Studies on mutant human insulin genes: identification and sequence analysis of a gene encoding [SerB24]insulin

A λ-Dynamics Investigation of Insulin Wakayama and Other A3 Variant Binding Affinities to the Insulin Receptor

Insulin Wakayama is a clinical insulin variant where a conserved valine at the third residue on insulin's A chain (ValA3) is replaced with a leucine (LeuA3), weakening insulin receptor (IR) binding by 140-500-fold. This severe impact on binding from a subtle modification has posed an intriguing problem for decades. Although experimental investigations of natural and unnatural A3 mutations have highlighted the sensitivity of insulin-IR binding at this site, atomistic explanations of these binding trends have remained elusive. We investigate this problem computationally using λ-dynamics free energy calculations to model structural changes in response to perturbations of the ValA3 side chain and to calculate associated relative changes in binding free energy (ΔΔGbind). The Wakayama LeuA3 mutation and seven other A3 substitutions were studied in this work. The calculated ΔΔGbind results showed high agreement compared to experimental binding potencies with a Pearson correlation of 0.88 and a mean unsigned error of 0.68 kcal/mol. Extensive structural analyses of λ-dynamics trajectories revealed that critical interactions were disrupted between insulin and the insulin receptor as a result of the A3 mutations. This investigation also quantifies the effect that adding an A3 Cδ atom or losing an A3 Cγ atom has on insulin's binding affinity to the IR. Thus, λ-dynamics was able to successfully model the effects of mutations to insulin's A3 side chain on its protein-protein interactions with the IR and shed new light on a decades-old mystery: the exquisite sensitivity of hormone-receptor binding to a subtle modification of an invariant insulin residue.

A λ-dynamics investigation of insulin Wakayama and other A3 variant binding affinities to the insulin receptor

Insulin Wakayama is a clinical insulin variant where a conserved valine at the third residue on insulin's A chain (ValA3) is replaced with a leucine (LeuA3), impairing insulin receptor (IR) binding by 140-500 fold. This severe impact on binding from such a subtle modification has posed an intriguing problem for decades. Although experimental investigations of natural and unnatural A3 mutations have highlighted the sensitivity of insulin-IR binding to minor changes at this site, an atomistic explanation of these binding trends has remained elusive. We investigate this problem computationally using λ-dynamics free energy calculations to model structural changes in response to perturbations of the ValA3 side chain and to calculate associated relative changes in binding free energy (ΔΔGbind). The Wakayama LeuA3 mutation and seven other A3 substitutions were studied in this work. The calculated ΔΔGbind results showed high agreement compared to experimental binding potencies with a Pearson correlation of 0.88 and a mean unsigned error of 0.68 kcal/mol. Extensive structural analyses of λ-dynamics trajectories revealed that critical interactions were disrupted between insulin and the insulin receptor as a result of the A3 mutations. This investigation also quantifies the effect that adding an A3 Cδ atom or losing an A3 Cγ atom has on insulin's binding affinity to the IR. Thus, λ-dynamics was able to successfully model the effects of subtle modifications to insulin's A3 side chain on its protein-protein interactions with the IR and shed new light on a decades-old mystery: the exquisite sensitivity of hormone-receptor binding to a subtle modification of an invariant insulin residue.

A Review of the Biosynthesis and Structural Implications of Insulin Gene Mutations Linked to Human Disease

The discovery of the insulin hormone over 100 years ago, and its subsequent therapeutic application, marked a key landmark in the history of medicine and medical research. The many roles insulin plays in cell metabolism and growth have been revealed by extensive investigations into the structure and function of insulin, the insulin tyrosine kinase receptor (IR), as well as the signalling cascades, which occur upon insulin binding to the IR. In this review, the insulin gene mutations identified as causing disease and the structural implications of these mutations will be discussed. Over 100 studies were evaluated by one reviewing author, and over 70 insulin gene mutations were identified. Mutations may impair insulin gene transcription and translation, preproinsulin trafficking and proinsulin sorting, or insulin-IR interactions. A better understanding of insulin gene mutations and the resultant pathophysiology can give essential insight into the molecular mechanisms underlying impaired insulin biosynthesis and insulin-IR interaction.

Human Pluripotent Stem Cells Go Diabetic: A Glimpse on Monogenic Variants

Diabetes, as one of the major diseases in industrial countries, affects over 350 million people worldwide. Type 1 (T1D) and type 2 diabetes (T2D) are the most common forms with both types having invariable genetic influence. It is accepted that a subset of all diabetes patients, generally estimated to account for 1-2% of all diabetic cases, is attributed to mutations in single genes. As only a subset of these genes has been identified and fully characterized, there is a dramatic need to understand the pathophysiological impact of genetic determinants on β-cell function and pancreatic development but also on cell replacement therapies. Pluripotent stem cells differentiated along the pancreatic lineage provide a valuable research platform to study such genes. This review summarizes current perspectives in applying this platform to study monogenic diabetes variants.

