UKPDS 21: low prevalence of the mitochondrial transfer RNA gene (tRNA(Leu(UUR))) mutation at position 3243bp in UK Caucasian type 2 diabetic patients

Mitochondrial DNA A3243G variant-associated retinopathy: Current perspectives and clinical implications

Cellular function and survival are critically dependent on the proper functionality of the mitochondrion. Neurodegenerative cellular processes including cellular adenosine triphosphate production, intermediary metabolism control, and apoptosis regulation are all mitochondrially mediated. The A to G transition at position 3243 in the mitochondrial MTTL1 gene that encodes for the leucine transfer RNA (m.3243A>G) causes a variety of diseases, including maternally inherited loss of hearing and diabetes syndrome (MIDD), mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes syndrome (MELAS). Ophthalmological findings-including posterior sub-capsular cataract, ptosis, external ophthalmoplegia, and pigmentary retinopathy- have all been associated with the m.3243A>G variant. Pigmentary retinopathy is, however, the most common ocular finding, occurring in 38% to 86% of cases. To date, little is known about the pathogenesis, natural history, and heteroplasmic and phenotypic correlations of m.3243A>G-associated pigmentary retinopathy. We summarize the current understanding of mitochondrial genetics and pathogenesis of some associated diseases. We then review the pathophysiology, histology, clinical features, treatment, and important ocular and systemic phenotypic manifestations of m.3243A>G variant associated retinopathy. Mitochondrial diseases require a multidisciplinary team approach to ensure effective treatment, regular follow-up, and accurate genetic counseling.

Mitochondrial mutation m.3243A>G associates with insulin resistance in non-diabetic carriers

AIM

This case-control study aimed to examine impairments in glucose metabolism in non-diabetic carriers of the mitochondrial mutation m.3243A>G by evaluating insulin secretion capacity and sensitivity.

METHODS

Glucose metabolism was investigated in 23 non-diabetic m.3243A>G carriers and age-, sex- and BMI-matched healthy controls with an extended 4-h oral glucose tolerance test (OGTT). Insulin sensitivity index and acute insulin response were estimated on the basis of the OGTT. This was accompanied by examination of body composition by dual-energy X-ray absorptiometry (DXA), maximum aerobic capacity and a Recent Physical Activity Questionnaire (RPAQ).

RESULTS

Fasting p-glucose, s-insulin and s-c-peptide levels did not differ between m.3243A>G carriers and controls. Insulin sensitivity index (BIGTT-S1) was significantly lower in the m.3243A>G carriers, but there was no difference in the acute insulin response between groups. P-lactate levels were higher in carriers throughout the OGTT. VO2max, but not BMI, waist and hip circumferences, lean and fat body mass%, MET or grip strength, was lower in mutation carriers. BIGTT-S1 remained lower in mutation carriers after adjustment for multiple confounding factors including VO2max in regression analyses.

CONCLUSIONS

Glucose metabolism in m.3243A>G carriers was characterized by reduced insulin sensitivity, which could represent the earliest phase in the pathogenesis of m.3243A>G-associated diabetes.

Identifying Pathogenic Variants of Monogenic Diabetes Using Targeted Panel Sequencing in an East Asian Population

PURPOSE

Monogenic diabetes is a specific type of diabetes in which precision medicine could be applied. In this study, we used targeted panel sequencing to investigate pathogenic variants in Korean patients clinically suspected to have monogenic diabetes.

METHODS

The eligibility criteria for inclusion were non-type 1 diabetes patients with an age of onset ≤ 30 years and a BMI (body mass index) ≤ 30 kg/m2. Among the 2,090 non-type 1 diabetes patients, 109 were suspected to have monogenic diabetes and subjected to genetic testing. We analyzed 30 monogenic diabetes genes using targeted panel sequencing. The pathogenicity of the genetic variants was evaluated according to the American College of Medical Genetics and Genomics and the Association for Molecular Pathology guidelines.

RESULTS

Among the 109 suspected monogenic diabetes patients, 23 (21.1%) patients harbored pathogenic/likely pathogenic variants. A total of 14 pathogenic/likely pathogenic variants of common maturity onset diabetes of the young (MODY) genes were identified in GCK, HNF1A, HNF4A, and HNF1B. Other pathogenic/likely pathogenic variants were identified in WFS1, INS, ABCC8 and FOXP3. The mitochondrial DNA 3243 A>G variant was identified in five participants. Patients with pathogenic/likely pathogenic variants had a significantly higher MODY probability, a lower BMI, and a lower C-peptide level than those without pathogenic/likely pathogenic variants (P=0.007, P=0.001, and P=0.012, respectively).

