Posttranslational cleavage of proinsulin is blocked by a point mutation in familial hyperproinsulinemia

Understanding Insulin in the Age of Precision Medicine and Big Data: Under-Explored Nature of Genomics

Insulin is amongst the human genome's most well-studied genes/proteins due to its connection to metabolic health. Within this article, we review literature and data to build a knowledge base of Insulin (INS) genetics that influence transcription, transcript processing, translation, hormone maturation, secretion, receptor binding, and metabolism while highlighting the future needs of insulin research. The INS gene region has 2076 unique variants from population genetics. Several variants are found near the transcriptional start site, enhancers, and following the INS transcripts that might influence the readthrough fusion transcript INS-IGF2. This INS-IGF2 transcript splice site was confirmed within hundreds of pancreatic RNAseq samples, lacks drift based on human genome sequencing, and has possible elevated expression due to viral regulation within the liver. Moreover, a rare, poorly characterized African population-enriched variant of INS-IGF2 results in a loss of the stop codon. INS transcript UTR variants rs689 and rs3842753, associated with type 1 diabetes, are found in many pancreatic RNAseq datasets with an elevation of the 3'UTR alternatively spliced INS transcript. Finally, by combining literature, evolutionary profiling, and structural biology, we map rare missense variants that influence preproinsulin translation, proinsulin processing, dimer/hexamer secretory storage, receptor activation, and C-peptide detection for quasi-insulin blood measurements.

The role of the unfolded protein response in diabetes mellitus

The endoplasmic reticulum (ER) plays a key role in the synthesis and modification of secretory and membrane proteins in all eukaryotic cells. Under normal conditions, these proteins are correctly folded and assembled in the ER. However, when cells are exposed to environmental factors such as overproduction of ER proteins, viral infections, or glucose deprivation, the secretory and membrane proteins can accumulate in unfolded or misfolded forms in the lumen of the ER, and consequently, cause stress in the ER. To maintain cellular homeostasis, cells induce several responses to ER stress. In mammalian cells, ER stress responses are induced by a diversity of signal pathways. There are three ER-located transmembrane proteins that play important roles in mammalian ER stress responses: activating transcription factor 6, inositol-requiring protein 1, and protein kinase RNA-like endoplasmic reticulum kinase. ER stress is linked to various diseases, including diabetes. This review highlights the particular importance of ER stress-responsive molecules in insulin biosynthesis, glyconeogenesis, insulin resistance, glucose intolerance, and pancreatic β-cell apoptosis. An understanding of the pathogenic mechanism of diabetes from the aspect of ER stress is crucial in formulating therapeutic strategies.

Insulin gene mutations and diabetes

Some mutations of the insulin gene cause hyperinsulinemia or hyperproinsulinemia. Replacement of biologically important amino acid leads to defective receptor binding, longer half-life and hyperinsulinemia. Three mutant insulins have been identified: (i) insulin Chicago (F49L or PheB25Leu); (ii) insulin Los Angeles (F48S or PheB24Ser); (iii) and insulin Wakayama (V92L or ValA3Leu). Replacement of amino acid is necessary for proinsulin processing results in hyperproinsulinemia. Four types have been identified: (i) proinsulin Providence (H34D); (ii) proinsulin Tokyo (R89H); (iii) proinsulin Kyoto (R89L); and (iv) proinsulin Oxford (R89P). Three of these are processing site mutations. The mutation of proinsulin Providence, in contrast, is thought to cause sorting abnormality. Compared with normal proinsulin, a significant amount of proinsulin Providence enters the constitutive pathway where processing does not occur. These insulin gene mutations with hyper(pro)insulinemia were very rare, showed only mild diabetes or glucose intolerance, and hyper(pro)insulinemia was the key for their diagnosis. However, this situation changed dramatically after the identification of insulin gene mutations as a cause of neonatal diabetes. This class of insulin gene mutations does not show hyper(pro)insulinemia. Mutations at the cysteine residue or creating a new cysteine will disturb the correct disulfide bonding and proper conformation, and finally will lead to misfolded proinsulin accumulation, endoplasmic reticulum stress and apoptosis of pancreatic β-cells. Maturity-onset diabetes of the young (MODY) or an autoantibody-negative type 1-like phenotype has also been reported. Very recently, recessive mutations with reduced insulin biosynthesis have been reported. The importance of insulin gene mutation in the pathogenesis of diabetes will increase a great deal and give us a new understanding of β-cell biology and diabetes. (J Diabetes Invest, doi: 10.1111/j.2040-1124.2011.00100.x, 2011).

