Evidence for alternate galactose oxidation in a patient with deletion of the galactose-1-phosphate uridyltransferase gene

[13C]-galactose breath test in a patient with galactokinase deficiency and spastic diparesis

Galactokinase deficiency is an inborn error of carbohydrate metabolism due to a block in the formation of galactose-1-phosphate from galactose. Although the association of galactokinase deficiency with formation of cataracts is well established, the extent of the clinical phenotype is still under investigation. We describe a 6-year-old female who was diagnosed with galactokinase deficiency due to cataract formation when she was 10 months of age and initially started on galactose-restricted diet at that time for 5 months. She developed gait abnormality at 4 years of age. Breath tests via measurement of 13C isotope in exhaled carbon dioxide following 13C-labeled galactose administration at carbon-1 and carbon-2 positions revealed oxidation rates within the normal range. The results in this patient strikingly contrast with the results of another patient with GALK1 deficiency that underwent breath testing with [1-14C]-galactose and [2-14C]-galactose. Extension of in vivo breath tests to other galactokinase patients is needed to better understand the pathophysiology of this disease.

Hereditary galactosemia

Hereditary galactosemia is an inborn error of carbohydrate metabolism. Galactose is metabolized by Leloir pathway enzymes; galactokinase (GALK), galactose-1-phosphate uridylyltransferase (GALT) and UDP-galactose 4-epimerase (GALE). The defects in these enzymes cause galactosemia in an autosomal recessive manner. The severe GALT deficiency, or classic galactosemia, is life-threatening in the newborn period. The treatment for classic galactosemia is dietary restriction of lactose. Although implementation of lactose restricted diet is efficient in resolving the acute complications, it is not sufficient to prevent long-term complications affecting the brain and female gonads, the two main target organs of damage. Implementation of molecular genetics diagnostic tools and GALT enzyme assays are instrumental in distinguishing classic galactosemia from clinical and biochemical variant forms of GALT deficiency. Better understanding of mechanisms responsible for the phenotypic variation even within the same genotype is essential to provide appropriate counseling for families. Utilization of a lactose restricted diet is also recommended for GALK deficiency and some rare forms of GALE deficiency. Novel modes of therapies are being explored; they may be beneficial if access issues to the affected tissues are circumvented and optimum use of therapeutic window is achieved.

Galactose-1-phosphate uridyltransferase deficiency: A literature review of the putative mechanisms of short and long-term complications and allelic variants

Galactosemia type 1 is an autosomal recessive disorder of galactose metabolism, determined by a deficiency in the enzyme galactose-1-phosphate uridyltransferase (GALT). GALT deficiency is classified as severe or variant depending on biochemical phenotype, genotype and potential to develop acute and long-term complications. Neonatal symptoms usually resolve after galactose-restricted diet; however, some patients, despite the diet, can develop long-term complications, in particular when the GALT enzyme activity results absent or severely decreased. The mechanisms of acute and long-term complications are still discussed and several hypotheses are presented in the literature like enzymatic inhibition, osmotic stress, endoplasmic reticulum stress, oxidative stress, defects of glycosylation or epigenetic modification. This review summarizes the current knowledge of galactosemia, in particular the putative mechanisms of neonatal and long-term complications and the molecular genetics of GALT deficiency.

Clinical profile and molecular characterization of Galactosemia in Brazil: identification of seven novel mutations

Background

Classical Galactosemia (CG) is an inborn error of galactose metabolism caused by the deficiency of the galactose-1-phosphate uridyltransferase enzyme. It is transmitted as an autosomal recessive disease and is typically characterized by neonatal galactose intolerance, with complications ranging from neonatal jaundice and liver failure to late complications, such as motor and reproductive dysfunctions. Galactosemia is also heterogeneous from a molecular standpoint, with hundreds of different mutations described in the GALT gene, some of them specific to certain populations, reflecting consequence of founder effect.

Methods

This study reviews the main clinical findings and depicts the spectrum of mutations identified in 19 patients with CG, six with Duarte Galactosemia and one with type 2 Galactosemia in Brazil. Some individuals were diagnosed through expanded newborn screening test, which is not available routinely to all newborns.

Results

The main classical Galactosemia mutations reported to date were identified in this study, as well as the Duarte variant and seven novel mutations - c.2 T > C (p.M1T), c.97C > A (p.R33S), c.217C > T (p.P73S), c.328 + 1G > A (IVS3 + 1G > A), c.377 + 4A > C (IVS4 + 4A > C), c.287_289delACA (p.N97del) and c.506A > C (p.Q169P). This was expected, given the high miscegenation of the Brazilian population.