Computational study of the activity, dynamics, energetics and conformations of insulin analogues using molecular dynamics simulations: Application to hyperinsulinemia and the critical residue B26

Due to the increasing prevalence of diabetes, finding therapeutic analogues for insulin has become an urgent issue. While many experimental studies have been performed towards this end, they have limited scope to examine all aspects of the effect of a mutation. Computational studies can help to overcome these limitations, however, relatively few studies that focus on insulin analogues have been performed to date. Here, we present a comprehensive computational study of insulin analogues-three mutant insulins that have been identified with hyperinsulinemia and three mutations on the critical B26 residue that exhibit similar binding affinity to the insulin receptor-using molecular dynamics simulations with the aim of predicting how mutations of insulin affect its activity, dynamics, energetics and conformations. The time evolution of the conformers is studied in long simulations. The probability density function and potential of mean force calculations are performed on each insulin analogue to unravel the effect of mutations on the dynamics and energetics of insulin activation. Our conformational study can decrypt the key features and molecular mechanisms that are responsible for an enhanced or reduced activity of an insulin analogue. We find two key results: 1) hyperinsulinemia may be due to the drastically reduced activity (and binding affinity) of the mutant insulins. 2) Y26_BS and Y26_BE are promising therapeutic candidates for insulin as they are more active than WT-insulin. The analysis in this work can be readily applied to any set of mutations on insulin to guide development of more effective therapeutic analogues.

Monogenic Diabetes: What It Teaches Us on the Common Forms of Type 1 and Type 2 Diabetes

To date, more than 30 genes have been linked to monogenic diabetes. Candidate gene and genome-wide association studies have identified > 50 susceptibility loci for common type 1 diabetes (T1D) and approximately 100 susceptibility loci for type 2 diabetes (T2D). About 1-5% of all cases of diabetes result from single-gene mutations and are called monogenic diabetes. Here, we review the pathophysiological basis of the role of monogenic diabetes genes that have also been found to be associated with common T1D and/or T2D. Variants of approximately one-third of monogenic diabetes genes are associated with T2D, but not T1D. Two of the T2D-associated monogenic diabetes genes-potassium inward-rectifying channel, subfamily J, member 11 (KCNJ11), which controls glucose-stimulated insulin secretion in the β-cell; and peroxisome proliferator-activated receptor γ (PPARG), which impacts multiple tissue targets in relation to inflammation and insulin sensitivity-have been developed as major antidiabetic drug targets. Another monogenic diabetes gene, the preproinsulin gene (INS), is unique in that INS mutations can cause hyperinsulinemia, hyperproinsulinemia, neonatal diabetes mellitus, one type of maturity-onset diabetes of the young (MODY10), and autoantibody-negative T1D. Dominant heterozygous INS mutations are the second most common cause of permanent neonatal diabetes. Moreover, INS gene variants are strongly associated with common T1D (type 1a), but inconsistently with T2D. Variants of the monogenic diabetes gene Gli-similar 3 (GLIS3) are associated with both T1D and T2D. GLIS3 is a key transcription factor in insulin production and β-cell differentiation during embryonic development, which perturbation forms the basis of monogenic diabetes as well as its association with T1D. GLIS3 is also required for compensatory β-cell proliferation in adults; impairment of this function predisposes to T2D. Thus, monogenic forms of diabetes are invaluable "human models" that have contributed to our understanding of the pathophysiological basis of common T1D and T2D.

Gene-specific function prediction for non-synonymous mutations in monogenic diabetes genes

The rapid progress of genomic technologies has been providing new opportunities to address the need of maturity-onset diabetes of the young (MODY) molecular diagnosis. However, whether a new mutation causes MODY can be questionable. A number of in silico methods have been developed to predict functional effects of rare human mutations. The purpose of this study is to compare the performance of different bioinformatics methods in the functional prediction of nonsynonymous mutations in each MODY gene, and provides reference matrices to assist the molecular diagnosis of MODY. Our study showed that the prediction scores by different methods of the diabetes mutations were highly correlated, but were more complimentary than replacement to each other. The available in silico methods for the prediction of diabetes mutations had varied performances across different genes. Applying gene-specific thresholds defined by this study may be able to increase the performance of in silico prediction of disease-causing mutations.

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).

