TRANSLATIONAL IMPACT

Abstract

UDP-galactose 4’ epimerase (GALE) plays key roles in the metabolism of dietary galactose, in the production of endogenous galactose when exogenous sources are lacking, and in maintaining the ratios of key substrates for glycoprotein and glycolipid biosynthesis. GALE catalyzes the interconversion of UDP-galactose and UDP-glucose in the final step of the Leloir pathway of galactose metabolism and, in humans, also interconverts UDP-N-acetylgalactosamine and UDP-N-acetylglucosamine. No patients completely lacking GALE activity have been found, suggesting that loss of GALE must be lethal. However, partial loss of GALE can be frequent, at least in some populations, and results in epimerase deficiency galactosemia. This spectrum disorder ranges in severity from benign to potentially lethal, depending, among other factors, on the degree and tissue-specificity of GALE impairment. The prognosis and treatment for patients with epimerase deficiency galactosemia are currently ill-defined, partly because affected individuals are often either missed or misdiagnosed, but also owing to the lack of an appropriate animal model.This paper reports the generation and initial characterization of the first whole-animal model of GALE deficiency, using the fruit fly Drosophila melanogaster. Drosophila lacking GALE died as embryos, confirming that GALE function is essential in developing animals. Larvae in which Drosophila GALE (dGALE) expression was conditionally inhibited by dGALE-specific RNA interference (RNAi) also died within days of GALE loss. Both types of affected embryos were rescued by expression of a human GALE (hGALE) transgene, showing the specificity of the mutation. To examine possible tissue specificity of the lethal GALE phenotype, conditional knockdown and hGALE transgene expression were used to show that GALE expression in the gut primordium and Malpighian tubules was both necessary and sufficient for Drosophila survival. Finally, like patients with generalized epimerase deficiency galactosemia, Drosophila with a partial loss of GALE activity survived in the absence of galactose, but succumbed during development if exposed to dietary galactose.These data establish the utility of the fly model of generalized GALE deficiency and set the stage for future studies to define the mechanism(s) and modifiers of outcome in epimerase deficiency galactosemia.