Clinical and molecular genetics of neonatal diabetes due to mutations in the insulin gene

Over the last decade our insight into the causes of neonatal diabetes has greatly expanded. Neonatal diabetes was once considered a variant of type 1 diabetes that presented early in life. Recent advances in our understanding of this disorder have established that neonatal diabetes is not an autoimmune disease, but rather is a monogenic form of diabetes resulting from mutations in a number of different genes encoding proteins that play a key role in the normal function of the pancreatic beta-cell. Moreover, a correct genetic diagnosis can affect treatment and clinical outcome. This is especially true for patients with mutations in the genes KCNJ11 or ABCC8 that encode the two protein subunits (Kir6.2 and SUR1, respectively) of the ATP-sensitive potassium channel. These patients can be treated with oral sulfonylurea drugs with better glycemic control and quality of life. Recently, mutations in the insulin gene (INS) itself have been identified as another cause of neonatal diabetes. In this article, we review the role of INS mutations in the pathophysiology of neonatal diabetes.

Insulin gene mutations resulting in early-onset diabetes: marked differences in clinical presentation, metabolic status, and pathogenic effect through endoplasmic reticulum retention

OBJECTIVE

Heterozygous mutations in the human preproinsulin (INS) gene are a cause of nonsyndromic neonatal or early-infancy diabetes. Here, we sought to identify INS mutations associated with maturity-onset diabetes of the young (MODY) or nonautoimmune diabetes in mid-adult life, and to explore the molecular mechanisms involved.

RESEARCH DESIGN AND METHODS

The INS gene was sequenced in 16 French probands with unexplained MODY, 95 patients with nonautoimmune early-onset diabetes (diagnosed at <35 years) and 292 normoglycemic control subjects of French origin. Three identified insulin mutants were generated by site-directed mutagenesis of cDNA encoding a preproinsulin-green fluorescent protein (GFP) (C-peptide) chimera. Intracellular targeting was assessed in clonal beta-cells by immunocytochemistry and proinsulin secretion, by radioimmunoassay. Spliced XBP1 and C/EBP homologous protein were quantitated by real-time PCR.

RESULTS

A novel coding mutation, L30M, potentially affecting insulin multimerization, was identified in five diabetic individuals (diabetes onset 17-36 years) in a single family. L30M preproinsulin-GFP fluorescence largely associated with the endoplasmic reticulum (ER) in MIN6 beta-cells, and ER exit was inhibited by approximately 50%. Two additional mutants, R55C (at the B/C junction) and R6H (in the signal peptide), were normally targeted to secretory granules, but nonetheless caused substantial ER stress.

CONCLUSIONS

We describe three INS mutations cosegregating with early-onset diabetes whose clinical presentation is compatible with MODY. These led to the production of (pre)proinsulin molecules with markedly different trafficking properties and effects on ER stress, demonstrating a range of molecular defects in the beta-cell.