Implications for the active form of human insulin based on the structural convergence of highly active hormone analogues

Insulin is a key protein hormone that regulates blood glucose levels and, thus, has widespread impact on lipid and protein metabolism. Insulin action is manifested through binding of its monomeric form to the Insulin Receptor (IR). At present, however, our knowledge about the structural behavior of insulin is based upon inactive, multimeric, and storage-like states. The active monomeric structure, when in complex with the receptor, must be different as the residues crucial for the interactions are buried within the multimeric forms. Although the exact nature of the insulin's induced-fit is unknown, there is strong evidence that the C-terminal part of the B-chain is a dynamic element in insulin activation and receptor binding. Here, we present the design and analysis of highly active (200-500%) insulin analogues that are truncated at residue 26 of the B-chain (B(26)). They show a structural convergence in the form of a new beta-turn at B(24)-B(26). We propose that the key element in insulin's transition, from an inactive to an active state, may be the formation of the beta-turn at B(24)-B(26) associated with a trans to cis isomerisation at the B(25)-B(26) peptide bond. Here, this turn is achieved with N-methylated L-amino acids adjacent to the trans to cis switch at the B(25)-B(26) peptide bond or by the insertion of certain D-amino acids at B(26). The resultant conformational changes unmask previously buried amino acids that are implicated in IR binding and provide structural details for new approaches in rational design of ligands effective in combating diabetes.

The inherited basis of diabetes mellitus: implications for the genetic analysis of complex traits

Diabetes encompasses a heterogeneous group of diseases, each with a substantial genetic component. We review the division of diabetes into different subtypes based on clinical phenotype, the fruitful pursuit of genes underlying monogenic forms of the disease, the successes and drawbacks of whole-genome linkage scans in type 1 and type 2 diabetes, and the recent identification of several diabetes genes by large association studies. We use the lessons learned from this extensive body of evidence to illustrate general implications for the genetic analysis of complex traits.

Genetic epidemiology of type 1 diabetes

Family and twin studies indicate that a substantial fraction of susceptibility to type 1 diabetes is attributable to genetic factors. These and other epidemiologic studies also implicate environmental factors as important triggers. Although the specific environmental factors that contribute to immune-mediated diabetes remain unknown, several of the relevant genetic factors have been identified using two main approaches: genome-wide linkage analysis and candidate gene association studies. This article reviews the epidemiology of type 1 diabetes, the relative merits of linkage and association studies, and the results achieved so far using these two approaches. Prospects for the future of type 1 diabetes genetics research are considered.

The role of the C-terminus of the insulin B-chain in modulating structural and functional properties of the hormone

Within the scope of structure-function studies on the proteohormone insulin, the role of the C-terminal segment B26-B30 for self-association and receptor interaction was analyzed. Insulin derivatives with modifications in the region B26-B30 were synthesized by trypsin-catalyzed coupling reactions of des-(B23-B30)-insulin with synthetic peptides. The peptides were obtained by Fmoc solid-phase peptide synthesis. Insulins with multiple amino acid-->glycine substitutions were examined to distinguish between the influence of the side chains and the influence of the main chain in positions B27-B30 on the self-association of the hormone. The analogues [GlyB27,B28,B29,B30]insulin and [GlyB27,B28,B30]insulin exhibit relative receptor affinities of 80% and self-associate. The successive extension of [AlaB26]des-(B27-B30)-insulin-B26-amide (relative receptor binding 273%) with amino acids corresponding to the native sequence B27-B30 showed the influence of the length of the B-chain on receptor affinity: the extension by B27-threonine amide reduces receptor binding to 71%, all further prolongations have only small effects on the binding. The effect of the B28-side chain on main-chain conformation, self-association and receptor binding was examined with [XB28]des-(B29-B30)-insulin-B28-amides (X = Phe, Gly, D-Pro). While the glycine and D-proline analogues (relative binding 104 and 143%, respectively) retain the self-association properties typical of insulin, [PheB28]des-(B29-B30)-insulin-B28-amide (relative binding 50%) shows diminished self-association. The backbone-modified insulin derivative [SarB26]des-(B27-B30)-insulin-B26-amide (sarcosine = N-methylglycine) exhibits an unexpectedly high receptor affinity of 1100% which demonstrates that the B26-amide hydrogen of the native hormone is not important for receptor binding.

