Paul S. Thornton

A Nonsense Mutation in the Inward Rectifier Potassium Channel Gene, Kir6.2, Is Associated With Familial Hyperinsulinism

ATP-sensitive potassium (KATP) channels are an essential component of glucose-dependent insulin secretion in pancreatic islet β-cells. These channels comprise the sulfonylurea receptor (SUR1) and Kir6.2, a member of the inward rectifier K+ channel family. Mutations in the SUR1 subunit are associated with familial hyperinsulinism (HI) (MIM:256450), an inherited disorder characterized by hyperinsulinism in the neonate. Since the Kir6.2 gene maps to human chromosome 11p15.1 (1,2), which also encompasses a locus for HI, we screened the Kir6.2 gene for the presence of mutations in 78 HI probands by single-strand conformation polymorphism (SSCP) and nucleotide sequence analyses. A nonsense mutation, Tyr→Stop at codon 12 (designated Y12X) was observed in the homozygous state in a single proband. 86Rb+ efflux measurements and single-channel recordings of COS-1 cells co-expressing SUR1 and either wild-type or Y12X mutant Kir6.2 proteins confirmed that KATP channel activity was abolished by this nonsense mutation. The identification of an HI patient homozygous for the Kir6.2/Y12X allele affords an opportunity to observe clinical features associated with mutations resulting in an absence of Kir6.2. These data provide evidence that mutations in the Kir6.2 sub-unit of the islet β-cell KATP channel are associated with the HI phenotype and also suggest that the majority of HI cases are not attributable to mutations in the coding region of the Kir6.2 gene.

Genetics of neonatal hyperinsulinism

Congenital hyperinsulinism (HI) is a clinically and genetically heterogeneous entity. The clinical heterogeneity is manifested by severity ranging from extremely severe, life threatening disease to very mild clinical symptoms, which may even be difficult to identify. Furthermore, clinical responsiveness to medical and surgical management is extremely variable. Recent discoveries have begun to clarify the molecular aetiology of this disease and thus the mechanisms responsible for this clinical heterogeneity are becoming more clear. Mutations in 4 different genes have been identified in patients with this clinical syndrome. Most cases are caused by mutations in either of the 2 subunits of the β cell ATP sensitive K+ channel (K ATP ), whereas others are caused by mutations in the β cell enzymes glucokinase and glutamate dehydrogenase. However, for as many as 50% of the cases, no genetic aetiology has yet been determined. The study of the genetics of this disease has provided important new information about β cell physiology. Although the clinical ramifications of these findings are still limited, in some situations genetic studies might greatly aid in patient management.

Clinical characteristics and biochemical mechanisms of congenital hyperinsulinism associated with dominant KATP channel mutations

Congenital hyperinsulinism is a condition of dysregulated insulin secretion often caused by inactivating mutations of the ATP-sensitive K+ (KATP) channel in the pancreatic beta cell. Though most disease-causing mutations of the 2 genes encoding KATP subunits, ABCC8 (SUR1) and KCNJ11 (Kir6.2), are recessively inherited, some cases of dominantly inherited inactivating mutations have been reported. To better understand the differences between dominantly and recessively inherited inactivating KATP mutations, we have identified and characterized 16 families with 14 different dominantly inherited KATP mutations, including a total of 33 affected individuals. The 16 probands presented with hypoglycemia at ages from birth to 3.3 years, and 15 of 16 were well controlled on diazoxide, a KATP channel agonist. Of 29 adults with mutations, 14 were asymptomatic. In contrast to a previous report of increased diabetes risk in dominant KATP hyperinsulinism, only 4 of 29 adults had diabetes. Unlike recessive mutations, dominantly inherited KATP mutant subunits trafficked normally to the plasma membrane when expressed in COSm6 cells. Dominant mutations also resulted in different channel-gating defects, as dominant ABCC8 mutations diminished channel responses to magnesium adenosine diphosphate or diazoxide, while dominant KCNJ11 mutations impaired channel opening, even in the absence of nucleotides. These data highlight distinctive features of dominant KATP hyperinsulinism relative to the more common and more severe recessive form, including retention of normal subunit trafficking, impaired channel activity, and a milder hypoglycemia phenotype that may escape detection in infancy and is often responsive to diazoxide medical therapy, without the need for surgical pancreatectomy.

