders 1 and 2. By scanning the whole Chromosome 4 of the five F1 founders, founders 1- and 2-specific haplotype block was found between D4Mit293 (20.6 Mbp) and D4Mit171 (22.4 Mbp). In this study we exploited the outbred nature of M. m. castaneus stock to identify a haplotype contributing to papilloma size on mouse Chromosome 4. These data illustrate the strength of using outbred mice in identification of the genetic component of a mouse complex trait such as the skin cancer-susceptibility phenotype.

Multi-stage chemical carcinogenesis in mouse skin: fundamentals and applications

For more than 60 years, the chemical induction of tumors in mouse skin has been used to study mechanisms of epithelial carcinogenesis and evaluate modifying factors. In the traditional two-stage skin carcinogenesis model, the initiation phase is accomplished by the application of a sub-carcinogenic dose of a carcinogen. Subsequently, tumor development is elicited by repeated treatment with a tumor-promoting agent. The initiation protocol can be completed within 1-3 h depending on the number of mice used; whereas the promotion phase requires twice weekly treatments (1-2 h) and once weekly tumor palpation (1-2 h) for the duration of the study. Using the protocol described here, a highly reproducible papilloma burden is expected within 10-20 weeks with progression of a portion of the tumors to squamous cell carcinomas within 20-50 weeks. In contrast to complete skin carcinogenesis, the two-stage model allows for greater yield of premalignant lesions, as well as separation of the initiation and promotion phases.

New chemically induced skin tumour susceptibility loci identified in a mouse backcross between FVB and dominant resistant PWK

Background

A variety of skin cancer susceptibility among mouse strains has allowed identification of genes responsible for skin cancer development. Fifteen Skts loci for skin tumour susceptibility have been mapped so far by using the two-stage skin carcinogenesis model [induced by 7.12-dimethylbenz(a)anthracene (DMBA)/12-O-tetradecanoylphorbol-13-acetate (TPA)]. A few responsible genes have been identified using wild-derived dominant resistant Mus spretus mice, and one has been confirmed as a low penetrance cancer susceptibility gene in a variety of human cancers.

Results

In the present study, we found that wild-derived PWK mice developed no tumour by treatment with the two-stage skin carcinogenesis protocol. This phenotype is dominant resistant when crossed with the highly susceptible strain FVB. By analyzing the F1 backcross generation between PWK and FVB, we found empirical evidence of significant linkage at the new loci Skts-fp1 on chromosome 4 and suggestive linkage on chromosomes 1, 3, 11, 12 and 14 for skin tumour susceptibility. Skts-fp1 includes the Skts7 interval, which was previously mapped by a Mus spretus and NIH backcross. We also observed suggestive linkage on chromosomes 1 and 2 in the female population only, while suggestive linkage on chromosomes 14 and 15 only was observed in the male population. A significant genetic interaction was seen between markers of D11Mit339 and D16Mit14.

Conclusion

Analysis of this new cross may facilitate the identification of genes responsible for mouse skin cancer susceptibility and may reveal their biological interactions.

Modulation of Patched-Associated Susceptibility to Radiation Induced Tumorigenesis by Genetic Background

We described previously a basal cell carcinoma (BCC) and medulloblastoma (MB) phenotype for CD1Ptch1(neo67/+) mice exposed to ionizing radiation. Ptch1 heterozygous mice mimic the predisposition to BCC and MB development of patients affected by nevoid BCC syndrome that inherit a mutant Patched (Ptch1) allele. To examine the impact of genetic background on development of BCCs and other tumors we used two outbred mouse lines characterized by extremely high, carcinogenesis-susceptible (Car-S), and low, carcinogenesis-resistant (Car-R), susceptibility to skin carcinogenesis. Crosses between Ptch1(neo67/+) mice and Car-S (F1S) or Car-R mice (F1R) were exposed to ionizing radiation. F1SPtch1(neo67/+) mice were highly susceptible to radiation-induced BCCs, whereas F1RPtch1(neo67/+) mice were completely resistant, indicating that tumor penetrance can be modulated by genetic background. Development of microscopic and macroscopic BCC lesions was influenced by Car-S and Car-R genotypes, suggesting a genetic-background effect on both initiation and progression of BCC. Susceptibility was additionally increased in N2 backcross mice (Car-S x F1SPtch1(neo67/+)), showing a contribution from recessive-acting Car-S modifiers. The modifying effects of Car-S-derived susceptibility alleles were tissue specific. In fact, despite higher susceptibility to BCC induction, Car-S-derived lines had lower MB incidence compared with CD1Ptch1(neo67/+) mice. BCC-associated somatic events were not influenced by genetic background, as shown by similar rate of wild-type Ptch1 loss in BCCs from F1SPtch1(neo67/+) (93%) and CD1Ptch1(neo67/+) mice (100%). Finally, microsatellite analysis of BCCs showed Ptch1 loss through interstitial deletion. These results are relevant to humans, in which BCC is the commonest malignancy, because this model system may be used to study genes modifying BCC development.

Loss of tyrosinase activity confers increased skin tumor susceptibility in mice

The tyrosinase (Tyr) gene encodes the enzyme tyrosinase that catalyses the conversion of L-tyrosine into DOPA (3,4-dihydroxyphenylalanine)-quinone. The albino mutation abrogates functional activity of tyrosinase resulting in deficiency of melanin pigment production in skin and retina. Tyr maps to a region in the central position of Chromosome 7 that contains a skin tumor-modifier locus. We rescued the albino mutation in transgenic mice to assess a possible role of Tyr gene in two-stage skin carcinogenesis. Transgenic expression of the functional Tyr(Cys) allele in albino mice (Tyr(Ser)) caused a reduction in skin papilloma multiplicity, in four independent experiments and at three dose levels of DMBA (9,10-dimethyl-1,2-benzanthracene). In vitro mechanistic studies demonstrated that transfection of the Tyr(Cys) allele in a human squamous cell carcinoma cell line (NCI-H520) increases tyrosinase enzyme activity and confers resistance to hydrogen peroxide-induced oxidative DNA damage. These results provide direct evidence that the Tyr gene can act as a skin cancer-modifier gene, whose mechanism of action may involve modulation of oxidative DNA damage.