Hexokinase: gene structure and mutations

Biallelic hexokinase 1 (HK1) variants causative of non-spherocytic haemolytic anaemia: A case series with emphasis on the HK1 promoter variant and literature review

The hexokinase (HK) enzyme plays a key role in red blood cell energy production. Hereditary non-spherocytic haemolytic anaemia (HNSHA) caused by HK deficiency is a rare disorder with only 12 different disease-associated variants identified. Here, we describe the clinical features and genotypes of four previously unreported patients with hexokinase 1 (HK1)-related HNSHA, yielding two novel truncating HK1 variants. The patients' phenotypes varied from mild chronic haemolytic anaemia to severe infantile-onset transfusion-dependent anaemia. Three of the patients had mild haemolytic disease caused by the common HK1 promoter c.-193A>G variant combined with an intragenic HK1 variant, emphasizing the importance of including this promoter variant in the haemolytic disease gene panels. HK activity was normal in a severely affected patient with a homozygous HK1 c.2599C>T, p.(His867Tyr) variant, but the affinity for ATP was reduced, hampering the HK function. In cases of HNSHA, kinetic studies should be considered in the functional studies of HK. We reviewed the literature of previously published patients to provide better insight into this rare disease and add to the understanding of genotype-phenotype correlation.

Hexokinase domain-containing protein-1 in metabolic diseases and beyond

Glucose phosphorylation by hexokinases (HKs) traps glucose in cells and facilitates its usage in metabolic processes dependent on cellular needs. HK domain-containing protein-1 (HKDC1) is a recently discovered protein with wide expression containing HK activity, first noted through a genome-wide association study (GWAS) to be linked with gestational glucose homeostasis during pregnancy. Since then, HKDC1 has been observed to be expressed in many human tissues. Moreover, studies have shown that HKDC1 plays a role in glucose homeostasis by which it may affect the progression of many pathophysiological conditions such as gestational diabetes mellitus (GDM), nonalcoholic steatohepatitis (NASH), and cancer. Here, we review the key studies contributing to our current understanding of the roles of HKDC1 in human pathophysiological conditions and potential therapeutic interventions.

A dominant mutation in hexokinase 1 (HK1) causes retinitis pigmentosa

PURPOSE

To identify the cause of retinitis pigmentosa (RP) in UTAD003, a large, six-generation Louisiana family with autosomal dominant retinitis pigmentosa (adRP).

METHODS

A series of strategies, including candidate gene screening, linkage exclusion, genome-wide linkage mapping, and whole-exome next-generation sequencing, was used to identify a mutation in a novel disease gene on chromosome 10q22.1. Probands from an additional 404 retinal degeneration families were subsequently screened for mutations in this gene.

RESULTS

Exome sequencing in UTAD003 led to identification of a single, novel coding variant (c.2539G>A, p.Glu847Lys) in hexokinase 1 (HK1) present in all affected individuals and absent from normal controls. One affected family member carries two copies of the mutation and has an unusually severe form of disease, consistent with homozygosity for this mutation. Screening of additional adRP probands identified four other families (American, Canadian, and Sicilian) with the same mutation and a similar range of phenotypes. The families share a rare 450-kilobase haplotype containing the mutation, suggesting a founder mutation among otherwise unrelated families.

CONCLUSIONS

We identified an HK1 mutation in five adRP families. Hexokinase 1 catalyzes phosphorylation of glucose to glucose-6-phosphate. HK1 is expressed in retina, with two abundant isoforms expressed at similar levels. The Glu847Lys mutation is located at a highly conserved position in the protein, outside the catalytic domains. We hypothesize that the effect of this mutation is limited to the retina, as no systemic abnormalities in glycolysis were detected. Prevalence of the HK1 mutation in our cohort of RP families is 1%.

