Chronic non-spherocytic haemolytic disorders associated with glucose-6-phosphate dehydrogenase variants

Establishing the Approach to the Diagnosis of Hemolytic Anemia in the Genetic Era: A Case Series

BACKGROUND

Hemolytic anemia is characterized by the premature destruction of red blood cells, a condition that ranges from chronic to life-threatening. Hereditary hemolytic anemias (HHAs) encompass a broad spectrum of disorders including hemoglobinopathies, enzymopathies, and membrane disorders. In India, hemoglobinopathies, notably thalassemia and sickle cell disease, are significant health concerns contributing to high morbidity and mortality rates. Despite many cases being clinically insignificant, these disorders exert a considerable public health burden due to their prevalence. Techniques like next-generation sequencing (NGS) and high-performance liquid chromatography (HPLC) have emerged as powerful tools for identifying and diagnosing HHAs. NGS enables comprehensive genetic analysis, pinpointing mutations associated with hemoglobinopathies and other forms of hereditary anemia. HPLC allows precise quantification and characterization of hemoglobin variants, which is crucial for diagnosing hemoglobinopathies.

AIMS AND OBJECTIVES

This study aimed to establish a refined approach for diagnosing hemolytic anemias and categorize different types of hemolytic anemia using state-of-the-art technologies for early and precise treatment interventions.

MATERIALS AND METHODS

This retrospective study was conducted in the Hematology Section of the Department of Pathology at Atal Bihari Vajpayee Government Medical College, Vidisha, Madhya Pradesh. The study included six patients diagnosed with hemolytic anemia based on comprehensive hematological, biochemical, and molecular evaluations.  Results: The retrospective analysis of six cases of hemolytic anemia highlighted the diagnostic approach utilized. Clinical presentations, physical examinations, routine hematological investigations, advanced diagnostic modalities, and hemoglobin electrophoresis were instrumental in identifying specific types of hemolytic anemias.

CONCLUSION

Despite the availability of advanced diagnostic techniques, basic hematological investigations remain the cornerstone in the initial evaluation of HHAs. Hemoglobin electrophoresis plays a pivotal role in confirming diagnoses. In some cases, subtle hematological findings necessitate thorough evaluation, including familial studies, to guide appropriate management strategies.

A computational study of structural analysis of Class I human glucose-6-phosphate dehydrogenase (G6PD) variants: Elaborating the correlation to chronic non-spherocytic hemolytic anemia (CNSHA)

Glucose-6-phosphate dehydrogenase (G6PD) deficiency is the most common human enzyme defect that affects more than 500 million people worldwide. Individuals affected with G6PD deficiency may occasionally suffer mild-to-severe chronic hemolytic anemia. Chronic non-spherocytic hemolytic anemia (CNSHA) is a potential result of the Class I G6PD variants. This comparative computational study attempted to correct the defect in variants structure by docking the AG1 molecule to selected Class I G6PD variants [G6PD^Nashville (Arg393His), G6PD^Alhambra (Val394Leu), and G6PD^Durham (Lys238Arg)] at the dimer interface and structural NADP⁺ binding site. It was followed by an analysis of the enzyme conformations before and after binding to the AG1 molecule using the molecular dynamics simulation (MDS) approach, while the severity of CNSHA was determined via root-mean-square deviation (RMSD), root-mean-square fluctuation (RMSF), hydrogen bonds, salt bridges, radius of gyration (Rg), solvent accessible surface area analysis (SASA), and principal component analysis (PCA). The results revealed that G6PD^Nashville (Arg393His) and G6PD^Durham (Lys238Arg) had lost the direct contact with structural NADP⁺ and salt bridges at Glu419 - Arg427 and Glu206 - Lys407 were disrupted in all selected variants. Furthermore, the AG1 molecule re-stabilized the enzyme structure by restoring the missing interactions. Bioinformatics approaches were also used to conduct a detailed structural analysis of the G6PD enzyme at a molecular level to understand the implications of these variants toward enzyme function. Our findings suggest that despite the lack of treatment for G6PDD to date, AG1 remains a novel molecule that promotes activation in a variety of G6PD variants.