The molecular genetics of diabetes mellitus

Diabetes mellitus is a clinically heterogeneous disorder which is characterized by hyperglycaemia due to an absolute or relative deficiency of insulin. Both genetic and non-genetic factors contribute to its development and, as such, it represents a multifactorial disorder. In addition, it may also be, in some instances, a polygenic disorder resulting from the cumulative effects of several genes with or without environmental factors. Serological and/or DNA markers for genes that confer susceptibility to the insulin-dependent form of the disorder (IDDM; type 1) have been identified in the HLA-D region of chromosome 6 and near the insulin gene on chromosome 11. Patients with non-insulin-dependent diabetes mellitus (NIDDM; type 2) make up a more heterogeneous group than those with IDDM and it is likely that in these patients similar clinical phenotypes may be produced by different genetic defects. The synthesis of either an abnormal insulin/proinsulin molecule or an abnormal insulin receptor can confer susceptibility to NIDDM. The genes encoding insulin and the insulin receptor are on chromosomes 11 and 19, respectively. In addition, studies of restriction fragment length polymorphism and disease associations suggest that two other genes may contribute to the development of NIDDM on chromosome 11, one near the insulin gene on the short arm of this chromosome and the other near the apolipoprotein A-I gene on the long arm. None of the susceptibility genes that have been identified to date causes diabetes in the absence of other genetic or non-genetic contributing factors, which is consistent with a multifactorial or polygenic origin for this disorder.

Genes of type 2 diabetes in beta cells

Type 2 diabetes is a complex polygenic metabolic disorder of epidemic proportions. This review provides a brief overview of the susceptibility genes in type 2 diabetes that primarily affect pancreatic 3 cells, with emphasis on their function and most relevant polymorphisms. We focus on calpain 10, the only susceptibility gene identified thus far through a positional cloning approach in subjects with diabetes.

A novel point mutation in the insulin gene giving rise to hyperproinsulinemia

A 58-yr-old obese white Caucasian male type 2 diabetic, entered into the UK Prospective Diabetes Study, was found to have raised fasting total proinsulin levels 708 pmol/L(-1) (normal range, 3-16 pmol/L(-1)) and normal specific plasma insulin level 29 pmol/L(-1) (normal range, 21-75 pmol/L(-1)). Immunoreactive plasma insulin, measured by RIA, was 503 pmol/L(-1). DNA was extracted, the insulin gene amplified by the PCR, and by direct sequencing, a novel point mutation, G1552C, was identified, which resulted in the substitution of proline (CCT) for arginine (CGT) at position 65. This prevented cleavage of the C-peptide A-chain dibasic cleavage site (lys-arg) by the processing protease in the pancreatic beta-cells. The plasma proinsulin and insulin levels were in accord with expression of both the wild-type and the mutant alleles. The G1552C mutation was not linked with diabetes, because it was present in a 37-yr-old nondiabetic daughter and not in a 35-yr-old daughter who had had gestational diabetes.

Non-insulin-dependent diabetes mellitus in childhood and adolescence

NIDDM in children and adolescents represents a heterogeneous group of disorders with different underlying pathophysiologic mechanisms. Most subtypes of NIDDM that occur in childhood are uncommon, but some, such as early onset of "classic" NIDDM, seem to be increasing in prevalence. This observed increase is thought to be caused by societal factors that lead to sedentary lifestyles and an increased prevalence of obesity. In adults, hyperglycemia frequently exists for years before a diagnosis of NIDDM is made and treatment is begun. Microvascular complications, such as retinopathy, are often already present at the time of diagnosis. Children are frequently asymptomatic at the time of diagnosis, so screening for this disorder in high-risk populations is important. Screening should be considered for children of high-risk ethnic populations with a strong family history of NIDDM with obesity or signs of hyperinsulinism, such as acanthosis nigricans. Even for children in these high-risk groups who do not yet manifest hyperglycemia, primary care providers can have an important role in encouraging lifestyle modifications that might delay or prevent onset of NIDDM.

Hyperproinsulinemia in Japan

Four families with hyperproinsulinemia found in Japan were described. The details of the first case, who was investigated by Kanazawa et al., were reported and the similarity of the first case to the following cases was shown. Arginine 65 of the proinsulin molecule might be a hot spot of the insulin gene. A possible abnormality of insulin release in affected individuals was disclosed by investigation of the family members of the first case.

