Fine structure of the human galactokinase GALK1 gene

The Central Role of Glucokinase in Glucose Homeostasis: A Perspective 50 Years After Demonstrating the Presence of the Enzyme in Islets of Langerhans

It is hypothesized that glucokinase (GCK) is the glucose sensor not only for regulation of insulin release by pancreatic β-cells, but also for the rest of the cells that contribute to glucose homeostasis in mammals. This includes other cells in endocrine pancreas (α- and δ-cells), adrenal gland, glucose sensitive neurons, entero-endocrine cells, and cells in the anterior pituitary. Glucose transport is by facilitated diffusion and is not rate limiting. Once inside, glucose is phosphorylated to glucose-6-phosphate by GCK in a reaction that is dependent on glucose throughout the physiological range of concentrations, is irreversible, and not product inhibited. High glycerol phosphate shuttle, pyruvate dehydrogenase, and pyruvate carboxylase activities, combined with low pentose-P shunt, lactate dehydrogenase, plasma membrane monocarboxylate transport, and glycogen synthase activities constrain glucose-6-phosphate to being metabolized through glycolysis. Under these conditions, glycolysis produces mostly pyruvate and little lactate. Pyruvate either enters the citric acid cycle through pyruvate dehydrogenase or is carboxylated by pyruvate carboxylase. Reducing equivalents from glycolysis enter oxidative phosphorylation through both the glycerol phosphate shuttle and citric acid cycle. Raising glucose concentration increases intramitochondrial [NADH]/[NAD⁺] and thereby the energy state ([ATP]/[ADP][Pi]), decreasing [Mg²⁺ADP] and [AMP]. [Mg²⁺ADP] acts through control of K_ATP channel conductance, whereas [AMP] acts through regulation of AMP-dependent protein kinase. Specific roles of different cell types are determined by the diverse molecular mechanisms used to couple energy state to cell specific responses. Having a common glucose sensor couples complementary regulatory mechanisms into a tightly regulated and stable glucose homeostatic network.

Laboratory diagnosis of galactosemia: a technical standard and guideline of the American College of Medical Genetics and Genomics (ACMG)

Disclaimer: These ACMG Standards and Guidelines are developed primarily as an educational resource for clinical laboratory geneticists to help them provide quality clinical laboratory genetic services. Adherence to these Standards and Guidelines is voluntary and does not necessarily assure a successful medical outcome. These Standards and Guidelines should not be considered inclusive of all proper procedures and tests or exclusive of other procedures and tests that are reasonably directed to obtaining the same results. In determining the propriety of any specific procedure or test, the clinical laboratory geneticist should apply his or her own professional judgment to the specific circumstances presented by the individual patient or specimen. Clinical laboratory geneticists are encouraged to document in the patient's record the rationale for the use of a particular procedure or test, whether or not it is in conformance with these Standards and Guidelines. They also are advised to take notice of the date any particular guideline was adopted, and to consider other relevant medical and scientific information that becomes available after that date. It also would be prudent to consider whether intellectual property interests may restrict the performance of certain tests and other procedures.Galactosemias are inherited disorders of galactose metabolism due to deficiency in one of the three enzymes involved in the Leloir pathway: galactose-1-phosphate uridyltransferase, galactokinase, and uridine diphosphate (UDP)-galactose-4'-epimerase. Galactose-1-phosphate uridyltransferase deficiency, or classic galactosemia, is the most frequent and the most severe of the three enzyme deficiencies; it is characterized by failure to thrive, liver failure, susceptibility to sepsis, and death, if untreated. Newborn screening for classic galactosemia has been implemented in all of the United States, while screening for galactokinase deficiency and UDP-galactose-4'-epimerase deficiency is not universal. Early identification and treatment of galactosemia leads to improved outcome. This document reviews the laboratory methods and best practices for the diagnosis of galactosemia.

Clinical evaluation and mutational analysis of GALK and GALE genes in patients with galactosemia in Greece: one novel mutation and two rare cases

BACKGROUND

Deficiencies of galactokinase (GALK) and UDP-epimerase (GALE) are implicated with galactose metabolic disorders. The aim of the study was the identification of mutations in GALK and GALE genes and clinical evaluation of patients.

METHODS

Five patients with GALK and five with GALE deficiency were picked up via the Neonatal Screening Program. Additionally, two females, 4 years old, were referred with late diagnosed galactosemia, as rare cases. Mutational analysis was conducted via Sanger sequencing, while in silico analysis tools were utilized for the novel mutation. Psychomotor and speech development tests were performed, as well.