Epidemiological shift of glucose-6-phosphate dehydrogenase mutations in northern Italy in the last 15 years

Glucose-6-phosphate dehydrogenase (G6PD) deficiency is an X-linked recessive hemolytic anemia caused by mutations in G6PD gene. The distribution and frequency of genetic variants differ depending on ethnicity and geographical areas. Because of new migrations different variants are now present in Europe. This retrospective study aims to identify variants among the G6PD deficient subjects referred since 2004 to IRCCS Ca' Granda Foundation Hospital in Milan. The subjects were divided into 3 groups: group 1 (2004-2008), group 2 (2009-2013), and group 3 (2014-2018). During 15 years a significant decrease of the Mediterranean and an important increase of the African, Asian, and uncommon variants (classified as Others) have been observed. Three new mutations were found: in group 2 heterozygosity for c.[1454G > A] (Gly485Asp) in an adult female with severe anemia, high bilirubin levels and G6PD activity of 0,69 (IU/gHb) and heterozygosity for c.[584A > G] (Gln195Arg) in an elderly woman of Italian origin showing only anemia and enzymatic activity of 1,54 (IU/gHb) were detected. In group 3 hemizygosity for c.[670A > T] (Ile224Phe) in an adult Chinese man without anemia but with total absence of G6PD activity was found. These data reflect the appearance of uncommon G6PD mutations in northern Italy, probably due to new migrations, as consequence G6PD characterization becomes a diagnostic issue.

G6PD Variants and Haemolytic Sensitivity to Primaquine and Other Drugs

Restrictions on the cultivation and ingestion of fava beans were first reported as early as the fifth century BC. Not until the late 19th century were clinical descriptions of fava-induced disease reported and soon after characterised as “favism” in the early 20th century. It is now well known that favism as well as drug-induced haemolysis is caused by a deficiency of the glucose-6-phosphate dehydrogenase (G6PD) enzyme, one of the most common enzyme deficiency in humans. Interest about the interaction between G6PD deficiency and therapeutics has increased recently because mass treatment with oxidative 8-aminoquinolines is necessary for malaria elimination. Historically, assessments of haemolytic risk have focused on the clinical outcomes (e.g., haemolysis) associated with either a simplified phenotypic G6PD characterisation (deficient or normal) or an ill-fitting classification of G6PD genetic variants. It is increasingly apparent that detailed knowledge of both aspects is required for a complete understanding of haemolytic risk. While more attention has been devoted recently to better phenotypic characterisation of G6PD activity (including the development of new point-of care tests), the classification of G6PD variants should be revised to be clinically useful in malaria eliminating countries and in populations with prevalent G6PD deficiency. The scope of this work is to summarize available literature on drug-induced haemolysis among individuals with different G6PD variants and to highlight knowledge gaps that could be filled with further clinical and laboratory research.

Fyn specifically Regulates the activity of red cell glucose-6-phosphate-dehydrogenase

Fyn is a tyrosine kinase belonging to the Src family (Src-Family-Kinase, SFK), ubiquitously expressed. Previously, we report that Fyn is important in stress erythropoiesis. Here, we show that in red cells Fyn specifically stimulates G6PD activity, resulting in a 3-fold increase enzyme catalytic activity (k_cat) by phosphorylating tyrosine (Tyr)-401. We found Tyr-401 on G6PD as functional target of Fyn in normal human red blood cells (RBC), being undetectable in G6PD deficient RBCs (G6PD-Mediterranean and G6PD-Genova). Indeed, Tyr-401 is located to a region of the G6PD molecule critical for the formation of the enzymatically active dimer. Amino acid replacements in this region are mostly associated with a chronic hemolysis phenotype. Using mutagenesis approach, we demonstrated that the phosphorylation status of Tyr401 modulates the interaction of G6PD with G6P and stabilizes G6PD in a catalytically more efficient conformation. RBCs from Fyn^-/-mice are defective in G6PD activity, resulting in increased susceptibility to primaquine-induced intravascular hemolysis. This negatively affected the recycling of reduced Prx2 in response to oxidative stress, indicating that defective G6PD phosphorylation impairs defense against oxidation. In human RBCs, we confirm the involvement of the thioredoxin/Prx2 system in the increase vulnerability of G6PD deficient RBCs to oxidation. In conclusion, our data suggest that Fyn is an oxidative radical sensor, and that Fyn-mediated Tyr-401 phosphorylation, by increasing G6PD activity, plays an important role in the physiology of RBCs. Failure of G6PD activation by this mechanism may be a major limiting factor in the ability of G6PD deficient RBCs to withstand oxidative stress.

