Twitcher mice with only a single active galactosylceramide synthase gene exhibit clearly detectable but therapeutically minor phenotypic improvements

Cross-breeding of mouse mutants, each defective in either synthesis (CGT knockout) or degradation (twitcher) of galactosylceramide, generates hybrids with a genotype of galc(-/-), cgt(+/-), in addition to doubly deficient mice. They are ideally suited to test the potential usefulness of limiting synthesis of the substrate as a treatment of genetic disorders due to degradative enzyme defects. The rate of accretion of galactosylceramide in the brain of CGT knockout carrier mice (cgt(+/-)) is approximately two-thirds of the normal, suggesting a gene-level compensation for the reduced gene dosage. Phenotype of twitcher mice with a single dose of normal cgt gene was indeed milder with statistical significance, albeit only slightly. Compared among 10 paired littermates, the difference in the life span was 7 ± 3.9 days (S.D.) and the difference in the maximum attained body weight was 1.9 ± 1.2 g (S.D.). Neuropathologists were able to distinguish blindly galc(-/-), cgt(+/-) mice from galc(-/-), cgt(+/+) mice. The brain psychosine level in galc(-/-), cgt(+/-) mice was also approximately two-thirds of the galc(-/-), cgt(+/+) mice. These observations indicate that reduction of galactosylceramide synthesis to two-thirds of the normal level results in minor but clearly detectable phenotypic improvements. Because of the detrimental consequences of drastic reduction in galactosylceramide synthesis that may be required for pragmatically meaningful improvements, this approach by itself is unlikely to be useful as the sole treatment but may be helpful as a supplement to other therapies.