RESULTS

The mutation p.Pro28Thr was identified in both alleles in GALK-deficient patients of Roma (gypsy) origin, whereas the novel p.Asn39Ser was detected in two non-Roma patients. In GALE-deficient patients benign and/or likely benign mutations were found. Psychomotor and speech delay were determined in the Roma GALK patients. In each of the late diagnosed females, four mutations were identified in all galactosemia-related genes.

CONCLUSIONS

The mutational spectrums of GALE- and GALK-deficient patients in Greece are presented for the first time along with a clinical evaluation. Mutational analysis in all galactosemia-related genes of symptomatic patients is highly recommended for future cases.

Epigenetic and SP1-mediated regulation is involved in the repression of galactokinase 1 gene in the liver of neonatal piglets born to betaine-supplemented sows

PURPOSE

In this study, we sought to investigate the effects of maternal betaine supplementation on the expression and regulation of GALK1 gene in the liver of neonatal piglets.

METHODS

Sixteen sows of two groups were fed control or betaine-supplemented diets (3 g/kg), respectively, throughout the pregnancy. Newborn piglets were individually weighed immediately after birth, and one male piglet close to mean body weight from the same litter was selected and killed before suckling. Serum samples of newborn piglets were analyzed for biochemical indexes, hormone and amino acid levels. Liver samples were analyzed for GALK1 expression by real-time PCR and western blotting, while GALK1 regulational mechanism was analyzed by methylated DNA immunoprecipitation, chromatin immunoprecipitation and microRNAs expression.

RESULTS

Betaine-exposed neonatal piglets had lower serum concentration of galactose, which was associated with significantly down-regulated hepatic GALK1 expression. The repression of GALK1 mRNA expression was associated with DNA hypermethylation and more enriched repression histone mark H3K27me3 on its promoter. Binding sites of SP1, GR and STAT3 were predicted on GALK1 promoter, and decreased SP1 protein content and lower SP1 binding to GALK1 promoter were detected in the liver of betaine-exposed piglets. Furthermore, the expression of miRNA-149 targeting GALK1 was up-regulated in the liver of betaine-exposed piglets, along with elevated miRNAs-processing enzymes Dicer and Ago2.

CONCLUSIONS

Our results suggest that maternal dietary betaine supplementation during gestation suppresses GALK1 expression in the liver of neonatal piglets, which involves complex gene regulation mechanisms including DNA methylation, histone modification, miRNAs expression and SP1-mediated transcriptional modulation.

Galactokinase deficiency induced cataracts in Indian infants: identification of 4 novel mutations in GALK gene

PURPOSE

To establish the incidence and molecular basis of type II galactosemia in Indian infants presenting with congenital cataracts.

METHODS

200 infants with congenital cataracts were screed for galactokinase (GALK) enzyme deficiency. GALK enzyme activity was measured using radioactive galactose-1-(14)C and mutations were studied using polymerase chain reaction (PCR), single strand conformational polymorphism (SSCP) and subsequent DNA sequencing.

RESULTS

16 (8%) out of 200 infants with congenital cataracts were found to be GALK deficient with male: female:: 9:7. A significantly reduced GALK activity of 0.13 ± 0.04 µmoles/h/mL was observed in the galactosemia patients compared to 0.232 ± 0.07 µmoles/h/mL in the normal controls. A total of 5 distinct mutations were identified in GALK gene in five different patients out of which four were novel mutations viz. S79F, S79Y, S205S and F275Y. The functional significance of the mutations and splicing defects associated with them were investigated using different computer based applications.

CONCLUSION

The study highlighted the importance of GALK gene analysis in diagnosis of galactosemia in Indian population. It also revealed that the mutational profile of Indian GALK patients differs significantly from other populations studied. Low mutation detection rate observed in this study may be due to the fact that SSCP is a quite indirect and non-fullproof method of mutation detection.

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.

A novel c.-22T>C mutation in GALK1 promoter is associated with elevated galactokinase phenotype

Background

Many genetic variations of GALK1 have been identified in the patients with galactokinase (GALK1) deficiency. However, the molecular characteristics of GALK1 in individuals with elevated GALK1 activity are relatively unknown.

Methods

We investigated the relationship between elevated GALK1 activity and the molecular GALK1 gene variations, and the molecular mechanism underlying elevated GALK1 activity. PCR products from 63 subjects, without any attenuation of galactose degradation enzymes, were sequenced to screen for nucleotide alterations in the GALK1 promoter.