CONCLUSIONS

Using targeted panel sequencing followed by pathogenicity evaluation, we were able to make molecular genetic diagnoses for 23 (21.1%) suspected monogenic diabetes patients. Lower BMI, higher MODY probability, and lower C-peptide levels were characteristics of these participants.

Mitochondrial dysfunction in type 2 diabetes mellitus: an organ-based analysis

Type 2 diabetes mellitus (T2DM) is a systemic disease characterized by hyperglycemia, hyperlipidemia, and organismic insulin resistance. This pathological shift in both circulating fuel levels and energy substrate utilization by central and peripheral tissues contributes to mitochondrial dysfunction across organ systems. The mitochondrion lies at the intersection of critical cellular pathways such as energy substrate metabolism, reactive oxygen species (ROS) generation, and apoptosis. It is the disequilibrium of these processes in T2DM that results in downstream deficits in vital functions, including hepatocyte metabolism, cardiac output, skeletal muscle contraction, β-cell insulin production, and neuronal health. Although mitochondria are known to be susceptible to a variety of genetic and environmental insults, the accumulation of mitochondrial DNA (mtDNA) mutations and mtDNA copy number depletion is helping to explain the prevalence of mitochondrial-related diseases such as T2DM. Recent work has uncovered novel mitochondrial biology implicated in disease progressions such as mtDNA heteroplasmy, noncoding RNA (ncRNA), epigenetic modification of the mitochondrial genome, and epitranscriptomic regulation of the mtDNA-encoded mitochondrial transcriptome. The goal of this review is to highlight mitochondrial dysfunction observed throughout major organ systems in the context of T2DM and to present new ideas for future research directions based on novel experimental and technological innovations in mitochondrial biology. Finally, the field of mitochondria-targeted therapeutics is discussed, with an emphasis on novel therapeutic strategies to restore mitochondrial homeostasis in the setting of T2DM.

New mtDNA Association Model, MutPred Variant Load, Suggests Individuals With Multiple Mildly Deleterious mtDNA Variants Are More Likely to Suffer From Atherosclerosis

The etiology of common complex diseases is multifactorial, involving both genetic, and environmental factors. A role for mitochondrial dysfunction and mitochondrial DNA (mtDNA) variation has been suggested in the pathogenesis of common complex traits. The aim of this study was to investigate a potential role of mtDNA variants in the development of obesity, diabetes, and atherosclerosis in the Polish population. Whole mtDNA sequences from 415 Polish individuals representing three disease cohorts and a control group were obtained using high-throughput sequencing. Two approaches for the assessment of mtDNA variation were applied, traditional mitochondrial haplogroup association analysis and the mutational or variant load model using the MutPred pathogenicity prediction algorithm for amino acid substitutions in humans. We present a possible association between mildly deleterious mtDNA variant load and atherosclerosis that might be due to having more than one likely mildly deleterious non-synonymous substitution. Moreover, it seems largely dependent upon a few common haplogroup associated variants with MutPred score above 0.5.

Rapid Detection of the mt3243A > G Mutation Using Urine Sediment in Elderly Chinese Type 2 Diabetic Patients

OBJECTIVE

In this study, we aimed to identify mt3243A > G mutation carriers in a group of Chinese elderly type 2 diabetic patients by a rapid and noninvasive diagnostic system.

METHODS

DNA was extracted from blood, saliva, and urine sediment samples. The mutation screening and quantitation of heteroplasmy were performed by high-resolution melting (HRM) curve and pyrosequencing, respectively. Patients with mt3243A > G mutation underwent a detailed audiometric, ophthalmologic, neurological, and cardiac examination.

RESULTS

Two patients (2/1041) carrying the mt3243A > G mutation were detected among all type 2 diabetic patients. In patient 1, the heteroplasmy was 0.8%, 2.8%, and 14.7% in peripheral blood leukocytes, saliva, and urine sediment, respectively. In patient 2, the heteroplasmy was 5.3%, 8.4%, and 37.7% in peripheral blood leukocytes, saliva, and urine sediment, respectively. Both of the two patients showed hearing impairment. Abnormal ophthalmologic conditions and hyperintensity on T2-weighted magnetic resonance images were showed in patient 1.

CONCLUSION

The occurrence of mt3243 A > G mutation was 0.2% in Chinese elderly type 2 diabetic patients. Moreover, detection of mt3243 A > G mutation in urine sediment with high-resolution melting (HRM) curve and pyrosequencing is feasible in molecular genetic diagnosis.