Genetics of insulin action

Insulin action is highly likely to be primarily genetically determined (given a permissive or facilitative environment, for example sufficient calorie availability), as shown by variations in ethnic distribution, evidence for familial transmission and genotypic responses to experimentally induced metabolic stresses. Further, it is likely that the genetic predisposition to insulin resistance is closely linked to (or perhaps synonymous with) the predisposition to develop overt NIDDM. Alternatively, in the development of diabetes, the genetic basis for insulin resistance may be necessary, but not sufficient, requiring a second major gene for beta-cell vulnerability (e.g. exhaustion, deterioration of function, amyloid deposition). The future examination of the genetics of insulin action depends in large measure on the method of assessment of insulin action that is selected and its consistent application to individuals, families and populations. The phenomenological approaches currently being used to describe and define insulin resistance could be identifying many different disorders, all leading to an apparent decrease or impairment of insulin action compared with that in 'normals'. Selection of any method for determining the presence of insulin resistance, together with selection of the threshold for 'present versus absent' is, at best, difficult. It is further complicated by the frequent association of insulin resistance with a wide range of disturbances, including hypertension, dyslipidaemia and glucose intolerance--the insulin resistance 'syndrome'. A number of possible loci and candidate genes controlling insulin action have been studied, and most have been ruled out as the probable underlying cause of the majority of cases of defective insulin action. Among those genes that are unlikely to be determinants of insulin resistance (except in a few rare cases of mutations) are those for insulin, the insulin receptor, glucose transporters and the genes for many specific enzymes. While these are unlikely to be responsible for insulin resistance, such potential genetic defects cannot be fully excluded using present methods. Direct gene sequencing of polymerase-chain-reaction amplified DNA may be the ultimate approach to identifying the critical defects underlying insulin resistance. Other candidate genes regulating insulin action are likely soon to come forth, such as those controlling the generation and function of the intracellular mediators of insulin action.(ABSTRACT TRUNCATED AT 400 WORDS)

A novel point mutation in the human insulin gene giving rise to hyperproinsulinemia (proinsulin Kyoto)

We have identified a 65-yr-old nonobese Japanese man with diabetes mellitus, fasting hyperinsulinemia (150-300 pM), and a reduced fasting C-peptide/insulin molar ratio of 2.5-3.0. Fasting hyperinsulinemia was also found in his son and daughter. Analysis of insulin isolated from the serum of the proband and his son by reverse-phase high performance liquid chromatography revealed a minor peak coeluting with human insulin and a major peak of proinsulin-like materials. The insulin gene of the patient was amplified by the polymerase chain reaction and the products were sequenced. A novel point mutation was identified in which guanine was replaced by thymine. The substitution gives rise to a new HindIII recognition site and results in the amino acid replacement of leucine for arginine at position 65. These results indicate that the amino-acid replacement prevents recognition of the C-peptide-A chain dibasic protease and results in an elevation of proinsulin-like materials in the circulation. Furthermore, in this family the proinsulin-like materials is due to a biosynthetic defect, inherited as an autosomal dominant trait. Rapid detection of this mutation can be accomplished by HindIII restriction enzyme mapping of polymerase chain reaction-generated DNA, which enables us to facilitate the diagnosis and screening.

Ionizing radiation and genetic risks. I. Epidemiological, population genetic, biochemical and molecular aspects of Mendelian diseases

This paper reviews the currently available information on naturally occurring Mendelian diseases in man; it is aimed at providing a background and framework for discussion of experimental data on radiation-induced mutations (papers II and III) and for the estimation of the risk of Mendelian disease in human populations exposed to ionizing radiation (paper IV). Current consensus estimates indicate that a total of about 125 per 10(4) livebirths are directly affected by one or another naturally occurring Mendelian disease (autosomal dominants, 95/10(4); X-linked ones, 5/10(4); and autosomal recessives, 25/10(4). These estimates are conservative and take into account conditions which are very rare and for which prevalence estimates are unavailable. Most, although not all, of the recognized "common" dominants have onset in adult ages while most sex-linked and autosomal recessives have onset at birth or in childhood. Autosomal dominant and X-linked diseases (i.e., the responsible mutant alleles) presumed to be maintained in the population due to a balance between mutation and selection are the ones which may be expected to increase in frequency as a result of radiation exposures. Viewed from this standpoint, the above assumption seems safe only for a small proportion of such diseases; for the remainder, there is no easy way to discriminate between different mechanisms that may be responsible or to rigorously exclude some in favor of some others. Mutations in genes that code for enzymic proteins are more often recessive in contrast to those that code for non-enzymic proteins, which are more often dominant. At the molecular level, with recessives, a wide variety of changes is possible and these include specific types of point mutations, small and large intragenic deletions, multilocus deletions and rearrangements. In the case of dominants, however, the kinds of recoverable point mutations and deletion-type changes are less extensive because of functional constraints. The mutational potential of genes varies, depending on the gene, its size, sequence content and arrangement, location and its normal functions, and can be grouped into three groups: those in which only point mutations have been found to occur, those in which only deletions or other gross changes have been recovered and those in which both kinds of changes are known. Molecular data are available for about 75 Mendelian conditions and these suggest that in approximately 50% of them, the changes categorized to date are point mutations and in the remainder, intragenic deletions or other gross changes; there does not seem to be any fundamental difference between dominants and recessives with respect to the underlying molecular defect.(ABSTRACT TRUNCATED AT 400 WORDS)

