Title: Familial hyperinsulinism and pancreatic β-cell ATP-sensitive potassium channels
Abstract: Familial hyperinsulinism and pancreatic β-cell ATP-sensitive potassium channels. Familial hyperinsulinism, also known as persistent hyperinsulinemic hypoglycemia of infancy (PHHI), is a genetic disease characterized by mild to severe hypoglycemia in the presence of inappropriately high levels of insulin. The recessive form is caused by mutations in the adenosine 5′-triphosphate (ATP)-sensitive K+ channel (KATP channel) present in the plasma membrane of pancreatic β-cells. This channel is formed by two subunits, the high-affinity sulfonylurea receptor, SUR1, and KIR6.2, a member of the inwardly rectifying family of K+ channels. KATP channels regulate insulin secretion by linking membrane excitability with glucose metabolism. Approximately 50 mutations, in both channel subunits, that abolish or alter the regulation of β-cell KATP channels have been identified in patients with the recessive form of PHHI. Familial hyperinsulinism and pancreatic β-cell ATP-sensitive potassium channels. Familial hyperinsulinism, also known as persistent hyperinsulinemic hypoglycemia of infancy (PHHI), is a genetic disease characterized by mild to severe hypoglycemia in the presence of inappropriately high levels of insulin. The recessive form is caused by mutations in the adenosine 5′-triphosphate (ATP)-sensitive K+ channel (KATP channel) present in the plasma membrane of pancreatic β-cells. This channel is formed by two subunits, the high-affinity sulfonylurea receptor, SUR1, and KIR6.2, a member of the inwardly rectifying family of K+ channels. KATP channels regulate insulin secretion by linking membrane excitability with glucose metabolism. Approximately 50 mutations, in both channel subunits, that abolish or alter the regulation of β-cell KATP channels have been identified in patients with the recessive form of PHHI. Persistent hyperinsulinemic hypoglycemia of infancy (PHHI), also known as familial hyperinsulinism or nesidioblastosis (OMIM:256450), is an inherited disorder of glucose metabolism that presents in newborns and infants, and is mainly characterized by inappropriately high insulin levels in the presence of low levels of blood glucose. PHHI was originally thought to be a relatively homogenous disorder, but over the last several years an underlying heterogeneity has become clear1Stanley C.A. Hyperinsulinism in infants and children.Pediatric Clinics North Am. 1997; 44: 363-374Abstract Full Text Full Text PDF PubMed Scopus (181) Google Scholar. It is now understood that PHHI can present with a quite variable clinical picture, has at least two different histopathologic forms, can be inherited in either a recessive or dominant manner, and can be caused by mutations in at least four genes2Aguilar-Bryan L. Bryan J. The molecular biology of ATP-sensitive potassium channels.Endocrine Rev. 1999; 20: 101-135Crossref PubMed Scopus (607) Google Scholar. From the clinical point of view, some patients present with severe hypoglycemia almost immediately after birth, while others show mild hypoglycemia weeks or months after birth3Permutt M.A. Nestorowicz A. Glaser B. Familial hyperinsulinism: An inherited disorder of spontaneous hypoglycemia in neonates and infants.Diabetes Rev. 1996; 4: 347-355Google Scholar. A biochemical diagnosis is based on abnormally high insulin levels in the presence of persisting hypoglycemia, low ketone bodies (a result of their suppression by elevated insulin), and a glucagon test that shows an increased glycemic response due to increased glycogen storage in the liver. After the diagnosis of PHHI has been established, immediate treatment is directed towards the maintenance of euglycemia usually by continuous administration of intravenous glucose at rates between 15 and 20 mg/kg/min. If hypoglycemia persists, the second line of treatment is the administration of compounds that inhibit insulin secretion. Two compounds, diazoxide and somatostatin (octreotide), are widely used4Glaser B. Hirsch H.J. Landau H. Persistent hyperinsulinemic hypoglycemia of infancy: Long-term octreotide treatment without pancreatectomy.J Pediatr. 