Review: UKPDS odyssey — 2001

The United Kingdom Prospective Diabetes Study, the most ambitious single study of the treatment and nature of type 2 diabetes to date has two and nature of type 2 diabetes to date has two aspects: (i) as a clinical trial it has established that intensified glucose and blood pressure control significantly reduce the micro- and macrovascular complications of type 2 diabetes; (ii) in addition, because of its size and duration and the density of the data collected, it provides unparalleled information on the nature and natural history of the disease. It has defined the major importance of hyperglycaemia and hypertension as risk factors for complications and with the trial results provides the rationale and the goals for therapy. However, identification of the progressive deterioration of beta-cell function and the relative inefficacy of currently available therapies has posed major challenges to the achievement of these goals. Like all classic studies it raises more questions for further investigation.

Pyrosequencing is an accurate and reliable method for the analysis of heteroplasmy of the A3243G mutation in patients with mitochondrial diabetes

Human mitochondrial DNA is a circular DNA molecule that encodes some of the proteins required for oxidative phosphorylation. Different mitochondrial DNA genotypes may coexist within a single cell, a condition known as heteroplasmy. An A-to-G transition at position 3243 of mitochondrial DNA (A3243G) can result in maternally inherited diabetes and deafness (mitochondrial diabetes). However, the commonly used methods of PCR restriction fragment length polymorphism and Sanger sequencing are neither sensitive nor reliable enough to detect this low level of heteroplasmy. Here, we developed a quantitative method based on pyrosequencing to analyze the heteroplasmy of the A3243G mutation in leukocyte DNA obtained from 83 persons of 15 unrelated pedigrees with mitochondrial diabetes. The accuracy and reliability of this method were also measured by comparing the results with those from high-resolution melting analysis, Sanger sequencing, and PCR restriction fragment length polymorphism with artificial heteroplasmy standard samples. The results showed that the accuracy of pyrosequencing was much higher than that of the other methods, and the limitation of heteroplasmy detection with this method reached 2%, based on our artificial control studies. An inverse correlation was found between the level of heteroplasmy and the age of the onset in our patients. This result suggested that the heteroplasmy of the A3243G mutation could become a significant prediction index for the onset of mitochondrial diabetes.

Endocrine disorders in mitochondrial disease

Endocrine dysfunction in mitochondrial disease is commonplace, but predominantly restricted to disease of the endocrine pancreas resulting in diabetes mellitus. Other endocrine manifestations occur, but are relatively rare by comparison. In mitochondrial disease, neuromuscular symptoms often dominate the clinical phenotype, but it is of paramount importance to appreciate the multi-system nature of the disease, of which endocrine dysfunction may be a part. The numerous phenotypes attributable to pathogenic mutations in both the mitochondrial (mtDNA) and nuclear DNA creates a complex and heterogeneous catalogue of disease which can be difficult to navigate for novices and experts alike. In this article we provide an overview of the endocrine disorders associated with mitochondrial disease, the way in which the underlying mitochondrial disorder influences the clinical presentation, and how these factors influence subsequent management.

A deaf mother and son with diabetes and renal failure

Chronic renal failure is a well-known complication of long-standing diabetes. Moreover, audiological abnormalities are a common feature of patients with end-stage renal disease. Severe deafness, however, is not a typical symptom in most patients with chronic renal failure and likewise in patients with diabetes mellitus. In this case report, we describe a young patient with insulin-dependant diabetes mellitus, severe deafness requiring hearing aid and chronic renal failure outlining typical clinical features of the maternally inherited diabetes with deafness syndrome. Genetic testing confirmed the presence of the m.3243A>G mutation.

Unusual occurrence of intestinal pseudo obstruction in a patient with maternally inherited diabetes and deafness (MIDD) and favorable outcome with coenzyme Q10

Maternally inherited diabetes and deafness (MIDD) has been related to an A to G transition in the mitochondrial tRNA Leu (UUR) gene at the base pair 3243. This subtype of diabetes is characterized by maternal transmission, young age at onset and bilateral hearing impairment. Besides diabetes and deafness, the main diagnostic features, a wide range of multisystemic symptoms may be associated with the A3243G mutation. Organs that are most metabolically active, such as muscles, myocardium, retina, cochlea, kidney and brain are frequently affected. Gastrointestinal tract symptoms are also common in patients with mitochondrial disease and constipation and diarrhea are the most frequent manifestations. However, there are few prior reports of intestinal pseudo obstruction in MIDD patients. Here we report the case of a patient with MIDD associated with the mtDNA A3243G mutation who developed chronic intestinal pseudo obstruction, and the introduction of Coenzyme Q10 as adjunctive therapy led to a solution of the pseudo obstruction.

