Experiment
To identify genetic determinants of skin cancer susceptibility and carcinoma development, a large backcross population of FVB/NJcl x (FVB/NJcl x MSM/Ms) F1 mice were subjected to DMBA/TPA-induced skin carcinogenesis. MSM/Ms mice are more resistant to chemically induced skin tumor development than the highly susceptible FVB/N strain.
(FVB/NJcl x MSM/Ms) male mice were backcrossed with female FVB/NJcl mice generating 121 F1 Tpr53 homozygous mice, p53+/+. Tpr53 deficient male mice (FVB/NJcl x MSM.Cg-Trp53tm1Sia/+) were backcrossed to female FVB/NJcl mice generating 107 Tpr53 heterozygous F1 backcross mice, p53+/-. Mice were genotyped using PCR assays.
Tumor development was monitored for a period of 40 weeks; the number of papillomas at 20 weeks as well as the presence of carcinomas at 40 weeks was documented. In addition papillomas were categorized into three groups based on diameter size, <2mm, 2-6mm and >6mm. All mice were genotyped using 107 SNP markers distributed evenly over the genome.
In the FVB/NJcl x (FVB/NJcl x MSM/Ms) F1 backcrossed mice the following significant linkages were detected on Chromosome 7:
Stmm1c, skin tumor modifier of MSM 1c, a measure of the total number of papillomas at 20 weeks, LOD=5.109 at 53.6 cM (D7SNP32, rs32219681). Seventy six percent of mice without tumors at this locus were heterzygous with FVB/MSM alleles.
Stmm2b, skin tumor modifier of MSM 2b, a measure of the total number of papillomas <2mm in diameter at 20 weeks, LOD=5.063 at 55.0 cM (D7SNP513, rs31348773). Seventy eight percent of mice without tumors at this locus were heterzygous with FVB/MSM alleles.
Stmm1d, skin tumor modifier of MSM 1d, a measure of the total number of papillomas measuring between 2 and 6mm in diameter at 20 weeks, LOD=3.921 at 53.6 cM (D7SNP32, rs32219681). Seventy five percent of mice without tumors at this locus were also heterzygous with FVB/MSM alleles. [Table 1.]
In the analysis of combined results from both crosses, 121 FVB/NJcl x (FVB/NJcl x MSM/Ms)F1 mice and 107 (FVB/NJcl x (FVB/NJcl x MSM.Cg-Tpr53tmSia1/+)F1, the following significant linkages on Chromosome 7 were detected:
Stmm1a, skin tumor modifier of MSM 1a, a measure of the total number of papillomas at 20 weeks, LOD=6.974 at 53.6cM (D7SNP32, rs32219681). Eighty pecent of mice without tumors were heterzygous with FVB/MSM alleles at this locus.
Stmm1b, skin tumor modifier of MSM 1b, a measure of the total number of papillomas <2mm at 20 weeks, LOD=4.869 at 53.6 cM (D7SNP32, rs32219681). Approximately 65 percent of mice without tumors at this locus were heterozygous with FVB/MSM alleles and 64 percent with tumors were homozygous with FVB/N alleles.
Stmm2a, skin tumor modifier of MSM 2a, a measure of the total number of papillomas measuring between 2 and 6mm in diameter at 20 weeks, LOD=5.316 at 64.0 cM (D7SNP6, rs3023159). Approximately 77 percent of mice without tumors at this locus were heterozygous with FVB/MSM alleles. [Table 1.]
Figures 3A-C, taken together, show that the same two markers on Chromosome 7, D7SNP32 at 53.6 and C7.loc64 at 64.0 cM, define the linkage peaks in three three papilloma size catgories (all sizes, <2mm, and 2-6mm). To narrow the identified regions, precise genotyping was performed with an additional 10 mircosatellite markers on Chromosome 7.
Highly significant linkage for papilloma multiplicity:
mapped between D7SNP507 and D7SNP513 (an interval 25.2 Mb in size) with a maximum LOD score of 6.974 at 53.6 cM with peak marker D7SNP32 and was labeled Stmm1, skin tumor modifier of MSM 1; and between D7SNP6 and D7Mit10 (an interval 10Mb in size) with a maximum LOD score of 6.780 at C7.loc64 at 64.0 cM and was labeled Stmm2, skin tumor modifier of MSM 2. [Fig 4A.]
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