Conclusions

This study expands the mutation spectrum in GALT gene and reinforces the importance of early diagnosis and introduction of dietary treatment, what is possible with the introduction of Galactosemia in neonatal screening programs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12881-016-0300-8) contains supplementary material, which is available to authorized users.

The eye and the skin in nonendocrine metabolic disorders

As metabolism is controlled by the input of genes and the environment, metabolic disorders result from some disturbance in the interaction between genes and environmental factors. Many metabolic disorders consist in congenital enzyme deficiencies, also known as "inborn errors of metabolism," that may be disabling or cause severe illness and death and are predominantly inherited in an autosomal recessive fashion. The deposit in cells and tissues of storage substances from errors in metabolic processes may produce a wide variety of disorders affecting different organs and functions, with different degrees of severity, and often present around the time of birth or early childhood. Distinctive ocular and skin manifestations accompany many metabolic diseases and may provide clues for their diagnosis and evolution.

A Novel Large Deletion Encompassing the Whole of the Galactose-1-Phosphate Uridyltransferase (GALT) Gene and Extending into the Adjacent Interleukin 11 Receptor Alpha (IL11RA) Gene Causes Classic Galactosemia Associated with Additional Phenotypic Abnormalities

Objective The characterization of a novel large deletion in the galactose-1-phosphate uridyltransferase (GALT) gene accounting for the majority of disease alleles in Cypriot patients with classic galactosemia. Methods DNA sequencing was used to identify the mutations followed by multiplex ligation-dependent probe amplification (MLPA) analysis in the cases suspected of harboring a deletion. In order to map the breakpoints of the novel deletion, a PCR walking approach was employed. A simple PCR assay was validated for diagnostic testing for the new deletion. Haplotype analysis was performed using microsatellite markers in the chromosomal region 9p. RT-PCR was used to study RNA expression in lymphoblastoid cell lines. Results The new deletion spans a region of 8489 bp and eliminates all GALT exons as well as the non-translated sequences of the adjacent interleukin 11 receptor alpha (IL11RA) gene. In addition, the deletion is flanked by a 6 bp block of homologous sequence on either side suggesting that a single deletion event has occurred, probably mediated by a recombination mechanism. Microsatellite marker analysis revealed the existence of a common haplotype. The RNA expression studies showed a lack of IL11RA transcripts in patients homozygous for the deletion. Conclusions We have identified and characterized a novel contiguous deletion which affects both the GALT enzyme and the IL11RA protein resulting in classic galactosemia with additional phenotypic abnormalities such as craniosynostosis, a feature that has been associated with defects in the IL11RA gene.

The adult galactosemic phenotype

BACKGROUND

Classic galactosemia is an autosomal recessive disorder due to galactose-1-phosphate uridyltransferase (GALT) deficiency. Newborn screening and early treatment do not completely prevent tremor, speech deficits, and diminished IQ in both sexes and premature ovarian insufficiency (POI) in women. Data on how individuals with galactosemia fare as adults will improve our ability to predict disease progression.

METHODS

Thirty-three adults (mean age = 32.6 ± 11.7 years; range = 18-59) with classic galactosemia, confirmed by genotype and undetectable GALT enzyme activity, were evaluated. Analyses assessed associations among age, genotype, clinical features and laboratory measures.

RESULTS

The sample included 17 men and 16 women. Subjects exhibited cataracts (21%), low bone density (24%), tremor (46%), ataxia (15%), dysarthria (24%), and apraxia of speech (9%). Subjects reported depression (39%) and anxiety (67%). Mean full scale IQ was 88 ± 20, (range = 55-122). All subjects followed a dairy-free diet and 75-80% reported low intake of calcium and vitamin D. Mean height, weight and body mass were within established norms. All female subjects had been diagnosed with POI. One woman and two men had had children. Logistic regression analyses revealed no associations between age, genotype or gender with IQ, tremor, ataxia, dysarthria, apraxia of speech or anxiety. Each 10- year increment of age was associated with a twofold increase in odds of depression.

CONCLUSIONS

Taken together, these data do not support the hypothesis that galactosemia is a progressive neurodegenerative disease. However, greater attention to depression, anxiety, and social relationships may relieve the impact of this disorder in adults.