Methylation and expression of human pepsinogen genes in normal tissues and their alteration in stomach cancer

In normal human tissues, pepsinogen A mRNA was expressed only in the fundic mucosa of the stomach, whereas pepsinogen C mRNA was expressed in all regions of the stomach mucosa and also in the proximal duodenal mucosa. The distributions of these mRNAs were consistent with those of pepsinogens A and C in the gastroduodenal mucosa. Methylation analysis of DNAs from normal tissues with methylation-sensitive restriction enzymes, HpaII and HhaI, revealed that pepsinogen A and C genes are hypomethylated in tissues producing pepsinogens A and C, suggesting a role of DNA methylation in the regulation of the differential expression of the genes for the two human pepsinogens during normal differentiation. In stomach cancer tissues and cancer cell lines, the expressions of the pepsinogen genes were decreased or lost, in good accordance with their pepsinogen productions. No gross structural changes of the pepsinogen genes were observed in these cancers, but the methylation patterns of the pepsinogen genes were found to be altered in different ways in different cancers. The functional significance of the altered methylation is unknown; however, these results suggest that considerable heterogeneity of the methylation patterns occurs in human stomach cancers.

Familial hypothyroidism caused by a nonsense mutation in the thyroid-stimulating hormone beta-subunit gene

Hereditary hypothyroidism caused by thyroid-stimulating hormone (TSH) deficiency is a rare autosomal recessive disease. Affected individuals show symptoms of severe mental and growth retardation that can be prevented by early administration of exogenous thyroid hormone. In this paper, we describe two related Greek families with three children affected by congenital TSH-deficient hypothyroidism. Sequence analysis of the TSH beta-subunit gene (TSHB) showed that the mutation responsible for the hypothyroidism in these families is a nonsense mutation in exon 2. This mutation is a G-to-T transversion at nucleotide 94 that destroys the only TaqI site in the TSHB-coding region and gives rise to a novel 8.5-kb TaqI fragment. Restriction analysis showed that the three affected children are homozygous for the 8.5-kb allele and that the four parents and two unaffected children are heterozygous. This mutation gives rise to a truncated peptide which includes only the first 11 of 118 amino acids of the mature TSHB peptide.

Ornithine transcarbamylase deficiency resulting from a C-to-T substitution in exon 5 of the ornithine transcarbamylase gene

To define the molecular basis for the TaqI site alteration in the ornithine transcarbamylase (OTC) (E.C.2.1.3.3) gene of a female patient with mild OTC deficiency, we used a combination of genomic amplification followed by direct sequencing and oligodeoxyribonucleotide hybridization. We obtained evidence for a C-to-T substitution in exon 5 (codon 141) of this gene. This mutation generates a stop codon, in place of Arg, at amino acid 109 of the mature OTC protein. The mutation arose, de novo, in a germ cell of one of the parents.