Identification of the NADP+ Structural Binding Site and Coenzyme Effect on the Fused G6PD::6PGL Protein from Giardia lamblia

Giardia lambia is a flagellated protozoan parasite that lives in the small intestine and is the causal agent of giardiasis. It has been reported that G. lamblia exhibits glucose-6-phosphate dehydrogenase (G6PD), the first enzyme in the pentose phosphate pathway (PPP). Our group work demonstrated that the g6pd and 6pgl genes are present in the open frame that gives rise to the fused G6PD::6PGL protein; where the G6PD region is similar to the 3D structure of G6PD in Homo sapiens. The objective of the present work was to show the presence of the structural NADP⁺ binding site on the fused G6PD::6PGL protein and evaluate the effect of the NADP⁺ molecule on protein stability using biochemical and computational analysis. A protective effect was observed on the thermal inactivation, thermal stability, and trypsin digestions assays when the protein was incubated with NADP⁺. By molecular docking, we determined the possible structural-NADP⁺ binding site, which is located between the Rossmann fold of G6PD and 6PGL. Finally, molecular dynamic (MD) simulation was used to test the stability of this complex; it was determined that the presence of both NADP⁺ structural and cofactor increased the stability of the enzyme, which is in agreement with our experimental results.

Molecular Characterization of G6PD Deficiency: Report of Three Novel G6PD Variants

G6PD deficiency is a monogenic, X-linked genetic defect with a worldwide prevalence of around 400 million people and an overall prevalence of 8.5% in India. Hemolytic anemia is encountered in only a small proportion of patients with G6PD variants and is usually triggered by some exogenous agent. Although G6PD deficiency was reported in India more than 50 years ago, there are very few studies on molecular characterization and phenotypic correlation in G6PD deficient patients. We aimed to study the epidemiology and correlate the phenotypic expression with molecular genotypes in symptomatic G6PD deficient patients. All symptomatic hemolytic anaemia patients with a possible etiology of G6PD deficiency based on the clinical, hematological and biochemical parameters and reduced G6PD enzyme levels were included in this study. Molecular analysis of the G6PD gene was done by direct Sanger sequencing. From a total of 38 patients with hemolytic anemia suspected for G6PD deficiency, 24 patients had reduced G6PD enzyme levels and were included for the molecular analysis and mutations in the G6PD gene were identified in 21 of them (83.3%). The different mutations identified in our study include 6 patients with c.131C > G (G6PD Orissa), 3 patients with c.563C > T (G6PD Mediterranean), two patients with c.825G > T (G6PD Bangkok), one patient each with c.208T > C (G6PD Namouru), c.487G > A (G6PD Mahidol), c.949G > A (G6PD Kerala-Kalyan), c.100 G > A (G6PD Chatham), c.1178C > G (G6PD Nashville), c.1361 G > A (G6PD Andalus) and 4 patients with novel mutations (2 patients with c.1186C > T and 1 patient each with c.1288-2A > T and c.1372C > T. No disease causing genetic variants were identified in the other three cases. Co-inheritance of other red cell and hemoglobin disorders can modify the clinical phenotype of G6PD patients and the diagnostic accuracy can be improved by molecular characterization of the variant.

A rare disorder or not? How a child with jaundice changed a nationwide regimen in the Netherlands

Due to global migration, there is an increased frequency of diseases, which used to be rare in Western countries. Here, we describe a striking case in order to create awareness for diseases that are known for decades but sometimes "forgotten" in Western countries, including glucose-6-phosphate dehydrogenase deficiency. We will discuss how everyday practice can lead to serious medical problems and present general recommendations to support.

