Galactose metabolism by the mouse with galactose-1-phosphate uridyltransferase deficiency

Galactokinase 1 is the source of elevated galactose-1-phosphate and cerebrosides are modestly reduced in a mouse model of classic galactosemia

Classic galactosemia (CG) arises from loss-of-function mutations in the Galt gene, which codes for the enzyme galactose-1-phosphate uridylyltransferase (GALT), a central component in galactose metabolism. The neonatal fatality associated with CG can be prevented by galactose dietary restriction, but for decades it has been known that limiting galactose intake is not a cure and patients often have lasting complications. Even on a low-galactose diet, GALT's substrate galactose-1-phosphate (Gal1P) is elevated and one hypothesis is that elevated Gal1P is a driver of pathology. Here we show that Gal1P levels were elevated above wildtype (WT) in Galt mutant mice, while mice doubly mutant for Galt and the gene encoding galactokinase 1 (Galk1) had normal Gal1P levels. This indicates that GALK1 is necessary for the elevated Gal1P in CG. Another hypothesis to explain the pathology is that an inability to metabolize galactose leads to diminished or disrupted galactosylation of proteins or lipids. Our studies reveal that levels of a subset of cerebrosides-galactosylceramide 24:1, sulfatide 24:1, and glucosylceramide 24:1-were modestly decreased compared to WT. In contrast, gangliosides were unaltered. The observed reduction in these 24:1 cerebrosides may be relevant to the clinical pathology of CG, since the cerebroside galactosylceramide is an important structural component of myelin, the 24:1 species is the most abundant in myelin, and irregularities in white matter, of which myelin is a constituent, have been observed in patients with CG. Therefore, impaired cerebroside production may be a contributing factor to the brain damage that is a common clinical feature of the human disease.

Time-based investigation of urinary metabolic markers for Type 2 diabetes: Metabolomics approach for diabetes management

Diabetes is considered one of the most important health emergencies worldwide and Egypt has 8.2 million diabetic patients according to the International Diabetes Federation report in 2017. The objective of this study was to monitor the time-course variation in the metabolic profile of diabetic rats to detect urinary metabolic biomarkers using the metabolomics approach. Type 2 diabetes was induced in male Wistar albino rats using a single intraperitoneal injection of 40 mg/kg of streptozotocin following oral administration of 10% fructose in drinking water for 3 weeks. Then, urine was collected for 24 h from rats at three time points (0, 2, and 4 weeks after confirmation of diabetes), and were analyzed by nuclear magnetic resonance (H¹ -NMR), followed by multivariate data analysis. The results from H¹ -NMR pointed out that d-glucose, taurine, l-carnitine, l-fucose, 1,5-anhydrosorbitol, and d-galactose levels showed consistent significant variation (p < 0.05) between the positive (diabetic) and negative (normal) controls during the whole experimental period. Also, with the disease progression, myoinositol, and l-phenylalanine levels were significantly altered (p < 0.05) after 2 weeks and this alteration was maintained till the end of the 4-week experimental period in the positive control group. From the results of the present study, it could be concluded that we cannot depend only on glucose levels for prognostic purposes since there are other metabolic disturbances in diabetes which need to be tracked for better disease prognosis.

Sodium Taurocholate Cotransporting Polypeptide (NTCP) Deficiency Hidden Behind Citrin Deficiency in Early Infancy: A Report of Three Cases

Sodium taurocholate cotransporting polypeptide (NTCP), a carrier protein encoded by the gene SLC10A1, is expressed in the basolateral membrane of the hepatocyte to uptake bile acids from plasma. As a new inborn error of bile acid metabolism, NTCP deficiency remains far from being well understood in terms of the clinical and molecular features. Citrin deficiency is a well-known autosomal recessive disease arising from SLC25A13 mutations, and in neonates or infants, this condition presents as transient intrahepatic cholestasis which usually resolves before 1 year of age. All the three patients in this paper exhibited cholestatic jaundice and elevated total bile acids in their early infancy, which were attributed to citrin deficiency by SLC25A13 genetic analysis. In response to feeding with lactose-free and medium-chain triglycerides-enrich formula, their clinical and laboratory presentations disappeared gradually while the hypercholanemia persisted, even beyond 1 year of age. On subsequent SLC10A1 analysis, they were all homozygous for the well-known pathogenic variant c.800C > T (p.Ser267Phe), and NTCP deficiency was thus definitely diagnosed. The findings in this paper indicated that NTCP deficiency could be covered up by citrin deficiency during early infancy; however, in citrin-deficient patients with intractable hypercholanemia following resolved cholestatic jaundice, NTCP deficiency should be taken into consideration.

Partial replacement of glucose by galactose in the post-weaning diet improves parameters of hepatic health

Replacing part of glucose with galactose in the post-weaning diet beneficially affects later life metabolic health in female mice. The liver is the main site of galactose metabolism, but the direct effects of this dietary intervention on the liver in the post-weaning period are not known. The aim of this study was to elucidate this. Weanling female mice (C57BL/6JRccHsd) were fed a starch containing diet with glucose (32 en%) monosaccharide (GLU), or a diet with glucose and galactose (1:1 both 16 en%) (GLU+GAL). Body weight, body composition, and food intake were determined weekly. After 3 weeks, mice were sacrificed, and serum and liver tissues were collected. Global hepatic mRNA expression was analyzed and hepatic triglyceride (TG) and glycogen contents were determined by enzymatic assays. Body weight and body composition were similar in both groups, despite higher food intake in mice on GLU+GAL diet. Hepatic TG content was lower in GLU+GAL-fed than GLU-fed females, while glycogen levels were unaffected. Analysis of global expression patterns of hepatic mRNA showed that mainly inflammation-related pathways were affected by the diet, which were predominantly downregulated in GLU+GAL-fed females compared to GLU-fed females. This reduction in inflammation in GLU+GAL-fed females was also reflected by decreased serum concentrations of acute phase protein Serum amyloid A 3. In conclusion, replacing part of glucose with galactose in the post-weaning diet reduces hepatic TG content and hepatic inflammation.

Acute and long-term outcomes in a Drosophila melanogaster model of classic galactosemia occur independently of galactose-1-phosphate accumulation

Classic galactosemia (CG) is a potentially lethal inborn error of metabolism that results from the 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 minimizes or resolves the acute sequelae of CG, but fails 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 the 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 suggests that other metabolite(s) of galactose, and/or other pathogenic factors, might be involved.

