A subtype of diabetes mellitus associated with a mutation of mitochondrial DNA

The epidemiology of mitochondrial disorders--past, present and future

A number of epidemiological studies of mitochondrial disease have been carried out over the last decade, clearly demonstrating that mitochondrial disorders are far more common than was previously accepted. This review summarizes current knowledge of the prevalence of human mitochondrial disorders--data that has important implications for the provision of health care and adequate resources for research into the pathogenesis and treatment of these disorders.

Varying loads of the mitochondrial DNA A3243G mutation in different tissues: implications for diagnosis

Testing for common mutations in mitochondrial DNA (mtDNA), including the A3243G MELAS (mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes) mutation, is routinely done in DNA isolated from blood. Since the blood level of the A3243G mutation may be low in probands and even lower in asymptomatic or oligosymptomatic maternal relatives, we assessed the proportion of A3243G mutant genomes in other accessible tissues. We studied five tissues (leukocytes, skin fibroblasts, hair roots, urinary sediment, and cheek mucosa) in 61 individuals from 22 families harboring the A3243G mutation. Although mutational loads varied widely among these tissues, as a rule DNA from urinary sediment had the highest and blood the lowest proportion of mutant genomes; individual hair roots differed markedly from one another; fibroblasts and cheek mucosa tended to have higher mutation loads than blood but lower than urinary sediment. In all individuals in whom the mutation was detectable in blood, it was also detected in other tissues. Conversely, in some individuals the mutation was undetectable in blood but clearly present in other tissues. We conclude that urinary sediment and cheek mucosa are tissues of choice for the diagnosis of mtDNA mutations, as they are easy to obtain and the mutation load is almost always greater than in blood.

Inhibition of mitochondrial gene transcription suppresses neurotensin secretion in the human carcinoid cell line BON

Mitochondria, organelles essential for ATP production, play a central role in a number of cellular functions, including the regulation of insulin secretion. Neurotensin (NT), an important regulatory intestinal hormone, has been implicated in fatty acid translocation, gut motility and secretion, and intestinal cell growth; however, mechanisms regulating NT secretion have not been entirely defined. The purpose of this study was to determine the effect of inhibition of mitochondrial gene transcription on NT secretion. BON cells, a novel human carcinoid cell line that produces and secretes NT peptide and expresses the gene encoding NT (designated NT/N), were treated with ethidium bromide (EB; 0.05, 0.1, and 0.4 microg/ml), an inhibitor of DNA and RNA synthesis, or vehicle over a time course (1-4 days). Cells were then stimulated with either ACh (100 microM) or phorbol 12 myristate,13-acetate (PMA, 10 nM) for 30 min. Media and cells were extracted, and NT peptide measured by RIA. Treatment with EB had no effect on BON cell viability or cell cycle distribution over the 4-day course. In contrast, EB treatment produced a dose-dependent reduction of mitochondrial gene expression; however, NT/N gene expression was not altered. Mitochondrial inhibition by EB treatment suppressed NT secretion induced by ACh and PMA, both in a dose-dependent manner. EB-mediated inhibition of NT secretion and mitochondrial gene expression was reversed with removal of EB. Our results demonstrate that inhibition of mitochondrial gene transcription suppresses both ACh- and PMA-stimulated NT release. These findings are the first to demonstrate that mitochondrial function is important for agonist-mediated NT secretion.

Increased Reactive Oxygen Species and Anti-Oxidative Response in Mitochondrial Cardiomyopathy

A 60-year-old woman was admitted for treatment of congestive heart failure. She had been diagnosed with diabetes mellitus when she was 23 years old, and she began to go deaf when she was 34 years old. She showed symptoms of heart failure at age 51 and was diagnosed with hypertrophic cardiomyopathy. Echocardiography showed progressive diffuse hypokinetic motion of the left ventricle and the left ventricular hypertrophy had gradually regressed. A mitochondrial transition mutation, A3243G, was detected in her peripheral leukocytes (9%) and in those of her 27-year-old son, who also has diabetes and deafness. Electron microscopy of an endomyocardial biopsy specimen showed proliferation and swelling of the mitochondria, and significant generation of reactive oxygen species (ROS), as well as marked induction of heme oxygenase-1, which is an adaptive enzyme to oxidative damage, were also observed in the myocardial tissue. These observations were more prominent than in other patients with heart failure of different etiology, which suggests that the increased ROS generation and anti-oxidative response were involved in the development of the mitochondrial cardiomyopathy.