Diagnostic approaches for inherited hemolytic anemia in the genetic era

Inherited hemolytic anemias (IHAs) are genetic diseases that present with anemia due to the increased destruction of circulating abnormal RBCs. The RBC abnormalities are classified into the three major disorders of membranopathies, hemoglobinopathies, and enzymopathies. Traditional diagnosis of IHA has been performed via a step-wise process combining clinical and laboratory findings. Nowadays, the etiology of IHA accounts for germline mutations of the responsible genes coding for the structural components of RBCs. Recent advances in molecular technologies, including next-generation sequencing, inspire us to apply these technologies as a first-line approach for the identification of potential mutations and to determine the novel causative genes in patients with IHAs. We herein review the concept and strategy for the genetic diagnosis of IHAs and provide an overview of the preparations for clinical applications of the new molecular technologies.

Glucose-6-Phosphate Dehydrogenase Deficiency

G6PD is a housekeeping gene expressed in all cells. Glucose-6-phosphate dehydrogenase (G6PD) is part of the pentose phosphate pathway, and its main physiologic role is to provide NADPH. G6PD deficiency, one of the commonest inherited enzyme abnormalities in humans, arises through one of many possible mutations, most of which reduce the stability of the enzyme and its level as red cells age. G6PD-deficient persons are mostly asymptomatic, but they can develop severe jaundice during the neonatal period and acute hemolytic anemia when they ingest fava beans or when they are exposed to certain infections or drugs. G6PD deficiency is a global health issue.

Safety of 8-aminoquinolines given to people with G6PD deficiency: protocol for systematic review of prospective studies

INTRODUCTION

A single dose or short course of primaquine given to people infected with malaria may reduce transmission of Plasmodium falciparum through its effects on gametocytes. Primaquine is also known to cause haemolysis in people with variants of glucose-6-phosphate dehydrogenase (G6PD) deficiency. The objective of this systematic review was to assess the risk of adverse effects in people with G6PD deficiency given primaquine or other 8-aminoquinoline (8AQ) as a single dose or short course (less than 7 days).

METHODS AND ANALYSIS

We will search the following databases: Cochrane Infectious Diseases Group Specialized Register, the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, EMBASE and LILACS. Prospective cohort studies, randomised and quasi-randomised trials that evaluated 8AQs for whatever reason in adults or children with a known G6PD deficiency will be included. Two authors will independently assess each study for eligibility, risk of bias and extract data.

ETHICS AND DISSEMINATION

This systematic review will be published in a peer-reviewed journal. Brief reports of the review findings will be disseminated directly to the appropriate audiences and the WHO Technical Expert Group in Malaria Chemotherapy. As no primary data collection will be undertaken, no additional formal ethical assessment and informed consent are required.

PROTOCOL REGISTRATION IN PROSPERO

The protocol is registered with PROSPERO, registration number CRD42013006518.

G6PD deficiency: global distribution, genetic variants and primaquine therapy

Glucose-6-phosphate dehydrogenase (G6PD) is a potentially pathogenic inherited enzyme abnormality and, similar to other human red blood cell polymorphisms, is particularly prevalent in historically malaria endemic countries. The spatial extent of Plasmodium vivax malaria overlaps widely with that of G6PD deficiency; unfortunately the only drug licensed for the radical cure and relapse prevention of P. vivax, primaquine, can trigger severe haemolytic anaemia in G6PD deficient individuals. This chapter reviews the past and current data on this unique pharmacogenetic association, which is becoming increasingly important as several nations now consider strategies to eliminate malaria transmission rather than control its clinical burden. G6PD deficiency is a highly variable disorder, in terms of spatial heterogeneity in prevalence and molecular variants, as well as its interactions with P. vivax and primaquine. Consideration of factors including aspects of basic physiology, diagnosis, and clinical triggers of primaquine-induced haemolysis is required to assess the risks and benefits of applying primaquine in various geographic and demographic settings. Given that haemolytically toxic antirelapse drugs will likely be the only therapeutic options for the coming decade, it is clear that we need to understand in depth G6PD deficiency and primaquine-induced haemolysis to determine safe and effective therapeutic strategies to overcome this hurdle and achieve malaria elimination.