1993; 123: 644-650Abstract Full Text PDF PubMed Scopus (131) Google Scholar. If this clinical treatment fails to normalize glucose levels a partial or subtotal pancreatectomy becomes the treatment of choice1Stanley C.A. Hyperinsulinism in infants and children.Pediatric Clinics North Am. 1997; 44: 363-374Abstract Full Text Full Text PDF PubMed Scopus (181) Google Scholar. Mild forms of the disease may go undiagnosed until eating habits change and an infant has less frequent meals and thus becomes hypoglycemic. Diazoxide or octreotide for a few months may be sufficient to normalize glucose levels, and clinical remission of symptoms with time has been observed3Permutt M.A. Nestorowicz A. Glaser B. Familial hyperinsulinism: An inherited disorder of spontaneous hypoglycemia in neonates and infants.Diabetes Rev. 1996; 4: 347-355Google Scholar. In a follow-up study on patients who were treated clinically, Leibowitz et al have shown that although patients appear to go into remission, the beta cell defect remains and a significant number of these individuals become diabetic when they reach puberty5Leibowitz G. Glaser B. Higazi A.A. Salameh M. Cerasi E. Landau H. Hyperinsulinemic hypoglycemia of infancy (nesidioblastosis) in clinical remission: High incidence of diabetes mellitus and persistent β-cell dysfunction at long-term follow-up.J Clin Endo Metab. 1995; 80: 386-392Crossref PubMed Google Scholar. The frequency of the recessive form of PHHI in the general population is low (1:50,000), but is reported to be as high as 1:2500 in inbred populations with a high frequency of consanguineous marriages6Mathew P.M. Young J.M. Abu-Osba Y.K. Mulhern B.D. Hamoudi S. Hamadan J.A. Sa'di A.R. Persistent neonatal hyperinsulinism.Clin Pediatr (Phila). 1988; 27: 148-151Crossref PubMed Scopus (90) Google Scholar. PHHI is genetically heterogeneous. Four genes have now been identified to cause the disease, and in approximately 50% of the cases that have been screened for the known recessive and dominant genes, the mutations have not been identified. Two mild dominant forms of PHHI have been described that are caused by mutations in the glucokinase and glutamate dehydrogenase genes, respectively. Loss of function mutations in glucokinase have been identified as a cause of maturity-onset diabetes of the young, MODY27Stoffel M. Patel P. Lo Y.M. Hattersley A.T. Lucassen A.M. Page R. Bell J.I. Bell G.I. Turner R.C. Wainscoat J.S. Missense glucokinase mutation in maturity-onset diabetes of the young and mutation screening in late-onset diabetes.Nat Genet. 1992; 2: 153-156Crossref PubMed Scopus (117) Google Scholar,8Velho G. Froguel P. Genetic determinants of non-insulin-dependent diabetes mellitus: Strategies and recent results.Diabetes Metabolism. 1997; 23: 7-17PubMed Google Scholar. Interestingly, a point mutation, V455 M, in glucokinase has now been identified as causing one of the mild dominant forms of PHHI. This mutation produces an enzyme with a Km that is ∼65% lower in comparison with wild-type glucokinase (2.9 mmol/L vs. 8.4 mmol/L). This better binding of glucose to glucokinase results in insulin release at a lower blood glucose concentration9Glaser B. Kesavan P. Heyman M. Davis E. Cuesta A. Buchs A. Stanley C.A. Thornton P.S. Permutt M.A. Matschinsky F.M. Herold K. Familial hyperinsulinism caused by an activating glucokinase mutation.N Engl J Med. 1998; 338: 226-230Crossref PubMed Scopus (489) Google Scholar. The second dominant form of PHHI presents with asymptomatic hyperammonemia and hypoglycemia1Stanley C.A. Hyperinsulinism in infants and children.Pediatric Clinics North Am. 1997; 44: 363-374Abstract Full Text Full Text PDF PubMed Scopus (181) Google Scholar. Five mutations in the glutamate dehydrogenase gene have been identified that give this phenotype10Stanley C.A. Lieu Y.K. Hsu B.Y. Burlina A.B. Greenberg C.R. Hopwood N.J. Perlman K. Rich B.H. Zammarchi E. Poncz M. Hyperinsulinism and hyperammonemia in infants with regulatory mutations of the glutamate dehydrogenase gene.N Engl J Med. 1998; 338: 1352-1357Crossref PubMed Scopus (587) Google Scholar. These dominant forms of PHHI respond to treatment with diazoxide indicating KATP channels are functional. The recessive and most common form of the