Hepatic microsomal glucose-6-phosphatase system and sudden infant death syndrome

Post-mortem genetic analysis of sudden unexplained death in a young cohort: a whole-exome sequencing study

Sudden unexplained death (SUD) constitutes a considerable portion of unexpected sudden death in the young. Molecular autopsy has proved to be an efficient diagnostic tool in the multidisciplinary management of SUD. Yet, many cases remain undiagnosed using the widely adopted targeted genetic screening strategies. Here, we investigated the genetic substrates of a young SUD cohort (18-40 years old) from China using whole-exome sequencing (WES), with the primary aim to identify novel SUD susceptibility genes. Within 255 previously acknowledged SUD-associated genes, 21 variants with likely functional effects (pathogenic/likely pathogenic) were identified in 51.9% of the SUD cases. More importantly, a set of 33 candidate genes associated with myopathy were identified to be novel susceptibility genes for SUD. Comparative analysis of the cumulative PHRED-scaled CADD score and polygenetic burden score showed that the amount and deleteriousness of variants in the 255 SUD-associated genes and the 33 candidate genes identified by this study were significantly higher compared with 289 randomly selected genes. A significantly higher genetic burden of rare variants (MAF < 0.1%) in the 33 candidate genes also highlighted putative roles of these genes in SUD. After incorporating these novel genes, the genetic testing yields of the current SUD cohort elevated from 51.9 to 66.7%. Our study expands understanding of the genetic variants underlying SUD and presents insights that improve the utility of genetic screenings.

Cardiac ion channelopathies and the sudden infant death syndrome

The sudden infant death syndrome (SIDS) causes the sudden death of an apparently healthy infant, which remains unexplained despite a thorough investigation, including the performance of a complete autopsy. The triple risk model for the pathogenesis of SIDS points to the coincidence of a vulnerable infant, a critical developmental period, and an exogenous stressor. Primary electrical diseases of the heart, which may cause lethal arrhythmias as a result of dysfunctioning cardiac ion channels (“cardiac ion channelopathies”) and are not detectable during a standard postmortem examination, may create the vulnerable infant and thus contribute to SIDS. Evidence comes from clinical correlations between the long QT syndrome and SIDS as well as genetic analyses in cohorts of SIDS victims (“molecular autopsy”), which have revealed a large number of mutations in ion channel-related genes linked to inheritable arrhythmogenic syndromes, in particular the long QT syndrome, the short QT syndrome, the Brugada syndrome, and catecholaminergic polymorphic ventricular tachycardia. Combining data from population-based cohort studies, it can be concluded that at least one out of five SIDS victims carries a mutation in a cardiac ion channel-related gene and that the majority of these mutations are of a known malignant phenotype.

Genetic variation in hepatic glucose-6-phosphatase system genes in cases of sudden infant death syndrome

Genetic deficiencies of the hepatic glucose-6-phosphatase system, either of the enzyme (G6PC1) or of the glucose-6-phosphate transporter (G6PT1), result in fasting hypoglycaemia. Low hepatic G6PC1 activities were previously reported in a few term sudden infant death syndrome (SIDS) infants and assumed to be due to G6PC1 genetic deficiencies. In preterm infants, failures of postnatal activation of G6PC1 expression suggest disordered development as a novel cause of decreased G6PC1 activity in SIDS. G6PC1 and G6PT1 functional and mutational analysis was investigated in SIDS and non-SIDS infants. G6PC1 hepatic activity was abnormally low in 98 SIDS (preterm, n=13; term, n=85), and non-SIDS preterm infants (n=35) compared to term non-SIDS infants (n=29) and adults (n=9). Mean glycogen levels were elevated, except in term non-SIDS infants. A novel G6PT1 promoter polymorphism, 259C --> T was found; the - 259*T allele frequency was greater in term SIDS infants (n=140) than in term control infants (n=119) and preterm SIDS infants (n=30). Heterozygous and homozygous prevalence of 259C --> T was 38.6% and 7.1%, respectively, in term SIDS infants. In cell-based expression systems, the presence of - 259T in the promoter decreased basal luciferase activity by 3.2-fold compared to - 259C. Glucose-6-phosphatase latency in hepatic microsomes was elevated (indicating decreased G6PT1 function) in heterozygous and homozygous - 259T states. Delayed postnatal appearance of hepatic glucose-6-phosphatase in infants makes them vulnerable to hypoglycaemic episodes and this may occur in some SIDS infants. However, SIDS may be an association of more complex phenotypes in which several genes interact with multiple environmental factors. A UK-wide DNA Biobank of samples from all infant deaths, with an accompanying epidemiological database, should be established by pathologists to allow cumulative data to be collected from multiple genetic investigations on the same large cohort of samples, with the aim of selection of the best combination of genetic markers to predict unexpected infant death.