Life and Death of Glucose-6-Phosphate Dehydrogenase (G6PD) Deficient Erythrocytes - Role of Redox Stress and Band 3 Modifications

SUMMARY

G6PD catalyzes the first, pace-making reaction of pentosephosphate cycle (PPC) which produces NADPH. NADPH maintains glutathione and thiol groups of proteins and enzymes in the reduced state which is essential for protection against oxidative stress. Individuals affected by G6PD deficiency are unable to regenerate reduced glutathione (GSH) and are undefended against oxidative stress. G6PD deficiency accelerates normal senescence and enhances the precocious removal of chronologically young, yet biologically old cells. The term hemolytic anemia is misleading because RBCs do not lyse but are removed by phagocytosis. Acute hemolysis by fava bean ingestion in G6PD deficient individuals (favism) is described being the best-studied natural model of oxidant damage. It bears strong analogies to hemolysis by oxidant drugs or chemicals. Membrane alterations observed in vivo during favism are superimposable to changes in senescent RBCs. In summary, RBC membranes isolated from favic patients contained elevated amounts of complexes between IgG and the complement fragment C3b/C3c and were prone to vesiculation. Anti-band 3 IgG reacted to aggregated band 3-complement complexes. In favism extensive clustering of band 3 and membrane deposition of hemichromes were also observed. Severely damaged RBCs isolated from early crises had extensive membrane cross-bonding and very low GSH levels and were phagocytosed 10-fold more intensely compared to normal RBCs.

Predicting the Kinetic Properties Associated with Redox Imbalance after Oxidative Crisis in G6PD-Deficient Erythrocytes: A Simulation Study

It is well known that G6PD-deficient individuals are highly susceptible to oxidative stress. However, the differences in the degree of metabolic alterations among patients during an oxidative crisis have not been extensively studied. In this study, we applied mathematical modeling to assess the metabolic changes in erythrocytes of various G6PD-deficient patients during hydrogen peroxide- (H(2)O(2)-) induced perturbation and predict the kinetic properties that elicit redox imbalance after exposure to an oxidative agent. Simulation results showed a discrepancy in the ability to restore regular metabolite levels and redox homeostasis among patients. Two trends were observed in the response of redox status (GSH/GSSG) to oxidative stress, a mild decrease associated with slow recovery and a drastic decline associated with rapid recovery. The former was concluded to apply to patients with severe clinical symptoms. Low V(max) and high K(mG6P) of G6PD were shown to be kinetic properties that enhance consequent redox imbalance.

Relating mutant genotype to phenotype via quantitative behavior of the NADPH redox cycle in human erythrocytes

BACKGROUND

The NADPH redox cycle plays a key role in antioxidant protection of human erythrocytes. It consists of two enzymes: glucose-6-phosphate dehydrogenase (G6PD) and glutathione reductase. Over 160 G6PD variants have been characterized and associated with several distinct clinical manifestations. However, the mechanistic link between the genotype and the phenotype remains poorly understood.

METHODOLOGY/PRINCIPAL FINDINGS

We address this issue through a novel framework (design space) that integrates information at the genetic, biochemical and clinical levels. Our analysis predicts three qualitatively-distinct phenotypic regions that can be ranked according to fitness. When G6PD variants are analyzed in design space, a correlation is revealed between the phenotypic region and the clinical manifestation: the best region with normal physiology, the second best region with a pathology, and the worst region with a potential lethality. We also show that Plasmodium falciparum, by induction of its own G6PD gene in G6PD-deficient erythrocytes, moves the operation of the cycle to a region of the design space that yields robust performance.

CONCLUSIONS/SIGNIFICANCE

In conclusion, the design space for the NADPH redox cycle, which includes relationships among genotype, phenotype and environment, illuminates the function, design and fitness of the cycle, and its phenotypic regions correlate with the organism's clinical status.

Using in silico models to simulate dual perturbation experiments: procedure development and interpretation of outcomes

Background

A growing number of realistic in silico models of metabolic functions are being formulated and can serve as 'dry lab' platforms to prototype and simulate experiments before they are performed. For example, dual perturbation experiments that vary both genetic and environmental parameters can readily be simulated in silico. Genetic and environmental perturbations were applied to a cell-scale model of the human erythrocyte and subsequently investigated.