Depletion of mitochondrial DNA causes impaired glucose utilization and insulin resistance in L6 GLUT4myc myocytes

Mitochondrial dysfunction contributes to a number of human diseases, such as hyperlipidemia, obesity, and diabetes. The mutation and reduction of mitochondrial DNA (mtDNA) have been suggested as factors in the pathogenesis of diabetes. To elucidate the association of cellular mtDNA content and insulin resistance, we produced L6 GLUT4myc myocytes depleted of mtDNA by long term treatment with ethidium bromide. L6 GLUT4myc cells cultured with 0.2 mug/ml ethidium bromide (termed depleted cells) revealed a marked decrease in cellular mtDNA and ATP content, concomitant with a lack of mRNAs encoded by mtDNA. Interestingly, the mtDNA-depleted cells showed a drastic decrease in basal and insulin-stimulated glucose uptake, indicating that L6 GLUT4myc cells develop impaired glucose utilization and insulin resistance. The repletion of mtDNA normalized basal and insulin-stimulated glucose uptake. The mRNA level and expression of insulin receptor substrate (IRS)-1 associated with insulin signaling were decreased by 76 and 90% in the depleted cells, respectively. The plasma membrane (PM) GLUT4 in the basal state was decreased, and the insulin-stimulated GLUT4 translocation to the PM was drastically reduced by mtDNA depletion. Moreover, insulin-stimulated phosphorylation of IRS-1 and Akt2/protein kinase B were drastically reduced in the depleted cells. Those changes returned to control levels after mtDNA repletion. Taken together, our data suggest that PM GLUT4 content and insulin signal pathway intermediates are modulated by the alteration of cellular mtDNA content, and the reductions in the expression of IRS-1 and insulin-stimulated phosphorylation of IRS-1 and Akt2/protein kinase B are associated with insulin resistance in the mtDNA-depleted L6 GLUT4myc myocytes.

Genes and pathophysiology of type 2 diabetes: more than just the Randle cycle all over again

The Randle cycle, which has been invoked to explain the reciprocal relationship between fatty acid oxidation and glucose oxidation, has long been implicated as a potential mechanism for hyperglycemia and type 2 diabetes mellitus (T2DM). Now genetic, functional genomic, and transgenic approaches have identified PPARgamma coactivators (PGC-1alpha and PGC-1beta) as key regulators of mitochondrial number and function. They regulate adaptive thermogenesis as well as glucose and fat oxidation in muscle and fat tissue, gluconeogenesis in liver, and even glucose-regulated insulin secretion in beta cells. PGC-1alpha and PGC-1beta mRNA levels and the mitochondrial genes they regulate are decreased in muscle of people with prediabetes and T2DM. A new report indicates that PGC-1alpha and PGC-1beta mRNA levels decrease with age in individuals with a genetic variant in PGC-1alpha, and these decreases correlate with alterations in whole-body glucose and fatty acid oxidation. These findings provide insights into how aging modifies genetic susceptibility to alterations in oxidative phosphorylation and T2DM.