Preparation of intact microsomes from cultured mammalian H4IIE cells

INTRODUCTION

Mammalian cell culture is widely used for the cloning and expression of insoluble proteins. The established methods of sub-cellular fractionation of tissues are not always directly suitable for the sub-cellular fractionation of cultured cells. In this study we have optimized the conditions for the preparation of microsomal fractions from cultured cells with the aim of isolating intact vesicles that are suitable for the assay of transport proteins and lumenal enzymes.

METHODS

H4IIE cell cultures were used as a convenient model with high latency of internal endoplasmic reticulum enzyme glucose-6-phosphatase towards mannose-6-phosphate. Also 7-ethoxyresorufin O-deethylase (EROD) activity was determined as a reflection of the state of monooxygenase system.

RESULTS

The variations in a number of homogenization strokes and buffer composition revealed that one homogenization stroke in glass homogenizer with 0.25 M sucrose, 5 mM HEPES, pH 7.4 buffer provides the best latency/activity ratio for homogenates, but for the isolation of microsomes the higher number of strokes (10) as well as low-osmotic buffer (5 mM HEPES, pH 7.4) are needed. However EROD activity is largely reduced in the preparations using buffers containing sucrose, so 5 mM HEPES buffer is recommended as the most suitable to study the microsomal reactions in H4IIE cells.

DISCUSSION

The isolation of microsomes was followed by the significant proteolytic breakdown of the glucose-6-phosphatase enzyme. It is recommended to use cell culture homogenates for assays when possible.

Identification of novel polymorphisms in the glucokinase and glucose-6-phosphatase genes in infants who died suddenly and unexpectedly

Sudden and unexpected infant deaths can be unexplained [sudden infant death syndrome (SIDS)] or explained (non-SIDS) but risk factors including lower birthweight are similar in both groups. Mutations in the glucokinase (GK) gene result in Maturity Onset Diabetes of the Young type 2 (MODY 2) and are associated with lower birthweight. Low hepatic glucose-6-phosphatase (G6PC1) expression occurs in both low birthweight and SIDS infants. We investigated whether polymorphisms are prevalent in the GK and G6PC1 genes in infants who died suddenly and unexpectedly. Mutation analysis was performed by polymerase chain reaction (PCR) and denaturing high-performance liquid chromatography (DHPLC) in samples from 126 infants who died suddenly and unexpectedly (78 SIDS, 48 non-SIDS) and from 70 healthy, living infants. G6PC1 promoter polymorphism significance was investigated by transfection of reporter gene constructs into a H4IIE cell line. Heterozygous GK polymorphisms were identified in 17.9% of SIDS and 20.8% of non-SIDS infants: two rare silent polymorphisms, Y215Y and S263S, in the coding region; a third rare polymorphism, -45G>A, in the hepatic promoter and the most prevalent polymorphism, c.484-29G>C, in a non-coding region upstream from the intron 4-exon 5 junction. A novel heterozygous polymorphism -77G>A in the G6PC1 promoter in 6.3% of non-SIDS and 2.9% of control infants decreased basal G6PC1 promoter activity (p<0.001). We describe three novel polymorphisms in the GK gene, S263S, -45G>A, and a common (14.3%) intronic substitution, c.484-29G>C, in infants who died suddenly and unexpectedly. We identified the first G6PC1 promoter polymorphism, which lowers expression, potentially increasing risk of hypoglycaemia and hence risk of sudden and unexpected death.