Viral genomes and arterial disease

Recent studies suggest that the initial stages of human atherogenesis may be defined as inordinate inflammatory-proliferative responses of intimal arterial cells, interacting with circulating lymphocytes and monocyte/macrophages, to multiple focal stimuli. The latter include transmembrane signal transductions induced by cytokines and growth factors as well as by activated immune cells releasing vasoregulatory molecules affecting local transarterial lipoprotein transport and metabolism. The observed discriminating cell proliferation and characteristic focal eccentric intimal thickening of spontaneous atheroma may thus result from the phenotypic expression of transformed cell clones with selective proliferative advantages and yet unaffected by tissue immune responses. A suggested mechanism for such cell transformation is the partial expression of widely distributed herpesvirus genomes, resulting in the induction of clonal expansion and enhancement of selective cell growth in transfected host cells. Major obstacles for the unambiguous laboratory demonstration of a direct cause/effect relationship between herpes induced cellular transformation and early human atheroma are (1) potential loss of recognizable viral transforming sequences in the host cell genome by the "hit and run" mechanism originally proposed by Skinner in 1976 and (2) irreversible cytopathic effects induced by these viruses in experimentally infected human cells in vitro, preventing any long-term proliferative or metabolic studies. The observation that immortalized cultured rabbit arterial cells retain for many generations marked mitogenic activity and accelerated lipoprotein uptake after herpesvirus transfection suggested to us the possibility of developing a reproducible in vivo laboratory model in inbred rabbits. To that end, discrete intraarterial injections of fragments of HSV-1 or HSV-2 genomes were made via specially designed catheters in temporarily isolated arterial segments of Watanabe heritable hyperlipemic rabbits. While normolipemic heterozygous animals developed segmental, highly localized, proliferative intimal tumors, containing over 95% HHF35 (+) smooth muscle cells with RAM 11 (+) macrophages and platelets attached to the endothelial surface in 30-60 days, no lesions were found in placebo-injected controls. When hyperlipemic homozygous rabbits were similarly tested, they manifested at injected loci larger intimal lesions containing abundant lipid-laden macrophages and smooth muscle cells before typical rabbit fatty streaks developed elsewhere. These findings suggest that selective transfection with viral genome sequences may indeed induce specific growth promoters for intimal arterial smooth muscle cells and thus play an important role during the initial stages of atherogenesis.

Insulin fragments as a carrier for peptide delivery across the blood-brain barrier

The possibility of using insulin (INS), which is transported into the brain by receptor-mediated transcytosis, as a peptide carrier for delivery across the blood-brain barrier (BBB) was investigated. After mice received an i.v. injection of horseradish peroxidase (HRP, M.W., 40,000) conjugated with INS, the HRP activity in the brain was higher than that after HRP injection. Since INS-HRP lowered the blood glucose level, we prepared insulin fragments by chemical and enzymatic procedures in an effort to find a carrier with no hypoglycemic activity. Seven fragments were synthesized taking the binding regions into consideration, but none showed any receptor binding affinity in cultures of bovine brain microvessel endothelial cells (BMEC). However, the fragment (F007) obtained by trypsin digestion showed high affinity and scarcely any hypoglycemic activity in mice even at a dose ten times the effective dose of insulin. These results suggest that this fragment may be useful as a carrier to transport therapeutic peptides across the BBB.

Allelic divergence in the human insulin gene provides evidence for intragenic recombination events in the non-coding regions: evidence for existence of new alleles

Intragenic polymorphism of the human insulin gene (INS) was investigated in Korean subjects. The 1.9 kb INS sequence, including the 5' to 3' flanking regions, was amplified using the polymerase chain reaction (PCR), and analyzed by direct sequencing. All nucleotide sequences in the coding regions were the same as INS sequences previously reported, and four nucleotides, at positions +216, +1045, +1367, and +1380 in the non-coding regions, were found to be polymorphic. In addition to the previously identified polymorphic alleles alpha 1 (A-C-C-C) and beta 1 (T-G-T-A), new nucleotide arrangements were also identified and designated alpha 4 (A-C-C-A), alpha 5 (A-G-C-C), alpha 6 (A-C-T-C), and beta 2 (T-C-C-C). It was concluded that the new alleles may originate by intragenic recombination within INS during chromosomal crossing-over between the alpha 1 and beta 1 alleles. The allele alpha 1 was the predominant form in our sample; the new variant alleles, as well as allele beta 1, appeared to be much less frequent in INSs genes of the Korean subjects studied. Furthermore, the new alleles were detected only in heterozygous form. These results suggest that intragenic recombination can account for allelic divergence in INS.

Prevalence of beta allele of the insulin gene in type II diabetes mellitus

Fifty-two patients and 36 controls were compared in a search for insulin gene variants among type II diabetic patients with fasting hyperinsulinemia (above 90 microU/ml) and a fasting C-peptide to insulin molar ratio between 1.11 and 1.50. Alpha and beta alleles of the insulin gene were characterized by restriction analysis of polymerase chain reaction (PCR) products and direct sequencing. The more frequent occurrence of the alpha allele of the insulin gene within the control population as compared with a prevalence of the beta allele in the diabetic patients (P, 0.05) was observed. The beta allele, usually described as the rare allele, seems to be associated with the disease.

