Accelerated appearance of chemically induced mammary carcinomas in obese yellow (Avy/A) (BALB/c X VY) F1hybrid mice

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Abstract
Latent periods and cumulative incidence of mammary carcinomas (MT) up to 50 wk after initial gavage with 7.12‐dimethylbenz[a]anthracene (DMBA) were determined in virgin yellow (Avy/A) and agouti (A/a) (BALB/cStCrlfC3Hf/Nctr X VY/WffC3Hf/ Nctr‐Avy) F 1 hybrid female mice. When subcutaneous masses reached 5–10 mm in diameter, the mice were killed and necropsied, and the tissues examined histologically. No MT were found in control mice. Cumulative MT incidence in the 1.5‐mg DMBA group (A) was 43% (41/95) among yellow mice, and 33% (32/96) among agoutis. In the 6.0‐mg DMBA group (B), corresponding MT incidences were 86% (83/96) and 71% (67/95). In group A, the first percentile of MT detection was 13.0 wk after initial carcinogen treatment in yellow mice; it was 18.0 wk in agoutis. Corresponding latent periods for the 20th percentile were 34.3 and 47.0 wk. In group B, latencies for the first percentile were 8.3 and 9.0 wk. Corresponding latencies for the 20th percentile were 15.3 and 16.0 wk. Within genotypes and dose groups, rates of weight gain of mice that developed MT and those that did not were similar. We conclude that MT induced by low doses of DMBA arise more rapidly in yellow mice than in nonyellow littermates. The absence of spontaneous MTs, acceleration of chemically induced MT formation at a low dose level that does not induce general toxicity, and availability of genetically identical (except for one gene) normal control animals make this experimental system suitable for development of an assay to efficiently test the carcinogenic potential of low dose levels of chemical substances.