The clinical variability of maternally inherited diabetes and deafness is associated with the degree of heteroplasmy in blood leukocytes

CONTEXT

Maternally inherited diabetes and deafness (MIDD) is a rare form of diabetes with a matrilineal transmission, sensorineural hearing loss, and macular pattern dystrophy due to an A to G transition at position 3243 of mitochondrial DNA (mtDNA) (m.3243A>G). The phenotypic heterogeneity of MIDD may be the consequence of different levels of mutated mtDNA among mitochondria in a given tissue.

OBJECTIVE

The aim of the present study was thus to ascertain the correlation between the severity of the phenotype in patients with MIDD and the level of heteroplasmy in the blood leukocytes.

PARTICIPANTS

The GEDIAM prospective multicenter register was initiated in 1995. Eighty-nine Europid patients from this register, with MIDD and the mtDNA 3243A>G mutation, were included. Patients with MELAS (mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes) or with mitochondrial diabetes related to other mutations or to deletions of mtDNA were excluded.

RESULTS

A significant negative correlation was found between levels of heteroplasmy and age of the patients at the time of sampling for molecular analysis, age at the diagnosis of diabetes, and body mass index. After adjustment for age at sampling for molecular study and gender, the correlation between heteroplasmy levels and age at the diagnosis of diabetes was no more significant. The two other correlations remained significant. A significant positive correlation between levels of heteroplasmy and HbA(1c) was also found and remained significant after adjustment for age at molecular sampling and gender.

CONCLUSIONS

These results support the hypothesis that heteroplasmy levels are at least one of the determinants of the severity of the phenotype in MIDD.

Genetics: how the UKPDS contributed to determining the genetic landscape of Type 2 diabetes

The identification and functional characterisation of genetic variants that either cause or predispose to diabetes is a major focus of biomedical research. The molecular basis is now known for the majority of monogenic forms of diabetes arising from pancreatic beta-cell dysfunction; however finding the genetic variants underlying susceptibility to Type 2 diabetes (T2DM) has been a greater technical, statistical and biological challenge. The advent of biology-agnostic approaches made possible by the improved arsenal of research platforms and genetic tools available has increased the number of known T2DM genes dramatically and provided important insights into the pathophysiology of T2DM. Over the past 18 months, the list of T2DM susceptibility genes has grown from three to close to 20, illustrating the substantial progress which has been made. These recent milestones have not only illustrated the limited knowledge we have of the pancreatic beta-cell, but have also reinforced our belief in the involvement of common genetic variants in the genes involved in monogenic forms of diabetes in the susceptibility to T2DM and have clearly shown a primary role for pancreatic beta-cell dysfunction in T2DM. Both of these concepts were explored in the early work of the UK Prospective Diabetes Study (UKPDS) genetics research groups.

Characterisation of the macular dystrophy in patients with the A3243G mitochondrial DNA point mutation with fundus autofluorescence

INTRODUCTION

The mitochondrial DNA A3243G point mutation is associated with a wide variety of systemic manifestations including a macular dystrophy. The characteristics of fundus autofluorescence (AF) in these patients are distinctive and have not been previously described.

METHODS

A complete history and ophthalmic examination, including fundus photography and autofluorescence imaging, was performed on twelve probands harbouring the A3243G point mutation.

RESULTS

Four patients had diabetes, 10/12 hearing loss, and 7/12 were visually symptomatic. A positive family history was present in 5/12. Fundus findings consisted of two primary phenotypes: discontinuous circumferentially oriented perifoveal atrophy (9/12) or an appearance consistent with pattern dystrophy (3/12). In both phenotypes pale deposits and pigment clumping were seen at the level of the retinal pigment epithelium, with occasional changes also noted outside the arcades and nasal to the optic nerve. Fundus AF imaging revealed decreased autofluorescence in areas of atrophy and increased AF of the pale subretinal deposits. In areas of the retina that appeared normal clinically, variable sized flecks of increased and decreased AF were present.

CONCLUSIONS

The mitochondrial DNA A3243G point mutation can result in disease with a variable presentation. Fundus autofluorescence reveals a recognisable phenotype in most cases that is different from other macular dystrophies.