Summary: In a GALT-deficient Drosophila model of classic galactosemia, Gal-1P accumulation is not required for compromised larval survival following galactose exposure or adult movement and fecundity phenotypes.

Identification of a Large SLC25A13 Deletion via Sophisticated Molecular Analyses Using Peripheral Blood Lymphocytes in an Infant with Neonatal Intrahepatic Cholestasis Caused by Citrin Deficiency (NICCD): A Clinical and Molecular Study

Background. Neonatal intrahepatic cholestasis caused by citrin deficiency (NICCD) is a Mendelian disorder arising from biallelic SLC25A13 mutations, and SLC25A13 genetic analysis was indispensable for its definite diagnosis. However, conventional SLC25A13 analysis could not detect all mutations, especially obscure large insertions/deletions. This paper aimed to explore the obscure SLC25A13 mutation in an NICCD infant. Methods. Genomic DNA was extracted to screen for 4 high-frequency SLC25A13 mutations, and then all 18 exons and their flanking sequences were analyzed by Sanger sequencing. Subsequently, cDNA cloning, SNP analyses, and semiquantitative PCR were performed to identify the obscure mutation. Results. A maternally inherited mutation IVS16ins3kb was screened out, and then cDNA cloning unveiled paternally inherited alternative splicing variants (ASVs) featuring exon 5 skipping. Ultimately, a large deletion c.329-1687_c.468+3865del5692bp, which has never been described in any other references, was identified via intensive study on the genomic DNA around exon 5 of SLC25A13 gene. Conclusions. An NICCD patient was definitely diagnosed as a compound heterozygote of IVS16ins3kb and c.329-1687_c.468+3865del5692bp. The large deletion enriched the SLC25A13 mutation spectrum, and its identification supported the concept that cDNA cloning analysis, along with other molecular tools such as semiquantitative PCR, could provide valuable clues, facilitating the identification of obscure SLC25A13 deletions.

GALK inhibitors for classic galactosemia

Classic galactosemia is an inherited metabolic disease for which, at present, no therapy is available apart from galactose-restricted diet. However, the efficacy of the diet is questionable, since it is not able to prevent the insurgence of chronic complications later in life. In addition, it is possible that dietary restriction itself could induce negative side effects. Therefore, there is a need for an alternative therapeutic approach that can avert the manifestation of chronic complications in the patients. In this review, the authors describe the development of a novel class of pharmaceutical agents that target the production of a toxic metabolite, galactose-1-phosphate, considered as the main culprit for the cause of the complications, in the patients.

Manganese-based superoxide dismutase mimics modify both acute and long-term outcome severity in a Drosophila melanogaster model of classic galactosemia

AIMS

The goal of this study was to use two manganese (Mn)-based superoxide dismutase (SOD) mimics to test the hypothesis that reactive oxygen species contribute to both acute and long-term outcomes in a galactose-1P uridylyltransferase (GALT)-null Drosophila melanogaster model of classic galactosemia.

RESULTS

We tested the impact of each of two Mn porphyrin SOD mimics, MnTnBuOE-2-PyP(5+), and MnTE-2-PyP(5+), (i) on survival of GALT-null Drosophila larvae reared in the presence versus absence of dietary galactose and (ii) on the severity of a long-term movement defect in GALT-null adult flies. Both SOD mimics conferred a significant survival benefit to GALT-null larvae exposed to galactose but not to controls or to GALT-null larvae reared in the absence of galactose. One mimic, MnTE-2-PyP(5+), also largely rescued a galactose-independent long-term movement defect otherwise seen in adult GALT-null flies. The survival benefit of both SOD mimics occurred despite continued accumulation of elevated galactose-1P in the treated animals, and studies of thiolated proteins demonstrated that in both the presence and absence of dietary galactose MnTE-2-PyP(5+) largely prevented the elevated protein oxidative damage otherwise seen in GALT-null animals relative to controls.

INNOVATION AND CONCLUSIONS

Our results confirm oxidative stress as a mediator of acute galactose sensitivity in GALT-null Drosophila larvae and demonstrate for the first time that oxidative stress may also contribute to galactose-independent adult outcomes in GALT deficiency. Finally, our results demonstrate for the first time that both MnTnBuOE-2-PyP(5+) and MnTE-2-PyP(5+) are bioavailable and effective when administered through an oral route in a D. melanogaster model of classic galactosemia.

Clinical, molecular and functional investigation on an infant with neonatal intrahepatic cholestasis caused by citrin deficiency (NICCD)

BACKGROUND AND OBJECTIVE

SLC25A13 analysis has provided reliable evidences for the definitive diagnosis of citrin deficiency (CD) in the past decade. Meanwhile, these studies generated some issues yet to be resolved, including the pathogenicity of SLC25A13 missense mutations and the mRNA product from the mutation c.615+5G>A. This study aims to investigate the effect of a novel missense mutation on the aspartate/glutamate carrier (AGC) function of citrin protein, and to explore the aberrant transcript from c.615+5G>A in the same CD infant.

METHODS AND RESULTS

By means of screening for prevalent SLC25A13 mutations and exons sequencing, the patient proved a compound heterozygote of c.615+5G>A and a novel c.1064G>A (p.Arg355Gln) mutation. An aberrant transcript with retention of the entire intron 6, r.[615+1_615+1789ins; 615+5 g>a] (GenBank accession number KJ128074), which was resulted from c.615+5G>A, was detected by RT-PCR and cDNA sequencing. After bioinformatic analyses of the novel missense mutation c.1064G>A, the growth abilities of three agc1Δ yeast strains were tested, which had been transformed with recombinant or empty vectors, respectively. Besides the bioinformatically pathogenic evidences, the growth ability of the agc1Δ strains transformed with mutant recombinant was the same as with empty vector, but significantly lower than that with normal control in functional analysis.

CONCLUSIONS

A CD infant was definitely diagnosed in this paper by a genetic, transcriptional and functional analysis of SLC25A13 gene. This study provided direct laboratory evidences supporting the splice-site nature of the c.615+5G>A mutation, and the novel c.1064G>A variation, which proved a pathogenic mutation bioinformatically and functionally, enriched the SLC25A13 mutation spectrum.

