Analysis of transcription factors key for mouse pancreatic development establishes NKX2-2 and MNX1 mutations as causes of neonatal diabetes in man

Understanding transcriptional regulation of pancreatic development is required to advance current efforts in developing beta cell replacement therapies for patients with diabetes. Current knowledge of key transcriptional regulators has predominantly come from mouse studies, with rare, naturally occurring mutations establishing their relevance in man. This study used a combination of homozygosity analysis and Sanger sequencing in 37 consanguineous patients with permanent neonatal diabetes to search for homozygous mutations in 29 transcription factor genes important for murine pancreatic development. We identified homozygous mutations in 7 different genes in 11 unrelated patients and show that NKX2-2 and MNX1 are etiological genes for neonatal diabetes, thus confirming their key role in development of the human pancreas. The similar phenotype of the patients with recessive mutations and mice with inactivation of a transcription factor gene support there being common steps critical for pancreatic development and validate the use of rodent models for beta cell development.

Biallelic PDX1 (insulin promoter factor 1) mutations causing neonatal diabetes without exocrine pancreatic insufficiency

AIMS

Recessive PDX1 (IPF1) mutations are a rare cause of pancreatic agenesis, with three cases reported worldwide. A recent report described two cousins with a homozygous hypomorphic PDX1 mutation causing permanent neonatal diabetes with subclinical exocrine insufficiency. The aim of our study was to investigate the possibility of hypomorphic PDX1 mutations in a large cohort of patients with permanent neonatal diabetes and no reported pancreatic hypoplasia or exocrine insufficiency.

METHODS

PDX1 was sequenced in 103 probands with isolated permanent neonatal diabetes in whom ABCC8, KCNJ11 and INS mutations had been excluded.

RESULTS

Sequencing analysis identified biallelic PDX1 mutations in three of the 103 probands with permanent neonatal diabetes (2.9%). One proband and his affected brother were compound heterozygotes for a frameshift and a novel missense mutation (p.A34fsX191; c.98dupC and p.P87L; c.260C>T). The other two probands were homozygous for novel PDX1 missense mutations (p.A152G; c.455C>G and p.R176Q; c.527G>A). Both mutations affect highly conserved residues located within the homeobox domain. None of the four cases showed any evidence of exocrine pancreatic insufficiency, either clinically, or, where data were available, biochemically. In addition a heterozygous nonsense mutation (p.C18X; c.54C>A) was identified in a fourth case.

CONCLUSIONS

This study demonstrates that recessive PDX1 mutations are a rare but important cause of isolated permanent neonatal diabetes in patients without pancreatic hypoplasia/agenesis. Inclusion of the PDX1 gene in mutation screening for permanent neonatal diabetes is recommended as a genetic diagnosis reveals the mode of inheritance, allows accurate estimation of recurrence risks and confirms the requirement for insulin treatment.

HNF1B deletions in patients with young-onset diabetes but no known renal disease

AIMS

Hepatocyte nuclear factor 1β (HNF1B) mutations cause a syndrome of renal cysts and diabetes, with whole gene deletions accounting for approximately 50% of cases. The severity of the renal phenotype is variable, from enlarged cystic kidneys incompatible with life to normal renal development and function. We investigated the prevalence of HNF1B deletions in patients with diabetes but no known renal disease.

METHODS

We tested 461 patients with familial diabetes diagnosed before 45 years, including 258 probands who met clinical criteria for maturity-onset diabetes of the young (two generations affected and at least one family member diagnosed under 25 years). A fluorescent polymerase chain reaction assay was used to analyse two intragenic polymorphic HNF1B markers and identify heterozygous patients who therefore did not have whole gene deletions. Those patients homozygous for both markers were then tested for an HNF1B deletion using multiplex ligation-dependent probe amplification.

RESULTS

Heterozygous HNF1B intragenic polymorphisms were identified in 337/461 subjects. Multiplex ligation-dependent probe amplification analysis showed an HNF1B gene deletion in three of the remaining 124 probands, all of whom met the criteria for maturity-onset diabetes of the young. Testing of their relatives identified three additional deletion carriers and ultrasound scanning showed renal developmental abnormalities in three of these six patients.

CONCLUSIONS

We estimate that HNF1B mutations account for < 1% of cases of maturity-onset diabetes of the young. Although HNF1B mutations are a rare cause of diabetes in the absence of known renal disease, a genetic diagnosis of renal cysts and diabetes syndrome is important as it raises the possibility of subclinical renal disease and the 50% risk of renal cysts and diabetes syndrome in the patient's offspring.

Insights into the pathogenicity of rare missense GCK variants from the identification and functional characterization of compound heterozygous and double mutations inherited in cis

OBJECTIVE

To demonstrate the importance of using a combined genetic and functional approach to correctly interpret a genetic test for monogenic diabetes.

RESEARCH DESIGN AND METHODS

We identified three probands with a phenotype consistent with maturity-onset diabetes of the young (MODY) subtype GCK-MODY, in whom two potential pathogenic mutations were identified: [R43H/G68D], [E248 K/I225M], or [G261R/D217N]. Allele-specific PCR and cosegregation were used to determine phase. Single and double mutations were kinetically characterized.

RESULTS

The mutations occurred in cis (double mutants) in two probands and in trans in one proband. Functional studies of all double mutants revealed inactivating kinetics. The previously reported GCK-MODY mutations R43H and G68D were inherited from an affected father and unaffected mother, respectively. Both our functional and genetic studies support R43H as the cause of GCK-MODY and G68D as a neutral rare variant.

CONCLUSIONS

These data highlight the need for family/functional studies, even for previously reported pathogenic mutations.