Results

The resulting steady state fluxes and concentrations, as well as dynamic responses to the perturbations were analyzed, yielding two important conclusions: 1) that transporters are informative about the internal states (fluxes and concentrations) of a cell and, 2) that genetic variations can disrupt the natural sequence of dynamic interactions between network components. The former arises from adjustments in energy and redox states, while the latter is a result of shifting time scales in aggregate pool formation of metabolites. These two concepts are illustrated for glucose-6 phosphate dehydrogenase (G6PD) and pyruvate kinase (PK) in the human red blood cell.

Conclusion

Dual perturbation experiments in silico are much more informative for the characterization of functional states than single perturbations. Predictions from an experimentally validated cellular model of metabolism indicate that the measurement of cofactor precursor transport rates can inform the internal state of the cell when the external demands are altered or a causal genetic variation is introduced. Finally, genetic mutations that alter the clinical phenotype may also disrupt the 'natural' time scale hierarchy of interactions in the network.

A novel G6PD mutation leading to chronic hemolytic anemia

Glucose-6-phosphate dehydrogenase (G6PD) deficiency is an important cause of hemolytic anemia worldwide. Severely affected patients have chronic hemolysis with exacerbations following oxidative stress. Mutations causing severe chronic non-spherocytic hemolytic anemia (CNSHA) commonly cluster in Exon 10, a region important for protein dimerization. An African-American male presented at age 2 weeks with pallor and jaundice, and was found to have hemolytic anemia with G6PD deficiency. His severe clinical course was inconsistent with the expected G6PD A(-) variant. DNA sequencing revealed two common mutations (A(-)) and a third novel Exon 10 mutation. This inherited haplotype represents a novel triple G6PD coding mutation causing chronic hemolysis.

Glucose-6-phosphate dehydrogenase deficiency

Glucose-6-phosphate dehydrogenase (G6PD) deficiency is the most common human enzyme defect, being present in more than 400 million people worldwide. The global distribution of this disorder is remarkably similar to that of malaria, lending support to the so-called malaria protection hypothesis. G6PD deficiency is an X-linked, hereditary genetic defect due to mutations in the G6PD gene, which cause functional variants with many biochemical and clinical phenotypes. About 140 mutations have been described: most are single base changes, leading to aminoacid substitutions. The most frequent clinical manifestations of G6PD deficiency are neonatal jaundice, and acute haemolytic anaemia, which is usually triggered by an exogenous agent. Some G6PD variants cause chronic haemolysis, leading to congenital non-spherocytic haemolytic anaemia. The most effective management of G6PD deficiency is to prevent haemolysis by avoiding oxidative stress. Screening programmes for the disorder are undertaken, depending on the prevalence of G6PD deficiency in a particular community.

G6PD deficiency: the genotype-phenotype association

Deficiency of glucose-6-phosphate dehydrogenase is a very common X-linked genetic disorder though most deficient people are asymptomatic. A number of different G6PD variants have reached polymorphic frequencies in different parts of the world due to the relative protection they confer against malaria infection. People, usually males, with deficient alleles are susceptible to neonatal jaundice, and acute hemolytic anemia, usually during infection, after treatment with certain drugs or after eating fava beans. Very rarely de novo mutations can arise causing the more severe condition of chronic nonspherocytic hemolytic anemia. Altogether 160 different mutations have been described. The majority of mutations cause red cell enzyme deficiency by decreasing enzyme stability. The polymorphic mutations affect amino acid residues throughout the enzyme and decrease the stability of the enzyme in the red cell, possibly by disturbing protein folding. The severe mutations mostly affect residues at the dimer interface or those that interact with a structural NADP molecule that stabilizes the enzyme.

Oxidant stress and haemolysis of the human erythrocyte

The erythrocyte is a highly specialised cell with a limited metabolic repertoire. As an oxygen shuttle, it must continue to perform this essential task while exposed to a wide range of environments on each vascular circuit, and to a variety of xenobiotics across its lifetime. During this time, it must continuously ward off oxidant stress on the haeme iron, the globin chain and on other essential cellular molecules. Haemolysis, the acceleration of the normal turnover of senescent erythrocytes, follows severe and irreversible oxidant injury. A detailed understanding of the molecular mechanisms underlying oxidant injury and its reversal, and of the clinical and laboratory features of haemolysis is important to the medical toxicologist. This review will also briefly review glucose-6-phosphate deficiency, a common but heterogeneous range of enzyme-deficient states, which impairs the ability of the erythrocyte to respond to oxidant injury.