Diagnosis and management of MELAS

Mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) is the most common maternally inherited mitochondrial disease. An A-->G mutation in the transfer RNA(Leu(UUR)) gene at position 3243 of the mitochondrial DNA accounts for most MELAS cases. The transient nature of the stroke-like episodes is reflected in abnormalities on neuroimaging. The cardinal laboratory abnormalities include elevated serum lactate during the acute episodes and respiratory enzyme defects in skeletal muscle. Muscle biopsy also helps confirm the diagnosis by identifying abnormal proliferation of mitochondria. Although current treatment options for MELAS are largely supportive, several therapeutic approaches have been attempted with limited success. Genetic counseling is an important component of patient management in MELAS. Newer reproductive technologies hold promise for reducing the recurrence of MELAS in subsequent generations. Advances in research into gene therapy offer hope of treatment for the future.

Common genetic basis between type 1 and type 2 diabetes mellitus indicated by interview-based assessment of family history

To investigate the intrafamilial clustering of type 1 and type 2 diabetes, an interview-based assessment of family history of diabetes was conducted. Outpatients with either type 1 (n = 23) or type 2 diabetes (n = 124), and non-diabetic subjects (n = 118) received an interview regarding the diabetic status of each of their family members. In patients with type 1 diabetes, 22% (5 out of 23) had a parental history of diabetes, and diabetes in these 5 parents was assessed as type 2 diabetes mellitus. The prevalence of parental diabetes in the type 1 diabetic probands (22%) was significantly higher (P < 0.05) than that in non-diabetic probands (7%, 8 out of 118). In probands with type 2 diabetes, the prevalence of parental diabetes was 39% (48 out of 124), which was significantly higher (P < 0.0005) than that in the non-diabetic probands (7%). In the type 2 diabetic probands, no significant difference was noted in the prevalence between paternal (19%, 23 out of 124) and maternal diabetes (23%, 28 out of 124), suggesting no preferential inheritance of maternal diabetes in this population. The present interview-based assessment of family history of diabetes suggested a common genetic basis between type 1 and type 2 diabetes.

PAX4 gene variations predispose to ketosis-prone diabetes

Ketosis-prone diabetes (KPD) is a rare form of type 2 diabetes, mostly observed in subjects of west African origin (west Africans and African-Americans), characterized by fulminant and phasic insulin dependence, but lacking markers of autoimmunity observed in type 1 diabetes. PAX4 is a transcription factor essential for the development of insulin-producing pancreatic beta-cells. Recently, a missense mutation (Arg121Trp) of PAX4 has been implicated in early and insulin deficient type 2 diabetes in Japanese subjects. The phenotype similarities between KPD and Japanese carriers of Arg121Trp have prompted us to investigate the role of PAX4 in KPD. We have screened 101 KPD subjects and we have found a new variant in the PAX4 gene (Arg133Trp), specific to the population of west African ancestry, and which predisposes to KPD under a recessive model. Homozygous Arg133Trp PAX4 carriers were found in 4% of subjects with KPD but not in 355 controls or 147 subjects with common type 2 or type 1 diabetes. In vitro, the Arg133Trp variant showed a decreased transcriptional repression of target gene promoters in an alpha-TC1.6 cell line. In addition, one KPD patient was heterozygous for a rare PAX4 variant (Arg37Trp) that was not found in controls and that showed a more severe biochemical phenotype than Arg133Trp. Clinical investigation of the homozygous Arg133Trp carriers and of the Arg37Trp carrier demonstrated a more severe alteration in insulin secretory reserve, during a glucagon-stimulation test, compared to other KPD subjects. Together these data provide the first evidence that ethnic-specific gene variants may contribute to the predisposition to this particular form of diabetes and suggest that KPD, like maturity onset diabetes of the young, is a rare, phenotypically defined but genetically heterogeneous form of type 2 diabetes.

The cellular fate of glucose and its relevance in type 2 diabetes

Type 2 diabetes is a complex disorder with diminished insulin secretion and insulin action contributing to the hyperglycemia and wide range of metabolic defects that underlie the disease. The contribution of glucose metabolic pathways per se in the pathogenesis of the disease remains unclear. The cellular fate of glucose begins with glucose transport and phosphorylation. Subsequent pathways of glucose utilization include aerobic and anaerobic glycolysis, glycogen formation, and conversion to other intermediates in the hexose phosphate or hexosamine biosynthesis pathways. Abnormalities in each pathway may occur in diabetic subjects; however, it is unclear whether perturbations in these may lead to diabetes or are a consequence of the multiple metabolic abnormalities found in the disease. This review is focused on the cellular fate of glucose and relevance to human type 2 diabetes.