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.

Streptozotocin interactions with pancreatic beta cells and the induction of insulin-dependent diabetes

The MSZ diabetic male mouse represents one of the most useful tools available to researchers interested in analyzing the consequences of insulin dependent diabetes in male mice. In contrast to the high mortality induced by single high doses of SZ, protracted administration of smaller SZ dosages yields a more stable diabetic condition. Moreover, in insulitis prone strains such as BKs, the model allows "synchronization" of beta cell destruction such that the inflammatory events occur on a predictable timescale. The MSZ-diabetic mouse represents a diabetic condition in which the primary etiopathologic effect is produced by an environmental toxin, and not by a genetically programmed loss of tolerance to beta cell specific antigens. In this regard, etiopathogenesis in the MSZ model is quite distinct from that underlying autoimmune type I diabetes in humans, NOD mice, and BB rats, and it is probably not appropriate to refer to pathogenesis in the MSZ model as one of "autoimmune insulitis" as has sometimes been done. The fact that insulitis in the MSZ model may not be "autoimmune," but may actually be a normal response to either tissue damage or to beta cells that have been structurally modified by a chemical, makes the model of special interest. Clearly, there is no single cause of insulin dependent diabetes, with disease induction representing a genetic susceptibility interacting with environmental triggers, such as toxins in the diet (including nitrosamines and fungal metabolites) as well as pathogenic viruses. The MSZ model will continue to be actively investigated because of insights it will afford regarding the genetic bases for susceptibility and resistance to diabetogenic environmental toxins. The model will be of further value by contributing to knowledge of the complicated interactions between pancreatic islet cells, other endocrine cells, and leukocytes in maintenance of glucose homeostasis.

Maturity-onset diabetes of the young (MODY)

This review summarized aspects of the widening scope, phenotypic expression, natural history, recognition, pathogeneses, and heterogenous nature of maturity-onset diabetes of the young (MODY), an autosomal dominant inherited subtype of NIDDM, which can be recognized at a young age. There are differences in metabolic, hormonal, and vascular abnormalities in different ethnic groups and even among Caucasian pedigrees. In MODY patients with low insulin responses, there is a delayed and decreased insulin and C-peptide secretory response to glucose from childhood or adolescence, even before glucose intolerance appears; it may represent the basic genetic defect. The nondiabetic siblings have had normal insulin responses for decades. The fasting hyperglycemia of some MODY has been treated successfully with sulfonylureas for more than 30 years. In a few, after years or decades of diabetes, the insulin and C-peptide responses to glucose are so low that they may resemble those of early Type I diabetes. The rate of progression of the insulin secretory defect over time does distinguish between these two types of diabetes. In contrast are patients from families who have very high insulin responses to glucose despite glucose intolerance and fasting hyperglycemia similar to those seen in patients with low insulin responses. In many of these patients, there is in vivo and in vitro evidence of insulin resistance. Whatever its mechanism, the compensatory insulin responses to nutrients must be insufficient to maintain normal carbohydrate tolerance. This suggests that diabetes occurs only in those patients who have an additional islet cell defect, i.e., insufficient beta cell reserve and secretory capacity. In a few MODY pedigrees with high insulin responses to glucose and lack of evidence of insulin resistance, an insulin is secreted which is a structurally abnormal, mutant insulin molecule that is biologically ineffective. No associations have been found between specific HLA antigens and MODY in Caucasian, black, and Asian pedigrees. Linkage studies of the insulin gene, the insulin receptor gene, the erythrocyte/Hep G2 glucose transporter locus, and the apolipoprotein B locus have shown no association with MODY. Vascular disease may be as prevalent as in conventional NIDDM. Because of autosomal dominant transmission and penetrance at a young age, MODY is a good model for further investigations of etiologic and pathogenetic factors in NIDDM, including the use of genetic linkage strategies to identify diabetogenic genes.