Glucose homeostasis in the newborn

Hepatic glucose production by glycogenolysis and gluconeogenesis is essential to maintain blood glucose levels, and the glucose-6-phosphatase system catalyses the terminal step of both pathways. Developmental delays in the postnatal up-regulation of hepatic glucose-6-phosphatase enzyme activity are common in preterm infants. Two groups of infants have been identified with failure of developmental regulation of glucose homeostasis. Firstly, up to 20% of preterm infants about to be discharged home are at risk of hypoglycaemia if a feed is delayed. Cortisol, corticotrophin and epinephrine levels are higher in the infants with severe and persistent hypoglycaemia, but insulin, glucagon and human growth hormone do not differ from normoglycaemic infants. Secondly, preterm infants with an inadequate glycaemic response to glucagon (30% at the time of discharge home) have relative fasting hyperglycaemia, hyperinsulinaemia, increased insulin:glucagon ratios and a lower insulin sensitivity index. Hormonal dysfunctions in preterm infants may contribute to failures in postnatal expression of hepatic enzymes.

Hypoglycemia: factitious and felonious

Factitious diseases are characterized by physical or psychologic symptoms that are voluntarily self-induced. These diseases are as old as mankind. Once called "malingerers," these patients must be distinguished from hysterics in whom symptoms are produced unconsciously. In factitious diseases, illness is produced by deliberate acts by the patient who when seeking medical help omits to mention them and may continue strenuously to deny them even when confronted with the evidence. Factitious diseases occur in patients who simulate or exaggerate symptoms or disability to obtain some kind of discernible personal gain or avoid an unpleasant situation; however, such actions may only produce disadvantages by exposing the patient to the risk of death or permanent injury. This has been described as Munchausen syndrome, which is probably a manifestation of severe psychiatric disease. The use of medicines or poisons to induce illness in others also produces a type of factitial disease and presents similar or greater difficulties in diagnosis. In both situations, the clinical history, ordinarily the most important clue to the correct diagnosis, is not only incomplete but often misleading. Sometimes referred to as Munchausen by proxy, this form of factitial disease may be impossible to distinguish from attempted murder or grievous bodily harm. The subtle differences between these disorders, if any, have not been discussed herein.

The molecular background of glycogen metabolism disorders

The molecular pathology of classical glycogen storage disorders, glycogen synthase deficiency and Fanconi-Bickel syndrome is reviewed. The isolation of the respective cDNAs, the chromosomal localization of the genes and the elucidation of the genomic organization enabled mutation analysis in most disorders. The findings have shed light on the multi-protein structure of the glucose-6-phosphatase system, the phosphorylase kinase enzymatic complex and the molecular background of the differential tissue expression in debranching enzyme deficiency. The immediate practical benefit of these studies is our extending ability to predict the outcome of clinical variants and to offer genetic counseling to most families. The elucidation of the tertiary structure of these proteins and their structure-function relationship poses major challenges for the future.

Conformational change of the catalytic subunit of glucose-6-phosphatase in rat liver during the fetal-to-neonatal transition

The glucose-6-phosphatase system was investigated in fetal rat liver microsomal vesicles. Several observations indicate that the orientation of the catalytic subunit is different in the fetal liver in comparison with the adult form: (i) the phosphohydrolase activity was not latent using glucose-6-phosphate as substrate, and in the case of other phosphoesters it was less latent; (ii) the intravesicular accumulation of glucose upon glucose-6-phosphate hydrolysis was lower; (iii) the size of the intravesicular glucose-6-phosphate pool was independent of the glucose-6-phosphatase activities; (iv) antibody against the loop containing the proposed catalytic site of the enzyme inhibited the phosphohydrolase activity in fetal but not in adult rat liver microsomes. Glucose-6-phosphate, phosphate, and glucose uptake could be detected by both light scattering and/or rapid filtration method in fetal liver microsomes; however, the intravesicular glucose-6-phosphate and glucose accessible spaces were proportionally smaller than in adult rat liver microsomes. These data demonstrate that the components of the glucose-6-phosphatase system are already present, although to a lower extent, in fetal liver, but they are functionally uncoupled by the extravesicular orientation of the catalytic subunit.