Heterozygous ABCC8 mutations are a cause of MODY

AIMS/HYPOTHESIS

The ABCC8 gene encodes the sulfonylurea receptor 1 (SUR1) subunit of the pancreatic beta cell ATP-sensitive potassium (K(ATP)) channel. Inactivating mutations cause congenital hyperinsulinism (CHI) and activating mutations cause transient neonatal diabetes (TNDM) or permanent neonatal diabetes (PNDM) that can usually be treated with sulfonylureas. Sulfonylurea sensitivity is also a feature of HNF1A and HNF4A MODY, but patients referred for genetic testing with clinical features of these types of diabetes do not always have mutations in the HNF1A/4A genes. Our aim was to establish whether mutations in the ABCC8 gene cause MODY that is responsive to sulfonylurea therapy.

METHODS

We sequenced the ABCC8 gene in 85 patients with a BMI <30 kg/m², no family history of neonatal diabetes and who were deemed sensitive to sulfonylureas by the referring clinician or were sulfonylurea-treated. All had tested negative for mutations in the HNF1A and HNF4A genes.

RESULTS

ABCC8 mutations were found in seven of the 85 (8%) probands. Four patients were heterozygous for previously reported mutations and four novel mutations, E100K, G214R, Q485R and N1245D, were identified. Only four probands fulfilled MODY criteria, with two diagnosed after 25 years and one patient, who had no family history of diabetes, as a result of a proven de novo mutation.

CONCLUSIONS/INTERPRETATION

ABCC8 mutations can cause MODY in patients whose clinical features are similar to those with HNF1A/4A MODY. Therefore, sequencing of ABCC8 in addition to the known MODY genes should be considered if such features are present, to facilitate optimal clinical management of these patients.

Sequencing PDX1 (insulin promoter factor 1) in 1788 UK individuals found 5% had a low frequency coding variant, but these variants are not associated with Type 2 diabetes

AIM

Genome-wide association studies have identified >30 common variants associated with Type 2 diabetes (>5% minor allele frequency). These variants have small effects on individual risk and do not account for a large proportion of the heritable component of the disease. Monogenic forms of diabetes are caused by mutations that occur in <1:2000 individuals and follow strict patterns of inheritance. In contrast, the role of low frequency genetic variants (minor allele frequency 0.1-5%) in Type 2 diabetes is not known. The aim of this study was to assess the role of low frequency PDX1 (also called IPF1) variants in Type 2 diabetes.

METHODS

We sequenced the coding and flanking intronic regions of PDX1 in 910 patients with Type 2 diabetes and 878 control subjects.

RESULTS

We identified a total of 26 variants that occurred in 5.3% of individuals, 14 of which occurred once. Only D76N occurred in >1%. We found no difference in carrier frequency between patients (5.7%) and control subjects (5.0%) (P=0.46). There were also no differences between patients and control subjects when analyses were limited to subsets of variants. The strongest subset were those variants in the DNA binding domain where all five variants identified were only found in patients (P=0.06).

CONCLUSION

Approximately 5% of UK individuals carry a PDX1 variant, but there is no evidence that these variants, either individually or cumulatively, predispose to Type 2 diabetes. Further studies will need to consider strategies to assess the role of multiple variants that occur in <1 in 1000 individuals.

Mutations in the hepatocyte nuclear factor-1β (HNF1B) gene are common with combined uterine and renal malformations but are not found with isolated uterine malformations

OBJECTIVE

Congenital uterine abnormalities are common and may be associated with developmental renal abnormalities. Mutations of the hepatocyte nuclear factor-1β (HNF1B) gene are associated with renal and uterine abnormalities. We aimed to study the role of HNF1B mutations in a cohort with congenital uterine abnormalities.

STUDY DESIGN

We tested 108 probands with uterine abnormalities for HNF1B mutations. We collected clinical information from patient records.

RESULTS

Nine of 108 women (8%) had a mutation or deletion in the HNF1B gene. Abnormal HNF1B was found in 18% of the 50 probands who had both uterine and renal abnormalities but in none of the 58 women with isolated uterine abnormalities.

CONCLUSION

Mutations of the HNF1B gene are found in women with both uterine and renal abnormalities but are rare in isolated uterine abnormalities. We suggest that HNF1B testing should be performed in patients with both renal and uterine abnormalities, but not in patients with isolated uterine abnormalities.

Permanent neonatal diabetes due to activating mutations in ABCC8 and KCNJ11

The ATP-sensitive potassium (K(ATP)) channel is composed of two subunits SUR1 and Kir6.2. The channel is key for glucose stimulated insulin release from the pancreatic beta cell. Activating mutations have been identified in the genes encoding these subunits, ABCC8 and KCNJ11, and account for approximately 40% of permanent neonatal diabetes cases. The majority of patients with a K(ATP) mutation present with isolated diabetes however some have presented with the Developmental delay, Epilepsy and Neonatal Diabetes syndrome. This review focuses on mutations in the K(ATP) channel which result in permanent neonatal diabetes, we review the clinical and functional effects as well as the implications for treatment.

Sequencing of candidate genes selected by beta cell experts in monogenic diabetes of unknown aetiology

CONTEXT

Approximately 39% of cases with permanent neonatal diabetes (PNDM) and about 11% with maturity onset diabetes of the young (MODY) have an unknown genetic aetiology. Many of the known genes causing MODY and PNDM were identified as being critical for beta cell function before their identification as a cause of monogenic diabetes.

OBJECTIVE

We used nominations from the EU beta cell consortium EURODIA project partners to guide gene candidacy.

SUBJECTS

Seventeen cases with permanent neonatal diabetes and 8 cases with maturity onset diabetes of the young.

MAIN OUTCOME MEASURES

The beta cell experts within the EURODIA consortium were asked to nominate 3 "gold", 3 "silver" and 4 "bronze" genes based on biological or genetic grounds. We sequenced twelve candidate genes from the list based on evidence for candidacy.