Results

Three nucleotide substitutions were identified: c.-179A>G, c.-27A>C, and c.-22T>C. With respect to the c.-22T>C mutation, GALK1 activity in 13 subjects with the T/C or C/C genotype was significantly higher than those in 50 subjects with the T/T genotype (p < 0.001). The dual luciferase reporter assay in Hep3B cells showed that the luciferase activity with the GALK1 promoter with the c.-22C mutant allele increased approximately 2.5-fold, compared to that with the c.-22T. A specific DNA-protein complex was observed in an electrophoretic mobility shift assay, with slightly higher affinity to c.-22C than to c.-22T.

Conclusion

The c.-22T>C mutation, which was observed frequently in individuals with elevated GALK1 activity, increased the expression of a reporter gene through enhanced binding of a currently unidentified nuclear protein. These results suggest that the elevated GALK1 activity resulted from enhanced gene expression, due to nucleotide variation within GALK1 promoter.

Molecular and biochemical characterization of the GALK1 gene in Korean patients with galactokinase deficiency

Galactokinase (GALK) deficiency is an autosomal recessive disorder characterized by elevation of blood galactose concentration and diminished galactose-1-phosphate, leading to the production of galactitol. To investigate the molecular defects of GALK1 gene and the biochemical characteristics of their mutant proteins, PCR-direct sequencing and in vitro expression analysis in Cos7 cells were performed in five Korean patients with GALK deficiency galactosemia. Four missense mutations (p.G137R, p.R256W, p.R277Q, and p.V281M) and one small insertion (c.850_851insG) were identified. Among four patients with severely reduced GALK activity, two were found to be homozygotes for p.R256W and the other two were compound heterozygotes for different molecular defects (p.G137R/p.R277Q and p.V281M/c.850_851insG). One Patient with moderately decreased GALK activity was heterozygous for p.R256W. Expression analysis in Cos7 cells confirmed that each of the mutations resulted in reduction of GALK activity and caused GALK deficiency.

Developmental regulation of galactokinase in suckling mouse liver by the Egr-1 transcription factor

The numerous changes in metabolic pathways that accompany liver development entail associated changes in gene expression. Egr-1 is a zinc-finger transcription factor that regulates genes involved in cellular growth, differentiation, stress response, and apoptosis in many cell types. Egr-1 is induced in liver regeneration in rodents, but its role in normal hepatocyte function has not been characterized. We examined the developmental expression of Egr-1 in mouse liver and found that its expression increased during the suckling period. In screening the sequences of the genes involved in lactose assimilation, we found that the galactokinase gene Glk contains four potential Egr-1 binding sites in its proximal promoter. A minimal promoter of 155 nucleotides encompassing the four Egr-1 sites exhibited activity in hepatoma cell lines by transient transfection assays. Moreover, co-transfection of an Egr-1 expression plasmid increased promoter activity. Finally, mutations introduced into three of the four Egr-1 binding sites decreased activity, whereas mutation of the remaining site increased promoter activity. These data tie Egr-1 and galactokinase together in a developmentally regulated chain to prepare the neonate for suckling.

An unexpectedly high frequency of hypergalactosemia in an immigrant Bosnian population revealed by newborn screening

In galactokinase (GALK) deficiency, galactose cannot be phosphorylated into galactose-1-phosphate, which leads to cataract formation. Neonatal screening for hypergalactosemia in Berlin has been performed by thin-layer chromatography since 1978, which detects classical galactosemia and GALK deficiency. Until 1991, GALK deficiency has not been identified in a total of approximately 260,000 samples. In contrast, from 1992 to 1999, nine patients were detected in a total of approximately 240,000 screened newborns. One Turkish patient was homozygous for two novel S142I/G148C GALK mutations in close proximity to the putative ATP-binding site of the enzyme. The other eight children were born to five families belonging to the Bosnian refugee population consisting of approximately 30,000 individuals who have arrived in Berlin since 1991. In two of these families, GALK deficiency was subsequently diagnosed in siblings who had cataract surgery at 4 and 5 y of age, respectively. In all these 10 Bosnian patients, a homozygous P28T mutation located near the active center of the enzyme was identified. We propose that neonatal screening of populations with a significant proportion of Bosnians and possibly other southeastern Europeans, e.g. Romani, should be particularly directed toward GALK deficiency, an inborn error of metabolism that is readily amenable to effective treatment.

