Classic galactosemia (CG) is a potentially lethal inborn error of metabolism that results from profound loss of galactose-1-phosphate uridylyltransferase (GALT), the second enzyme in the Leloir pathway of galactose metabolism. Neonatal detection and dietary restriction of galactose minimize or resolve the acute sequelae of CG, but fail to prevent the long-term complications experienced by a majority of patients. One of the substrates of GALT, galactose-1-phosphate (Gal-1P), accumulates to high levels in affected infants, especially following milk exposure, and has been proposed as the key mediator of acute and long-term pathophysiology in CG. However, studies of treated patients demonstrate no association between red blood cell Gal-1P level and long-term outcome severity. Here, we used genetic, epigenetic, and environmental manipulations of a Drosophila melanogaster model of CG to test the role of Gal-1P as a candidate mediator of outcome in GALT-deficiency. Specifically, we both deleted and knocked down the gene encoding galactokinase (GALK) in control and GALT-null Drosophila and assessed acute and long-term outcomes of the resulting animals in the presence and absence of dietary galactose. GALK is the first enzyme in the Leloir pathway of galactose metabolism and is responsible for generating Gal-1P in humans and Drosophila. Our data confirmed that, as expected, loss of GALK lowered or eliminated Gal-1P accumulation in GALT-null animals. However, we saw no concomitant rescue of larval survival or adult climbing or fecundity phenotypes. Instead, we saw that loss of GALK itself was not benign and in some cases phenocopied or exacerbated the outcome seen in GALT-null animals. These findings strongly contradict the long-standing hypothesis that Gal-1P alone underlies pathophysiology of acute and long-term outcomes in GALT-null Drosophila and suggest that other metabolite(s) of galactose, and/or other pathogenic factors, may be involved.
- Received August 26, 2015.
- Accepted August 17, 2016.
- © 2016. Published by The Company of Biologists Ltd
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