disease is caused by mutations in SUR1 and KIR6.2, the subunits of the pancreatic β-cell ATP-sensitive K+ channel. From the histopathologic point of view, PHHI was initially thought to result from nesidioblastosis following the report by Brown and Young of neoformation of islets in the pancreas of a newborn patient with severe hypoglycemia11Brown R.E. Young R.B. A possible role for the exocrine pancreas in the pathogenesis of neonatal leucine-sensitive hypoglycemia.Am J Dig Dis. 1970; 15: 65-72Crossref PubMed Scopus (36) Google Scholar. Nesidioblastosis was used to identify the disorder over the next decade, and it was not until the early 1980s that Jaffe, Hashida and Yunis12Jaffe R. Hashida Y. Yunis E.J. Pancreatic pathology in hyperinsulinemic hypoglycemia of infancy.Lab Invest. 1980; 42: 356-365PubMed Google Scholar and Rahier et al13Rahier J. Falt K. Muntefering H. Becker K. Gepts W. Falkmer S. The basic structural lesion of persistent neonatal hypoglycaemia with hyperinsulinism: Deficiency of pancreatic D cells or hyperactivity of B cells?.Diabetologia. 1984; 26: 282-289Crossref PubMed Scopus (137) Google Scholar, using specific staining techniques and by studying age-matched controls, determined that nesidioblastosis was present in both hypoglycemic and control groups. Following these observations and confirmation of the lack of specificity of nesidioblastosis, others have identified nesidioblastosis in diseases like MEN1, pancreatitis and cystic fibrosis. Recent observations have identified at least two histopathologic forms of PHHI: focal and diffuse14Goossens A. Gepts W. Saudubray J.M. Bonnefont J.P. Nihoul F. Heitz P.U. Kloppel G. Diffuse and focal nesidioblastosis. A clinicopathological study of 24 patients with persistent neonatal hyperinsulinemic hypoglycemia.Am J Surg Pathol. 1989; 13: 766-775Crossref PubMed Scopus (159) Google Scholar. The focal form is described by the presence of a well defined area within normal pancreatic tissue showing β-cells with enlarged nuclei and increased cytoplasmic volume. Enlarged cells with similar morphology, indicative of increased metabolic and secretory activity, are widely scattered throughout the pancreas in the diffuse form of PHHI. The differentiation of these two types is important as the focal form of the disease, present in ∼30 to 50% of cases that require surgery, responds well to partial rather than total pancreatectomy with the patient becoming euglycemic after removal of the hyperplastic lesion. Considerable effort is being directed at understanding the origin of the focal form of PHHI. SUR1 and KIR6.2 map to a region of chromosome 11, where other genes show loss of heterozygosity as a result of "imprinting." In some cases of the recessive form of PHHI, loss of heterozygosity appears to result from a loss of the maternal allele through a somatic deletion. PHHI can result when apparent homozygosity is achieved by combination of this imprinting effect with a genomic mutation in either of the paternal SUR1 alleles. At this time, no mutations in KIR6.2 have been found that reach the homozygous state by this mechanism15de Lonlay P. Fournet J. Rahier J. Gross-Morand M. Poggi-Travert F. Foussier V. Bonnefont J. Brusset M. Brunelle F. Robert J. Nihoul-Fekete C.J.S. Junien C. Somatic deletion of the imprinted 11p15 region in sporadic persistent hyperinsulinemic hypoglycemia of infancy is specific of focal adenomatous hyperplasia and endorses partial pancreatectomy.J Clin Invest. 1997; 100: 802-807Crossref PubMed Scopus (259) Google Scholar,16Verkarre V. De Fournet J.C. Lonlay P. Gross-Morand M.S. Devillers M. Rahier J. Brunelle F. Robert J.J. Nihoul-Fekete C. Saudubray J.M. Junien C. Paternal mutation of the sulfonylurea receptor (SUR1) gene and maternal loss of 11p15 imprinted genes lead to persistent hyperinsulinism in focal adenomatous hyperplasia.J Clin Invest. 1998; 102: 1286-1291Crossref PubMed Scopus (246) Google Scholar. The first suggestion that the recessive form of PHHI might be a channelopathy was the mapping of the SUR1 gene to the short arm of chromosome 11 (11p15.1). This location was within the region where previous family studies by Ben Glaser, Heddy Landau, Alan Permutt17Glaser B. Chiu K.C. Anker R. Nestorowicz A. Landau H. Ben-Bassat H. Shlomai Z. Kaiser N. Thornton P.S. Stanley C.A. Spielman R.S. Gogolin-Ewens K. Cerasi E. Baker L. Rice J. Donis-Keller H. Permutt M.A. Familial hyperinsulinism maps to chromosome 11p14–15.1, 30 cM centromeric to the insulin gene.Nat Genet. 