Accumulation of pathogenic ΔmtDNA induced deafness but not diabetic phenotypes in mito-mice

Mito-mice carrying various proportions of deletion mutant mtDNA (DeltamtDNA) were generated by introduction of the DeltamtDNA from cultured cells into fertilized eggs of C57BL/6J (B6) strain mice. Great advantages of mito-mice are that they share exactly the same nuclear-genome background, and that their genetic variations are restricted to proportions of pathogenic DeltamtDNA. Since accumulation of DeltamtDNA to more than 75% induced respiration defects, the disease phenotypes observed exclusively in mito-mice carrying more than 75% DeltamtDNA should be due to accumulated DeltamtDNA. In this study, we focused on the expressions of hearing loss and diabetic phenotypes, since these common age-associated abnormalities have sometimes been reported to be inherited maternally and to be associated with pathogenic mutant mtDNAs. The results showed that accumulation of exogenously introduced DeltamtDNA was responsible for hearing loss, but not for expression of diabetic phenotypes in mito-mice.

High-sensitivity detection of the A3243G mutation of mitochondrial DNA by a combination of allele-specific PCR and peptide nucleic acid-directed PCR clamping

BACKGROUND

The A3243G mutation of mitochondrial DNA (mtDNA) is involved in many common diseases, including diabetes mellitus and mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS). For detection of this mutation, allele-specific PCR is highly sensitive but requires strict control of PCR conditions; it thus is not adequate for a routine clinical test. We aimed to develop a routinely available PCR method for quantitative detection of low-level heteroplasmy of the A3243G mutation.

METHODS

Quantitative allele-specific PCR for the A3243G mutation was performed in the presence of peptide nucleic acid (PNA), in which PNA is complementary to the wild-type mtDNA, with one primer having a 3' end matched to nucleotide position 3243 of the mutant.

RESULTS

With our method, amplification of wild-type mtDNA was suppressed 7000-fold compared with amplification of the mutant mtDNA under a broad range of conditions: DNA, 5-100 ng; annealing temperature, 61-66 degrees C; and PNA, 1.5-3.5 micromol/L. Hence, 0.1% heteroplasmy of the A3243G mutation can be reliably quantified by this method. Blood samples form 40 healthy volunteers showed <0.06% heteroplasmy, suggesting that 0.1% is diagnostically significant.

CONCLUSIONS

PNA maintains the specificity of allele-specific PCR over a wide range of conditions, which is important for routine clinical testing.

Accumulation of somatic mutation in mitochondrial DNA and atherosclerosis in diabetic patients

A point mutation of mitochondrial DNA at nucleotide position 3243 A to G is responsible for the genetic cause of diabetes. Otherwise, this mutation is also reported to occur as a somatic mutation, possibility because of oxidative stress. Because diabetes may cause oxidative stress, we hypothesized that accumulation of the somatic A3243G mutation in mitochondrial DNA may be accelerated by diabetes. DNA was extracted from blood samples of 290 nondiabetic healthy subjects (aged 0-60 years) and from 383 type 2 diabetic patients (aged 18-80 years). Then, the extent of somatic A3243G mutation in total mitochondrial DNA was detected by real-time polymerase chain reaction (PCR) using the TaqMan probe. The genotyping of ACE I/D or p22phox C242T was done by PCR or PCR-restriction fragment length polymorphism. Although the level of the A3243G mutation was negligible in the newborn group, it increased in healthy subjects aged 20-29 and 41-60 years. In diabetic patients, the mutational rate increased along with age and the duration of diabetes. In the middle-aged group (41-60 years old), the A3243G mutation accumulates fourfold higher in the diabetic patients than in the healthy subjects. Moreover, multiple regression analysis revealed that the most critical factor associated with this mutation in diabetic patients was the duration of diabetes. Furthermore, the genotype of DD, DI-CC (ACE-p22phox) has the highest mutational rate and the thickest intima-media thickness of the carotid artery. In conclusion, diabetes accelerates the accumulation of the somatic A3243G mutation in mitochondrial DNA, and this somatic mutation may be a marker for the duration of diabetes and atherosclerosis.