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

The first GalT gene knockout (KO) mouse model for Classic Galactosemia (OMIM 230400) accumulated some galactose and its metabolites upon galactose challenge, but was seemingly fertile and symptom free. Here we constructed a new GalT gene-trapped mouse model by injecting GalT gene-trapped mouse embryonic stem cells into blastocysts, which were later implanted into pseudo-pregnant females. High percentage GalT gene-trapped chimera obtained were used to generate heterozygous and subsequently, homozygous GalT gene-trapped mice. Biochemical assays confirmed total absence of galactose-1 phosphate uridylyltransferase (GALT) activity in the homozygotes. Although the homozygous GalT gene-trapped females could conceive and give birth when fed with normal chow, they had smaller litter size (P=0.02) and longer time-to-pregnancy (P=0.013) than their wild-type littermates. Follicle-stimulating hormone levels of the mutant female mice were not significantly different from the age-matched, wild-type females, but histological examination of the ovaries revealed fewer follicles in the homozygous mutants (P=0.007). Administration of a high-galactose (40% w/w) diet to lactating homozygous GalT gene-trapped females led to lethality in over 70% of the homozygous GalT gene-trapped pups before weaning. Cerebral edema, abnormal changes in the Purkinje and the outer granular cell layers of the cerebellum, as well as lower blood GSH/GSSG ratio were identified in the galactose-intoxicated pups. Finally, reduced growth was observed in GalT gene-trapped pups fed with normal chow and all pups fed with high-galactose (20% w/w) diet. This new mouse model presents several of the complications of Classic Galactosemia and will be useful to investigate pathogenesis and new therapies.

Diversity of approaches to classic galactosemia around the world: a comparison of diagnosis, intervention, and outcomes

Without intervention, classic galactosemia is a potentially fatal disorder in infancy. With the benefit of early diagnosis and dietary restriction of galactose, the acute sequelae of classic galactosemia can be prevented or reversed. However, despite early and lifelong dietary treatment, many galactosemic patients go on to experience serious long-term complications including cognitive disability, speech problems, neurological and/or movement disorders and, in girls and women, ovarian dysfunction. Further, there remains uncertainty surrounding what constitutes a 'best practice' for treating this disorder. To explore the extent and implications of this uncertainty, we conducted a small but global survey of healthcare providers who follow patients with classic galactosemia, seeking to compare established protocols for diagnosis, intervention, and follow-up, as well as the outcomes and outcome frequencies seen in the patient populations cared for by these providers. We received 13 survey responses representing five continents and 11 countries. Respondents underscored disparities in approaches to diagnosis, management and follow-up care. Notably, we saw no clear relationship between differing approaches to care and long-term outcomes in the populations studied. Negative outcomes occurred in the majority of cases regardless of when treatment was initiated, how tightly galactose intake was restricted, or how closely patients were monitored. We document here what is, to our knowledge, the first global comparison of healthcare approaches to classic galactosemia. These data reinforce the idea that there is currently no one best practice for treating patients with classic galactosemia, and underscore the need for more extensive and statistically powerful comparative studies to reveal potential positive or negative impacts of differing approaches.

Impact of selected inborn errors of metabolism on prenatal and neonatal development

In general, data regarding maturational processes of different metabolic pathways in the very vulnerable fetal and neonatal period are rare. This review is to substantiate the impact of selected inborn errors of metabolism on this critical period of life and their clinical manifestation. Significant adaptation of mitochondrial/energy-, carbohydrate-, lysosomal-, and amino acid-metabolism occurs during early prenatal and neonatal development. In utero, metabolic environment has an impact on the development of the fetus as well as fetal organ maturation. Defects of distinct metabolic pathways could therefore already be of significant relevance in utero and for clinical manifestations in the early fetal and neonatal period. Disturbances of these pathways may influence intrauterine growth and health. Production of a toxic intrauterine milieu, energy-deficiency, modification of membrane function, or disturbance of the normal intrauterine expression of genes may be responsible for fetal compromise and developmental disorders. Three categories of metabolic disorders will be discussed: the "intoxication type" (classical galactosemia, ornithine transcarbamylase deficiency, and "maternal phenylketonuria"), the "storage type" (Morbus Niemann Pick type C), and the "energy deficient type" (including long-chain fatty acid oxidation disorders, pyruvate dehydrogenase deficiency, and respiratory chain defects). For these disorders, the pathophysiology of early manifestation, special aspects regarding the prenatal and neonatal period, and diagnostic as well as therapeutic options are presented.

A Drosophila melanogaster model of classic galactosemia

Classic galactosemia is a potentially lethal disorder that results from profound impairment of galactose-1-phosphate uridylyltransferase (GALT). Despite decades of research, the underlying pathophysiology of classic galactosemia remains unclear, in part owing to the lack of an appropriate animal model. Here, we report the establishment of a Drosophila melanogaster model of classic galactosemia; this is the first whole-animal genetic model to mimic aspects of the patient phenotype. Analogous to humans, GALT-deficient D. melanogaster survive under conditions of galactose restriction, but accumulate elevated levels of galactose-1-phosphate and succumb during larval development following galactose exposure. As in patients, the potentially lethal damage is reversible if dietary galactose restriction is initiated early in life. GALT-deficient Drosophila also exhibit locomotor complications despite dietary galactose restriction, and both the acute and long-term complications can be rescued by transgenic expression of human GALT. Using this new Drosophila model, we have begun to dissect the timing, extent and mechanism(s) of galactose sensitivity in the absence of GALT activity.

Galactose toxicity in animals

In most organisms, productive utilization of galactose requires the highly conserved Leloir pathway of galactose metabolism. Yet, if this metabolic pathway is perturbed due to congenital deficiencies of the three associated enzymes, or an overwhelming presence of galactose, this monosaccharide which is abundantly present in milk and many non-dairy foodstuffs, will become highly toxic to humans and animals. Despite more than four decades of intense research, little is known about the molecular mechanisms of galactose toxicity in human patients and animal models. In this contemporary review, we take a unique approach to present an overview of galactose toxicity resulting from the three known congenital disorders of galactose metabolism and from experimental hypergalactosemia. Additionally, we update the reader about research progress on animal models, as well as advances in clinical management and therapies of these disorders.