Comparison and correlation of binding mode of ATP in the kinase domains of Hexokinase family

Hexokinases (HKs) are the enzymes that catalyses the ATP dependent phosphorylation of Hexose sugars to Hexose-6-Phosphate (Hex-6-P). There exist four different forms of HKs namely HK-I, HK-II, HK-III and HK-IV and all of them share a common ATP binding site core surrounded by more variable sequence that determine substrate affinities. Although they share a common binding site but they differ in their kinetic functions, hence the present study is aimed to analyze the binding mode of ATP. The analysis revealed that the four ATP binding domains are showing 13 identical, 7 similar and 6 dissimilar residues with similar structural conformation. Molecular docking of ATP into the kinase domains using Molecular Operating Environment (MOE) soft ware tool clearly showed the variation in the binding mode of ATP with variable docking scores. This probably explains the variable phosphorylation rates among hexokinases family.

First mutation in the red blood cell-specific promoter of hexokinase combined with a novel missense mutation causes hexokinase deficiency and mild chronic hemolysis

BACKGROUND

Hexokinase is one of the key enzymes of glycolysis and catalyzes the phosphorylation of glucose to glucose-6-phosphate. Red blood cell-specific hexokinase is transcribed from HK1 by use of an erythroid-specific promoter. The aim of this study was to investigate the molecular basis for hexokinase deficiency in a patient with chronic hemolysis.

DESIGN AND METHODS

Functional studies were performed using transient transfection of HK promoter constructs in human K562 erythroleukemia cells. The DNA-protein interaction at the promoter of hexokinase was studied using electrophoretic mobility shift assays with nuclear extracts from K562 cells. DNA analysis and reverse transcriptase polymerase chain reaction were performed according to standardized procedures.

RESULTS

On the paternal allele we identified two novel mutations in cis in the erythroid-specific promoter of HKI: -373A>C and -193A>G. Transfection of promoter reporter constructs showed that the -193A>G mutation reduced promoter activity to 8%. Hence, -193A>G is the first mutation reported to affect red blood cell-specific hexokinase specific transcription. By electrophoretic mobility shift assays we showed that in vitro binding of c-jun to an AP-1 binding site was disrupted by this mutation. Subsequent chromatin-immunoprecipitation assays demonstrated that c-jun binds this region of the promoter in vivo. On the maternal allele we identified a novel missense mutation in exon 3: c.278G>A, encoding an arginine to glutamine substitution at residue 93, affecting both hexokinase-1 and red cell specific-hexokinase. In addition, this missense mutation was shown to compromise normal pre-mRNA processing.

CONCLUSIONS

We postulate that reduced erythroid transcription of HK1 together with aberrant splicing of both hexokinase-1 and red cell specific-hexokinase results in hexokinase deficiency and mild chronic hemolysis.

A mutation in an alternative untranslated exon of hexokinase 1 associated with hereditary motor and sensory neuropathy -- Russe (HMSNR)

Hereditary Motor and Sensory Neuropathy -- Russe (HMSNR) is a severe autosomal recessive disorder, identified in the Gypsy population. Our previous studies mapped the gene to 10q22-q23 and refined the gene region to approximately 70 kb. Here we report the comprehensive sequencing analysis and fine mapping of this region, reducing it to approximately 26 kb of fully characterised sequence spanning the upstream exons of Hexokinase 1 (HK1). We identified two sequence variants in complete linkage disequilibrium, a G>C in a novel alternative untranslated exon (AltT2) and a G>A in the adjacent intron, segregating with the disease in affected families and present in the heterozygote state in only 5/790 population controls. Sequence conservation of the AltT2 exon in 16 species with invariable preservation of the G allele at the mutated site, strongly favour the exonic change as the pathogenic mutation. Analysis of the Hk1 upstream region in mouse mRNA from testis and neural tissues showed an abundance of AltT2-containing transcripts generated by extensive, developmentally regulated alternative splicing. Expression is very low compared with ubiquitous Hk1 and all transcripts skip exon1, which encodes the protein domain responsible for binding to the outer mitochondrial membrane, and regulation of energy production and apoptosis. Hexokinase activity measurement and immunohistochemistry of the peripheral nerve showed no difference between patients and controls. The mutational mechanism and functional effects remain unknown and could involve disrupted translational regulation leading to increased anti-apoptotic activity (suggested by the profuse regenerative activity in affected nerves), or impairment of an unknown HK1 function in the peripheral nervous system (PNS).