RESULTS

Sequencing ISL1, LMX1A, MAFA, NGN3, NKX2.2, NKX6.1, PAX4, PAX6, SOX2, SREBF1, SYT9 and UCP2 did not identify any pathogenic mutations.

CONCLUSION

Further work is needed to identify novel causes of permanent neonatal diabetes and maturity onset diabetes of the young utilising genetic approaches as well as further candidate genes.

Severe insulin resistance and intrauterine growth deficiency associated with haploinsufficiency for INSR and CHN2: new insights into synergistic pathways involved in growth and metabolism

OBJECTIVE

Digenic causes of human disease are rarely reported. Insulin via its receptor, which is encoded by INSR, plays a key role in both metabolic and growth signaling pathways. Heterozygous INSR mutations are the most common cause of monogenic insulin resistance. However, growth retardation is only reported with homozygous or compound heterozygous mutations. We describe a novel translocation [t(7,19)(p15.2;p13.2)] cosegregating with insulin resistance and pre- and postnatal growth deficiency. Chromosome translocations present a unique opportunity to identify modifying loci; therefore, our objective was to determine the mutational mechanism resulting in this complex phenotype.

RESEARCH DESIGN AND METHODS

Breakpoint mapping was performed by fluorescence in situ hybridization (FISH) on patient chromosomes. Sequencing and gene expression studies of disrupted and adjacent genes were performed on patient-derived tissues. RESULTS Affected individuals had increased insulin, C-peptide, insulin-to-C-peptide ratio, and adiponectin levels consistent with an insulin receptoropathy. FISH mapping established that the translocation breakpoints disrupt INSR on chromosome 19p15.2 and CHN2 on chromosome 7p13.2. Sequencing demonstrated INSR haploinsufficiency accounting for elevated insulin levels and dysglycemia. CHN2 encoding beta-2 chimerin was shown to be expressed in insulin-sensitive tissues, and its disruption was shown to result in decreased gene expression in patient-derived adipose tissue.

CONCLUSIONS

We present a likely digenic cause of insulin resistance and growth deficiency resulting from the combined heterozygous disruption of INSR and CHN2, implicating CHN2 for the first time as a key element of proximal insulin signaling in vivo.

Wolcott-Rallison syndrome is the most common genetic cause of permanent neonatal diabetes in consanguineous families

CONTEXT AND OBJECTIVE

Mutations in EIF2AK3 cause Wolcott-Rallison syndrome (WRS), a rare recessive disorder characterized by early-onset diabetes, skeletal abnormalities, and liver dysfunction. Although early diagnosis is important for clinical management, genetic testing is generally performed after the full clinical picture develops. We aimed to identify patients with WRS before any other abnormalities apart from diabetes are present and study the overall frequency of WRS among patients with permanent neonatal diabetes.

RESEARCH DESIGN AND METHODS

The coding regions of EIF2AK3 were sequenced in 34 probands with infancy-onset diabetes with a clinical phenotype suggestive of WRS (n = 28) or homozygosity at the WRS locus (n = 6).

RESULTS

Twenty-five probands (73.5%) were homozygous or compound heterozygous for mutations in EIF2AK3. Twenty of the 26 mutations identified were novel. Whereas a diagnosis of WRS was suspected before genetic testing in 22 probands, three patients with apparently isolated diabetes were diagnosed after identifying a large homozygous region encompassing EIF2AK3. In contrast to nonconsanguineous pedigrees, mutations in EIF2AK3 are the most common known genetic cause of diabetes among patients born to consanguineous parents (24 vs. < 2%). Age at diabetes onset and birth weight might be used to prioritize genetic testing in the latter group.

CONCLUSIONS

WRS is the most common cause of permanent neonatal diabetes mellitus in consanguineous pedigrees. In addition to testing patients with a definite clinical diagnosis, EIF2AK3 should be tested in patients with isolated neonatal diabetes diagnosed after 3 wk of age from known consanguineous families, isolated populations, or countries in which inbreeding is frequent.

Testing for monogenic diabetes among children and adolescents with antibody-negative clinically defined Type 1 diabetes

AIMS

Monogenic diabetes is frequently misdiagnosed as Type 1 diabetes. We aimed to screen for undiagnosed monogenic diabetes in a cohort of children who had a clinical diagnosis of Type 1 diabetes but were pancreatic autoantibody-negative.

METHODS

We studied 252 patients diagnosed clinically with Type 1 diabetes between 6 months and 17 years of age. Pancreatic autoantibodies [islet cell autoantibodies (ICA), glutamic acid decarboxylase antibodies (GADA) and/or insulinoma-associated antigen-2 antibodies (IA2A)] were absent in 25 cases (9.9%). The most frequent genes involved in monogenic diabetes [KCNJ11 and INS for neonatal diabetes and HNF1A and HNF4A for maturity-onset diabetes of the young (MODY)] were directly sequenced.

RESULTS

Two of the 25 (8%) antibody-negative patients had de novo heterozygous mutations in INS; c.94G>A (G32S) and c.265C>T (R89C). The two patients presented with non-ketotic hyperglycaemia at 8 and 11 months of age. In contrast, the four antibody-positive patients who presented at a similar age (6-12 months) had a more severe metabolic derangement, manifested as ketosis in all four cases, with ketoacidosis in two. At ages 15 and 5 years, both INS mutation patients were prescribed a replacement dose of insulin with good glycaemic control [glycated haemoglobin (HbA(1c)) 7.0 and 7.2%]. No mutations were found in KCNJ11, HNF1A or HNF4A.

CONCLUSIONS

The identification of patients with monogenic diabetes from children with clinically defined Type 1 diabetes may be helped by clinical criteria including the absence of pancreatic autoantibodies.