Evidence of cataplerosis in a patient with neonatal classical galactosemia presenting as citrin deficiency

Classical galactosemia is an autosomal recessive disorder caused by a deficiency of the enzyme galactose-1-phosphate uridyltransferase. Undoubtedly, some of the short term complications are linked to the toxic effects of the accumulated abnormal metabolites (galactose-1-phosphate and galactitol). However, the physiopathology of neonatal liver failure remains unclear. We report the case of a 7-week-old girl who was first diagnosed with liver failure, hypoprotidaemia, ascites and generalized edemas. High citrulline (293 micromol/L), on initial plasma amino acid, suggested the diagnosis of citrin deficiency. As the citric acid cycle intermediates were non-detectable (oxoglutarate, succinate and citrate), a cataplerotic state was suspected. As a result, citrate (as an anaplerotic treatment) induced a clear improvement in her liver function. Four weeks later, this patient was switched to a galactose-free formula (as recommended in citrin deficiency with galactosemia) and her pathological status returned to normal. Citrin deficiency was later ruled out by molecular biology studies; then we reintroduced a galactose-containing formula which re-evoked rapidly vomiting, galactose aversion and hepatic cytolysis and the diagnosis of classical galactosemia was established. Our case clearly shows that cataplerosis could play a role in the pathophysiology of the neonatal liver disease observed in classical galactosemia.

ARHI: A new target of galactose toxicity in Classic Galactosemia

In humans, deficiency of galactose-1-phosphate uridyltransferase (GALT) activity can lead to a potentially lethal disease called Classic Galactosemia. Although a galactose-restricted diet can prevent the acute lethality associated with the disorder, chronic complications persist in many well-treated patients. Approximately 85% of young women with Classic Galactosemia experience hypergonadotropic hypogonadism and premature ovarian failure (POF). Others suffer from mental retardation, growth restriction, speech dyspraxia, and ataxia. Despite decades of intense biochemical characterization, little is known about the molecular etiology, as well as the chronology of the pathological events leading to the poor outcomes. Several hypotheses have been proposed, most of which involved the accumulation of the intermediates and/or the deficit of the products, of the blocked GALT pathway. However, none of these hypotheses satisfactorily explained the absence of patient phenotypes in the GALT-knockout mice. Here we proposed that the gene encoded the human tumor suppressor gene aplysia rashomolog I (ARHI) is a target of toxicity in Classic Galactosemia, and because ARHI gene is lost in rodents in through evolution, it thus accounts for the lack of clinical phenotypes in the GALT-knockout mice.

Involvement of endoplasmic reticulum stress in a novel Classic Galactosemia model

Inherited deficiency of galactose-1-phosphate uridyltransferase (GALT) activity in humans leads to a potentially lethal disorder called Classic Galactosemia. It is well known that patients often accumulate high levels of galactose metabolites such as galactose-1-phosphate (gal-1-p) in their tissues. However, specific targets of gal-1-p and other accumulated metabolites remain uncertain. In this study, we developed a new model system to study this toxicity using primary fibroblasts derived from galactosemic patients. GALT activity was reconstituted in these primary cells through lentivirus-mediated gene transfer. Gene expression profiling showed that GALT-deficient cells, but not normal cells, responded to galactose challenge by activating a set of genes characteristic of endoplasmic reticulum (ER) stress. Western blot analysis showed that the master regulator of ER stress, BiP, was up-regulated at least threefold in these cells upon galactose challenge. We also found that treatment of these cells with galactose, but not glucose or hexose-free media reduced Ca2+ mobilization in response to activation of Gq-coupled receptors. To explore whether the muted Ca2+ mobilization is related to reduced inositol turnover, we discovered that gal-1-p competitively inhibited human inositol monophosphatase (hIMPase1). We hypothesize that galactose intoxication under GALT-deficiency resulted from accumulation of toxic galactose metabolite products, which led to the accumulation of unfolded proteins, altered calcium homeostasis, and subsequently ER stress.

Metabolic fate of administered [13C]galactose in tissues of galactose-1-phosphate uridyl transferase deficient mice determined by nuclear magnetic resonance

The pattern of distribution of galactose and its metabolites was determined in tissues of mice deficient in galactose-1-phosphate uridyl transferase (G/G) 4 h after the administration of 1mg/g of [13C]galactose. Labeled galactose was found in all the tissues examined, the highest amounts in liver and kidney. Each of the tissues had its own pattern of labeling of galactose-1-phosphate (gal-1-P), galactitol and galactonate. [13C]gal-1-P and galactonate concentration was highest in liver while [13C]galactitol was higher in kidney and heart than in other tissues. Muscle had the lowest amounts of these compounds. In contrast, no galactose was found in tissues of normal mice (N/N) except for a minute amount in muscle. No [13C]gal-1-P was found in liver, kidney or brain and only minute amounts in heart and muscle of N/N animals. Barely detectible, labeled galactitol was observed in these tissues except liver, where none was found. [13C]Galactonate was formed in liver comparable to G/G mice. Almost all of the accumulating 13C isotope was found in liver and kidney glucose and lactate in the normal animals. [13C]Glucose and lactate was also found in liver of the G/G animals, but to a lesser extent than in normals, indicating the presence of a pathway in G/G animals for circumventing the block at GALT for the normal conversion of galactose to glucose.

Classical galactosaemia revisited

Classical galactosaemia (McKusick 230400) is an: autosomal recessive disorder of galactose metabolism, caused by a deficiency of the enzyme galactose-1-phosphate uridyltransferase (GALT; EC 2.7.712). Most patients present in the neonatal period, after ingestion of galactose, with jaundice, hepatosplenomegaly, hepatocellular insufficiency, food intolerance, hypoglycaemia, renal tubular dysfunction, muscle hypotonia, sepsis and cataract. The gold standard for diagnosis of classical galactosaemia is measurement of GALT activity in erythrocytes. Gas-chromatographic determination of urinary sugars and sugar alcohols demonstrates elevated concentrations of galactose and galactitol. The only therapy for patients with classical galactosaemia is a galactose-restricted diet, and initially all galactose must be removed from the diet as soon as the diagnosis is suspected. After the neonatal period, a lactose-free diet is advised in most countries, without restriction of galactose-containing fruit and vegetables. In spite of the strict diet, long-term complications such as retarded mental development, verbal dyspraxia, motor abnormalities and hypergonadotrophic hypogonadism are frequently seen in patients with classical galactosaemia. It has been suggested that these complications may result from endogenous galactose synthesis or from abnormal galactosylation. Novel therapeutic strategies, aiming at the prevention of galactose 1-phosphate production, should be developed. In the meantime, the follow-up protocol for patients with GALT deficiency should focus on early detection, evaluation and, if possible, early intervention in problems of motor, speech and cognitive development.