The structural and molecular biology of type I galactosemia: Enzymology of galactose 1-phosphate uridylyltransferase

Reduced galactose 1-phosphate uridylyltransferase (GALT) activity is associated with the genetic disease type I galactosemia. This results in an increase in the cellular concentration of galactose 1-phosphate. The accumulation of this toxic metabolite, combined with aberrant glycoprotein and glycolipid biosynthesis, is likely to be the major factor in molecular pathology. The mechanism of GALT was established through classical enzymological methods to be a substituted enzyme in which the reaction with UDP-glucose results in the formation of a covalent, UMP-histidine adduct in the active site. The uridylated enzyme can then react with galactose 1-phosphate to form UDP-galactose. The structure of the enzyme from Escherichia coli reveals a homodimer containing one zinc (II) and one iron (II) ion per subunit. This enzymological and structural knowledge provides the basis for understanding the biochemistry of this critical step in the Leloir pathway. However, a high-resolution crystal structure of human GALT is required to assist greater understanding of the effects of disease-associated mutations.

Mutation database for the galactose-1-phosphate uridyltransferase (GALT) gene

Classical galactosemia is an autosomal recessive disorder caused by mutations in the galactose-1-phosphate uridyltransferase (GALT) gene. Our group developed a disease-specific database containing all of the reported sequence variants in GALT (Available at: http://arup.utah.edu/database/galactosemia/GALT_welcome.php; Last accessed: 13 April 2007). Currently the database contains a total of 229 sequence variants, of which 196 are mutations (including nine novel mutations identified in our laboratory), 31 polymorphisms in both introns and exons, and two variants of unknown or uncertain significance. All sequence variants have been verified for their position within the GALT gene and named following standard nomenclature. Sequence variants are reported with accompanying information on protein effect, classification of mutation vs. polymorphism, mutation type (when applicable) based on how each was first described in the literature, and accompanying link to pertinent publication. Unpublished variants are described with relevant clinical information that supports their classification as causative of the disease vs. polymorphisms. Other features of this database include disease information, relevant links for galactosemia and literature, reference sequences, ability to query by various criteria, and submit of novel variations to the database. This free online scientific resource was developed with the clinical laboratory in mind to serve as a reference and repository for novel findings that are periodically collected, verified, and updated into the database.

Evidence for function of UDP galactose pyrophosphorylase in mice with absent galactose-1-phosphate uridyltransferase

Mice with deletion of the galactose-1-phosphate uridyltransferase (GALT) gene were examined for their ability to form (13)C labeled hepatic UDP glucose from administered 1-(13)C galactose. NMR analysis of urinary acetaminophen glucuronide, which is derived from hepatic UDP glucose showed (13)C enrichment after concomitant administration of (13)C galactose and acetaminophen. The finding is consistent with the function of UDP galactose pyrophosphorylase as an alternate pathway of galactose metabolism.

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.

Characterization of an unusual deletion of the galactose-1-phosphate uridyl transferase (GALT) gene

PURPOSE

We previously reported a deletion of the Galactose-1-Phosphate Uridyl Transferase (GALT) gene. This deletion can cause apparent homozygosity for variants located on the opposite allele, potentially resulting in a discrepancy between the biochemical phenotype and the apparent genotype in an individual. The purpose of this study was to determine the deletion breakpoints, allowing the development of a rapid and reliable molecular test for the mutation.

METHODS

A Polymerase Chain Reaction walking strategy was used to map the 5' and 3' breakpoints. The junction fragment was amplified and sequenced to precisely characterize the deletion breakpoints.

RESULTS

The deletion has a bipartite structure involving two large segments of the GALT gene, while retaining a short internal segment of the gene. Molecular characterization allowed the development of a deletion specific Polymerase Chain Reaction-based assay. In 25 individuals who had a biochemical carrier galactosemia phenotype, but tested negative for 8 common GALT gene variants, 3 carried this deletion.

CONCLUSION

This deletion occurs at an appreciable frequency and should be considered when there is a discrepancy between the genotype and biochemical phenotype. Many of the individuals carrying the allele were of Ashkenazi Jewish ancestry suggesting that the deletion may be a common cause of galactosemia in that population.

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.

Prevention of a molecular misdiagnosis in galactosemia

PURPOSE

The polymerase chain reaction is generally used for mutational analysis of the galactose-1-phosphate uridyl transferase (GALT) gene in the diagnosis of galactosemia. This method is problematic when used in families of Ashkenazi Jewish descent.