Postnatal regression of glucose transport in a patient with glycogen storage disease type 1b

Decreased 2-deoxyglucose (2-DOG) uptake is well described in the neutrophils of patients with glycogen storage disease type 1b (GSD 1b). We report a patient with GSD 1b who presented with a normal antenatal and perinatal 2-deoxyglucose uptake that showed a slow regression during the first months of life. These indicate limitations of 2-deoxyglucose uptake in the diagnosis of GSD 1b. While it appears that low uptake rate below 0.25 nmol/min in 10(6) cells is of significance, normal uptake does not rule out the presence of the disease. It seems that antenatal diagnosis of GSD 1b cannot be made by measurement of 2-deoxyglucose uptake in the fetal neutrophils.

Immunocytochemical detection of the microsomal glucose-6-phosphatase in human brain astrocytes

Using an antibody raised against the catalytic subunit of glucose-6-phosphatase, this enzyme was immunolocalized in many astrocytes in 20 normal human brains. Double immunofluorescence studies showed co-localization of glial fibrillary acidic protein (GFAP) with glucose-6-phosphatase in astrocytes. However, not all GFAP-positive cells were also glucose-6-phosphatase positive, indicating that some astrocytes do not contain demonstrable expression of this enzyme. Reactive astrocytes in a variety of abnormal brains were strongly glucose-6-phosphatase positive, but neoplastic astrocytes were often only weakly positive. Expression of the enzyme could not be demonstrated in radial glia, neurons or oligodendroglia. Astrocytes normally contain glycogen and the demonstration that some astrocytes also contain glucose-6-phosphatase indicates that they are competent for both glycogenolysis and gluconeogenesis, which may be critical for neuronal welfare.

Abnormal expression of glucose-6-phosphatase in preterm infants

The hepatic microsomal glucose-6-phosphatase enzyme was studied in liver samples from 76 premature infants including 15 victims of sudden infant death syndrome. The data obtained were compared with glucose-6-phosphatase activity in liver samples from 95 term infants. In the majority of preterm infants up to 350 days of age the activity of the glucose-6-phosphatase enzyme was at or below the extreme low limit of the normal range in term infants. The premature infants with the lowest hepatic microsomal glucose-6-phosphatase activities are likely to be at risk of hypoglycaemic episodes during periods of relative starvation or stress.

Human microsomal glucose-6-phosphatase system

The discovery of glucose-6-phosphatase (EC 3.1.3.9) and of its physiological function in releasing glucose from the liver are discussed briefly. The identification by the Coris of glucose-6-phosphatase deficiency as the underlying defect in certain cases of glycogenosis (type I glycogenosis; von Gierke disease) is described. Characteristics of the catalyst, with a focus on its multiplicity of functions and multicomponent character, are considered with an emphasis on the human liver enzyme. Pioneering studies from the author's laboratory leading to the characterization of two variants of type I glycogenosis, types Ib and Ic, are described.

Identification, purification and genetic deficiencies of the glucose-6-phosphatase system transport proteins

Hepatic microsomal glucose-6-phosphatase (Glc-6-P'ase) is a complex multicomponent system containing at least three transport proteins, in addition to the catalytic subunit and a Ca2+ binding regulatory protein. The transport proteins have been designated T1 the glucose-6-phosphate transport protein, T2 a phosphate/pyrophosphate transport protein and T3 a glucose transport protein. Diagnosis of the genetic deficiencies of these transport proteins at present requires a complex kinetic analysis of the Glc-6-P'ase system as a whole. Here we describe the progress to date in our attempts to identify, purify and clone each transport protein with the ultimate aim of isolating specific cDNA probes for each transport protein which can be used for the diagnosis of types 1b, 1c and the putative 1d glycogen storage diseases.

