Galactose Metabolism by the Mouse with Galactose-1-Phosphate Uridyltransferase Deficiency

Abstract

The ability of mice deficient in galactose-1-phosphate uridyltransferase (GALT) to metabolize galactose was determined in animals weaned to a mouse chow diet for a 4-wk period. When given [14 C]galactose intraperitoneally, these animals slowly oxidized the sugar, excreting only 5.5% of the dose as 14 CO 2 in 4 h, whereas normal animals excreted 39.9%. These results mimic those seen in human galactosemic patients given isotopic galactose. When given 10 mol of [1-13 C]galactose, normal animals excrete small amounts of labeled galactose and galactonate but no galactitol in urine whereas GALT-deficient mice excrete significant amounts of all of these as labeled compounds in urine. When challenged with galactose, only about 20% of the dose is excreted in urine, and even on the chow diet, significant amounts of galactose, galactonate, and galactitol are excreted in urine. These compounds are also found to be present in liver, kidney, and brain, except that galactonate is not found in brain. Galactose-1-phosphate accumulates in red blood cells to levels found in humans exposed to large amounts of galactose, and galactose-1-phosphate is found in increased amounts in liver, kidney, and brain of GALT-deficient animals. There was no difference in the hepatic concentration of uridine diphosphate galactose and uridine diphosphate glucose between normal and GALT-deficient mice. The explanation for the presence of galactose and its conversion products in tissues and urine of affected mice appears to be related to the presence of approximately 1.75% of galactose-containing carbohydrates in the chow, which becomes bioavailable to mice. Despite the presence of galactose and its metabolites in tissues and urine and impaired ability to oxidize the sugar, the GALT-deficient animals are indistinguishable from normal animals and do not exhibit the phenotype of humans with GALT-deficiency galactosemia. Abbreviations GALT, galactose-1-phosphate uridyltransferase Gal-1-P, galactose-1-phosphate N/N, normal mice G/N, heterozygous normal and GALT deficient G/G, homozygous for a deficiency of GALT UDPgal, uridine diphosphate galactose UDPglu, uridine diphosphate glucose BSTFA, N,O-bis-(trimethylsilyl)trifluoroacetamide TMCS, trimethylchlorosilane CHOP, Children's Hospital of Philadelphia GC-MS, gas chromatography-mass spectroscopy After more than 60 y of experience with the diagnosis and dietary therapy of galactosemia caused by GALT deficiency, the pathophysiology of the disorder has remained unclear (1). Enigmatic has been the origin of both the rapid development of liver, kidney, eye, and central nervous toxicity occurring in newborns exposed to large amounts of galactose in milk and the long-term diet-independent complications of cognitive impairment, speech and learning disorders, ataxia, and primary ovarian failure in females. To enhance the study of the pathogenesis of GALT deficiency and further our understanding of the cause of the galactosemic phenotype, we made mice deficient in GALT activity using gene-targeting techniques (2).We...

Involvement of endoplasmic reticulum stress in a novel Classic Galactosemia model

“…Faced with the lack of overt galactose toxicity in the GALTknockout mouse model [15][16][17], researchers nonetheless proposed that galactose-1-phosphate (gal-1-p) plays a major role in causing acute toxicity syndrome and chronic complications [44][45][46]. It has been recognized that patients with inherited deficiency of galactokinase (GALK) do not accumulate gal-1-p in their tissues, nor manifest either the acute toxicity syndrome or chronic complications seen in the GALT-deficient patients [3,10,47,48].…”

“…The lack of explicit galactose toxicity in the GALT-knockout mouse model [15][16][17] has prompted us to focus on cell models of human origin. In this study, we examined how GALTdeficient and GALT-positive cells behave differently in the presence of galactose.…”

“…Yet the in vivo targets of this presumably toxic intermediate have never been identified. Concurrently, the lack of overt galactose toxicity in the GALT-knockout mouse model has continued to hamper the efforts to delineate the molecular mechanism of this disease [15][16][17].…”

Subfertility and growth restriction in a new galactose-1 phosphate uridylyltransferase (GALT) - deficient mouse model

“…20,21 This seminal work re-affirmed the suspected pathogenic role of gal-1P, product of GALK, in GALT-deficiency, but did not reveal the mechanism of the in vivo toxicity of gal-1P leading to speech dyspraxia and POI. The first GalT gene-knockout (KO) mouse model for Classic Galactosemia [22][23][24] had moderate accumulation of galactose and gal-1P upon galactose challenge, but no overt human disease phenotypes. [22][23][24] The first dGALT-deficient Drosophila melanogaster model had arrested larval development with a high-galactose diet.…”

“…The first GalT gene-knockout (KO) mouse model for Classic Galactosemia [22][23][24] had moderate accumulation of galactose and gal-1P upon galactose challenge, but no overt human disease phenotypes. [22][23][24] The first dGALT-deficient Drosophila melanogaster model had arrested larval development with a high-galactose diet. 25 Impaired geotaxic response was also seen in these flies despite dietary restriction of galactose.…”