Pathophysiology of impaired ovarian function in galactosaemia

Classical galactosaemia is an inherited inborn error of the major galactose assimilation pathway, caused by galactose-1-phosphate uridyltransferase (GALT) deficiency. Many GALT mutations have been described, with different clinical consequences. In severe forms, newborns present with a life-threatening, acute toxic syndrome that rapidly regresses under a galactose-restricted diet. However, long-term complications, particularly cognitive and motor abnormalities, as well as hypergonadotrophic hypogonadism in female patients are still unavoidable. The pathogenesis of galactose-induced ovarian toxicity remains unclear but probably involves galactose itself and its metabolites such as galactitol and UDP-galactose. Possible mechanisms of ovarian damage include direct toxicity of galactose and metabolites, deficient galactosylation of glycoproteins and glycolipids, oxidative stress and activation of apoptosis. As there is no aetiological treatment, clinical management of ovarian failure in galactosaemic patients principally relies on hormonal replacement therapy to induce pubertal development and to prevent bone loss and other consequences of estrogen deprivation. Further investigations will be necessary to better understand the metabolic flux of galactose through its biochemical pathways and the mechanisms of these secondary complications. The aim of this article is to present an extensive review on the pathogenesis and clinical management of galactose-induced premature ovarian failure.

Relationship between UDP-Galactose 4′-Epimerase Activity and Galactose Sensitivity in Yeast

UDP-galactose 4'-epimerase (GALE) catalyzes the final step of the highly conserved Leloir pathway of galactose metabolism. Loss of GALE in humans results in a variant form of the metabolic disorder, galactosemia. Loss of GALE in yeast results in galactose-dependent growth arrest. Although the role of GALE in galactose metabolism has been recognized for decades, the precise relationship between GALE activity and galactose sensitivity has remained unclear. Here we have explored this relationship by asking the following. 1) Is GALE rate-limiting for galactose metabolism in yeast? 2) What is the relationship between GALE activity and galactose-dependent growth arrest in yeast? 3) What is the relationship between GALE activity and the abnormal accumulation of galactose metabolites in yeast? To answer these questions we engineered a strain of yeast in which GALE was doxycycline-repressible and studied these cells under conditions of intermediate GALE expression. Our results demonstrated a smooth linear relationship between galactose metabolism and GALE activity over a range from 0 to approximately 5% but a steep threshold relationship between growth rate in galactose and GALE activity over the same range. The relationship between abnormal accumulation of metabolites and GALE activity was also linear over the range from 0 to approximately 5%, suggesting that if the abnormal accumulation of metabolites underlies galactose-dependent growth-arrest in GALE-impaired yeast, either the impact of individual metabolites must be synergistic and/or the threshold of sensitivity must be very steep. Together these data reveal important points of similarity and contrast between the roles of GALE and galactose-1-phosphate uridylyltransferase in galactose metabolism in yeast and provide a framework for future studies in mammalian systems.

8-hydroxy-2-desoxyguanosine serum concentrations as a marker of DNA damage in patients with classical galactosaemia

BACKGROUND

Classical galactosaemia is caused by a deficiency of galactose-1-phosphate uridyl transferase, resulting in high galactose (Gal), galactose-1-phosphate (Gal-1-P) and galactitol blood levels. Galactose/lactose restriction intake is the only treatment. 8-hydroxy-2-desoxyguanosine (8-OHdG) is a marker of oxidized DNA damage.

AIM

Since galactosaemia outcome is closely related to restriction of Gal intake, we aimed to evaluate correlations between Gal-1-P, total antioxidant status (TAS) and 8-OHdG blood levels in galactosaemic patients on poor or strict diet.

METHODS

Venous blood samples were obtained from galactosaemic patients (n = 11) on poor diet (group A) and after 30 d on strict diet (group B). Twenty-eight healthy children were the controls. Gal-1-P and TAS were evaluated in their blood spectrophotometrically and 8-OHdG with an immunoassay.

RESULTS

TAS was significantly decreased (905 +/- 112 micromol/l) in patients on a "loose diet" (group A) as compared to those when restored to their diet (group B) (1,340 +/- 112 micromol/l, p < 0.001) and controls (1,558 +/- 115 micromol/l, p < 0.001). As expected, Gal-1-P levels were remarkably increased in group A. 8-OHdG level was twofold higher (0.25 +/- 0.03 ng/ml) in group A than that of group B (0.11 +/- 0.04 ng/ml) and threefold higher than that of the controls (0.08 +/- 0.02 ng/ml). TAS and Gal-1-P inversely correlated to 8-OHdG (r= -0.802, p < 0.001), whereas Gal-1-P positively correlated to 8-OHdG (r = 0.820, p < 0.001) in all the groups.

CONCLUSION

a) Low TAS and high Gal-1-P levels are implicated with high 8-OHdG blood levels in galactosaemic patients; b) 8-OHdG may be a sensitive biomarker of DNA damage in patients with classical galactosaemia.

Intracellular galactose-1-phosphate accumulation leads to environmental stress response in yeast model

In humans, deficiency of galactose-1-phosphate uridyltransferase (GALT) can lead a metabolic disorder Classic Galactosemia. Although the biochemical abnormalities associated with this disease have been described in detail, few attempts have been made to characterize the pathogenic mechanisms of this disorder at the molecular level. Here we report the use of high-throughput DNA microarray to examine how galactose affects gene expression in isogenic yeast models that are deficient in either galactokinase (GALK) or GALT, two enzymes which are essential for normal galactose metabolism. We confirmed that the growth of our GALT-deficient, but not GALK-deficient yeast strain ceased 4 h after challenge with 0.2% galactose. Such inhibition was not associated with a reduction of ATP content and was reversible after removal of galactose from medium. We compared the gene expression profiles of the GALT-deficient and GALK-deficient cells in the presence/absence of galactose. We revealed that in the absence of galactose challenge, a subset of genes involved in RNA metabolism was expressed at a level 3-fold lower in the GALT-deficient cells. Upon galactose challenge, significantly more genes involved in various aspects of RNA metabolism and almost all ribosomal protein genes were downregulated in the GALT-deficient, but not GALK-deficient cells. Remarkably, genes involved in inositol biosynthesis and turnover were exclusively induced at high level in the galactose-intoxicated GALT-deficient cells. Our data thus suggested that RNA metabolism, ribosome biogenesis, and inositol metabolism were likely targets for galactose-1-phosphate, a toxic intermediate that is uniquely accumulated under GALT-deficiency.

