Mapping of new skin tumor susceptibility loci by a phenotype-driven congenic approach

Abstract

As cancer susceptibility varies among mouse strains, mouse models are powerful tools for the identification of genes responsible for cancer development. Several cancer susceptibility loci have been mapped by genetic analysis using cancer-resistant and cancer-susceptible mouse strains. However, only a few corresponding genes for these loci have been identified, because most of the cancer susceptibility loci are low-penetrance alleles. We reported previously that wild-derived PWK mice showed no tumor development on treatment with the two-stage skin carcinogenesis protocol [induced by 7.12-dimethylbenz(a)anthracene (DMBA)/12-O-tetradecanoylphorbol-13-acetate (TPA)], and that this phenotype is dominant-resistant when crossed with the highly susceptible strain FVB. From the analysis of the F1 backcross generation between PWK and FVB, we have mapped the new significant locus Skts-fp1 on chromosome 4. In the present study, congenic strains were generated with the PWK resistance allele in the FVB background using a phenotype-driven approach, and sought to narrow down the candidate loci and find the responsible gene(s). One of the resistant mice in the N₆ generation carried the remaining PWK allele on chromosomes 4, 7 and 11, and an association study using the progeny of this mouse suggested that the locus on chromosome 11 may affect the cancer susceptibility locus on chromosome 7. On the other hand, no skin tumor susceptibility locus was mapped on chromosome 11 as examined in N2 progeny. These findings suggest that there is at least one tumor-resistance gene on chromosome 7, the function of which could be regulated by gene(s) located on chromosome 11.

Keywords: cancer susceptibility; mouse model; squamous cell carcinoma; two stage skin carcinogenesis.

Figure 1.

Interval mapping of skin tumor susceptibility loci. The number of papillomas 12 weeks after initiation was used as a trait. (A) Whole-genome scan of FxFP backcross, and (B) genome scan of chromosomes 4, 7, and 11 in N₇ congenic mice. N₇ congenic mice were divided into two groups according to the genotype at the (C) D7Mit31 locus or (D) D11Mit338 locus, and interval mapping was performed for each group separately. The solid lines indicate FVB homozygous at (C) D7Mit31 or at (D) D11Mit338. Dashed lines indicate heterozygous at (C) D7Mit31 or at (D) D11Mit338.

Figure 2.

Association between genotypes of resistant loci and multiplicity of papilloma 12 weeks after the initial treatment. Means (lines) and distribution of individual mice (circles) are shown. F indicates FVB homozygous, FP indicates heterozygous. (A) Congenic mice in the N₇ generation derived from N₆−32. (B) FxFP backcross mouse. Asterisks next to the number of mice represent the significance of differences compared to mice with the FVB homozygous genotype at all three loci. *P<0.05, **P<0.01.

Figure 3.

Analysis of allelic imbalance on chromosome 7. Frequencies of allelic imbalances in skin tumor tissues obtained from N₅-N₇ congenic mice (A-C) or from FxFP mice (D-F). The results obtained from (A) all of the N₅-N₇ congenic mice, (B) N₅-N₇ congenic mice with FVB homozygous genotype, or (C) with FP heterozygous genotype at the D11Mit338 locus, (D) all of the FxFP mice, (E) FxFP mice with FVB homozygous genotype, or (F) with FP heterozygous genotype at the D11Mit338 locus are shown. Filled squares and open squares indicate allelic imbalances in favor of the FVB allele and the PWK allele, respectively. Dashed lines indicate the position of D7Mit31.