METHODS

We amplified the GALT gene from leukocyte DNA followed by allele specific oligonucleotide hybridization, DNA sequencing and Southern Blot analysis to determine the mutant alleles causing galactosemia in a representative Jewish family.

RESULTS

The proband's diagnosis of galactosemia was confirmed by high levels of erythrocyte galactose-1-phosphate, absence of erythrocyte GALT activity and impaired total body oxidation of galactose to expired CO2. Initial molecular analysis of GALT alleles in the family showed homozygosity for a K285N missense mutation in the proband, homozygosity for N314D in the mother and heterozygosity for N314D and K285N in the father. These results contradicted Mendelian logic. Southern blot hybridization with GALT cDNA proved the presence of a complex 5 kb GALT deletion in the proband and her mother's DNA enabling a corrected genotype.

CONCLUSIONS

Since a deletion of the GALT gene is a common mutation causing galactosemia among Ashkenazim Jewish families, this deletion should be suspected and tested for by genomic hybridization or by using primers specific for the 5 kb deletion.

The effect of diet on total antioxidant status, erythrocyte membrane Na+,K+-ATPase and Mg2+-ATPase activities in patients with classical galactosaemia

OBJECTIVE

Classical galactosaemia is characterized by high levels of galactose-1-phosphate (Gal-1-P), galactose and galactitol. In vitro studies have shown modulation of the rat brain Na+,K+-ATPase and Mg2+-ATPase activities by Gal-1-P. The aim of this study was to evaluate the erythrocyte membrane Na+,K+-ATPase and Mg2+-ATPase activities in galactosaemic patients and to correlate them to Gal-1-P, total antioxidant status (TAS) and membrane protein content (PC).

PATIENTS AND METHODS

Nine patients (N=9) originally on "loose diet" (group B) were requested to follow their diet strictly (group A). Twelve healthy children were the controls (group C). The activities of the enzymes, TAS and Gal-1-P in blood were determined spectrophotometrically. In the in vitro study, erythrocyte membranes from controls were preincubated with Gal-1-P (300 microM), and then with l-cysteine (0.83 mM) or reduced glutathione (0.83 mM) whereas these from the patients with the antioxidants only.

RESULTS

Na+,K+-ATPase, Mg2+-ATPase, TAS and PC were significantly (P<0.001) reduced (0.31+/-0.03, 1.7+/-0.2 micromol Pi/hxmg protein, 0.89+/-0.02 mmol/l, 36.8+/-2.0 g/l, respectively) in group B as compared with those of group A (0.58+/-0.06, 2.5+/-0.2 micromol Pi/hxmg protein, 1.41+/-0.11 mmol/l, 51.5+/-3.1g/l, respectively) and controls (0.67+/-0.05, 3.2+/-0.2 micromol Pi/hxmg protein, 1.65+/-0.12 mmol/l, 64.0+/-3.5 g/l, respectively). Gal-1-P levels in group B was significantly higher than those in group A and controls. Positive correlation coefficients were found between the enzyme activities, PC and TAS whereas Gal-1-P inversely correlated to the enzyme activities. Incubation of the erythrocyte membranes from the patients with the antioxidants failed to restore the activities of inhibited enzymes, whereas the inhibition by Gal-1-P in controls was reversed.

CONCLUSIONS

High blood Gal-1-P concentrations resulted in low TAS and PC. The inhibition of Na+,K+-ATPase and Mg2+-ATPase may be due to the presence of free radicals and/or the elevated Gal-1-P.

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.

Suckling rat brain regional distribution of Na+,K+-atpase activity in the in vitro galactosaemia: the effect of L-cysteine and glutathione

Inhibition of Na+,K+-ATPase activity causes edema and cell death in central nervous system. We determined the in vitro effects of galactose-l-phosphate (Gal-1-P), galactitol (Galtol) and galactose (Gal) (mix A = classical galactosaemia) or Galtol and Gal (mix B = galactokinase deficiency galactosaemia), on Na+,K+-ATPase activity in suckling rat brain frontal cortex, hippocampus or hypothalamus homogenates. Gal-1-P or Galtol alone at different concentrations, significantly inhibited Na+,K+-ATPase whereas Gal activated the enzyme in all investigated brain regions. Both mix A and mix B inactivated the enzyme by 20-30% (p < 0.001) in all studied areas. L-Cysteine (Cys) and glutathione (GSH) supplementation in mix B not only reversed the enzyme inhibition but also resulted in an activation of 50-60%, (p < 0.001) in all brain areas. Their presence in mix A also activated the inhibited Na+,K+-ATPase in hippocampus and hypothalamus to a lower degree, whereas Cys reversed the frontal cortex enzyme activity to control value only. These findings indicate that oxidation of the enzyme critical groups may be involved in galactosaemia, producing inhibitory effect. This phenomenon is reversed by antioxidants Cys and GSH, implying that free radicals may be implicated in the observed enzyme inactivation.