1994; 7: 185-188Crossref PubMed Scopus (102) Google Scholar,18Glaser B. Chiu K.C. Liu L. Anker R. Nestorowicz A. Cox N.J. Landau H. Kaiser N. Thornton P.S. Stanley C.A. Cerasi E. Baker L. Donis-Keller H. Permutt M.A. Recombinant mapping of the familial hyperinsulinism gene to an 0.8 cM region on chromosome 11p15.1 and demonstration of a founder effect in Ashkenazi Jews.Hum Mol Genet. 1995; 4: 879-886Crossref PubMed Scopus (30) Google Scholar and Pamela Thomas19Thomas P.M. Cote G.J. Hallman D.M. Mathew P.M. Homozygosity mapping of the gene for familial persistent hyperinsulinemic hypoglycemia of infancy to chromosome 11p.Am J Hum Genet. 1995; 56: 416-421PubMed Google Scholar had localized the gene(s) for the recessive form of PHHI. Subsequent studies showed that the SUR1 and KIR6.2 genes were clustered in this region Figure 1. This cluster has been completely sequenced and is available as part of the sequence of the chromosome 11p14.3 PAC clone, pDJ239b22 (Genbank accession #s AC003969 & U90583). This clone contains a considerable amount of flanking sequence, which should facilitate investigation of the regulatory sequences that control expression of these genes. The region specifying both genes spans ∼90 Kb of DNA. SUR1 (OMIM 600509) encodes the high affinity sulfonylurea receptor (1581 or 1582 amino acids)20Aguilar-Bryan L. Nichols C.G. Wechsler S.W. Clement IV, Jp Boyd III, Ae Gonzalez G. Herrera-Sosa H. Nguy K. Bryan J. Nelson D.A. Cloning of the beta cell high-affinity sulfonylurea receptor: A regulator of insulin secretion.Science. 1995; 268: 423-426Crossref PubMed Scopus (1242) Google Scholar. The intronless KCNJ11 gene encoding the 390 amino acid inward rectifier, KIR6.2 (OMIM 600937), is 4900 base pairs 3′ of the end of the SUR1 gene21Inagaki N. Gonoi T. Clement IV, Jp Namba N. Inazawa J. Gonzalez G. Aguilar-Bryan L. Seino S. Bryan J. Reconstitution of IKATP: An inward rectifier subunit plus the sulfonylurea receptor.Science. 1995; 270: 1166-1170Crossref PubMed Scopus (1565) Google Scholar. A preliminary identification of the basal promoter sequences is available22Ashfield R. Ashcroft S.J. Cloning of the promoters for the beta-cell ATP-sensitive K-channel subunits Kir6.2 and SUR1.Diabetes. 1998; 47: 1274-1280PubMed Google Scholar. We have compared the structure of the SUR1-KIR6.2 gene cluster with the SUR2-KIR6.1 gene cluster elsewhere23Aguilar-Bryan L. Clement IV, Jp Gonzalez G. Kunjilwar K. Babenko A. Bryan J. Towards understanding the assembly and structure of KATP channels.Physiol Rev. 1998; 78: 227-245Crossref PubMed Scopus (495) Google Scholar. A definitive connection between the recessive form of PHHI and KATP channels was made by Dunne et al, who demonstrated the absence of ATP-sensitive K+ channel activity in the β-cells of a patient homozygous for a mutation in exon 35 that resulted in truncation of SUR1 in the second nucleotide binding fold24Dunne M.J. Kane C. Shepherd R.M. Sanchez J.A. James R.F.L. Johnson P.R.V. Aynsley-Green A. Lu S. Clement IV., J.P. Lindley K.J. Seino S. Aguilar-Bryan L. Familial persistent hyperinsulinemic hypoglycemia of infancy and mutations in the sulfonylurea receptor.N Engl J Med. 1997; 336: 703-706Crossref PubMed Scopus (219) Google Scholar. The patient was not responsive to diazoxide, requiring continuous infusion of glucose (18 mg/kg/min) to remain euglycemic. Subtotal (95%) pancreatectomy did not significantly reduce insulin levels and removal of 99% of the pancreas was necessary to control hypoglycemia. Electrical recording from isolated β-cells from this patient failed to show KATP channel activity while voltage-gated Ca2+ channels were spontaneously active. As in the five infants studied earlier25Kane C. Shepherd R.M. Squires P.E. Johnson P.R. James R.F. Milla P.J. Aynsley-Green A. Lindley K.J. Dunne M.J. Loss of functional KATP channels in pancreatic beta-cells causes persistent hyperinsulinemic