Clinical features, diagnosis and management of maternally inherited diabetes and deafness (MIDD) associated with the 3243A>G mitochondrial point mutation

Maternally inherited diabetes and deafness (MIDD) affects up to 1% of patients with diabetes but is often unrecognized by physicians. It is important to make an accurate genetic diagnosis, as there are implications for clinical investigation, diagnosis, management and genetic counselling. This review summarizes the range of clinical phenotypes associated with MIDD; outlines the advances in genetic diagnosis and pathogenesis of MIDD; summarizes the published prevalence data and provides guidance on the clinical management of these patients and their families.

Etiology of early-onset type 2 diabetes in Indians: islet autoimmunity and mutations in hepatocyte nuclear factor 1alpha and mitochondrial gene

CONTEXT

Indians are at high risk of developing type 2 diabetes mellitus (T2DM) at an early age, despite their lower body mass index. Studies on the etiology of patients presenting as early-onset T2DM in this racial group are not available.

OBJECTIVE

The objective was to delineate the clinical features in young Indian patients with T2DM and to determine the role of mutations in the hepatocyte nuclear factor 1alpha (HNF1alpha) gene [MODY3 (maturity-onset diabetes of the young, type 3)], mitochondrial A3243G mutation, and islet autoimmunity in its etiology.

DESIGN

This was an observational cohort study.

SETTING

The setting was an outpatient diabetes clinic in a teaching hospital.

PATIENTS

Ninety-six consecutive young patients with T2DM (onset, <or=30 yr) were included in the study.

INTERVENTIONS

Glutamic acid decarboxylase and insulinoma antigen 2 antibodies, mitochondrial A3243G mutation, and the common HNF1alpha mutation P291fsinsC were measured in all patients. The entire HNF1alpha gene was studied for mutations in 32 subjects with onset less than 25 yr or with normal weight. The common HNF1alpha A98V polymorphism was studied in 91 patients.

RESULTS

The patients were clinically heterogeneous, with 42% having a normal body mass index. Glutamic acid decarboxylase antibodies were present in three (3%) subjects and mitochondrial A3243G mutation in one (1%) subject. The P291fsinsC mutation was not detected in any patient. A MODY3 mutation (R200W) was detected in one patient (3%). In this family, diabetes cosegregated with the R200W mutation in the proband and his youngest brother but not in three paternal uncles. The Val 98 allele was associated with T2DM (allele frequency, 0.14 vs. 0.03 in controls; odds ratio, 5.2; P < 0.001).

CONCLUSIONS

Despite a significant proportion of young Indian patients with T2DM having normal weight, islet autoimmunity, A3243G mitochondrial, and HNF1alpha gene mutations were infrequent.

Diabetes mellitus associado à mutação mitocondrial A3243G: freqüência e caracterização clínica

Diabetes mitocondrial é freqüentemente associado à mutação mitocondrial A3243G. A prevalência desse subtipo de diabetes na população diabética varia de 0,5 a 3%, dependendo do grupo populacional estudado. OBJETIVO: Examinar a freqüência e o quadro clínico do diabetes associado com a mutação mitocondrial A3243G em pacientes brasileiros com tolerância a glicose alterada. MÉTODOS: A população estudada foi composta por 78 indivíduos portadores de diabetes mellitus tipo 1 (grupo I), 148 diabéticos tipo 2 (grupo II), 15 diabéticos tipo 1 ou tipo 2 portadores de disacusia (grupo III) e 492 indivíduos da comunidade nipo-brasileira com vários graus de intolerância a glicose. O DNA foi extraído de leucócitos do sangue periférico e a mutação A3243G foi determinada através da amplificação por PCR e digestão por Apa 1. Em alguns pacientes, o DNA também foi extraído da mucosa oral e folículo capilar. A mutação A3243G foi identificada em três indivíduos, todos do grupo III, resultando em uma prevalência de 0,4%. Os carreadores da mutação apresentavam diagnóstico do diabetes em idade jovem, índice de massa corpórea normal ou baixo e requerimento de insulina. CONCLUSÃO: Diabetes mitocondrial é um subtipo raro de diabetes em nossa população e deve ser investigado naqueles indivíduos portadores de diabetes e surdez.