Dynamic simulation of red blood cell metabolism and its application to the analysis of a pathological condition

Background

Cell simulation, which aims to predict the complex and dynamic behavior of living cells, is becoming a valuable tool. In silico models of human red blood cell (RBC) metabolism have been developed by several laboratories. An RBC model using the E-Cell simulation system has been developed. This prototype model consists of three major metabolic pathways, namely, the glycolytic pathway, the pentose phosphate pathway and the nucleotide metabolic pathway. Like the previous model by Joshi and Palsson, it also models physical effects such as osmotic balance. This model was used here to reconstruct the pathology arising from hereditary glucose-6-phosphate dehydrogenase (G6PD) deficiency, which is the most common deficiency in human RBC.

Results

Since the prototype model could not reproduce the state of G6PD deficiency, the model was modified to include a pathway for de novo glutathione synthesis and a glutathione disulfide (GSSG) export system. The de novo glutathione (GSH) synthesis pathway was found to compensate partially for the lowered GSH concentrations resulting from G6PD deficiency, with the result that GSSG could be maintained at a very low concentration due to the active export system.

Conclusion

The results of the simulation were consistent with the estimated situation of real G6PD-deficient cells. These results suggest that the de novo glutathione synthesis pathway and the GSSG export system play an important role in alleviating the consequences of G6PD deficiency.

Glucose-6-phosphate dehydrogenase Guadalajara--a case of chronic non-spherocytic haemolytic anaemia responding to splenectomy and the role of splenectomy in this disorder

Glucose-6-phosphate dehydrogenase (G6PD) is an enzyme of the pentose phosphate shunt pathway a major function of which is to prevent cellular oxidative damage. Deficiency in red blood cells is associated with a number of varied clinical manifestations. Chronic non-spherocytic haemolytic anaemia is uncommon but is usually characterized by chronic haemolysis, often with severe anaemia. In the past splenectomy in this condition has been thought to be of questionable benefit. We report a case of G6PD Guadalajara where splenectomy produced transfusion independence and have reviewed the literature. Those cases with exon 10 mutations often have a severe clinical phenotype, which responds to splenectomy. This procedure should be considered in this condition.

Parallel analysis of mutant human glucose 6-phosphate dehydrogenase in yeast using PCR colonies

We demonstrate a highly parallel strategy to analyze the impact of single nucleotide mutations on protein function. Using our method, it is possible to screen a population and quickly identify a subset of functionally interesting mutants. Our method utilizes a combination of yeast functional complementation, growth competition of mutant pools, and polymerase colonies. A defined mutant human glucose-6-phosphate-dehydrogenase library was constructed which contains all possible single nucleotide missense mutations in the eight-residue glucose-6-phosphate binding peptide of the enzyme. Mutant human enzymes were expressed in a zwf1 (gene encoding yeast homologue) deletion strain of Saccharomyces cerevisiae. Growth rates of the 54 mutant strains arising from this library were measured in parallel in conditions selective for active hG6PD. Several residues were identified which tolerated no mutations (Asp200, His201 and Lys205) and two (Ile199 and Leu203) tolerated several substitutions. Arg198, Tyr202, and Gly204 tolerated only 1-2 specific substitutions. Generalizing from the positions of tolerated and non-tolerated amino acid substitutions, hypotheses were generated about the functional role of specific residues, which could, potentially, be tested using higher resolution/lower throughput methods.

