Hunting for a hypoglycemia gene: severe neonatal hypoglycemia in a consanguineous family

Short-chain 3-hydroxyacyl-CoA dehydrogenase deficiency: the clinical relevance of an early diagnosis and report of four new cases

Short-chain 3-hydroxyacyl-CoA dehydrogenase (HADH, SCHAD) deficiency (OMIM #231530) represents a recently described disorder of mitochondrial fatty acid beta-oxidation, with less than ten cases described worldwide. The main clinical presentation of this metabolic disease is different from other inherited defects of fatty acid β-oxidation as the hypoglycemia is associated with hyperinsulinism. We present the clinical, biochemical and molecular findings of four new Caucasian patients with HADH deficiency. These new cases contribute to a more comprehensive description of the phenotype, diagnostic biomarkers and treatment options for this poorly defined disease.

Congenital hyperinsulinism due to mutations in HNF4A and HADH

Mutations in the HADH and HNF4A genes are rare causes of diazoxide responsive congenital hyperinsulinism (CHI). This chapter details the phenotype known to be associated with mutations in these genes. Additionally, the authors give a brief overview of the role of these genes in glucose physiology and the possible mechanisms of CHI in patients with mutations in these genes.

Alanine in HI: a silent mutation cries out!

It is a widely held paradigm in molecular biology that a change in the third base of a codon is silent in terms of expression. In this investigation, results are presented that challenge that paradigm, at least in terms of one polymorphism in KCNJ11, which is one of five genes that have been implicated in the disorder Hyperinsulinism of Infancy. In two cohorts of Australian patients, an uneven distribution of KCNJ11 SNP's was observed. A silent polymorphism at codon 190 was over-represented in the patients who responded well to medical treatment and under-represented in those that required radical surgical intervention. In an attempt to investigate this polymorphism, it was expressed in vitro and western blot analysis showed that there were virtually no bands from the homozygous variant samples, while strong bands were seen in normal controls. The human genome is highly redundant in terms of tRNA species for each amino acids but enigmatically under-represents a number of specific codons. The polymorphism in question occurs within one such codon. We propose that the presence of a base change at the third position of codon that is not represented by a corresponding anti-codon within the human nuclear tRNA leads to a decreased rate of expression of the protein.

The role of ATP sensitive channels in insulin secretion and the implications in persistent hyperinsulinemic hypoglycaemia of infancy (PHHI)

Persistent Hyperinsulinemic Hypoglycaemia of Infancy (PHHI) is a metabolic syndrome of unregulated insulin secretion. It is a heterogenous disease with causes linked to mutations of the ATP sensitive potassium channels of the beta cell, as well as to metabolism in the beta cell. 5 candidate genes--ABCC8, KCNJ11, GCK, GLUD1 and SCHAD have been implicated in the disease so far, however the aetiology of the disease remains unknown in up to 50% of all patients. We genotyped 43 subjects with PHHI (20 surgically treated and 23 medically treated) for disease associated mutations in the candidate genes. Mutations on ABCC8 were identified in 16 of the 20 (80%) of the surgically treated patients. One putative mutation was identified in the medically treated cohort. The polymorphism E23K on KCNJ11 that is associated with NIDDM was differentially distributed in the 2 cohorts. We discuss the mutations identified, emphasise the importance of the K-ATP channel in physiological processes and discuss the possibility that the disease is caused by mutations in other genes associated with insulin release, glucose metabolism in the beta cell or beta cell apoptosis and survival. We propose that these processes must be explored in order to further our understanding of PHHI.

Glucagon: the effects of its excess and deficiency on insulin action

AIM

To review the role that glucagon plays in physiology, physiopathology and clinical medicine.

DATA SYNTHESIS

Glucagon assays employing specific radioimmunoassay (RIA) techniques are now widely used to characterize pathologic conditions where the effect of the excess or deficiency of glucagon on insulin actions might play a role. Glucagon excess counteracts the action of insulin on glucose metabolism by stimulating glycogenolysis and gluconeogenesis. Aside from glucagon excess in association with glucagonoma, glucagon excess is found in several metabolic disturbances. In diabetes mellitus, hyperglycaemia is the consequence of the glycogenolytic and gluconeogenic effects of glucagon excess occurring in the setting of a relative insulin deficiency (i.e. Type 2 diabetes), whereas excess of glucagon and absent insulin levels are typical features of diabetic ketoacidosis. Although plasma glucagon levels of patients with diabetes are usually increased relative to the prevailing plasma glucose concentrations, it is a paradox that in those patients glucagon levels fail to rise when hypoglycaemia develops. Since glucagon release is considered the primary defence against insulin-induced hypoglycaemia, the defective response of glucagon to hypoglycaemia may favour the development of severe hypoglycaemia. Such defective response to hypoglycaemia in diabetes can be regarded as a condition of selective glucagon deficiency the mechanisms of which remain to be elucidated.