The molecular basis of the genetic deficiencies of five of the components of the glucose-6-phosphatase system: improved diagnosis

The understanding of type 1 glycogen storage diseases (GSDs) has been greatly hindered by a lack of knowledge of the molecular basis of glucose-6-phosphatase (Glc-6-P'ase). The problem has been the complete failure of many laboratories, including our own, to purify to homogeneity a single polypeptide with high levels of Glc-6-P'ase activity. The best preparations to date all contain five or six different polypeptide bands and have specific activities in the range 17-50 mumoles/min per milligram. The two major reasons for failure have been that Glc-6-P'ase is extremely difficult to solubilise from the microsomal membrane (large amounts of detergents are needed) and that it is not a single polypeptide as originally thought, but a multicomponent system. Recent studies of patients with type 1 GSD have proved that Glc-6-P'ase comprises at least five different polypeptides. Four of the proteins have now been purified and three have been cloned. We have assayed the Glc-6-P'ase system in over 600 human biopsy samples and developed microassays to diagnose deficiencies of each of the proteins. Ways of avoiding possible problems which have the potential to lead to the wrong diagnosis will be discussed.

Impairment of the activity of the hepatic microsomal glucose-6-phosphatase system in three preterm infants

Three preterm infants born at 26-30 weeks' gestation who died between 103 and 266 days after birth were found to have elevated hepatic glycogen levels. Kinetic analysis of the hepatic microsomal glucose-6-phosphatase system demonstrated that one infant had abnormally low levels of activity of the glucose-6-phosphatase enzyme (partial type 1a glycogen storage disease) and two had deficiencies of T2, a microsomal phosphate/pyrophosphate transport protein (type 1c glycogen storage disease). In all three cases glycogen storage disease was not suspected prior to death even though both hypo- and hyperglycaemic episodes were recorded in the first 15 days after birth indicating that they had somewhat disordered blood glucose regulation. In the infant with low glucose-6-phosphatase enzyme activity, abnormal development of the glucose-6-phosphatase enzyme cannot be ruled out. This is the first description of abnormalities in the glucose-6-phosphatase system in preterm infants.

The molecular basis of the type 1 glycogen storage diseases

Microsomal glucose-6-phosphatase catalyses the last step in liver glucose production. Glucose-6-phosphatase deficiency, now termed type 1 glycogen storage disease, was first described almost 40 years ago but until recently very little was known about the molecular basis of the various type 1 glycogen storage diseases. Recently we have shown that at least six different proteins are needed for normal glucose-6-phosphatase activity in liver. Four of the proteins have been purified and three cloned. Study of the type 1 glycogen storage diseases has stimulated investigations of the mechanisms of small molecule transport across the endoplasmic reticulum membrane and demonstrated the existence of novel endoplasmic reticulum transport proteins for glucose and phosphate.

Analysis of human hepatic microsomal glucose-6-phosphatase in clinical conditions where the T2 pyrophosphate/phosphate transport protein is absent

The availability of a rare set of human hepatic microsomes in which T2, a pyrophosphate/phosphate transport protein of the glucose-6-phosphatase system, has been shown immunologically to be completely absent, has permitted further characterization of multicomponent glucose-6-phosphatase (EC 3.1.3.9). Pyrophosphatase activity in intact microsomes was found to be totally absent, but was normal in disrupted microsomes. However, Pi did not accumulate within the lumen of the microsomes when glucose 6-phosphate was the substrate. This was not as predicted if there is only one transport protein in the endoplasmic reticulum capable of transporting Pi, produced by glucose-6-phosphatase, out of the lumen. The results suggest that the pyrophosphate/phosphate transport system of human hepatic endoplasmic reticulum must be more complex than previously thought, as it must comprise at least two protein components.