An updated review of the long-term neurological effects of galactosemia

Classical galactosemia is an autosomal recessive condition in which there is near total absence of the activity of galactose-1-phosphate uridyltransferase. Patients with this condition have substantial motor, cognitive, and psychiatric impairments despite dietary treatment. A characteristic pattern of biochemical abnormalities is observed in patients with this disorder. Galactose-1-phosphate, the substrate of galactose-1-phosphate uridyltransferase, accumulates within cells, and surplus galactose is reduced to galactitol or oxidized to galactonate. Using sophisticated mass spectrometry, these compounds as well as free galactose can be measured in plasma and in urine. It is clear that initiation of dietary restriction of galactose in the newborn period produces reversal of hepatic, renal, brain, and immune dysfunction, along with reduction of the accumulated galactose metabolites. However, the neurologist should be aware that chronic and progressive neurologic impairments occur even in patients spared these neonatal symptoms. The purpose of this review is to summarize current information about neurologic complications of galactosemia and what is known, and still unknown, about its pathophysiology.

UDP-galactose pyrophosphorylase in mice with galactose-1-phosphate uridyltransferase deficiency

UDP-glucose pyrophosphorylase (E.C. 2.7.7.9), encoded by ugp, provides UDP-glucose which is critical to the synthesis of glycogen, and also catalyzes the reaction between UTP and galactose-1-phosphate, yielding UDP-galactose. This activity of UDP-gal pyrophosphorylase (UDP-galPP) suggests a role in an alternate pathway for galactose metabolism in patients with deficiency of galactose-1-phosphate uridyltransferase (GALT). We examined the effects of GALT deficiency and dietary galactose on UDP-glucose pyrophosphorylase (UDP-gluPP) and UDP-galactose pyrophosphorylase activity and ugp expression in liver of mice with homozygous deletion of the critical regions of galt. Activity with glucose-1-phosphate as substrate was significantly higher than that with galactose-1-phosphate. In liver from mice with GALT deficiency (G/G), UDP-galPP activity appeared to be lower than that measured in liver from control (N/N) animals. This difference disappeared when the N/N tissue homogenate was dialyzed to remove residual UDP-glucose, confirming that careful elimination of residual GALT activity is necessary, since GALT has 1000-fold greater activity toward galactose-1-phosphate than that of UDP-galPP in liver homogenates. Prior exposure to conventional mouse chow, high galactose chow, and high glucose chow did not alter UDP-glu PP or UDP-galPP activity. Steady state UGP mRNA levels were determined in tissues from normal and G/G animals. UGP expression was highest in liver, and did not differ by genotype or exposure to high galactose chow. UDP-galPP activity may account for unexplained ability to oxidize galactose in animals with no GALT activity, but is insufficient to alter accumulation of galactose metabolites.

Differential roles of the Leloir pathway enzymes and metabolites in defining galactose sensitivity in yeast

The metabolism of galactose via enzymes of the Leloir pathway: galactokinase, galactose-1-P uridylyltransferase, and UDP galactose-4'-epimerase, is a process that has been conserved from Escherichia coli through humans. Impairment of this pathway in patients results in the disease galactosemia. Despite decades of study, the underlying pathophysiology in galactosemia remains unknown. Here we have defined the functional and metabolic implications of impaired galactose metabolism in yeast, by asking two questions: (1) What is the impact of loss of each of the three Leloir enzymes on the ability of cells to metabolize galactose, and on their sensitivity to galactose, and (2) what is the relationship between gal-1P and galactose-sensitivity in yeast? Our results demonstrate that only transferase-null cells are able to deplete their medium of galactose; deletion of kinase or epimerase halts this process. In contrast, only kinase-null cultures grow well in glycerol/ethanol medium despite the addition of galactose; both transferase and epimerase-null yeast arrest growth under these conditions. Indeed, epimerase-null yeast arrest growth at galactose concentrations 10-fold lower than do their transferase-null counterparts. Secondary deletion of kinase relieves growth arrest in both strains. Finally, rather than a continuous relationship between gal-1P and growth arrest, we observed a threshold level of gal-1P (approximately 10 nmol/mg cell DM) above which both transferase-null and epimerase-null cultures could not grow. These results both confirm and significantly extend prior knowledge of galactose metabolism in yeast, and set the stage for future studies into the mediators and mechanism of Leloir-impaired galactose sensitivity in eukaryotes.

Galactitol and galactonate accumulation in heart and skeletal muscle of mice with deficiency of galactose-1-phosphate uridyltransferase

Under conditions of dietary galactose loading, mice deficient in galactose-1-phosphate uridyltransferase (GALT) accumulate large amounts of galactitol and galactonate in heart and skeletal muscle. In contrast to liver, brain, and kidney, which form little galactitol when GALT-deficient animals (G/G) ingest a 40% galactose diet, heart and skeletal muscle galactitol reaches 22.90+/-1.62 (M+/-SE) and 38.88+/-2.62 micromol/g tissue, respectively, levels 40-100 times that of galactose-1-phosphate (Gal-1-P). Sixteen-day-old suckling G/G mice accumulate galactitol in heart and to a lesser extent, in skeletal muscle. Heart and skeletal muscle of G/G mice also form galactonate, with levels comparable to that of liver, which was presumed previously to be the only tissue capable of converting galactose to galactonate under conditions of loading. The data suggest that heart and skeletal muscle play a role in disposition of galactose when GALT activity is impaired, contributing a large share to urinary galactitol and galactonate excretion. The ability of heart and muscle to form galactonate may also contribute to the G/G mouse's ability to slowly oxidize galactose to CO2, since the compound is an intermediate in an alternate route for galactose disposition.

Insights into the pathogenesis of galactosemia

In humans, the absence of galactose-1-phosphate uridyltransferase (GALT) leads to significant neonatal morbidity and mortality which are dependent on galactose ingestion, as well as long-term complications of primary ovarian failure and cognitive dysfunction, which are diet independent. The creation of a knockout mouse model for GALT deficiency was aimed at providing an organism in which metabolic challenges and gene manipulation could address the enigmatic pathophysiologic questions raised by humans with galactosemia. Instead, the mouse represents a biochemical phenotype without evidence of clinical morbidity. The similarities and differences between mice and humans with galactosemia are explored from metabolite, enzyme, and process points of view. The mouse both produces and oxidizes galactose in a manner similar to humans. It differs in brain accumulation of galactitol. Future directions for exploration of this enigmatic condition are discussed.