Diabetes mellitus with mitochondrial gene mutations in Japan

Diabetes mellitus due to the mitochondrial DNA 3243(A-G) mutation is reported to represent 0.5-1% of the general diabetic population in Japan. To further elucidate the clinical symptoms and course of diabetes mellitus with the 3243 mutation, we undertook a nationwide cross-sectional case-finding study and observational study of a genetically defined subject group. One hundred sixteen Japanese diabetic patients with the mutation were registered and analyzed. The patients had a higher maternal inheritance of diabetes or deafness, short stature, thin habitus, and early middle-aged onset of diabetes or deafness. Eighty-six percent of the patients required insulin therapy because of progressive insulin secretory defect. Although half of the patients had the phenotype of type 1 diabetes or slowly progressive type 1 diabetes, the patients lacked the presence of autoantibodies to glutamic acid decarboxylase. Diabetes in the mothers was characterized by early middle-aged onset, reduction in the insulin secretory capacity, early requirement of insulin therapy, and increases in the daily insulin dose. The heteroplasmic ratio of the 3243 mutation in leukocytes was low. The patients had mitochondria-related complications such as sensorineural deafness, cardiomyopathy, cardiac conductance disorders, encephalomyopathy, macular pattern dystrophy, and mental disorders. The patients also had advanced microvascular complications. 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 also diabetic.

Accumulation of somatic mutation in mitochondrial DNA and atherosclerosis in diabetic patients

A point mutation of mitochondrial DNA at nucleotide position 3243 A to G is responsible for the genetic cause of diabetes. Otherwise, this mutation is also reported to occur as a somatic mutation, possibility because of oxidative stress. Because diabetes may cause oxidative stress, we hypothesized that accumulation of the somatic A3243G mutation in mitochondrial DNA may be accelerated by diabetes. DNA was extracted from blood samples of 290 nondiabetic healthy subjects (aged 0-60 years) and from 383 type 2 diabetic patients (aged 18-80 years). Then, the extent of somatic A3243G mutation in total mitochondrial DNA was detected by real-time polymerase chain reaction (PCR) using the TaqMan probe. The genotyping of ACE I/D or p22phox C242T was done by PCR or PCR-restriction fragment length polymorphism. Although the level of the A3243G mutation was negligible in the newborn group, it increased in healthy subjects aged 20-29 and 41-60 years. In diabetic patients, the mutational rate increased along with age and the duration of diabetes. In the middle-aged group (41-60 years old), the A3243G mutation accumulates fourfold higher in the diabetic patients than in the healthy subjects. Moreover, multiple regression analysis revealed that the most critical factor associated with this mutation in diabetic patients was the duration of diabetes. Furthermore, the genotype of DD, DI-CC (ACE-p22phox) has the highest mutational rate and the thickest intima-media thickness of the carotid artery. In conclusion, diabetes accelerates the accumulation of the somatic A3243G mutation in mitochondrial DNA, and this somatic mutation may be a marker for the duration of diabetes and atherosclerosis.

Mitochondrial oxidative stress and mitochondrial DNA

Mitochondria produce reactive oxygen species (ROS) under physiological conditions in association with activity of the respiratory chain in aerobic ATP production. The production of ROS is essentially a function of O2 consumption. Hence, increased mitochondrial activity per se can be an oxidative stress to cells. Furthermore, production of ROS is markedly enhanced in many pathological conditions in which the respiratory chain is impaired. Because mitochondrial DNA, which is essential for execution of normal oxidative phosphorylation, is located in proximity to the ROS-generating respiratory chain, it is more oxidatively damaged than is nuclear DNA. Cumulative damage of mitochondrial DNA is implicated in the aging process and in the progression of such common diseases as diabetes, cancer, and heart failure.