Fabry disease: six gene rearrangements and an exonic point mutation in the alpha-galactosidase gene

Fabry disease, an X-linked recessive disorder of glycosphingolipid catabolism, results from the deficient activity of the lysosomal hydrolase, alpha-galactosidase. Southern hybridization analysis of the alpha-galactosidase gene in affected hemizygous males from 130 unrelated families with Fabry disease revealed six with different gene rearrangements and one with an exonic point mutation resulting in the obliteration of an Msp I restriction site. Five partial gene deletions were detected ranging in size from 0.4 to greater than 5.5 kb. Four of these deletions had breakpoints in intron 2, a region in the gene containing multiple Alu repeat sequences. A sixth genomic rearrangement was identified in which a region of about 8 kb, containing exons 2 through 6, was duplicated by a homologous, but unequal crossover event. The Msp I site obliteration, which mapped to exon 7, was detected in an affected hemizygote who had residual enzyme activity. Genomic amplification by the polymerase chain reaction and sequencing revealed that the obliteration resulted from a C to T transition at nucleotide 1066 in the coding sequence. This point mutation, the first identified in Fabry disease, resulted in an arginine356 to tryptophan356 substitution which altered the enzyme's kinetic and stability properties. The detection of these abnormalities provided for the precise identification of Fabry heterozygotes, thereby permitting molecular pedigree analysis in these families which revealed paternity exclusions and the first documented new mutations in this disease.

Diagnosis of genetic disorders at the DNA level

In the past 10 years considerable progress has been made in the diagnosis of hereditary disorders at the DNA level. Many monogenic disorders can now be examined at the gene level; such examination has led to a better understanding of the molecular basis of these disorders and made carrier detection and prenatal diagnosis possible. Each year, more and more monogenic disorders can be added to the list of diseases that can be diagnosed by DNA analysis. Future research will be devoted to the identification of genes responsible for other known monogenic hereditary disorders, the elucidation of the molecular lesion associated with chromosomal abnormalities, and the characterization of the genes and gene defects involved in the common multifactorial diseases. The goal of diagnosis is the identification of the genetic defect in affected patients, persons destined to be affected, and carriers.

Unprocessed insulin proreceptors due to point mutation at the cleavage site

Two sisters presented with severe insulin resistance and markedly decreased insulin binding to erythrocytes, cultured fibroblasts and transformed lymphocytes. The dose-response curve of insulin-stimulated amino acid uptake in the fibroblasts was shifted to the right. The molecular weight of the insulin receptor on the transformed lymphocytes from the patients was 210,000 and could not be dissociated to alpha- and beta-subunits by dithiothreitol treatment. However, the proreceptor was cleaved by trypsin and this led to the production of alpha-subunit with normal insulin binding. We performed cDNA sequence analysis of the cleavage site of the insulin proreceptor from the patients. The polymerase chain reaction was used to obtain a large amount of cDNA coding for the region including the interconnecting site. A thermostable DNA polymerase, Taq polymerase, successfully produced enough cDNA for the region to be sequenced. The results showed an AGG (Arg) to AGT (Ser) point mutation, resulting in the change of the interconnecting sequence of the two subunits from -Arg-Lys-Arg-Arg- to -Arg-Lys-Arg-Ser-. These results suggest that the tertiary structure change of the cleavage site leads to production of unprocessed insulin proreceptors.

A mutant human proinsulin is secreted from islets of Langerhans in increased amounts via an unregulated pathway

A coding mutation in the human insulin gene (His-B10----Asp) is associated with familial hyperproinsulinemia. To model this syndrome, we have produced transgenic mice that express high levels of the mutant prohormone in their islets of Langerhans. Strain 24-6 mice, containing about 100 copies of the mutant gene, were normoglycemic but had marked increases of serum human proinsulin immunoreactive components. Biosynthetic studies on isolated islets revealed that approximately 65% of the proinsulin synthesized in these mice was the human mutant form. Unlike the normal endogenous mouse proinsulin, which was almost exclusively handled via a regulated secretory pathway, up to 15% of the human [Asp10]proinsulin was rapidly secreted after synthesis via an unregulated or constitutive pathway, and approximately 20% was degraded within the islet cells. The secreted human [Asp10]proinsulin was not processed proteolytically. However, the processing of the normal mouse and human mutant proinsulins within the islets from transgenic mice was not significantly impaired. These findings suggest that the hyperproinsulinemia of the patients is the result of the continuous secretion of unprocessed mutant prohormone from the islets via this alternative unregulated pathway.

