Strain Differences in Susceptibility to Tar-Induced Skin Tumors in Mice

Abstract

We have already shown that certain strains of mice are more susceptible than others to tumors of the lung, both spontaneous and tar-induced. There is evidence also that these differences in susceptibility segregate out in appropriate crosses as would be expected if they were dependent upon Mendelian units. The present report concerns tar-induced tumors of the skin in mice.

The technique was the one usually employed in such experiments. 1 A coal tar residue was applied to the skin of the mouse in the interscapular region 3 times per week for 4 months. After a latent period papillomas may arise which sometimes develop into carcinoma or malignant growths may develop without a marked papillomatous stage. The course of the experiment was followed by external examination. The final diagnosis was based upon microscopic sections.

Five groups of mice were used. Two were inbred strains which have been employed extensively in previous experiments. There were 100 mice from the agouti strain, No. 1194, and 80 mice from selected lines derived from Bagg albinos. A third lot of 23 mice were also Bagg stock but of different descent. They were grouped separately because they had not been tested for lung tumor susceptibility and turned out later to react differently in respect to skin tumors. A fourth group of 71 mice, pink-eyed, dilute brown in color, came from a very highly inbred strain, No. 62. The last group of 54 mice were hybrids of mixed ancestry but were uniform in being heterozygous for the dominant, character hairless.

Strain 62 lived a much shorter time than the other 4 groups. By the 350th day no tumor-free mice were living so that no further change in tumor rate was possible. At this time 51.8 ±4.5% of the mice which had survived the tarring had carcinoma. This rate was about the same as that of the hairless mice (54.7 ± 4.6%) but differed significantly from Strain 1194 (25.3 ±3.0%) and the Bagg (a) group (17.1 ± 2.9%). The latter 2 do not differ significantly from each other. The small group of Bagg (b) stock were negative throughout the experiment. To make a mathematical comparison we may assume that if another animal had been included it would have been cancerous. On this basis the Bagg (b) mice would have a tumor incidence of 4.3% with a probable error of 2.9. This is significantly lower than the preceding rates.

The last 4 strains lived about twice as long as No. 62. By the end of the experiment the rate of the hairless had risen to 73.6 ± 4.1 %. Of the 1194 mice, 42.1 ± 3.4% had developed epitheliomas and of the Bagg (a) group 34.2 ± 3.7% had done so. The 2 latter rates do not differ significantly from each other but are different from the hairless and Bagg (b) groups. The rate of Strain 62 at 350 days was significantly greater than that ever attained by either Bagg group. Evidently there are marked differences in susceptibility to tar-induced skin tumors among these strains of mice.