The role of chemicals in the etiology of diabetes mellitus

Diabetes mellitus is a collection of diseases that culminate in defects in carbohydrate metabolism and result in inappropriate hyperglycemia. Most individuals with this disease can be categorized into 1 of 2 groups, depending upon their insulin requirements. Individuals who require insulin therapy to maintain life have been designated as having insulin-dependent diabetes mellitus (IDDM), while individuals who can live without insulin treatment have been classified as having noninsulin-dependent diabetes mellitus (NIDDM). The single most important pathological finding in IDDM is a substantial reduction in the number of insulin-secreting pancreatic beta cells. Compelling experimental and epidemiological evidence indicates that, at least in some forms of IDDM, environmental factors, such as chemical toxins, play an important role in the etiology of this disease. Chemical toxins can precipitate IDDM through a variety of mechanisms. They can poison beta cells directly and cause the destruction of a critical mass of beta cells; alternatively, they can trigger autoimmune processes directed against beta cells; or, finally, they can augment the diabetogenic properties of another agent, such as a virus, to hasten the onset of clinical manifestations. In NIDDM, impaired beta cell function appears to be the initial event observed at the onset of this syndrome. Functional defects similar to those seen in NIDDM can be produced in laboratory animals using a specific beta cell toxin. These animals develop permanent glucose intolerance and insulin resistance as a consequence of beta cell alteration. A variety of other chemicals also have been found to produce glucose intolerance; however, this condition is transient and is resolved when the chemical is removed.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Insulin Wakayama: familial mutant insulin syndrome in Japan

We describe a family from Japan displaying the mutant insulin syndrome with hyperinsulinaemia and an increased insulin: C-peptide molar ratio. Serum insulin isolated from several family members showed reduced in vitro biological activity, and analysis by high performance liquid chromatography revealed a peak co-eluting with human insulin and a second species of increased hydrophobicity co-migrating with the previously reported Insulin Wakayama. The insulin genes from the propositus were cloned and sequenced, revealing one normal allele; the second allele, encoding a leucine for valine amino acid substitution at position 3 of the insulin A chain, was similar to that previously described for Insulin Wakayama. Synthesized [LeuA3] insulin showed 0.14% of receptor binding activity on rat adipocytes and a 10-fold prolonged half-life in a somatostatin-infused dog compared with human insulin. The finding of the same mutant gene in two unrelated Japanese families suggests that Insulin Wakayama may be discovered in additional Japanese families with hyperinsulinaemia and/or diabetes.

Structurally abnormal insulin in a diabetic patient. Characterization of the mutant insulin A3 (Val----Leu) isolated from the pancreas

We have recently identified a diabetic patient with marked fasting hyperinsulinemia. Family study revealed that the abnormality was an autosomal dominant trait. High-performance liquid chromatography (HPLC) profile of the patient's serum insulin showed that she had an abnormal insulin in addition to a normal insulin. We have purified her insulin(s) from the specimen of her pancreas, which was biopsied during an operation of cholelithiasis. Insulin was also immunologically purified from the serum of her portal vein. The reverse-phase HPLC analysis revealed that the ratios of normal to abnormal insulin in the pancreas, portal vein, and peripheral vein were 5:4, 4:5, and 1:7, respectively. Radioreceptor assay for insulin using guinea pig kidney membrane revealed that the binding activities of the normal component insulin, the abnormal component insulin and her pancreatic insulin containing both components were 100, 5, and 50% of standard human insulin, respectively. The biological activities of the normal component, the abnormal component and her pancreatic insulin to stimulate glucose oxidation in rat adipocytes were found to be 100, 8, and 60% of standard human insulin, respectively. Analysis of amino acid sequences of the abnormal insulin purified from her pancreas strongly suggested the substitution of leucine for valine at the third position of the A chain, A3 (Val----Leu).

DNA analysis in human disease

The analysis of human DNA using recombinant DNA technology is fast becoming an integral part of the diagnosis, assessment, and prevention of inherited and somatic genetic disease. The rationale underlying these methods of analysis is discussed, and the nature and extent of mutational change in heritable disorders and neoplastic development is outlined.

Diagnosis of genetic disease using recombinant DNA

Recombinant DNA technology promises to make an important contribution to the analysis and diagnosis of inherited human disease. Direct detection and analysis of various genetic defects at the DNA level are now possible using cloned gene or oligonucleotide probes. In addition, the use of restriction fragment length polymorphisms associated with linked DNA segments should permit not only the diagnosis of hitherto undetectable disease states but also the chromosomal localization of the loci responsible. The eventual isolation of disease loci should lead to a better understanding of the molecular basis of inherited disease.

Diabetes due to secretion of a structurally abnormal insulin (insulin Wakayama). Clinical and functional characteristics of [LeuA3] insulin

