Dietary N‐3 fatty acids do not affect induction of Ha‐ras mutations in mammary glands of NMU‐treated rats

The purpose of this study was to determine whether dietary n‐3 polyunsaturates (PUFA) can affect the frequency of N‐nitrosomethylurea (NMU)‐induced Ha‐ras mutations in virgin female F344 rat mammary glands. Two groups of 15 rats each were fed isocaloric diets containing either 23% w/w corn oil or corn oil plus menhaden oil (1:1) at starting 14 d before NMU administration (day 50 of age) and continuing for 13 wk. Mammary gland samples were taken serially at 3, 5, 9, and 13 wk post‐NMU treatment. Total cellular DNA was isolated and analyzed by a newly devised enriched amplification procedure that involves predigestion of normal Ha‐ras alleles at codon 12, amplification of the mutant alleles, and restriction fragment length polymorphism analysis. Selective amplification enabled the detection of Ha‐ras mutations as early as 3 wk post–NMU treatment. Approximately 40‐50% of all glands and 75% of all rats tested had the Ha‐ras codon 12 mutation. No significant differences were found between the two dietary groups at any time point, indicating that the mammary tumor‐inhibiting effect of n‐3 PUFA is probably not exerted at the level of the Ha‐ras activation step in NMU tumorigenesis.