HNF1B mutations associate with hypomagnesemia and renal magnesium wasting

Mutations in hepatocyte nuclear factor 1B (HNF1B), which is a transcription factor expressed in tissues including renal epithelia, associate with abnormal renal development. While studying renal phenotypes of children with HNF1B mutations, we identified a teenager who presented with tetany and hypomagnesemia. We retrospectively reviewed radiographic and laboratory data for all patients from a single center who had been screened for an HNF1B mutation. We found heterozygous mutations in 21 (23%) of 91 cases of renal malformation. All mutation carriers had abnormal fetal renal ultrasonography. Plasma magnesium levels were available for 66 patients with chronic kidney disease (stages 1 to 3). Striking, 44% (eight of 18) of mutation carriers had hypomagnesemia (<1.58 mg/dl) compared with 2% (one of 48) of those without mutations (P < 0.0001). The median plasma magnesium was significantly lower among mutation carriers than those without mutations (1.68 versus 2.02 mg/dl; P < 0.0001). Because hypermagnesuria and hypocalciuria accompanied the hypomagnesemia, we analyzed genes associated with hypermagnesuria and detected highly conserved HNF1 recognition sites in FXYD2, a gene that can cause autosomal dominant hypomagnesemia and hypocalciuria when mutated. Using a luciferase reporter assay, we demonstrated HNF1B-mediated transactivation of FXYD2. These results extend the phenotype of HNF1B mutations to include hypomagnesemia. HNF1B regulates transcription of FXYD2, which participates in the tubular handling of Mg(2+), thus describing a role for HNF1B not only in nephrogenesis but also in the maintenance of tubular function.

Permanent neonatal diabetes mellitus due to a C96Y heterozygous mutation in the insulin gene. A case report

CONTEXT

Neonatal diabetes is a rare disorder with an incidence of 1 in 215,000-500,000 live births with 50% of them having permanent neonatal diabetes mellitus.

CASE REPORT

We present a case of permanent neonatal diabetes mellitus due to a C96Y (c.287G>A; p.Cys96Tyr) heterozygous mutation in the insulin (INS) gene. Both the patient and his father (who had childhood-onset insulin-requiring diabetes) were found to be carriers of a heterozygous missense mutation C96Y in exon 3 of the INS gene. It has been hypothesized that these mutations disrupt the folding of the proinsulin molecule and result in a misfolded protein or retention of the protein in the endoplasmic reticulum, resulting in endoplasmic reticulum stress and beta cell apoptosis. Subjects with this form of diabetes will need lifelong insulin therapy.

CONCLUSION

Insulin gene mutations appear to be an important cause of neonatal diabetes worldwide. This is the first report of a case from the Indian subcontinent. It is important to carry out genetic tests for mutations linked to pancreatic beta cell dysfunction in all patients with persistent neonatal diabetes mellitus in order to decide on therapy.

Insulin mutation screening in 1,044 patients with diabetes: mutations in the INS gene are a common cause of neonatal diabetes but a rare cause of diabetes diagnosed in childhood or adulthood

OBJECTIVE

Insulin gene (INS) mutations have recently been described as a cause of permanent neonatal diabetes (PND). We aimed to determine the prevalence, genetics, and clinical phenotype of INS mutations in large cohorts of patients with neonatal diabetes and permanent diabetes diagnosed in infancy, childhood, or adulthood.

RESEARCH DESIGN AND METHODS

The INS gene was sequenced in 285 patients with diabetes diagnosed before 2 years of age, 296 probands with maturity-onset diabetes of the young (MODY), and 463 patients with young-onset type 2 diabetes (nonobese, diagnosed <45 years). None had a molecular genetic diagnosis of monogenic diabetes.

RESULTS

We identified heterozygous INS mutations in 33 of 141 probands diagnosed at <6 months, 2 of 86 between 6 and 12 months, and none of 58 between 12 and 24 months of age. Three known mutations (A24D, F48C, and R89C) account for 46% of cases. There were six novel mutations: H29D, L35P, G84R, C96S, S101C, and Y103C. INS mutation carriers were all insulin treated from diagnosis and were diagnosed later than ATP-sensitive K(+) channel mutation carriers (11 vs. 8 weeks, P < 0.01). In 279 patients with PND, the frequency of KCNJ11, ABCC8, and INS gene mutations was 31, 10, and 12%, respectively. A heterozygous R6C mutation cosegregated with diabetes in a MODY family and is probably pathogenic, but the L68M substitution identified in a patient with young-onset type 2 diabetes may be a rare nonfunctional variant.

CONCLUSIONS

We conclude that INS mutations are the second most common cause of PND and a rare cause of MODY. Insulin gene mutation screening is recommended for all diabetic patients diagnosed before 1 year of age.

Partial and whole gene deletion mutations of the GCK and HNF1A genes in maturity-onset diabetes of the young

AIMS/HYPOTHESIS

Heterozygous mutations of glucokinase (GCK) and hepatocyte nuclear factor-1 alpha (HNF1A; also known as hepatic transcription factor 1 [TCF1]) genes are the most common cause of MODY. Genomic deletions of the HNF1B (also known as TCF2) gene have recently been shown to account for one third of mutations causing renal cysts and diabetes syndrome. We investigated the prevalence of partial and whole gene deletions in UK patients meeting clinical criteria for GCK or HNF-1alpha/-4alpha MODY and in whom no mutation had been identified by sequence analysis.

METHODS

A multiplex ligation-dependent probe amplification (MLPA) assay was developed using synthetic oligonucleotide probes for 30 exons of the GCK, HNF1A and HNF4A genes.