Insulin resistance by unprocessed insulin proreceptors point mutation at the cleavage site

Failure to cleave the interconnecting site between alpha- and beta-subunit produced insulin proreceptors in the plasma membranes which had markedly low affinity to insulin, leading to extreme insulin resistance in a patient. We performed cDNA sequence analysis of the cleavage site of the insulin proreceptor from the patient. Polymerase chain reaction was used to obtain large amount of cDNA coding for the region including the interconnecting site. A thermostable DNA polymerase, Taq polymerase, successfully produced enough amount of cDNA of the region to be sequenced. The results showed AGG (Arg) to AGT (Ser) point mutation, resulting in the change of interconnecting sequence of the two subunits from -Arg-Lys-Arg-Arg- to -Arg-Lys-Arg-Ser-. These results suggest that the tertial structure change of the cleavage site leads to production of unprocessed insulin proreceptors.

Nonsense and missense mutations in hemophilia A: estimate of the relative mutation rate at CG dinucleotides

Hemophilia A is an X-linked disease of coagulation caused by deficiency of factor VIII. Using cloned cDNA and synthetic oligonucleotide probes, we have now screened 240 patients and found CG-to-TG transitions in an exon in nine. We have previously reported four of these patients; and here we report the remaining five, all of whom were severely affected. In one patient a TaqI site was lost in exon 23, and in the other four it was lost in exon 24. The novel exon 23 mutation is a CG-to-TG substitution at the codon for amino acid residue 2166, producing a nonsense codon in place of the normal codon for arginine. Similarly, the exon 24 mutations are also generated by CG-to-TG transitions, either on the sense strand producing nonsense mutations or on the antisense strand producing missense mutations (Arg to Gln) at position 2228. The novel missense mutations are the first such mutations observed in association with severe hemophilia A. These results provide further evidence that recurrent mutations are not uncommon in hemophilia A, and they also allow us to estimate that the extent of hypermutability of CG dinucleotides is 10-20 times greater than the average mutation rate for hemophilia A.

Hypomethylation and expression of pepsinogen A genes in the fundic mucosa of human stomach

We have examined the correlation between the extents of methylation and expression of pepsinogen A genes in normal human tissues. Expression of pepsinogen A mRNA was detected only in the fundic mucosa of the stomach and both CCGG and GCGC sites in the genes region were less methylated in the fundic mucosa than in other non-expressing tissues. Thus, there was an inverse correlation between the extents of methylation and expression of pepsinogen A genes and the role of DNA methylation in the regulation of pepsinogen A genes expression during normal differentiation was suggested.

The CpG dinucleotide and human genetic disease

Reports of single base-pair mutations within gene coding regions causing human genetic disease were collated. Thirty-five per cent of mutations were found to have occurred within CpG dinucleotides. Over 90% of these mutations were C----T or G----A transitions, which thus occur within coding regions at a frequency 42-fold higher than that predicted from random mutations. These findings are consistent with methylation-induced deamination of 5-methyl cytosine and suggest that methylation of DNA within coding regions may contribute significantly to the incidence of human genetic disease.

Use of a synthetic peptide antigen to generate antisera reactive with a proteolytic processing site in native human proinsulin: demonstration of cleavage within clathrin-coated (pro)secretory vesicles

Polyclonal antibodies reactive with a cleavage site in human proinsulin (HPI) (C-peptide-A-chain junction) have been raised (rabbit, guinea pig) using a synthetic peptide antigen coupled with keyhole limpet hemocyanin. These antisera recognize native HPI and des-31,32-HPI equally well but react 20-50 times less well with des-64,65-HPI, the intermediate cleaved at the C-peptide-A-chain junction and lacking the Lys-Arg pair. The guinea pig antisera did not recognize insulin but reacted weakly with C peptide at high concentrations; the rabbit antisera reacted with neither insulin nor C peptide. Immunocytochemical studies with human islet tissue localized the immunoreactivity of these antisera to clathrin-coated (pro)secretory vesicles derived from the trans Golgi, indicating that cleavage of the C-peptide-A-chain junction of proinsulin occurs mainly, if not exclusively, in this compartment of the beta cell.