Mitochondrial Disease—Its Impact, Etiology, and Pathology

Mitochondria are ubiquitous organelles that are intimately involved in many cellular processes, but whose principal task is to provide the energy necessary for normal cell functioning and maintenance. Disruption of this energy supply can have devastating consequences for the cell, organ, and individual. Over the last two decades, mutations in both mitochondrial DNA (mtDNA) and nuclear DNA have been identified as causative in a number of well-characterized clinical syndromes, although for mtDNA mutations in particular, this relationship between genotype and phenotype is often not straightforward. Despite this, a number of epidemiological studies have been undertaken to assess the prevalence of mtDNA mutations and these have highlighted the impact that mtDNA disease has on both the community and individual families. Although there has been considerable improvement in the diagnosis of mitochondrial disorders, disappointingly this has not been matched by developments toward effective treatment. Nevertheless, our understanding of mitochondrial biology is gathering pace and progress in this area will be crucial to devising future treatment strategies. In addition to mitochondrial disease, evidence for a central role of mitochondria in other processes, such as aging and neurodegeneration, is slowly accumulating, although their role in cancer remains controversial. In this chapter, we discuss these issues and offer our own views based on our cumulative experience of investigating and managing these diseases over the last 20 years.

Mitochondrial diabetes and its lessons for common Type 2 diabetes

Multiple pathogenic pathways are able to deregulate glucose homoeostasis leading to diabetes. The 3243A>G mutation in the mtDNA (mitochondrial DNA)-encoded tRNALeu,UUR gene was found by us to be associated with a particular diabetic subtype, designated MIDD (maternally inherited diabetes and deafness). This mutation causes an imbalance in the mitochondrion between proteins encoded by the nuclear and mitochondrial genomes, resulting in a gradual deterioration of glucose homoeostasis during life. Remarkably, carriers of the 3243A>G mutation are generally not obese. The mutation also results in enhanced radical production by mitochondria. We propose that this mutation leads to the development of diabetes due to an inappropriate storage of triacylglycerols within adipocytes. The result is a fatty acid-induced deterioration of pancreatic β-cell function. In combination with an enhanced radical production in the β-cell due to the mutation, this leads to an age-dependent, accelerated decline in insulin production. In common Type 2 (non-insulin-dependent) diabetes, which is generally associated with obesity, a decline in mitochondrial function in adipose cells seems to result in an inappropriate scavenging of fatty acids by β-oxidation. As a consequence, a systemic overload with fatty acids occurs, leading to an enhanced decline in β-cell function due to lipotoxicity.

Mitochondrial DNA mutations in human disease

The human mitochondrial genome is extremely small compared with the nuclear genome, and mitochondrial genetics presents unique clinical and experimental challenges. Despite the diminutive size of the mitochondrial genome, mitochondrial DNA (mtDNA) mutations are an important cause of inherited disease. Recent years have witnessed considerable progress in understanding basic mitochondrial genetics and the relationship between inherited mutations and disease phenotypes, and in identifying acquired mtDNA mutations in both ageing and cancer. However, many challenges remain, including the prevention and treatment of these diseases. This review explores the advances that have been made and the areas in which future progress is likely.

Genetics of type 2 diabetes mellitus

The clinical picture of type 2 diabetes mellitus (T2DM) is formed by impairment in insulin secretion and resistance to insulin action. As a result of intensive efforts of the scientists around the world mutations and polymorphisms in a number of genes were linked with monogenic and polygenic forms of T2DM. Two major strategies were used in this research: genome scanning and the candidate gene approach. Monogenic forms, despite their rarity, constitute a field where substantial progress has been made in the dissection of the molecular background of T2DM. Monogenic forms of T2DM with profound defect in insulin secretion include subtypes of maturity onset diabetes of the young (MODY), maternally inherited diabetes with deafness (MIDD) caused by mitochondrial mutations, and rare cases resulting from insulin gene mutations. The majority of proteins associated with MODY are transcription factors, such as hepatocyte nuclear factor 4alpha (HNF-4alpha), HNF-1alpha, insulin promoter factor-1 (IPF-1), HNF-1beta, and NEUROD1. They influence expression of the other genes through regulation of mRNA synthesis. Only MODY2 form is associated with glucokinase, a key regulatory enzyme of the beta cell. There are striking differences in the clinical picture of MODY associated with glucokinase and MODY associated with transcription factors. Three monogenic forms of T2DM characterized by severe insulin resistance are the consequence of mutations in the PPARgamma, ATK2, and insulin receptor genes. Patients with monogenic T2DM, particularly with MODY, sometimes, develop discrete extra-pancreatic phenotypes; for example, lipid abnormalities or a variety of cystic renal diseases. Efforts aiming to identify genes responsible for more common, polygenic forms of T2DM were less effective. These forms of T2DM have a middle/late age of onset and occur with both impaired insulin secretion and insulin resistance. Their clinical picture is created by the interaction of environmental and genetic factors, such as frequent polymorphisms of many genes, not just of one. These polymorphisms may be localized in the coding or regulatory parts of the genes and are present, although with different frequencies, in T2DM patients as well as in healthy populations. Sequence differences in a few genes have been associated, so far, with complex, polygenic forms of T2DM, for example, calpain 10, PPARgamma, KCJN11, and insulin. In addition, some evidence exists that genes, such as adiponectin, IRS-1, and some others may also influence the susceptibility to T2DM. It is expected that in the nearest future more T2DM susceptibility genes will be identified.