Mg2+-ATPase activity in suckling rat brain regions in galactosaemia in vitro. l-Cysteine and glutathione effects

Mg2+-ATPase activity is implicated with Mg2+ homeostasis, maintaining high brain intracellular Mg2+ content. We determined the in vitro effects of galactose-1-phosphate (Gal-1-P), galactitol (Galtol) and galactose (Gal) {mix A=Gal-1-P(2 mM)+Galtol(2 mM)+Gal(4 mM) concentrations commonly found in patients with classical galactosaemia} or Galtol and Gal {mix B=Galtol(2 mM)+Gal(1 mM) concentrations usually measured in patients with galactokinase deficiency galactosaemia} on Mg2+-ATPase activity in suckling rat brain frontal cortex, hippocampus or hypothalamus homogenates. Gal-1-P significantly (p<0.001) enhanced enzyme activity in all the brain areas measured, whereas Galtol and Gal failed to cause any effect in the same regions. Mix A remarkably (p<0.001) stimulated Mg2+-ATPase in the studied areas. On the contrary, mix B had no effect. The supplementation of antioxidant l-cysteine (Cys) or reduced Glutathione (GSH) in mix A failed to reverse to normal the activated enzyme in frontal cortex and hypothalamus, while they significantly reduced Mg2+-ATPase activation in hippocampus.

CONCLUSIONS

(a) Gal-1-P enormously activated Mg2+-ATPase in all the studied brain regions, (b) Mix A, also, excessively activated the enzyme in the same areas, (c) the production of free radicals may be implicated with the enzyme activation and (d) Cys or GSH significantly decreased the activated hippocampal Mg2+-ATPase.

Short-term exogenous galactose supplementation does not influence rate of appearance of galactose in patients with classical galactosemia

INTRODUCTION

Recently, evidence has been presented that adult patients with classical galactosemia have higher than expected galactose tolerance. This may be caused by a decrease of endogenous galactose production with ageing. Alternatively, suppression of endogenous galactose production by exogenous galactose might be implicated. The aim of this study was to determine if the rate of appearance of galactose is suppressed by exogenous galactose.

MATERIALS AND METHODS

Two adult patients with classical galactosemia and three healthy control subjects were given a primed continuous infusion of D-[1-13C]galactose to determine the rate of appearance of galactose (GAR, expressed as micromol/kg/h) before and during additional galactose supplementation. After initial assessment of GAR (GAR1), GAR was determined during doubled (GAR2) or quadrupled (GAR4) galactose infusion.

RESULTS

GAR1 was 2.48 and 2.44 in patients 1 and 2, and 0.46, 0.34, and 0.39 in control subjects 1, 2, and 3, respectively. GAR(2) was 2.43 and 2.13 in patients 1 and 2, and 0.57, 0.38, and 0.47 in control subjects 1, 2, and 3, respectively. In patient 1 the experiment was repeated during quadrupled galactose infusion. Here GAR1 was 3.01 and GAR4 was 3.26.

CONCLUSIONS

No significant differences between GAR before and during additional galactose infusion were found in patients and in control subjects. GAR1 was significantly higher in patients than in control subjects. We conclude that the rate of appearance of galactose is not influenced by exogenous galactose, at least under short-term conditions, in patients with classical galactosemia and in control subjects.