A superactive insulin: [B10-aspartic acid]insulin(human)

The genetic basis for a case of familial hyperproinsulinemia has been elucidated recently. It involves a single point mutation in the proinsulin gene resulting in the substitution of aspartic acid for histidine-10 of the B chain of insulin. We have synthesized a human insulin analogue, [AspB10]insulin, corresponding to the mutant proinsulin and evaluated its biological activity. [AspB10]Insulin displayed a binding affinity to insulin receptors in rat liver plasma membranes that was 534 +/- 146% relative to the natural hormone. In lipogenesis assays, the synthetic analogue exhibited a potency that was 435 +/- 144% relative to insulin, which is statistically not different from its binding affinity. Reversed-phase HPLC indicated that the synthetic analogue is more apolar than natural insulin. We suggest that the observed properties reflect changes in the conformation of the analogue relative to natural insulin, which result in a stronger interaction with the insulin receptor. Thus, a single substitution of an amino acid residue of human insulin has resulted in a superactive hormone.

Precursors to regulatory peptides: their proteolytic processing

Precursors to regulatory peptides undergo maturation processes which include proteolytic processing. The enzymes involved in this process remove the hydrophobic peptide located at the amino-terminus of the precursor. Endoprotease cleavage also occurs at single and two adjacent basic residues, this is followed by a removal of basic residues located at the C-terminus of the peptides by a carboxypeptidase-like enzyme.

A mutation in the B chain coding region is associated with impaired proinsulin conversion in a family with hyperproinsulinemia

Gruppuso et al. [Gruppuso, P.A., Gordon, P., Kahn, C. R., Cornblath, M., Zeller, W. P. & Schwartz, R. (1984) N. Engl. J. Med. 311, 629-634] have recently described a family in which hyperproinsulinemia is inherited in an autosomal dominant pattern, suggesting a structural abnormality in the proinsulin molecule as the basis for this disorder. However, unlike two previous kindreds with a similar syndrome, the serum proinsulin-like material in this family did not appear to be an intermediate conversion product but instead behaved like normal human proinsulin by several criteria. To further characterize this disorder we isolated and sequenced the insulin gene of the propositus. Leukocyte DNA was cloned into lambda-WES and recombinants containing the two insulin alleles, lambda MD41 and lambda MD51, were isolated by plaque hybridization. DNA sequencing of lambda MD51 showed that it contained the normal coding sequence for human preproinsulin. Sequence analysis of lambda MD41, however, revealed a single nucleotide substitution in the codon for residue 10 of proinsulin (CAC----GAC) that predicts the exchange of aspartic acid for histidine in the insulin B chain region. This mutation was also found in an insulin allele cloned from a second affected family member (propositus's father). These results, along with the linkage analysis of Elbein et al. [Elbein, S.C., Gruppuso, P., Schwartz, R., Skolnick, M. & Permutt, M.A. (1985) Diabetes 34, 821-824], strongly implicate this mutation as the cause of the hyperproinsulinemia in this family. Inhibition of the conversion of proinsulin to insulin may be related to altered folding and/or self-association properties of the [Asp10]proinsulin.

Molecular basis of hemophilia B: a defective enzyme due to an unprocessed propeptide is caused by a point mutation in the factor IX precursor

A mutant factor IX, designated factor IXCambridge, was isolated from a patient with hemophilia B. This protein includes an 18-residue propeptide attached to the NH2 terminus of factor IX. A point mutation at residue -1, from an arginine to a serine, precludes cleavage of the propeptide by a processing protease and interferes with gamma-carboxylation of the factor IX, indicating the importance of the leader sequence in substrate recognition by the vitamin K-dependent carboxylase. This represents an example of an enzyme defect due to the presence of a point mutation in a precursor protein (preproenzyme) that is the cause of a human hereditary disease. This defect will serve as a prototype for understanding the molecular basis of some forms of hemophilia and other hereditary enzyme deficiencies.