Recommendations from the Pediatric Endocrine Society for Evaluation and Management of Persistent Hypoglycemia in Neonates, Infants, and Children

Recommendations from the Pediatric Endocrine Society for Evaluation and Management of Persistent Hypoglycemia in Neonates, Infants, and Children Paul S. Thornton, MB, BCh, Charles A. Stanley, MD, Diva D. De Leon, MD, MSCE, Deborah Harris, PhD, Morey W. Haymond, MD, Khalid Hussain, MD, MPH, Lynne L. Levitsky, MD, Mohammad H. Murad, MD, MPH, Paul J. Rozance, MD, Rebecca A. Simmons, MD, Mark A. Sperling, MBBS, David A. Weinstein, MD, MMSc, Neil H. White, MD, and Joseph I. Wolfsdorf, MB, BCh

A syndrome of congenital hyperinsulinism and hyperammonemia

This report describes two patients from unrelated families with an unusual syndrome of hyperinsulinism plus hyperammonemia. The diagnosis of hyperinsulinism was based on the demonstration of fasting hypoglycemia with inappropriately elevated insulin levels, inappropriately low beta-hydroxybutyrate and free fatty acid levels, and inappropriately large glycemic response to the administration of glucagon. In both patients, plasma ammonium levels were persistently elevated and unaffected by protein feeding, protein restriction, or benzoate therapy. Plasma and urinary amino acids, urinary organic acids, and urinary orotic acid levels were not consistent with any of the urea cycle enzyme defects or other hyperammonemic disorders. These two patients appear to represent a unique form of congenital hyperinsulinism distinct from the previously described autosomal dominant and autosomal recessive variants. We speculate that the underlying defect involves a site that is common to the amino acid regulation of both insulin secretion in pancreatic beta-cells and urea synthesis in the liver.

Familial hyperinsulinism with apparent autosomal dominant inheritance: Clinical and genetic differences from the autosomal recessive variant

We describe three families with hypoglycemia caused by familial hyperinsulinism (HI) in whom vertical transmission of the disorder occurred, suggesting autosomal dominant (AD) inheritance. We therefore examined the relationship between the apparent AD disorder and the more common autosomal recessive (AR) form of HI, which has recently been linked to the sulfonylurea receptor on chromosome 11p15.1. The clinical features of the 11 patients with AD HI were milder than those seen in 14 patients with AR HI. Hypoglycemia was readily controlled with either diet alone or with diazoxide in 10 of 11 patients with AD HI but in none of those with the AR form. In one large pedigree, analysis of genomic DNA with polymorphic simple sequence repeat markers excluded linkage of AD HI to the SUR locus in a dominant manner. The possibility of linkage to the SUR locus could not be absolutely excluded in the two smaller pedigrees. None of the published mutations of the SUR gene identified in patients with AR HI were detected in the patients with the AD form. We conclude that the AD form of hyperinsulinism is phenotypically different from the AR variant. The identification of more families with this form of HI may make it possible to locate the responsible gene by the use of linkage analysis.

Re-Evaluating "Transitional Neonatal Hypoglycemia": Mechanism and Implications for Management

A Committee of the Pediatric Endocrine Society was recently asked by xxx to develop guidelines for evaluation and management of hypoglycemia in neonates, infants, and children. To aid in formulating recommendations for neonates, in this review, we analyzed available data on the brief period of hypoglycemia which commonly is observed in normal newborns during the transition from fetal to extrauterine life, hereafter referred to as transitional neonatal hypoglycemia in normal newborns. The goal was to better understand the mechanism underlying this phenomenon in order to formulate recommendations for recognizing neonates requiring diagnosis and treatment during the first days of life for disorders causing severe and persistent hypoglycemia. It has long been known that plasma glucose concentrations are lower in the first 1–3 days of life in normal newborn infants than at later ages. Not until the 1960s was it appreciated that hypoglycemia in neonates could sometimes be symptomatic and, as in older infants and children, cause seizures or permanent brain damage (1, 2). Although studies in laboratory animals have demonstrated postnatal developmental changes in specific enzymes involved in hepatic gluconeogenesis and ketogenesis (3, 4), it is unclear that such changes adequately explain transitional neonatal hypoglycemia in human newborns or if other mechanisms may be involved (5, 6). A National Institutes of Health conference outlined many of the “gaps in knowledge” about neonatal hypoglycemia and lamented the lack of a rational basis for defining hypoglycemia in neonates (7). For this re-evaluation of transitional neonatal hypoglycemia in normal newborns, we used the strategy routinely employed by pediatric endocrinologists for evaluation of hypoglycemia in older infants and children. This strategy, based on an examination of the major metabolic fuel and hormone responses to hypoglycemia, makes it possible to discover the mechanism of hypoglycemia and to make a specific diagnosis of the underlying cause (Figure; available at www.jpeds.com) (8). We reviewed published data in normal newborns on metabolic fuel and hormone responses during the period of transitional neonatal hypoglycemia. We focused on mean responses as being most likely representative of normal newborns, recognizing the possibility of heterogeneity, particularly with regard to peri-partum stresses and feeding practices. We found that transitional neonatal hypoglycemia most closely resembles known genetic forms of congenital hyperinsulinism, which cause a lowering of the plasma glucose threshold for suppression of insulin secretion. This conclusion is based on strong evidence supported by two or more independent reports and provides a novel perspective on both the diagnosis and management of hypoglycemia in the first several days after birth. Figure Hypoglycemia diagnosis based on plasma metabolic fuel responses. Measurement of major fuels (lactate as a gluconeogenic substrate, FFA from adipose tissue lipolysis, and beta-hydroxybutyrate as the major ketone from hepatic ketogenesis) at a time of hypoglycemia ...