Inherited metabolic diseases in the sudden infant death syndrome

All sudden, unexpected infant deaths presenting during a two year period within a defined geographical area in Avon and north Somerset were investigated for inherited metabolic disease. Of 95 deaths, 88 were classified as cases of sudden infant death syndrome (SIDS). In addition to the normal postmortem investigations, samples of cerebrospinal fluid, urine, vitreous humour, and skin were collected for metabolic studies. No abnormal organic acid metabolites were found in the fluids from the 88 cases of SIDS. Fatty acid oxidation was assessed in skin fibroblasts from 70 cases of SIDS, but no examples of medium chain acyl CoA dehydrogenase (MCAD) deficiency were found. One case with abundant glycogen in the liver was subsequently diagnosed as having glycogen storage disease type 1c. These findings suggest that the incidence of MCAD deficiency and other metabolic diseases in SIDS is much lower than previously claimed.

Transverse topology of glucose-6-phosphatase in rat hepatic endoplasmic reticulum

Antibodies raised against purified components of glucose-6-phosphatase were used to study the transmembrane orientation of the complex. Measurements of glucose-6-phosphatase activities and immunoblot analysis of sealed microsomes and detergent-solubilized microsomes after treatment with proteases suggested that most of the catalytic subunit resides within the lumen of the endoplasmic reticulum. In contrast, other components of glucose-6-phosphatase are accessible to the cytoplasm. Treatment of the partially purified glucose-6-phosphatase enzyme with glycopeptide N-glycosidase indicated that the catalytic subunit of the enzyme was a glycoprotein.

Kinetic and immunologic evidence for the absence of glucose-6-phosphatase in early human chorionic villi and term placenta

The existence of the enzyme glucose-6-phosphatase (G6Pase) in early and term human placenta was investigated by comparing the characteristics of placental microsomal glucose 6-phosphate (G6P) hydrolytic activity and liver G6Pase. Placental microsomes exhibited similar apparent Km values for G6P and beta-glycerophosphate in intact and deoxycholate-treated microsomes, heat stability at acidic pH, low latency of mannose 6-phosphate hydrolysis, very low activity of pyrophosphate: glucose phosphotransferase, and undetectable [U-14C]G6P transport into the placental microsomes, all of which indicated that specific G6Pase activity does not exist in placenta. Immunological evidence of the absence of both 36.5 kDa and T2 proteins, which represent the G6Pase catalytic protein and the phosphate/pyrophosphate transporter protein, respectively, confirmed that early and term human placenta are devoid of the multicomponent G6Pase enzyme.

Calcium activates glucose-6-phosphatase in intact rat hepatic microsomes

The effects of Ca2+ on the microsomal glucose-6-phosphatase activity were investigated. Evidence is provided that increases by Ca2+ in both the pyrophosphatase and the glucose-6-phosphate-hydrolysing activities are due to an increase in microsomal transport capacity of T2, the phosphate/pyrophosphate-transport protein.

Sudden, unexpected, natural death in childhood

One thousand nine hundred and fifty four autopsies performed at British Columbia's Children's Hospital during a 7-year period were reviewed to determine the causes of sudden unexpected natural death in the age group from birth to 17 years. Of the 126 cases found, the largest group, 86 cases, was sudden infant death syndrome (SIDS). Nine deaths were the result of infection: 4 cases of H. influenza meningitis, 2 cases of meningococcemia, 2 cases of acute epiglottitis, and 1 case of necrotizing tracheobronchitis. Epilepsy, ruptured AV malformations, and brain tumors combined to make up an equally large group of 9 cases. Cardiac lesions were the third largest group, 6 cases. The three groups that posed the most difficulty in assigning a cause of death were (a) the group that were like SIDS yet had other confounding features, (b) the group in which metabolic death was suspected but not proven, and (c) death in epilepsy.

Diagnosis of glycogen storage disease

Glycogen storage diseases are associated with more than 15 different enzyme deficiencies and can be clinically divided mainly into two groups, those that affect primarily the liver and those that affect principally the muscle. In this report each glycogenosis has been clinically and biochemically documented and possibilities for an accurate and prompt diagnosis of the various types have been summarized. Most of the patients suffering from type II, type III, type IV and type VIa can easily be diagnosed by analysis of peripheral blood cells without the need for tissue biopsies. First trimester diagnosis using chorionic villi is feasible for severe forms of the glycogenoses, type IIa, type IIIa and type IV.