hypoglycemia of infancy.Nat Med. 1996; 2: 1344-1347Crossref PubMed Scopus (214) Google Scholar, β-cell cytosolic Ca2+ levels were elevated beyond control cell values (83 vs. 115 nmol/L) and this elevation was assumed to initiate insulin secretion. A parallel mutation engineered into hamster SUR1 failed to generate KATP channel activity when cotransfected into COS cells with wild-type KIR6.2. Biochemical studies on the parallel mutation showed the truncated receptor was produced and retained high affinity sulfonylurea binding activity. Preliminary results indicate that the truncated receptor is able to associate with and cophotolabel KIR6.2. For the KIR6.2 gene, three mutations have been identified that cause PHHI Figure 2. Thomas, Ye and Lightner first reported a KIR6.2 mutation, a leu→pro change, L147P, near the extracellular side of the second transmembrane helix (M2), to cause PHHI26Thomas P. Ye Y. Lightner E. Mutations of the pancreatic islet inward rectifier also lead to familial persistent hyperinsulinemic hypoglycemia of infancy.Hum Mol Genet. 1996; 5: 1809-1812Crossref PubMed Scopus (365) Google Scholar. This mutation was identified in a child of Iranian origin, the progeny of a first cousin marriage. Diagnosis of PHHI was based on an insulin level openface> 30 mU/mL with a glucose level <30 mg/dL and a requirement openface> 15 mg glucose kg/min to maintain euglycemia. Substitution of this proline into M2 abolishes KATP channel activity when it is engineered into KIR6.2 and expressed with wild-type SUR1. It is not clear if KIR6.2L147P can fold correctly and assemble with SUR1, as we have been unable to show co-photolabeling with 125I-iodoazidoglibenclamide (Aguilar-Bryan, unpublished data). Nestorowicz et al described a nonsense mutation in KIR6.2 that truncated the protein after 12 amino acids, Y12X27Nestorowicz A. Inagaki N. Gonoi T. Schoor K.P. Wilson B.A. Glaser B. Landau H. Stanley C.A. Thornton P.S. Seino S. Permutt M.A. A nonsense mutation in the inward rectifier potassium channel gene, Kir6.2, is associated with familial hyperinsulinism.Diabetes. 1997; 46: 1743-1748Crossref PubMed Scopus (0) Google Scholar. The patient was homozygous for this mutation. When Y12X was engineered into KIR6.2, as expected for a 12 residue peptide that is missing all of the elements required to form the K+ channel pore, it did not form a functional channel when coexpressed with wild-type SUR1. Sharma and Aguilar-Bryan have identified a third KIR6.2 mutant, a trp→arg change, W91R, near the external side of M1 (unpublished data). This mutation was identified in a newborn, the product of a first cousin marriage of Palestinian descent. Clinical treatment with diazoxide and somatostatin was not successful and a partial pancreatectomy was performed at two weeks of age; a second resection was necessary four weeks later. When coexpressed with wild-type SUR1, KIR6.2W91R failed to produce active channels. Fifty mutations in the SUR1 gene have been identified in PHHI patients. These mutations can be divided into two main groups based on the severity of the disease Figure 3. Point mutations that result in amino acid substitutions can produce mild forms of the disorder, while nonsense mutations or those that alter splice sites and result in truncation of the receptor are associated with severe cases. Studies on cells expressing the G1479R missense mutant of SUR1, identified in a patient with a mild case of familial hyperinsulinism, suggests adenosine 5′-diphosphate (ADP) rather than ATP is the physiologic regulator of KATP channels28Nichols C.G. Shyng S.L. Nestorowicz A. Glaser B. Clement IV, Jp Gonzalez G. Aguilar-Bryan L. Permutt M.A. Bryan J. Adenosine diphosphate as an intracellular regulator of insulin secretion.Science. 1996; 272: 1785-1787Crossref PubMed Scopus (454) Google Scholar. This mutation in NBF2 was identified by Dr. Ann Nestorowicz in Dr. Alan Permutt's laboratory in a patient of Iraqi and Moroccan Jewish extraction diagnosed with PHHI by Drs. Heddy Landau and Benjamin Glaser (Hebrew University, Hadassah Medical Center, Jerusalem). The patient is a compound heterozygote, with the G1479R allele on one chromosome and a second, unidentified allele on the other. In excised membrane patches MgADP antagonizes the inhibitory action of ATP on KATP channels. In the G1479R channels this antagonizing action is strongly reduced28Nichols C.G. Shyng S.L. Nestorowicz A. Glaser B. Clement IV, Jp Gonzalez G. Aguilar-Bryan L. Permutt M.A. Bryan J. Adenosine diphosphate as an intracellular regulator of insulin secretion.Science. 