Galactose content of baby food meats: considerations for infants with galactosemia

Treatment of galactosemia requires a galactose-restricted diet. Although meats are not traditionally thought of as a dietary carbohydrate source, small amounts may be present in free form and/or bound to proteins or lipids. The purpose of this study was to determine the free and bound galactose contents of baby food meats. Galactose was assayed using high-performance liquid chromatography. The free galactose content of baby food meats ranged from 0 to 0.031 mg/100 g. No statistically significant differences in free galactose content were found among the meats. Bound galactose was found in all analyzed baby food meats, ranging from 0.065 to 0.148 mg/100 g. The mean galactose content of BeechNut chicken (St. Louis, MO) was significantly less than that found in Gerber (Fremont, MI) and Heinz (Pittsburgh, PA) brands of chicken, beef, and turkey, and Gerber lamb and veal. Based on current recommendations, all examined baby food meats would be acceptable for infants with galactosemia.

Metabolism of 13C galactose by lymphoblasts from patients with galactosemia determined by NMR spectroscopy

In order to assess the pathways by which galactose is metabolized by galactose-1-phosphate uridyltransferase (GALT) deficient cells, lymphoblasts from 10 galactosemic patients with defined genotypes (six Q188R homozygotes, two S153L homozygotes, and two with homozygous deletions) were incubated with 1mM 1- or 2-13C galactose for 2.5 and 5 h. The 13C-labeled metabolites were identified and quantified using nuclear magnetic resonance and the results were compared to that obtained with cells from eight normal individuals. Cells from galactosemic patients formed two to three times the galactose-1-phosphate (Gal-1P) in normal cells, no difference being observed between the various genotypes. Galactitol formation was not significantly different from normal cells. No labeled galactonate was detected. Cells with the Q188R and S135L mutations formed both labeled uridine diphosphogalactose (UDPgal) and uridine diphosphoglucose (UDPglu), but to a lesser extent than normals, whereas cells with the GALT deletion did not. The pattern of 13C enrichment of the ribose carbons of adenosine monophosphate upon incubation of the normal cells with 1-13C galactose paralleled that found for incubations with 1-13C glucose, which is consistent with galactose disposition through the Leloir pathway to glucose and its subsequent metabolism to ribose. Cells with the GALT deletion formed no detectable labeled ribose, whereas cells from a patient homozygous for Q188R mutation formed labeled ribose in a pattern similar to normal albeit with lower enrichment. The results suggest that there is residual GALT activity and function of the Leloir pathway in the presence of the Q188R as well as S135L mutation.

Clinical features of galactokinase deficiency: a review of the literature

Galactokinase deficiency (McKusick 230200) is a rare autosomal recessive inborn error of galactose metabolism. Cataract and, rarely, pseudotumor cerebri caused by galactitol accumulation seem to be the only consistently reported abnormalities in this disorder. We performed a literature search to obtain information on the clinical spectrum of galactokinase deficiency. A total of 25 publications were traced describing 55 galactokinase-deficient patients. Cataract was reported in most patients. Clinical abnormalities other than cataract were reported in 15 (35%) out of 43 cases on which information was available. However, all symptoms were reported infrequently and a causal relationship with the galactokinase deficiency is unlikely. As cataract and pseudotumor cerebri appear to be the sole complications of galactokinase deficiency, the outcome for patients with galactokinase deficiency is much better than for patients with classical galactosaemia (McKusick 230400), a more common autosomal recessive disorder of galactose metabolism caused by galactose-1-phosphate uridyltransferase (GALT; EC 2.7.7.12) deficiency. Long-term follow-up of patients with this disorder has shown that, in spite of a severely galactose-restricted diet, most patients develop abnormalities such as a disturbed mental and/or motor development, dyspraxia and hypergonadotropic hypogonadism. Endogenous production of galactose has been considered an important aetiological factor. Although damage may well occur in utero, available evidence suggests that damage will continue after birth. Inhibition of galactokinase may then be a promising approach for controlling damage in GALT-deficient patients.

Identification of galactitol and galactonate in red blood cells by gas chromatography/mass spectrometry

BACKGROUND

Because the products of alternate pathways of galactose metabolism, galactitol and galactonate are important in galactosemia, we sought to identify these compounds in red blood cells (RBC).

METHODS

RBC extracts were trimethylsilylated (TMS) and analyzed by gas chromatography/mass spectrometry (GC/MS).

RESULTS

The presence of both galactitol and galactonate was identified in RBC of 15 galactosemic and 13 normal subjects by their mass spectra and chromatographic comparisons with both unlabeled and 13C labeled standards. The levels in RBC of galactosemics appear to be much higher than those of normal subjects.

CONCLUSION

The determination of these compounds in RBC along with galactose-1-phosphate (gal-1-P) in the same procedure provides the potential for their use in better monitoring of diet therapy in galactosemic patients.

The mousetrap: what we can learn when the mouse model does not mimic the human disease

In recent years, mouse models for human metabolic diseases have become commonplace because the information gained from in vivo study of biochemical pathways is invaluable, and many metabolic diseases are relatively easy to recreate in mice through gene knockout technology in embryonic stem cells. In certain cases, however, the knockout mice may reproduce only some of the human disease phenotype, may be more severely affected than human cases, or may have no clinical phenotype at all. Under these circumstances, the disease pathology can become more complex, causing the researcher to evaluate basic differences in mouse and human biology as well as questions of genetic background, alternate pathways, and possible gene interactions. This review is a brief analysis of gene knockout models for Lesch-Nyhan syndrome, Lowe syndrome, X-linked adrenoleukodystrophy, Fabry disease, galactosemia, glycogen storage disease type II, metachromatic leukodystrophy, and Tay-Sachs disease, which produce a biochemical model of disease but often do not reproduce clinical symptoms. These mice may be useful for studying the biochemical and physiological pathways in which certain metabolites function toward embryonic and fetal development, as well as specific functions in various organs, and they may provide an inexpensive and useful model system for development of new therapeutic techniques.

Oxidation of galactose by galactose-1-phosphate uridyltransferase-deficient lymphoblasts

The ability of EB virus-transformed lymphoblasts with undetectable galactose-1-phosphate uridyltransferase (GALT) from 15 galactosaemic patients to oxidize [1-(14)C]galactose to 14CO2 was compared to that of cells from 7 normal subjects. The oxidation of galactose but not of glucose was markedly diminished by cells from Q188R homozygous galactosaemic patients but was not absent. After 2.5 h these cells liberated 14CO2 at nearly 3% and at 5 h up to 9% of normal. Cells from patients homozygous for the S135L mutation produced much larger amounts of 14CO2 (15-17% of normal) and were distinguishable from the Q188R homozygous cells. A cell line with a homozygous deletion of the GALT gene oxidized galactose at 7% of the normal rate, suggesting that pathways(s) other than GALT exist in these cells as well as Q188R homozygous cells for oxidation of galactose to CO2. Concentration dependence studies are consistent with the presence of a pathway that is unsaturable or has a very high Km The ability of 10(7) lymphoblasts with the S135L genotype to oxidize more than 7% of the sugar to 14CO2 in 5 h suggests the presence of residual GALT despite the inability to detect the activity by enzymatic analysis.