RESULTS

Partial or whole gene deletions were identified in 1/29 (3.5%) probands using the GCK MLPA assay and 4/60 (6.7%) of probands using the HNF1A/-4A MLPA assay. Four different deletions were detected: GCK exon 2, HNF1A exon 1, HNF1A exons 2 to 10 and HNF1A exons 1 to 10. An additional Danish pedigree with evidence of linkage to HNF1A had a deletion of exons 2 to 10. Testing other family members confirmed co-segregation of the deletion mutations with diabetes in the pedigrees.

CONCLUSIONS/INTERPRETATION

Large deletions encompassing whole exons can cause GCK or HNF-1alpha MODY and will not be detected by sequencing. Gene dosage assays, such as MLPA, are a useful adjunct to sequence analysis when a diagnosis of MODY is strongly suspected.

Insulin gene mutations as a cause of permanent neonatal diabetes

We report 10 heterozygous mutations in the human insulin gene in 16 probands with neonatal diabetes. A combination of linkage and a candidate gene approach in a family with four diabetic members led to the identification of the initial INS gene mutation. The mutations are inherited in an autosomal dominant manner in this and two other small families whereas the mutations in the other 13 patients are de novo. Diabetes presented in probands at a median age of 9 weeks, usually with diabetic ketoacidosis or marked hyperglycemia, was not associated with beta cell autoantibodies, and was treated from diagnosis with insulin. The mutations are in critical regions of the preproinsulin molecule, and we predict that they prevent normal folding and progression of proinsulin in the insulin secretory pathway. The abnormally folded proinsulin molecule may induce the unfolded protein response and undergo degradation in the endoplasmic reticulum, leading to severe endoplasmic reticulum stress and potentially beta cell death by apoptosis. This process has been described in both the Akita and Munich mouse models that have dominant-acting missense mutations in the Ins2 gene, leading to loss of beta cell function and mass. One of the human mutations we report here is identical to that in the Akita mouse. The identification of insulin mutations as a cause of neonatal diabetes will facilitate the diagnosis and possibly, in time, treatment of this disorder.

Permanent neonatal diabetes caused by dominant, recessive, or compound heterozygous SUR1 mutations with opposite functional effects

Heterozygous activating mutations in the KCNJ11 gene encoding the pore-forming Kir6.2 subunit of the pancreatic beta cell K(ATP) channel are the most common cause of permanent neonatal diabetes (PNDM). Patients with PNDM due to a heterozygous activating mutation in the ABCC8 gene encoding the SUR1 regulatory subunit of the K(ATP) channel have recently been reported. We studied a cohort of 59 patients with permanent diabetes who received a diagnosis before 6 mo of age and who did not have a KCNJ11 mutation. ABCC8 gene mutations were identified in 16 of 59 patients and included 8 patients with heterozygous de novo mutations. A recessive mode of inheritance was observed in eight patients with homozygous, mosaic, or compound heterozygous mutations. Functional studies of selected mutations showed a reduced response to ATP consistent with an activating mutation that results in reduced insulin secretion. A novel mutational mechanism was observed in which a heterozygous activating mutation resulted in PNDM only when a second, loss-of-function mutation was also present.

Mutations in ATP-sensitive K+ channel genes cause transient neonatal diabetes and permanent diabetes in childhood or adulthood

Transient neonatal diabetes mellitus (TNDM) is diagnosed in the first 6 months of life, with remission in infancy or early childhood. For approximately 50% of patients, their diabetes will relapse in later life. The majority of cases result from anomalies of the imprinted region on chromosome 6q24, and 14 patients with ATP-sensitive K+ channel (K(ATP) channel) gene mutations have been reported. We determined the 6q24 status in 97 patients with TNDM. In patients in whom no abnormality was identified, the KCNJ11 gene and/or ABCC8 gene, which encode the Kir6.2 and SUR1 subunits of the pancreatic beta-cell K(ATP) channel, were sequenced. K(ATP) channel mutations were found in 25 of 97 (26%) TNDM probands (12 KCNJ11 and 13 ABCC8), while 69 of 97 (71%) had chromosome 6q24 abnormalities. The phenotype associated with KCNJ11 and ABCC8 mutations was similar but markedly different from 6q24 patients who had a lower birth weight and who were diagnosed and remitted earlier (all P < 0.001). K(ATP) channel mutations were identified in 26 additional family members, 17 of whom had diabetes. Of 42 diabetic patients, 91% diagnosed before 6 months remitted, but those diagnosed after 6 months had permanent diabetes (P < 0.0001). K(ATP) channel mutations account for 89% of patients with non-6q24 TNDM and result in a discrete clinical subtype that includes biphasic diabetes that can be treated with sulfonylureas. Remitting neonatal diabetes was observed in two of three mutation carriers, and permanent diabetes occurred after 6 months of age in subjects without an initial diagnosis of neonatal diabetes.

Origin of de novo KCNJ11 mutations and risk of neonatal diabetes for subsequent siblings

CONTEXT

Activating mutations in the KCNJ11 gene, which encodes the Kir6.2 subunit of the pancreatic beta-cell K(ATP) channel, result in permanent and transient neonatal diabetes. The majority of KCNJ11 mutations are spontaneous, but the parental origin of these mutations is not known.

OBJECTIVE

Our objective was to determine the parental origin of de novo KCNJ11 mutations and investigate the possibility of mosaicism in transmitting parents.

DESIGN

We identified 68 index cases with a KCNJ11 mutation where neither parent was known to be affected. DNA was available from both parents of 41 probands. The parental origin of the mutation was determined in 18 families by examination of pedigrees, microsatellite analysis, or allele-specific PCR.