Histopathology of Congenital Hyperinsulinism: Retrospective Study with Genotype Correlations

The majority of the most severe cases of congenital hyperinsulinism (HI) are caused by defects in the β-cell adenosine triphosphate (ATP)-sensitive potassium channel and usually require pancreatectomy to control blood sugar levels. In contrast to the recent advances in understanding the pathophysiology and genetic bases of HI, the histologic classification of this condition remains controversial. A recent proposal to classify the HI pancreata into diffuse and focal forms has drawn much interest because of its relative simplicity and its good correlation with the genetic abnormalities. We undertook a retrospective study to determine whether this classification scheme could be applied to 38 pancreata resected for HI at our institution. We also obtained leukocyte genomic DNA from 29 cases and screened the exons of ABCC8 and KCNJ11 genes for the presence of mutations. Nineteen cases (50.0%) were histologically classified as diffuse HI and 14 cases (36.8%) were categorized as focal form. The mutational analysis revealed that 14 of the 16 diffuse cases analyzed had either homozygous or compound heterozygous mutations of ABCC8 or KCNJ11 and 7 of 10 focal cases had only the paternally inherited mutations, consistent with the previous observations. Two patients (5.3%) had normal pancreatic histology but had persistent hypoglycemia postoperatively, leaving the possibility of residual focal lesion. Three of 38 cases (7.9%) did not fit well into either diffuse or focal category. Two cases differed from the described pattern for the diffuse form in that the nuclear enlargement was confined to a single area of the pancreas. The other case had a focal lesion but β-cell nuclear enlargement was present in nonadjacent areas. Mutations for typical diffuse or focal HI were not identified in two of these three equivocal cases. We conclude from this study that nearly 90% of HI cases can be classified into either a diffuse or a focal form. However, a small percentage of cases represented a diagnostic challenge.

Familial and sporadic hyperinsulinism: Histopathologic findings and segregation analysis support a single autosomal recessive disorder

We evaluated the possible genetic contribution to hyperinsulinism in a series of patients seen during the past 15 years. Of 26 families, 5 (19%) had more than one child affected (multiplex family). There were no apparent differences between patients in the 5 multiplex and 21 simplex families, clinically, biochemically, or on histologic examination of the pancreatic specimens. The families studied had a total of 63 offspring; the 26 index patients had 37 siblings, 6 of whom were affected. After four patients with hyperinsulinism caused by adenoma were excluded from the study, segregation analysis was carried out to test the data for agreement with results expected if familial and isolated hyperinsulinism represented a single disease with recessive mode of inheritance and a segregation ratio of 0.25. Excellent agreement was found between the observed number of affected siblings (20) and the expected number (19.65), with a segregation ratio of 0.254. The results were consistent with the hypothesis that in most or all cases, hyperinsulinism is inherited as an autosomal recessive disease. There was no evidence of distinct familial and sporadic types.

Molecular and immunohistochemical analyses of the focal form of congenital hyperinsulinism.

Congenital hyperinsulinism is a rare pancreatic endocrine cell disorder that has been categorized histologically into diffuse and focal forms. In focal hyperinsulinism, the pancreas contains a focus of endocrine cell adenomatous hyperplasia, and the patients have been reported to possess paternally inherited mutations of the ABCC8 and KCNJ11 genes, which encode subunits of an ATP-sensitive potassium channel (KATP). In addition, the hyperplastic endocrine cells show loss of maternal 11p15, where imprinted genes such as p57kip2 reside. In order to evaluate whether all cases of focal hyperinsulinism are caused by this mechanism, 56 pancreatectomy specimens with focal hyperinsulinism were tested for the loss of maternal allele by two methods: immunohistochemistry for p57kip2 (n=56) and microsatellite marker analysis (n=27). Additionally, 49 patients were analyzed for KATP mutations. Out of 56 focal lesions, 48 demonstrated clear loss of p57kip2 expression by immunohistochemistry. The other eight lesions similarly showed no nuclear labeling, but the available tissue was not ideal for definitive interpretation. Five of these eight patients had paternal KATP mutations, of which four demonstrated loss of maternal 11p15 within the lesion by microsatellite marker analysis. All of the other three without a paternal KATP mutation showed loss of maternal 11p15. KATP mutation analysis identified 32/49 cases with paternal mutations. There were seven patients with nonmaternal mutations whose paternal DNA material was not available, and one patient with a mutation that was not present in either parents DNA. These eight patients showed either loss of p57kip2 expression or loss of maternal 11p15 region by microsatellite marker analysis, as did the remaining nine patients with no identifiable KATP coding region mutations. The combined results from the immunohistochemical and molecular methods indicate that maternal 11p15 loss together with paternal KATP mutation is the predominant causative mechanism of focal hyperinsulinism.