DIETARY METHIONINE DOES NOT REDUCE PENETRANCE IN CURLY TAIL MICE BUT CAUSES A PHENOTYPE-SPECIFIC DECREASE IN EMBRYONIC GROWTH

The mouse mutation, curly tail, has incomplete penetrance and variable expression. Approximately 60% of the mice have a curly tail (CT), fro m which up to 20% may have lumbosacral spina bifida. Approximately 40% are normal, with a straight tail (ST). We tested whether L-methionine , which reduces the penetrance of neural tube defects in the Axd mouse mutant, has beneficial effects in the curly tail mutant. A single inj ection of L-methionine (200-1600 mg/kg body wt) on d 9 of pregnancy ha d no effect on the embryos, whereas there was a minor increase in pene trance at the highest dose. Chronic supplementation of L-methionine vi a the drinking water (1554 mg . kg body wt(-1). d(-1)) did not shift p enetrance. However, it decreased the weight of d 13 embryos from ST da ms but not of those from CT darns. This phenotype-specific difference in response was evident and most unexpected. Mice from curly tail and other inbred strains were subjected to an L-methionine loading test an d serum homocysteine assay. The different strains varied in their basa l serum homocysteine concentrations, and they had proportionate signif icant increases after L-methionine loading. In CT and ST mice, basal s erum homocysteine concentrations as well as the levels after loading w ere similar to each other and intermediate in the range of the mice te sted. We conclude that L-methionine does not reduce penetrance in the curly fail mouse and that this strain reflects no derangement in L-met hionine handling.