RESULTS

A nonsignificant excess of paternally derived mutations was found with 13 of 18 (72%) shown to have arisen on the paternal allele. There was no evidence to suggest an association with increased age at conception. In two families, there were half-siblings with permanent neonatal diabetes born to an unaffected father, suggesting germline mosaicism that was confirmed by the presence of the R201C mutation in one father's semen. Somatic mosaicism was detected in one unaffected mother, and this mutation will also be present in her germ cells.

CONCLUSION

De novo KCNJ11 mutations can arise either during gametogenesis or embryogenesis. The possibility of germline mosaicism means that future siblings are at increased risk of neonatal diabetes, and we recommend that molecular genetic testing is routinely offered at birth for subsequent siblings of children with de novo KCNJ11 mutations.

Hepatocyte nuclear factor-1 beta mutations cause neonatal diabetes and intrauterine growth retardation: support for a critical role of HNF-1beta in human pancreatic development

AIM

The transcription factor hepatocyte nuclear factor-1beta (HNF-1beta) is expressed in rodent pancreatic progenitor cells, where it is an important member of the genetic hierarchy that regulates the generation of pancreatic endocrine and exocrine cells. The recent description of an HNF-1beta mutation in a patient with neonatal diabetes suggests that HNF-1beta may also play a key role in human pancreatic B-cell development. We aimed to investigate the role of HNF-1beta mutations in neonatal diabetes and also the impact of HNF-1beta mutations on fetal growth.

METHODS

We sequenced the HNF-1beta gene in 27 patients with neonatal diabetes in whom other known genetic aetiologies had been excluded. Birth weight was investigated in 21 patients with HNF-1beta mutations.

RESULTS

A heterozygous HNF-1beta mutation, S148L, was identified in one patient with neonatal diabetes diagnosed at 17 days, which rapidly resolved only to relapse at 8 years. This patient had pancreatic atrophy, mild exocrine insufficiency and low birth weight (1.83 kg at 40 weeks' gestation). Intrauterine growth was markedly reduced in patients born to unaffected mothers with a median birth weight of 2.4 kg (range 1.8-3.3) (P = 0.006), median centile weight 3 (0.008-38), and 69% were small for gestational age.

CONCLUSION

HNF-1beta mutations are a rare cause of neonatal diabetes as well as pancreatic exocrine and endocrine dysfunction. Low birth weight is a common feature of patients with HNF-1beta mutations and is consistent with reduced insulin secretion in utero. These findings support a key role for HNF-1beta in early pancreatic progenitor cells in man.

HLA genotyping supports a nonautoimmune etiology in patients diagnosed with diabetes under the age of 6 months

Children with permanent diabetes are usually assumed to have type 1 diabetes. It has recently been shown that there are genetic subgroups of diabetes that are often diagnosed during the neonatal period but may present later. A recent Italian study proposed that type 1 diabetes is rare before 6 months of age. We aimed to examine genetic susceptibility to type 1 diabetes in patients diagnosed with diabetes before the age of 2 years. We analyzed HLA class II genotypes, markers of autoimmune diabetes, in 187 children with permanent diabetes diagnosed at <2 years of age. Of the 79 subjects diagnosed at <6 months of age, 41% (95% CI 0.30-0.51) had type 1 diabetes-associated high-risk genotypes, a proportion similar to that in healthy population control subjects (44%, P=0.56). This group included 32 patients with mutations in the KCNJ11 gene, which encodes Kir6.2 (44% high-risk HLA class II genotypes), and 47 in whom the etiology of diabetes was unknown (38% high-risk HLA class II genotypes). Of 108 patients diagnosed between 6 and 24 months of age, 93% (0.86-0.99) had high-risk HLA class II genotypes compared with 44% of the population control subjects (P<0.0001). We conclude that infants diagnosed with diabetes before 6 months of age are unlikely to have autoimmune type 1 diabetes and are most likely to have a monogenic etiology.

Mutations in KCNJ11, which encodes Kir6.2, are a common cause of diabetes diagnosed in the first 6 months of life, with the phenotype determined by genotype

AIMS/HYPOTHESIS

Heterozygous activating mutations in KCNJ11, which encodes the Kir6.2 subunit of the pancreatic ATP-sensitive potassium (K(ATP)) channel, cause both permanent and transient neonatal diabetes. A minority of patients also have neurological features. The identification of a KCNJ11 mutation has important therapeutic implications, as many patients can replace insulin injections with sulfonylurea tablets. We aimed to determine the age of presentation of patients with KCNJ11 mutations and to examine if there was a relationship between genotype and phenotype.

SUBJECTS AND METHODS

KCNJ11 was sequenced in 239 unrelated patients from 21 countries, who were diagnosed with permanent diabetes before 2 years of age.

RESULTS

Thirty-one of the 120 patients (26%) diagnosed in the first 26 weeks of life had a KCNJ11 mutation; no mutations were found in the 119 cases (0%) diagnosed after this age. Fourteen different heterozygous mutations were identified, with the majority resulting from de novo mutations. These include seven novel mutations: H46Y, R50Q, G53D C166Y, K170T, L164P and Y330S. All 11 probands with the most common mutation, R201H, had isolated diabetes. In contrast, developmental delay in addition to diabetes was seen in four of five probands with the V59M mutation and two of four with the R201C mutation. Five patients with developmental delay, epilepsy and neonatal diabetes (DEND) syndrome had unique mutations not associated with other phenotypes.

CONCLUSIONS/INTERPRETATION

KCNJ11 mutations are a common cause of permanent diabetes diagnosed in the first 6 months and all patients diagnosed in this age group should be tested. There is a strong genotype-phenotype relationship with the mutation being an important determinant of associated neurological features.