CONCLUSION

The most common condition associated with glucagon excess or deficiency is diabetes mellitus. Glucagon excess contributes to hyperglycaemia whereas reduced glucagon response to insulin-induced hypoglycaemia promotes severe hypoglycaemia. It is expected that drugs that are able to reduce glucagon secretion in concert with strategies directed to recover glucagon secretion to hypoglycaemia might contribute to improve the overall glycaemic control in diabetes.

Genetics and pathophysiology of hyperinsulinism in infancy

Hyperinsulinism in infancy (HI) is a condition of neonates and early childhood. For many years the pathophysiology of this potentially lethal disorder was unknown. Advances in the genetics, histopathology and molecular physiology of this disease have now provided insights into the causes of beta-cell dysfunction and revealed levels of diversity far in excess of our previous knowledge. These include defects in ion channel subunit genes and mutations in several enzymes associated with beta-cell metabolism and anaplerosis. In most cases, beta-cell pathophysiology leads to an alteration in the function of ATP-sensitive K(+) channels. This can manifest as 'channelopathies' of K(ATP) channels through gene defects in ABCC8 and KCNJ11 (Ch.11p15); or as a result of 'metabolopathies' through defects in the genes encoding glucokinase (GCK, Ch.7p15-p13), glutamate dehydrogenase (GLUD1, Ch.10q23.3) and short-chain L-3-hydroxyacyl-CoA dehydrogenase (HADHSC, Ch.4q22-q26). This review focuses upon the relationship between the causes of HI and therapeutic options.

Familial hyperinsulinemic hypoglycemia caused by a defect in the SCHAD enzyme of mitochondrial fatty acid oxidation

Inappropriately elevated insulin secretion is the hallmark of persistent hyperinsulinemic hypoglycemia of infancy (PHHI), also denoted congenital hyperinsulinism. Causal mutations have been uncovered in genes coding for the beta-cell's ATP-sensitive potassium channel and the metabolic enzymes glucokinase and glutamate dehydrogenase. In addition, one hyperinsulinemic infant was recently found to have a mutation in the gene encoding short-chain 3-hydroxyacyl-CoA dehydrogenase (SCHAD), an enzyme participating in mitochondrial fatty acid oxidation. We have studied a consanguineous family with severe neonatal hypoglycemia due to increased insulin levels and where well-established genetic causes of hyperinsulinism had been eliminated. A genome-wide, microsatellite-based screen for homozygous chromosomal segments was performed. Those regions that were inherited in accordance with the presupposed model were searched for mutations in genes encoding metabolic enzymes. A novel, homozygous deletion mutation was found in the gene coding for the SCHAD enzyme. The mutation affected RNA splicing and was predicted to lead to a protein lacking 30 amino acids. The observations at the molecular level were confirmed by demonstrating greatly reduced SCHAD activity in the patients' fibroblasts and enhanced levels of 3-hydroxybutyryl-carnitine in their blood plasma. Urine metabolite analysis showed that SCHAD deficiency resulted in specific excretion of 3-hydroxyglutaric acid. By the genetic explanation of our family's cases of severe hypoglycemia, it is now clear that recessively inherited SCHAD deficiency can result in PHHI. This finding suggests that mitochondrial fatty acid oxidation influences insulin secretion by a hitherto unknown mechanism.

Hyperinsulinism in Infancy: From Basic Science to Clinical Disease

Dunne, Mark J., Karen E. Cosgrove, Ruth M. Shepherd, Albert Aynsley-Green, and Keith J. Lindley. Hyperinsulinism in Infancy: From Basic Science to Clinical Disease. Physiol Rev 84: 239–275, 2004; 10.1152/physrev.00022.2003.—Ion channelopathies have now been described in many well-characterized cell types including neurons, myocytes, epithelial cells, and endocrine cells. However, in only a few cases has the relationship between altered ion channel function, cell biology, and clinical disease been defined. Hyperinsulinism in infancy (HI) is a rare, potentially lethal condition of the newborn and early childhood. The causes of HI are varied and numerous, but in almost all cases they share a common target protein, the ATP-sensitive K+channel. From gene defects in ion channel subunits to defects in β-cell metabolism and anaplerosis, this review describes the relationship between pathogenesis and clinical medicine. Until recently, HI was generally considered an orphan disease, but as parallel defects in ion channels, enzymes, and metabolic pathways also give rise to diabetes and impaired insulin release, the HI paradigm has wider implications for more common disorders of the endocrine pancreas and the molecular physiology of ion transport.