Molecular characterization of six unrelated Italian patients affected by pyrimidine 5'-nucleotidase deficiency

Pyrimidine 5'-nucleotidase deficiency is a rare autosomal recessive disorder characterized by haemolytic anaemia, marked basophilic stippling and accumulation of pyrimidine nucleotides within the erythrocytes. The gene encoding for this enzyme (P5'N-1) has been cloned recently, and seven mutations have so far been identified in 11 unrelated families. We describe the haematological and molecular characteristics of six unrelated Italian patients affected by pyrimidine 5'-nucleotidase deficiency (one from northern and five from southern Italy). The sequence of the complete P5'N-1 gene showed the presence of four different new mutations: a missense mutation AAT-AGT at codon 190 (Asn-Ser), one splicing mutation (IVS9-1 g-c) and two frameshift mutations, DelG576 and InsGG743. Although the molecular defect was homozygous in all patients but one, parents' consanguinity could be confirmed in only one case. InsGG743 was detected in two cases, and DelG576 was found in three patients originating from southern Italy, suggesting a possible geographical distribution of the genetic defect. Haematological data showed the presence of peripheral spherocytosis in all cases, although only one had a concomitant membrane defect. An increase in serum ferritin levels was observed in the splenectomized patients, suggesting that the iron status of these subjects should be monitored and that they should be investigated for potential additional risk factors for iron accumulation.

Murine hexose-6-phosphate dehydrogenase: a bifunctional enzyme with broad substrate specificity and 6-phosphogluconolactonase activity

Murine hexose-6-phosphate dehydrogenase has been purified from liver microsomes by affinity chromatography on 2('),5(')-ADP-Sepharose. The purified enzyme has 6-phosphogluconolactonase activity and glucose-6-phosphate dehydrogenase activity and has a native molecular mass of 178 kDa and a subunit molecular mass of 89 kDa. Glucose 6-phosphate, galactose 6-phosphate, 2-deoxyglucose 6-phosphate, glucosamine 6-phosphate, and glucose 6-sulfate are substrates for murine hexose-6-phosphate dehydrogenase, with either NADP or deamino-NADP as coenzyme. This study confirms that hexose-6-phosphate dehydrogenase is a bifunctional enzyme which can catalyze the first two reactions of the pentose phosphate pathway.

In silico model-driven assessment of the effects of single nucleotide polymorphisms (SNPs) on human red blood cell metabolism

The completion of the human genome project and the construction of single nucleotide polymorphism (SNP) maps have lead to significant efforts to find SNPs that can be linked to pathophysiology. In silico models of complete biochemical reaction networks relate a cell's individual reactions to the function of the entire network. Sequence variations can in turn be related to kinetic properties of individual enzymes, thus allowing an in silico model-driven assessment of the effects of defined SNPs on overall cellular functions. This process is applied to defined SNPs in two key enzymes of human red blood cell metabolism: glucose-6-phosphate dehydrogenase and pyruvate kinase. The results demonstrate the utility of in silico models in providing insight into differences between red cell function in patients with chronic and nonchronic anemia. In silico models of complex cellular processes are thus likely to aid in defining and understanding key SNPs in human pathophysiology.

Iron status and HFE genotype in erythrocyte pyruvate kinase deficiency: study of Italian cases

We evaluated the iron status and searched for mutations C282Y and H63D in the hereditary hemochromatosis gene (HFE) in 34 pyruvate kinase (PK)-deficient patients from 29 unrelated families. Nine had received multiple transfusions. Thirteen of the 25 nontransfused patients displayed increased serum ferritin concentration, in the absence of conditions known to raise this parameter. HFE genotype was abnormal in 9 of 34 patients. The allele frequency was 1.8% for mutation 845G--> (C282Y) and 16.1% for mutation 187C-->G (H63D). Nontransfused subjects with abnormal genotype had serum ferritin and transferrin saturation values significantly higher than those with wild-type genotype. Of the 12 adult nontransfused patients with increased iron status parameters, 1 was C282Y homozygous, 1 compound heterozygous for C282Y and H63D, 3 H63D heterozygous, and 7 had a normal HFE genotype. Serum ferritin and transferrin saturation were not related to hemoglobin, reticulocytes, and bilirubin concentration. At multivariate analysis serum ferritin was independently associated with age and gender, but not with splenectomy and HFE genotypes. The retrospective evaluation of the iron status profile of 10 patients (3 with abnormal and 7 with wild-type HFE genotype) with at least 10 years follow-up showed that overt iron accumulation requiring iron chelation had occurred only in the 3 patients (2 of whom were splenectomized) with the mutated HFE gene.