KCNJ11 activating mutations are associated with developmental delay, epilepsy and neonatal diabetes syndrome and other neurological features

Heterozygous activating mutations in the gene encoding for the ATP-sensitive potassium channel subunit Kir6.2 (KCNJ11) have recently been shown to be a common cause of permanent neonatal diabetes. Kir6.2 is expressed in muscle, neuron and brain as well as the pancreatic beta-cell, so patients with KCNJ11 mutations could have a neurological phenotype in addition to their diabetes. It is proposed that some patients with KCNJ11 mutations have neurological features that are part of a discrete neurological syndrome termed developmental Delay, Epilepsy and Neonatal Diabetes (DEND), but there are also neurological consequences of chronic or acute diabetes. We identified KCNJ11 mutations in four of 10 probands with permanent neonatal diabetes and one affected parent; this included the novel C166F mutation and the previously described V59M and R201H. Four of the five patients with mutations had neurological features: the patient with the C166F mutation had marked developmental delay, severe generalised epilepsy, hypotonia and muscle weakness; mild developmental delay was present in the patient with the V59M mutation; one patient with the R201H mutation had acute and chronic neurological consequences of cerebral oedema and another had diabetic neuropathy from chronic hyperglycaemia. In conclusion, the clinical features in these patients support the existence of a discrete neurological syndrome with KCNJ11 mutations. The severe DEND syndrome was seen with the novel C166F mutation and mild developmental delay with the V59M mutation. These features differ markedly from the neurological consequences of acute or chronic diabetes.

Mutations in hepatocyte nuclear factor-1beta and their related phenotypes

BACKGROUND

Hepatocyte nuclear factor-1 beta (HNF-1beta) is a widely distributed transcription factor which plays a critical role in embryonic development of the kidney, pancreas, liver, and Mullerian duct. Thirty HNF-1beta mutations have been reported in patients with renal cysts and other renal developmental disorders, young-onset diabetes, pancreatic atrophy, abnormal liver function tests, and genital tract abnormalities.

METHODS

We sequenced the HNF-1beta gene in 160 unrelated subjects with renal disease, 40% of whom had a personal/family history of diabetes.

RESULTS

Twenty three different heterozygous HNF-1beta mutations were identified in 23/160 subjects (14%), including 10 novel mutations (V61G, V110G, S148L, K156E, Q176X, R276Q, S281fsinsC, R295P, H324fsdelCA, Q470X). Seven (30%) cases were proven to be due to de novo mutations. Renal cysts were found in 19/23 (83%) patients (four with glomerulocystic kidney disease, GCKD) and diabetes in 11/23 (48%, while three other families had a family history of diabetes. Only 26% of families met diagnostic criteria for maturity-onset diabetes of the young (MODY) but 39% had renal cysts and diabetes (RCAD). We found no clear genotype/phenotype relationships.

CONCLUSION

We report the largest series to date of HNF-1beta mutations and confirm HNF-1beta mutations as an important cause of renal disease. Despite the original description of HNF-1beta as a MODY gene, a personal/family history of diabetes is often absent and the most common clinical manifestation is renal cysts. Molecular genetic testing for HNF-1beta mutations should be considered in patients with unexplained renal cysts (including GCKD), especially when associated with diabetes, early-onset gout, or uterine abnormalities.

Relapsing diabetes can result from moderately activating mutations in KCNJ11

Neonatal diabetes can either remit and hence be transient or else may be permanent. These two phenotypes were considered to be genetically distinct. Abnormalities of 6q24 are the commonest cause of transient neonatal diabetes (TNDM). Mutations in KCNJ11, which encodes Kir6.2, the pore-forming subunit of the ATP-sensitive potassium channel (K(ATP)), are the commonest cause of permanent neonatal diabetes (PNDM). In addition to diabetes, some KCNJ11 mutations also result in marked developmental delay and epilepsy. These mutations are more severe on functional characterization. We investigated whether mutations in KCNJ11 could also give rise to TNDM. We identified the three novel heterozygous mutations (G53S, G53R, I182V) in three of 11 probands with clinically defined TNDM, who did not have chromosome 6q24 abnormalities. The mutations co-segregated with diabetes within families and were not found in 100 controls. All probands had insulin-treated diabetes diagnosed in the first 4 months and went into remission by 7-14 months. Functional characterization of the TNDM associated mutations was performed by expressing the mutated Kir6.2 with SUR1 in Xenopus laevis oocytes. All three heterozygous mutations resulted in a reduction in the sensitivity to ATP when compared with wild-type (IC(50) approximately 30 versus approximately 7 microM, P-value for is all <0.01); however, this was less profoundly reduced than with the PNDM associated mutations. In conclusion, mutations in KCNJ11 are the first genetic cause for remitting as well as permanent diabetes. This suggests that a fixed ion channel abnormality can result in a fluctuating glycaemic phenotype. The multiple phenotypes associated with activating KCNJ11 mutations may reflect their severity in vitro.

Mutations in PTF1A cause pancreatic and cerebellar agenesis

Individuals with permanent neonatal diabetes mellitus usually present within the first three months of life and require insulin treatment. We recently identified a locus on chromosome 10p13-p12.1 involved in permanent neonatal diabetes mellitus associated with pancreatic and cerebellar agenesis in a genome-wide linkage search of a consanguineous Pakistani family. Here we report the further linkage analysis of this family and a second family of Northern European descent segregating an identical phenotype. Positional cloning identified the mutations 705insG and C886T in the gene PTF1A, encoding pancreas transcription factor 1alpha, as disease-causing sequence changes. Both mutations cause truncation of the expressed PTF1A protein C-terminal to the basic-helix-loop-helix domain. Reporter-gene studies using a minimal PTF1A deletion mutant indicate that the deleted region defines a new domain that is crucial for the function of this protein. PTF1A is known to have a role in mammalian pancreatic development, and the clinical phenotype of the affected individuals implicated the protein as a key regulator of cerebellar neurogenesis. The essential role of PTF1A in normal cerebellar development was confirmed by detailed neuropathological analysis of Ptf1a(-/-) mice.