Molecular mechanisms of mitochondrial diabetes (MIDD)

Mitochondria provide cells with most of the energy in the form of adenosine triphosphate (ATP). Mitochondria are complex organelles encoded both by nuclear and mtDNA. Only a few mitochondrial components are encoded by mtDNA, most of the mt-proteins are nuclear DNA encoded. Remarkably, the majority of the known mutations leading to a mitochondrial disease have been identified in mtDNA rather than in nuclear DNA. In general, the idea is that these pathogenic mutations in mtDNA affect energy supply leading to a disease state. Remarkably, different mtDNA mutations can associate with distinct disease states, a situation that is difficult to reconcile with the idea that a reduced ATP production is the sole pathogenic factor. This review deals with emerging insight into the mechanism by which the A3243G mutation in the mitochondrial tRNA (Leu, UUR) gene associates with diabetes as major clinical expression. A decrease in glucose-induced insulin secretion by pancreatic beta-cells and a premature aging of these cells seem to be the main process by which this mutation causes diabetes. The underlying mechanisms and variability in clinical presentation are discussed.

Mitochondrial diabetes: molecular mechanisms and clinical presentation

Mutations in mitochondrial DNA (mtDNA) associate with various disease states. A few mtDNA mutations strongly associate with diabetes, with the most common mutation being the A3243G mutation in the mitochondrial DNA-encoded tRNA(Leu,UUR) gene. This article describes clinical characteristics of mitochondrial diabetes and its molecular diagnosis. Furthermore, it outlines recent developments in the pathophysiological and molecular mechanisms leading to a diabetic state. A gradual development of pancreatic beta-cell dysfunction upon aging, rather than insulin resistance, is the main mechanism in developing glucose intolerance. Carriers of the A3243G mutation show during a hyperglycemic clamp at 10 mmol/l glucose a marked reduction in first- and second-phase insulin secretion compared with noncarriers. The molecular mechanism by which the A3243G mutation affects insulin secretion may involve an attenuation of cytosolic ADP/ATP levels leading to a resetting of the glucose sensor in the pancreatic beta-cell, such as in maturity-onset diabetes of the young (MODY)-2 patients with mutations in glucokinase. Unlike in MODY2, which is a nonprogressive form of diabetes, mitochondrial diabetes does show a pronounced age-dependent deterioration of pancreatic function indicating involvement of additional processes. Furthermore, one would expect that all mtDNA mutations that affect ATP synthesis lead to diabetes. This is in contrast to clinical observations. The origin of the age-dependent deterioration of pancreatic function in carriers of the A3243G mutation and the contribution of ATP and other mitochondrion-derived factors such as reactive oxygen species to the development of diabetes is discussed.

Clinical features of diabetes mellitus with the mitochondrial DNA 3243 (A-G) mutation in Japanese: maternal inheritance and mitochondria-related complications

Diabetes mellitus with the mitochondrial DNA 3243(A-G) mutation is reported to represent 0.5-2.8% of the general diabetic population. Since the characterization of diabetes with the mutation is still incomplete, we undertook a nation-wide case-finding study of genetically defined patients using questionnaires in Japan. One hundred and thirteen Japanese diabetic patients with the mutation were registered and analyzed. The patients had a high prevalence of maternal inheritance of diabetes and deafness, short and thin stature, and showed an early middle-aged onset of diabetes and deafness. Eighty-six percent of the patients required insulin therapy due to the progressive insulin secretory defect. Glucose intolerance of the mothers was associated with an early middle-aged onset of diabetes, reduction in the insulin secretory capacity, early requirement of insulin therapy, and increases in the daily insulin dose. The heteroplasmic concentrations of the 3243 mutation in leukocytes were low and declined with aging. The patients had advanced microvascular complications, and mitochondria-related complications such as cardiomyopathy, cardiac conductance disorders, neuromuscular symptoms, neuropsychiatric disturbance, and macular pattern dystrophy. Thus, this study has revealed that: (1) diabetes mellitus with the 3243 mutation is a subtype of diabetes mellitus with mitochondria-related complications; and (2) insulin secretory ability is more severely impaired in the patients whose mothers were glucose intolerance.