A genetic factor for age-related cataract: identification and characterization of a novel galactokinase variant, "Osaka," in Asians

Galactokinase (GALK) deficiency is an autosomal recessive disorder characterized by hypergalactosemia and cataract formation. Through mass screening of newborn infants, we identified a novel and prevalent GALK variant (designated here as the "Osaka" variant) associated with an A198V mutation in three infants with mild GALK deficiency. GALK activity and the amount of immunoreactive protein in the mutant were both 20% of normal construct in expression analysis. The K(m) values for galactose and ATP-Mg(2+) in erythrocytes with homozygous A198V were similar to those of the healthy adult control subjects. A population study for A198V revealed prevalences of 4.1% in Japanese and 2.8% in Koreans, lower incidence in Taiwanese and Chinese, no incidence in blacks and whites from the United States, and a significantly high frequency (7.8%; P < .023) in Japanese individuals with bilateral cataract. This variant probably originated in Japanese and Korean ancestors and is one of the genetic factors that causes cataract in elderly individuals.

Molecular basis of disorders of human galactose metabolism: past, present, and future

Molecular cloning and characterization of all three human galactose-metabolic genes have led to the identification of a number of mutations which result in three forms of galactosemia which are caused by kinase (GALK), transferase (GALT), or epimerase (GALE) deficiency. We review here recent developments in the molecular characterization of all three disorders of human galactose metabolism. Recent progress in the biochemical and/or structural analyses of the GALT and GALE proteins has complemented human mutational studies. Interestingly, genotype/phenotype correlations have been modest as in some other Mendelian disorders. We discuss possible reasons for this apparent paradox. Finally, we note the panethnic nature of galactosemia and suggest a hypothesis for it.

Novel mutations in 13 probands with galactokinase deficiency

Galactokinase is an essential enzyme in the metabolism of galactose. Patients with deficiencies in galactokinase exhibit early-onset cataracts. We examined the sequence of the human galactokinase gene (GK1) from 13 patients exhibiting galactokinase deficiency and identified 12 novel mutations. One of the mutations occurred in six of the 13 probands examined, and the remaining 11 were unique mutations. Expression of each of the mutant GK1 genes in Xenopus oocytes resulted in very low galactokinase activity levels. These results provide important information regarding the types of GK1 mutations that occur in the human population.

A founder mutation in the GK1 gene is responsible for galactokinase deficiency in Roma (Gypsies)

Galactokinase deficiency is an inborn error in the first step of galactose metabolism. Its major clinical manifestation is the development of cataracts in the first weeks of life. It has also been suggested that carriers of the deficiency are predisposed to presenile cataracts developing at age 20-50 years. Newborn screening data suggest that the gene frequency is very low worldwide but is higher among the Roma in Europe. Since the cloning of the galactokinase gene (GK1) in 1995, only two disease-causing mutations, both confined to single families, have been identified. Here we present the results of a study of six affected Romani families from Bulgaria, where index patients with galactokinase deficiency have been detected by the mass screening. Genetic linkage mapping placed the disease locus on 17q, and haplotype analysis revealed a small conserved region of homozygosity. Using radiation hybrid mapping, we have shown that GK1 is located in this region. The founder Romani mutation identified in this study is a single nucleotide substitution in GK1 resulting in the replacement of the conserved proline residue at amino acid position 28 with threonine (P28T). The P28T carrier rate in this endogamous population is approximately 5%, suggesting that the mutation may be an important cause of early childhood blindness in countries with a sizeable Roma minority.

Analysis of a 14-kb fragment containing a putative cell wall gene and a candidate for the ARA1, arabinose kinase, gene from chromosome IV of Arabidopsis thaliana

An Arabidopsis thaliana genomic DNA fragment of 14kb has been characterized in the framework of the E.S.S.A. programme. Computational and molecular approaches identified three novel gene sequences coding, respectively, for a protein of unknown function, a putative membrane-anchored cell wall protein and an arabinose kinase gene corresponding to the locus ARA1. The latter two genes named AtSEB1 and AtISA1 have been characterized in detail. They are very different in their organization, codon usage and level of expression. Homologues of AtSEB1 and AtISA1 have been identified. Sequence comparisons showed that the former genes contained a long 5' extension coding for an N-terminal domain probably specifying subcellular localization. Cloning and sequencing of the cognate cDNA for the AtISA1 homologue in A. thaliana, named GAL1, indicate that it encodes for a galactokinase-like protein. Our results highlight the integrative outcome of a systematic sequencing project in which links between biochemically and genetically characterized mutants, ESTs and genomic sequence data are generated.