Changes in muscle mRNAs for hexokinase, phosphofructokinase-1 and glycogen synthase in acute and persistent hypoglycemia induced by tolbutamide in chickens

To elucidate the specificity of glucose metabolism in chicken skeletal muscle, changes in mRNA levels of hexokinase I (HKI), hexokinase II (HKII), phosphofructokinase-1 (PFK-1) and glycogen synthase (GS) were characterized in acute and persistent hypoglycemia induced by tolbutamide administration. In acute hypoglycemia, induced by a single dose of tolbutamide (100 mg/kg body mass), HKII, PFK-1 and GS mRNA levels remained unchanged; however, levels of HKI mRNA and glucose transporter 1 (GLUT1) were significantly increased 4 h after administration. In persistent hypoglycemia, induced by sequential administration of tolbutamide (100 mg/kg body mass) 3 times a day for 5 days, GS mRNA was significantly increased at day 5, while HKI, HKII and PFK-1 mRNA levels remained unchanged. These results suggest that HKI is responsible for glucose transport into skeletal muscle in acute hypoglycemia and that glucose preferentially enters the glycogenic pathway before the glycolytic pathway in persistently hypoglycemic chickens.

The energy-less red blood cell is lost: erythrocyte enzyme abnormalities of glycolysis

The red blood cell depends solely on the anaerobic conversion of glucose by the Embden-Meyerhof pathway for the generation and storage of high-energy phosphates, which is necessary for the maintenance of a number of vital functions. Many red blood cell enzymopathies have been described that disturb the erythrocyte's integrity, shorten its cellular survival, and result in hemolytic anemia. By far the majority of these enzymopathies are hereditary in nature. In this review, we summarize the current knowledge regarding the genetic, biochemical, and structural features of clinically relevant red blood cell enzymopathies involved in the Embden-Meyerhof pathway and the Rapoport-Luebering shunt.

Gene expression and biological significance of hexokinase in erythroid cells

Red blood cells (RBCs) express two hexokinase (HK) isoforms, HK-I and HK-R. Both isozymes are generated from the HK-I gene by use of an alternate promoter. Gene structure and exon-intron organization of the HK-I gene have been elucidated from a sequence of three contiguous genomic clones localized at human chromosome 10. The sequence spans about 131 kb, and consists of 25 exons, which include 6 testis- and 1 erythroid-specific exons. HK-R has been shown as an erythroid-specific isozyme whose expression is turned on in the early erythroid-progenitors and is significantly induced during their differentiation. HK-R unfolds major HK activity in immature RBCs and is rapidly degraded during the maturation process. HK-I has a porin-binding domain in its N-terminus. Recent studies have shown that HK isozymes with a porin-binding domain play a role in mitochondrial integrity, suggesting that HK-I-deficient erythroid cells might be eliminated by apoptosis. It is most likely that RBCs are most labile as a result of HK-I/R deficiency since the HK-I gene but not the other isozyme genes are expressed in fetal and adult RBCs.

HK Utrecht: missense mutation in the active site of human hexokinase associated with hexokinase deficiency and severe nonspherocytic hemolytic anemia

Hexokinase deficiency is a rare autosomal recessive disease with a clinical phenotype of severe hemolysis. We report a novel homozygous missense mutation in exon 15 (c.2039C>G, HK [hexokinase] Utrecht) of HK1, the gene that encodes red blood cell-specific hexokinase-R, in a patient previously diagnosed with hexokinase deficiency. The Thr680Ser substitution predicted by this mutation affects a highly conserved residue in the enzyme's active site that interacts with phosphate moieties of adenosine diphosphate, adenosine triphosphate (ATP), and inhibitor glucose-6-phosphate. We correlated the molecular data to the severe clinical phenotype of the patient by means of altered enzymatic properties of partially purified hexokinase from the patient, notably with respect to Mg(2+)-ATP binding. These kinetic properties contradict those obtained from a recombinant mutant brain hexokinase-I with the same Thr680Ser substitution. This contradiction thereby stresses the valuable contribution of studying patients with hexokinase deficiency to achieve a better understanding of hexokinase's key role in glycolysis.