Maternal transmission of diabetes

Type 2 diabetes mellitus represents a heterogeneous group of conditions characterized by impaired glucose homeostasis. The disorder runs in families but the mechanism underlying this is unknown. Many, but not all, studies have suggested that mothers are excessively implicated in the transmission of the disorder. A number of possible genetic phenomena could explain this observation, including the exclusively maternal transmission of mitochondrial DNA (mtDNA). It is now apparent that mutations in mtDNA can indeed result in maternally inherited diabetes. Although several mutations have been implicated, the strongest evidence relates to a point substitution at nucleotide position 3243 (A to G) in the mitochondrial tRNA(leu(UUR)) gene. Mitochondrial diabetes is commonly associated with nerve deafness and often presents with progressive non-autoimmune beta-cell failure. Specific treatment with Coenzyme Q10 or L-carnitine may be beneficial. Several rodent models of mitochondrial diabetes have been developed, including one in which mtDNA is specifically depleted in the pancreatic islets. Apart from severe, pathogenic mtDNA mutations, common polymorphisms in mtDNA may contribute to variations of insulin secretory capacity in normal individuals. Mitochondrial diabetes accounts for less than 1% of all diabetes and other mechanisms must underlie the maternal transmission of Type 2 diabetes. Possibilities include the role of maternally controlled environments, imprinted genes and epigenetic phenomena.

Mitochondrial diabetes, DIDMOAD and other inherited diabetes syndromes

Inherited diabetes syndromes are individually rare but collectively make up a significant proportion of patients attending diabetes clinics, some of whom have multiple handicaps. This chapter focuses on syndromes in which major advances have been made in our understanding of the underlying molecular genetics. These conditions demonstrate novel genetic mechanisms such as maternal inheritance and genetic imprinting. They are also fascinating as they aid our understanding of insulin metabolism, both normal and abnormal. As the causative genes are identified, future issues will be the availability of genetic testing, their contribution to the genetic heterogeneity of the more common types of diabetes, and functional studies of the relevant proteins. It is probable that other subtypes of diabetes will be identified as the relevant metabolic pathways are characterized. This is an exciting time to be a diabetes physician as diabetology returns to being a diagnostic rather than a mainly management-based speciality.

Level of heteroplasmy for the mitochondrial mutation A3243G correlates with age at onset of diabetes and deafness

The mitochondrial mutation A3243G has been shown to be associated with a syndrome of diabetes mellitus and sensorineural hearing loss. Using a solid-phase-based sequencing method we have investigated the relation between the proportion of mutant mitochondrial genomes and the time of disease onset among members of three families where the mutation segregates. A striking association was observed between the level of heteroplasmy and time of onset of disease, particularly hearing loss. Accordingly, this syndrome shares features of diseases caused by dynamic mutations in that variable transmission of the level of heteroplasmy between generations influences disease severity.

Mitochondrial gene mutations in gestational diabetes mellitus

Mitochondrial DNA mutations have been implicated in many diseases including diabetes mellitus. Although gestational diabetes mellitus (GDM) has been suggested to have genetic determinant and to be etiologically indistinct with non-insulin-dependent diabetes mellitus (NIDDM), its association with mitochondrial gene mutations is still unknown. In this study, 137 patients with GDM and 292 non-diabetic pregnant controls were examined for mitochondrial DNA mutations from the nucleotide 3130-4260 encompassing tRNA-Leu gene and adjacent NADH dehydrogenase 1 gene by polymerase chain reaction, single-stranded conformation polymorphism, restriction fragment length polymorphism and DNA sequencing. One heteroplasmic mutation at the position of 3398 (T-C), which changed a highly conserved methionine to threonine in NADH dehydrogenase subunit 1, was identified in 2.9% GDM patients but not in the controls, indicating its association with GDM (P = 0.01). Two novel mutations, a heteroplasmic C3254A and a homoplasmic A3399T, were also found in GDM subjects, the functional meaning of which merits further investigation. G3316A and T3394C mutations implicated in NIDDM, were seen at higher frequencies in patients with GDM than the controls. Our results suggest that mitochondrial DNA mutations may contribute to the development of GDM in some patients.