We have identified a non-insulin-dependent diabetic patient with fasting hyperinsulinemia (90 microU/ml), an elevated insulin:C-peptide molar ratio (1.68; normal, 0.05-0.20), normal insulin counterregulatory hormone levels, and an adequate response to exogenously administered insulin. Insulin-binding antibodies were absent from serum, erythrocyte insulin receptor binding was normal, and greater than 90% of circulating immunoreactive insulin coeluted with 125I-labeled insulin on gel filtration. The patient's insulin diluted in parallel with a human standard in the insulin radioimmunoassay, confirming close molecular similarity. The patient's insulin was purified from serum and shown to possess both reduced binding and ability to stimulate glucose uptake and oxidation in vitro. Analysis of the patient's insulin by high-performance liquid chromatography (HPLC) revealed two products: 7.3% of insulin immunoreactivity coeluted with the human standard, while the remaining 92.7% eluted as a single peak with increased hydrophobicity. Family studies confirmed the presence of hyperinsulinemia in four of five relatives in three generations, with secretion of an abnormal insulin documented by HPLC in the three tested. Leukocyte DNA was harvested from the propositus and the insulin gene cloned. One allele was normal, but the other displayed a thymine for guanine substitution at nucleotide position 1298 from the putative cap site, resulting in a leucine for valine substitution at position 3 of the insulin A chain. Insulin Wakayama is therefore identified as [LeuA3] insulin.

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

Familial hyperproinsulinemia is characterized by the accumulation of proinsulin-like material (PLM) in the plasma of affected patients. This disorder is inherited in an autosomal dominant fashion. The accumulation of PLM is thought to be due to the impaired conversion of proinsulin to insulin. Although PLM has been suggested to have an amino acid substitution, it has been impossible to locate and identify a substituted amino acid, due to the difficulty in isolating sufficient amounts of PLM from plasma samples. Therefore, we analyzed leukocyte DNA from one member of a proinsulinemic family, and we found a point mutation that changed guanine to adenine in the insulin gene. This transition implies that a substitution of histidine for arginine has occurred at amino acid position 65. Furthermore, it indicates that arginine at 65 is essential for the conversion of proinsulin to insulin. Our results suggest a novel mechanism by which disease can be incurred: a heritable disorder can result from a posttranslational processing abnormality caused by a point mutation.

Heterogeneity within type II and MODY diabetes

The heterogeneity within Type II diabetes (NIDDM) and within Maturity-Onset type Diabetes of Young people (MODY), a subset of NIDDM which is inherited in an autosomal dominant fashion, is discussed. Aspects of the definition and phenotypic expression of MODY are reviewed. Within NIDDM there are differences in patterns of inheritance between subgroups. HLA antigen associations are not found in most NIDDM populations but exist in three specific population groups with Type II diabetes. Within NIDDM and within MODY there are differences in the magnitude of insulin responses to glucose, differences in target tissue responsiveness to insulin in vivo, and differences in receptor and post-receptor effects of insulin. Structurally abnormal variant and biologically defective insulin molecules have been found in some Type II diabetic patients and in members of certain MODY families. The presence or absence of obesity may mark heterogeneous groups of Type II diabetic patients, in addition to the importance of obesity in uncovering an insulin secretory defect by causing insulin resistance. There is heterogeneity in susceptibility to vascular disease within NIDDM and MODY. The natural history of carbohydrate metabolism and of insulin secretory responses to glucose in early Type I diabetes and in MODY with low insulin secretory responses are illustrated and similarities and dissimilarities compared and contrasted. Failure to recognize young patients with MODY may contribute to incorrect diagnosis, management, and assignment of prognosis of this form of diabetes in the young by many practicing physicians. The recognition that Type I or insulin-dependent diabetes (IDDM) and Type II or noninsulin-dependent (NIDDM) differ from each other not only phenotypically but also in etiology and pathogenesis led the National Diabetes Data Group (NDDG) to devise the present nomenclature and classification of diabetes mellitus. These were adopted by the World Health Organization. As suggested by the NDDG report, the classification should be reexamined periodically to reflect improved understanding of the disease, to stimulate further research, and to be of help to practicing physicians.

Familial hyperinsulinemia due to a structurally abnormal insulin. Definition of an emerging new clinical syndrome

We have identified a patient with mild diabetes, marked fasting hyperinsulinemia (89 to 130 microU of insulin per milliliter), and a reduced fasting C-peptide: insulin molar ratio of 1.11 to 1.50 (normal, greater than 4). The patient responded normally to exogenous insulin. However, her endogenous immunoreactive insulin showed reduced biologic activity during a glucose-clamp study with hyperglycemia and a reduced ability to bind to the insulin receptor and stimulate glucose transport in vitro. Family studies showed that five additional relatives in three generations had variable degrees of glucose intolerance, marked hyperinsulinemia, and a reduced peripheral C-peptide:insulin molar ratio. Restriction-endonuclease cleavage of DNA isolated from circulating leukocytes in the patient and in family members with hyperinsulinemia revealed loss of the MboII recognition site in one allele of the insulin gene--consistent with a point mutation at position 24 or 25 in the insulin B chain. Other studies using high-pressure liquid chromatography and detailed gene analysis have identified the defect as a serine for phenylalanine substitution at position 24 of the insulin B chain. The secretion of a structurally abnormal insulin should be considered in patients with hyperinsulinemia who respond normally to exogenous insulin and have a reduced C-peptide:insulin molar ratio. Glucose tolerance may range from relatively normal to overtly diabetic.