Activating mutations in the KCNJ11 gene encoding the ATP-sensitive K+ channel subunit Kir6.2 are rare in clinically defined type 1 diabetes diagnosed before 2 years

We have recently shown that permanent neonatal diabetes can be caused by activating mutations in KCNJ11 that encode the Kir6.2 subunit of the beta-cell ATP-sensitive K(+) channel. Some of these patients were diagnosed after 3 months of age and presented with ketoacidosis and marked hyperglycemia, which could have been diagnosed as type 1 diabetes. We hypothesized that KCNJ11 mutations could present clinically as type 1 diabetes. We screened the KCNJ11 gene for mutations in 77 U.K. type 1 diabetic subjects diagnosed under the age of 2 years. One patient was found to be heterozygous for the missense mutation R201C. She had low birth weight, was diagnosed at 5 weeks, and did not have a high risk predisposing HLA genotype. A novel variant, R176C, was identified in one diabetic subject but did not cosegregate with diabetes within the family. In conclusion, we have shown that heterozygous activating mutations in the KCNJ11 gene are a rare cause of clinically defined type 1 diabetes diagnosed before 2 years. Although activating KCNJ11 mutations are rare in patients diagnosed with type 1 diabetes, the identification of a KCNJ11 mutation may have important treatment implications.

Structure of the C-terminal RING finger from a RING-IBR-RING/TRIAD motif reveals a novel zinc-binding domain distinct from a RING

The really interesting new gene (RING) family of proteins contains over 400 members with diverse physiological functions. A subset of these domains is found in the context of the RING-IBR-RING/TRIAD motifs which function as E3 ubiquitin ligases. Our sequence analysis of the C-terminal RING (RING2) from this motif show that several metal ligating and hydrophobic residues critical for the formation of a classical RING cross-brace structure are not present. Thus, we determined the structure of the RING2 from the RING-IBR-RING motif of HHARI and showed that RING2 has a completely distinct topology from classical RINGs. Notably, RING2 binds only one zinc atom per monomer rather than two and uses a different hydrophobic network to that of classical RINGs. Additionally, this RING2 topology is novel, bearing slight resemblance to zinc-ribbon motifs around the zinc site and is different from the topologies of the zinc binding sites found in RING and PHDs. We demonstrate that RING2 acts as an E3 ligase in vitro and using mutational analysis deduce the structural features required for this activity. Further, mutations in the RING-IBR-RING of Parkin cause a rare form of Parkinsonism and these studies provide an explanation for those mutations that occur in its RING2. From a comparison of the RING2 structure with those reported for RINGs, we infer sequence determinants that allow discrimination between RING2 and RING domains at the sequence analysis level.

Permanent neonatal diabetes due to paternal germline mosaicism for an activating mutation of the KCNJ11 Gene encoding the Kir6.2 subunit of the beta-cell potassium adenosine triphosphate channel

Activating mutations in the KCNJ11 gene encoding for the Kir6.2 subunit of the beta-cell ATP-sensitive potassium channel have recently been shown to be a common cause of permanent neonatal diabetes. In 80% of probands, these are isolated cases resulting from de novo mutations. We describe a family in which two affected paternal half-siblings were found to be heterozygous for the previously reported R201C mutation. Direct sequencing of leukocyte DNA showed that their clinically unaffected mothers and father were genotypically normal. Quantitative real-time PCR analysis of the father's leukocyte DNA detected no trace of mutant DNA. These results are consistent with the father being a mosaic for the mutation, which is restricted to his germline. This is the first report of germline mosaicism in any form of monogenic diabetes. The high percentage of permanent neonatal diabetes cases due to de novo KCNJ11 mutations suggests that germline mosaicism may be common. The possibility of germline mosaicism should be considered when counseling recurrence risks for the parents of a child with an apparently de novo KCNJ11 activating mutation.

Activating mutations in the gene encoding the ATP-sensitive potassium-channel subunit Kir6.2 and permanent neonatal diabetes

BACKGROUND

Patients with permanent neonatal diabetes usually present within the first three months of life and require insulin treatment. In most, the cause is unknown. Because ATP-sensitive potassium (K(ATP)) channels mediate glucose-stimulated insulin secretion from the pancreatic beta cells, we hypothesized that activating mutations in the gene encoding the Kir6.2 subunit of this channel (KCNJ11) cause neonatal diabetes.

METHODS

We sequenced the KCNJ11 gene in 29 patients with permanent neonatal diabetes. The insulin secretory response to intravenous glucagon, glucose, and the sulfonylurea tolbutamide was assessed in patients who had mutations in the gene.

RESULTS

Six novel, heterozygous missense mutations were identified in 10 of the 29 patients. In two patients the diabetes was familial, and in eight it arose from a spontaneous mutation. Their neonatal diabetes was characterized by ketoacidosis or marked hyperglycemia and was treated with insulin. Patients did not secrete insulin in response to glucose or glucagon but did secrete insulin in response to tolbutamide. Four of the patients also had severe developmental delay and muscle weakness; three of them also had epilepsy and mild dysmorphic features. When the most common mutation in Kir6.2 was coexpressed with sulfonylurea receptor 1 in Xenopus laevis oocytes, the ability of ATP to block mutant K(ATP) channels was greatly reduced.

CONCLUSIONS

Heterozygous activating mutations in the gene encoding Kir6.2 cause permanent neonatal diabetes and may also be associated with developmental delay, muscle weakness, and epilepsy. Identification of the genetic cause of permanent neonatal diabetes may facilitate the treatment of this disease with sulfonylureas.