1996; 272: 1785-1787Crossref PubMed Scopus (454) Google Scholar. Co-expression of SUR1G1479R with wild-type KIR6.2 produces K+ channels that are inhibited by ATP4-, or MgATP, in excised patches with nearly the same IC50 as native or wild-type reconstituted channels; however, these channels are poorly activated by MgADP28Nichols C.G. Shyng S.L. Nestorowicz A. Glaser B. Clement IV, Jp Gonzalez G. Aguilar-Bryan L. Permutt M.A. Bryan J. Adenosine diphosphate as an intracellular regulator of insulin secretion.Science. 1996; 272: 1785-1787Crossref PubMed Scopus (454) Google Scholar. The SUR1G1479R/KIR6.2 channels are also poorly activated by conditions of metabolic inhibition that strongly activate wild-type channels. We have concluded that in PHHI β-cells the G1479R channels are blocked by ATP and apparently fail to respond to fluctuations in ADP. The result implies that fluctuations of [ATP]i alone are not sufficient to trigger insulin secretion in either the recombinant channels during metabolic inhibition or in PHHI patients and indicate ADP is a critical factor. Approximately 25% of the mutations identified in recessive cases of PHHI appear to have lost this stimulatory response to ADP2Aguilar-Bryan L. Bryan J. The molecular biology of ATP-sensitive potassium channels.Endocrine Rev. 1999; 20: 101-135Crossref PubMed Scopus (607) Google Scholar,29Shyng S.L. Ferrigni T. Shepard J.B. Nestorowicz A. Glaser B. Permutt M.A. Nichols C.G. Functional analyses of novel mutations in the sulfonylurea receptor 1 associated with persistent hyperinsulinemic hypoglycemia of infancy.Diabetes. 1998; 47: 1145-1151Crossref PubMed Scopus (136) Google Scholar. Patients with these SUR1 mutations usually respond well to a high carbohydrate diet, diazoxide or octreotide treatment. A number of mutations identified with severe cases of PHHI cause truncation of the receptor. As noted above for the exon 35 mutation reconstitution of the engineered mutant receptor with wild-type KIR6.2 does not produce active KATP channels. The reason for this failure is not clear. We have looked at recombinant receptors engineered with smaller truncations (one example with 49 amino acids truncated from the SUR1 C-terminus is shown in Figure 4) in order to determine whether smaller deletions can be tolerated without loss of channel function. 86Rb+ efflux data Figure 4a show that deletion of 49 amino acids from the C-terminus of SUR1 reduces channel activity to background levels equivalent to that observed with SUR1 or KIR6.2 alone21Inagaki N. Gonoi T. Clement IV, Jp Namba N. Inazawa J. Gonzalez G. Aguilar-Bryan L. Seino S. Bryan J. Reconstitution of IKATP: An inward rectifier subunit plus the sulfonylurea receptor.Science. 1995; 270: 1166-1170Crossref PubMed Scopus (1565) Google Scholar. Comparison of the photolabeling pattern of the SUR1αδC49 receptor with and without KIR6.2 indicates the truncated receptors have not undergone maturation. This result suggests that truncation of the C-terminus of SUR1 interferes with trafficking of the channel complex to the plasma membrane. It is now understood that mutations in SUR1 and KIR6.2 can result in the loss of KATP channel activity in pancreatic β-cells and cause a recessive form of PHHI. These results emphasize the important role of ionic mechanisms in control of insulin secretion. Efforts to understand how individual mutations result in loss of channel activity provide insight into the molecular mechanism(s) of regulation of this family of channels and their expression on the cell surface. This work was supported by grants from the American Diabetes Association (ADA) to Dr. Aguilar-Bryan, and NIH grants DK44311, DK50750, and DK52771 to Dr. J. Bryan. Dr. Aguilar-Bryan is a T.C. Chao Scholar. We thank Li-Zhen Song for technical assistance, and other members of the Baylor group for encouragement.