Human insulin B24 (Phe----Ser). Secretion and metabolic clearance of the abnormal insulin in man and in a dog model

We have already demonstrated that a hyperinsulinemic, diabetic subject secreted an abnormal insulin in which serine replaced phenylalanine B24 (Shoelson S., M. Fickova, M. Haneda, A. Nahum, G. Musso, E. T. Kaiser, A. H. Rubenstein, and H. Tager. 1983. Proc. Natl. Acad. Sci. USA. 80:7390-7394). High performance liquid chromatography analysis now shows that the circulating insulin in several other family members also consists of a mixture of the abnormal human insulin B24 (Phe----Ser) and normal human insulin in a ratio of approximately 9.5:1 during fasting. Although all affected subjects show fasting hyperinsulinemia, only the propositus and her father are overtly diabetic. Analysis of the serum insulin from two nondiabetic siblings revealed that normal insulin increased from approximately 2 to 15% of total serum insulin after the ingestion of glucose and that the proportion of the normal hormone plateaued or fell while the level of total insulin continued to rise. Animal studies involving the graded intraportal infusion of equimolar amounts of semisynthetic human [SerB24]-insulin and normal human insulin in pancreatectomized dogs (to simulate the secretion of insulin due to oral glucose in man) also showed both a rise in the fraction of normal insulin that reached the periphery and the attainment of a brief steady state in this fraction while total insulin levels continued to rise. Separate experiments documented a decreased hepatic extraction, a decreased metabolic clearance rate, and an increased plasma half-life of human [SerB24]-insulin within the same parameters as those determined for normal human insulin. These results form a basis for considering (a) the differential clearance of low activity abnormal insulins and normal insulin from the circulation in vivo, and (b) the causes of hyperinsulinemia in both diabetic and nondiabetic individuals who secrete abnormal human insulins.

DNA restriction fragment length polymorphisms and heterozygosity in the human genome

A list is presented of published reports of DNA polymorphisms found in the human genome by restriction enzyme analysis. While the list indicates the large number of restriction fragment length polymorphisms (RFLPs) detected to date, the information collated is insufficient to permit an estimate of heterozygosity for the genome as a whole. Data from our laboratory are therefore also presented on RFLPs detected using a random sample of cloned DNA segments. Such an analysis has permitted a first unbiassed estimate of heterozygosity for the human genome. Since this figure is an order of magnitude higher than previous estimates derived from protein data, the majority of polymorphic variation present in the human genome must, by implication, occur in noncoding sequences. In addition it was confirmed that enzymes containing the dinucleotide CpG in their recognition sequences detect more polymorphic variation than those that do not contain a CpG. Also presented are the clinical applications of DNA polymorphisms in the diagnosis of human genetic disease.

Familial hyperproinsulinemia. Two cohorts secreting indistinguishable type II intermediates of proinsulin conversion

Familial hyperproinsulinemia, a hereditary syndrome in which individuals secrete high amounts of 9,000-mol wt proinsulin-like material, has been identified in two unrelated cohorts. Separate analysis of the material from each of the two cohorts had suggested that the proinsulin-like peptide was a conversion intermediate in which the C-peptide remained attached to the insulin B-chain in one case, whereas it was a conversion intermediate in which the C-peptide remained attached to the insulin A-chain in the other. To reinvestigate this apparent discrepancy, we have now used chemical, biochemical, immunochemical, and physical techniques to compare in parallel the structures of the immunoaffinity chromatography-purified, proinsulin-like peptides isolated from the serum of members of both families. Our results show that affected individuals in both cohorts secrete two-chained intermediates of proinsulin conversion in which the COOH-terminus of the C-peptide is extended by the insulin A-chain and from which the insulin B-chain is released by oxidative sulfitolysis. Analysis of the conversion intermediates by reverse-phase high-performance liquid chromatography using two different buffer systems showed that the proinsulin-related peptides from both families elute at a single position very near that of the normal intermediate des-Arg31, Arg32-proinsulin. Further, treatment of these peptides with acetic anhydride prevented trypsin-catalyzed cleavage of the C-peptide from the insulin A-chain, a result demonstrating the presence of Lys64 and the absence of Arg65 in both abnormal forms. We conclude that individuals from both cohorts with familial hyperproinsulinemia secret very similar or identical intermediates of proinsulin conversion in which the C-peptide remains attached to the insulin A chain and in which Arg65 has been replaced by another amino acid residue.