Mediators of galactose sensitivity in UDP-galactose 4'-epimerase-impaired mammalian cells

UDP-galactose 4'-epimerase (GALE) catalyzes the final step in the Leloir pathway of galactose metabolism, interconverting UDP-galactose and UDP-glucose. Unlike its Escherichia coli counterpart, mammalian GALE also interconverts UDP-N-acetylgalactosamine and UDP-N-acetylglucosamine. Considering the key roles played by all four of these UDP-sugars in glycosylation, human GALE therefore not only contributes to the Leloir pathway, but also functions as a gatekeeper overseeing the ratios of important substrate pools required for the synthesis of glycosylated macromolecules. Defects in human GALE result in the disorder epimerase-deficiency galactosemia. To explore the relationship among GALE activity, substrate specificity, metabolic balance, and galactose sensitivity in mammalian cells, we employed a previously described GALE-null line of Chinese hamster ovary cells, ldlD. Using a transfection protocol, we generated ldlD derivative cell lines that expressed different levels of wild-type human GALE or E. coli GALE and compared the phenotypes and metabolic profiles of these lines cultured in the presence versus absence of galactose. We found that GALE-null cells accumulated abnormally high levels of Gal-1-P and UDP-Gal and abnormally low levels of UDP-Glc and UDP-GlcNAc in the presence of galactose and that human GALE expression corrected each of these defects. Comparing the human GALE- and E. coli GALE-expressing cells, we found that although GALE activity toward both substrates was required to restore metabolic balance, UDP-GalNAc activity was not required for cell proliferation in the presence of otherwise cytostatic concentrations of galactose. Finally, we found that uridine supplementation, which essentially corrected UDP-Glc and, to a lesser extent UDP-GlcNAc depletion, enabled ldlD cells to proliferate in the presence of galactose despite the continued accumulation of Gal-1-P and UDP-Gal. These data offer important insights into the mechanism of galactose sensitivity in epimerase-impaired cells and suggest a potential novel therapy for patients with epimerase-deficiency galactosemia.

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.

Extended [13C]galactose oxidation studies in patients with galactosemia

Since patients with galactose-1-phosphate uridyltransferase (GALT) deficiency have considerable endogenous galactose formation and only limited urinary excretion of galactose metabolites, there must be mechanisms for disposal of the sugar. Otherwise, a steady-state could not be maintained and there would be continuous body accumulation of galactose and alternate pathway products. Previous studies quantitating the amount of galactose handled by oxidation to CO2 focused on short collection periods of expired air after administering isotopically labeled galactose mainly designed for discerning differences in the capacity to oxidize the sugar in relation to genotype. Assuming that there may be more extensive oxidation than that observed in short-term studies in order to dispose the daily galactose burden, we have examined the amount of [1-13C]galactose oxidized to 13CO2 over a 24-h period after either a single bolus or continuous IV administration by 11 patients with classic galactosemia including patients homozygous for the Q188R gene mutation. As much as 58% of the administered galactose was oxidized to 13CO2 in 24 h. The pathways involved remain to be determined but a significant amount may be metabolized by non-GALT pathways since a patient homozygous for gene deletion had an oxidative capability. We conclude that classic patients have the ability to slowly oxidize galactose to CO2 in 24 h in amounts comparable to that which a normal handles in approximately one-fifth the time. This capacity enables the galactosemic to maintain a balance of galactose disposal with the galactose burden imposed by endogenous formation and dietary intake.

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.

Renal excretion of galactose and galactitol in patients with classical galactosaemia, obligate heterozygous parents and healthy subjects

The age dependence of galactose and galactitol excretion was assessed in overnight-fasted galactose-1-phosphate uridyltransferase-deficient patients under dietary treatment (ages 4-34 years; n = 51), obligate heterozygous parents (ages 25-71 years; n = 49) and healthy subjects (ages 3-58 years; n = 215). Urine concentrations were analysed by stable-isotope dilution gas chromatography mass spectrometry. There was considerable interindividual variability. The intraindividual variation, however, was not age-dependent and was rather low. Excretion estimates were calculated from the creatinine-related concentrations using weight-, age- and sex-related creatinine excretion rates. Experimental evidence is presented underscoring the problems inherent in random sampling and substantiating the primary endogenous origin of galactose and galactitol in postabsorptive urine samples. Age-dependent excretion estimates were best fitted to a simple growth-related model assuming an exponential decrease with age until adulthood. According to the model, mean postabsorptive galactose and galactitol excretion in healthy subjects was similar and decreased exponentially from about 1.2 micromol/kg body weight per day in infants to about 0.2 micromol/kg body weight per day in adults. Excretion in heterozygotes was normal. In galactosaemic patients, galactose excretion was in the normal range. Galactitol excretion, however, was enhanced over 50-fold and decreased from a mean estimate of about 64 micromol/kg body weight per day in infants to about 23 micromol/kg body weight per day in adults. The results are discussed with respect to the significance of galactose and galactitol excretion for whole-body galactose removal and with respect to the applicability of urinary galactitol analysis for metabolic monitoring in galactosaemia.

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.