Galactose metabolism in normal human lymphoblasts studied by (1)H, (13)C and (31)P NMR spectroscopy of extracts

The development of tools to follow and quantitate the fate of galactose in mammalian cells is crucial to the study and understanding of the inherited disorders of galactose metabolism. In this study we incubated normal human lymphoblasts with 1- or 2-(13)C galactose for 2.5 or 5 h and prepared TCA extracts of the cells. The various galactose metabolites were identified and quantified using a combination of proton, carbon and phosphorus NMR spectra. Galactose-1-phosphate (gal-1P), uridine diphosphogalactose, uridine diphosphoglucose and galactitol were present in the extracts. Average levels for gal-1P were around 10 nmol/mg protein and for uridine diphosphoglucose, uridine diphosphogalactose and galactitol in the range of 0.5-2 nmol/mg protein. Galactonate was never found in any conditions. Percentage labeling could be estimated for gal-1P and for the ribose carbons of AMP. The labeling agrees with a conversion of galactose to glucose through the Leloir pathway.

Relationship between genotype, activity, and galactose sensitivity in yeast expressing patient alleles of human galactose-1-phosphate uridylyltransferase

Impairment of the human enzyme galactose-1-phosphate uridylyltransferase (GALT) results in the potentially lethal disorder galactosemia; the biochemical basis of pathophysiology in galactosemia remains unknown. We have applied a yeast expression system for human GALT to test the hypothesis that genotype will correlate with GALT activity measured in vitro and with metabolite levels and galactose sensitivity measured in vivo. In particular, we have determined the relative degree of functional impairment associated with each of 16 patient-derived hGALT alleles; activities ranged from null to essentially normal. Next, we utilized strains expressing these alleles to demonstrate a clear inverse relationship between GALT activity and galactose sensitivity. Finally, we monitored accumulation of galactose-1-P, UDP-gal, and UDP-glc in yeast expressing a subset of these alleles. As reported for humans, yeast deficient in GALT, but not their wild type counterparts, demonstrated elevated levels of galactose 1-phosphate and diminished UDP-gal upon exposure to galactose. These results present the first clear evidence in a genetically and biochemically amenable model system of a relationship between GALT genotype, enzyme activity, sensitivity to galactose, and aberrant metabolite accumulation. As such, these data lay a foundation for future studies into the underlying mechanism(s) of galactose sensitivity in yeast and perhaps other eukaryotes, including humans.

Galactose metabolism in mice with galactose-1-phosphate uridyltransferase deficiency: sucklings and 7-week-old animals fed a high-galactose diet

Mice deficient in galactose-1-phosphate uridyltransferase (GALT) demonstrate abnormal galactose metabolism but no obvious clinical phenotype. To further dissect the pathways of galactose metabolism in these animals, galactose oxidation and metabolite levels were studied in 16-day-old sucklings and the effect of a 4 week prior exposure to a 40% glucose or 40% galactose diet was determined in 7-week-old mice. Suckling GALT-deficient (G/G) mice slowly oxidized [1-14C]galactose to 14CO2, 4.0% of the dose when fed and 7.9% when fasted compared to normal animals 38.3 and 36.4% in 4 h, respectively. Plasma of G/G sucklings contained 11.1 mM galactose and erythrocyte galactose 1-phosphate levels were 28.2 and 31.9 mg/dl packed cells. Galactose, galactitol, galactonate, and galactose 1-phosphate were found in G/G suckling mouse tissues. The tissue galactose concentrations were 10% or less of that in plasma, suggesting that there was limited cellular entry of galactose. In 7-week-old fasted mice with 4 weeks prior exposure to glucose or galactose-containing diet, 4-h oxidation was 12.9 and 15.0% of the administered radiolabeled galactose, respectively. Normal animals oxidized 33.9 and 37.9% of the dose when fed the same diets, respectively. The ability of G/G mice to oxidize galactose in the absence of GALT activity suggests the presence of alternate metabolic pathways for galactose disposition. G/G mice fed the galactose-free 40% glucose diet had erythrocyte galactose 1-phosphate levels ranging from 6.4 to 17.7 mg/dl packed cells and detectable galactose and galactose metabolites in tissues, suggesting that these animals endogenously produced galactose. The plasma of 40% galactose-fed G/G mice contained 9.1 mM galactose with red blood cell galactose 1-phosphate averaging 43.6 mg/dl. Tissues of these animals also contained high levels of galactose and galactose 1-phosphate. Liver contained over 4 micromol/g galactonate but little galactitol. Despite the elevated galactose and galactose 1-phosphate, the animals tolerated the high-galactose diet and were indistinguishable from normal animals, exhibiting no manifestations of galactose toxicity seen in human GALT-deficient galactosemia. The data suggest that high galactose 1-phosphate levels do not cause galactose toxicity and that high galactitol in combination with galactose 1-phosphate may be a prerequisite. Absence of GALT appears necessary but insufficient to produce human galactosemic phenotype.

Functional analysis of the mouse galactose-1-phosphate uridyl transferase (GALT)promoter

Galactose-1-phosphate uridyltransferase (GALT) is expressed in most tissues, but the near total absence of catalytic activity in humans with the disease galactosemia leads to specific organ dysfunction, the pathophysiology of which remains an enigma. To characterize the transcriptional regulation of the mouse GALT gene, we isolated and sequenced over 3 kb of a 5'-flanking sequence and functionally characterized the region using in vitro transient transfection and in transgenic mice. A minimal promoter of 145 bp was found to function in both HepG2 cells and NS20Y mouse neuroblastoma cells. The minimal promoter contains regions of homology to the corresponding rat and human GALT genes. In transgenic mice expressing a luciferase transgene under control of a 1.9-kb fragment of the mGALT promoter region, reporter activity was found in most tissues, with higher than expected reporter levels in neonatal brain. To determine if high galactose levels in tissues could induce promoter activity, we bred the mGALT:luciferase transgene into a line of mice in which the GALT gene function has been eliminated by homologous recombination. High tissue levels of galactose and metabolites did not induce reporter activity above background. The studies show that GALT transcriptional regulation is complex and not directly induced by substrate levels.