Paradoxical decrease of mitochondrial DNA deletions in epithelial cells of active ulcerative colitis patients

Ulcerative colitis (UC) is a condition characterized by chronic inflammation targeted at the epithelial layer. In addition to being involved in immune phenomena, UC epithelial cells exhibit decreased oxidation of butyrate, downregulation of oxidative pathway regulatory genes, and overexpression of mitochondrial (mt) genes. We investigated whether these events, which translate an altered energy metabolism, are associated with an abnormal pattern of mtDNA deletions. Highly purified colonocytes were isolated from surgically resected control, involved and uninvolved inflammatory bowel disease mucosa. The frequency, type, and number of mtDNA deletions were assessed by PCR amplification, Southern blot analysis, and cloning and sequencing of amplified DNA fragments. The 4977 mtDNA deletion was less frequent in UC than control and Crohn's disease (CD) epithelium, regardless of patient age. Several other deletions were detected, but all were less common in UC than control and CD cells. The frequency, variety, and number of mtDNA deletions were invariably lower in colonocytes isolated from inflamed mucosa than in autologous cells from noninflamed mucosa. In conclusion, in the absence of inflammation, UC colonocytes exhibit an mtDNA deletion pattern similar to that of control cells, indicating a normal response to physiological levels of oxidative stress. In active inflammation, when oxidative stress increases, the frequency, variety, and number of mtDNA deletions decrease. Because comparable abnormalities are absent in active CD, the mtDNA deletion pattern of active UC suggests that colonocytes respond uniquely to inflammation-associated stress in this condition.

Grossesse chez une patiente atteinte de cytopathie mitochondriale

We report a case of a pregnant woman with a mitochondrial disorder affecting the energy-generating pathway of oxidative phosphorylation which was suggested when the patient presented the progressive clinical phenotype of a proximal tubular renal insufficiency, a muscular weakness of extremities, a bilateral optic neuropathy and a brain magnetic resonance imaging suggesting diffuse leucoencephalopathy. Her diagnosis was made on the basis of abnormal mitochondria on a muscle biopsy and of spectrophotometric deficiencies of the complexes I, II+III and IV of the respiratory chain. No specific molecular mutation could be detected. Her pregnancy was complicated by a severe preeclampsia, an insulin requiring gestational diabetes and a worrying renal failure which precipitated the premature delivery by cesarean section at 30 weeks gestation. The clinical course of the female neonate weighing 1030 grams was uneventful. At two Years of age she showed no sign of mitochondrial disease. But the postpartum course of the mother was complicated by seizures and a terminal renal failure leading presently to dialysis, but requiring a kidney transplantation in the near future.

Prevalence of the mitochondrial DNA A1555G mutation in sensorineural deafness patients in island Southeast Asia

A mtDNA A1555G base substitution in a highly conserved region of the 12S rRNA gene has been reported to be the main cause of aminoglycoside induced deafness. This mutation is found in approximately 3% of Japanese and 0.5-2.4% of European sensorineural deafness patients. We report a high prevalence (5.3%) of the A1555G mutation in sensorineural deafness patients in Sulawesi (Indonesia). Our result confirms the importance of determining the prevalence of the mtDNA A1555G mutation in different populations, and the need for mutation detection before the administration of aminoglycoside antibiotics.

The contribution of mitochondria to common disorders

Mitochondrial dysfunction secondary to mitochondrial and nuclear DNA mutations has been associated with energy deficiency in multiple organ systems and a variety of severe, often fatal, clinical syndromes. Although the production of energy is indeed the primary function of mitochondria, attention has also been directed toward their role producing reactive oxygen and nitrogen species and the subsequent widespread deleterious effects of these intermediates. The generation of toxic reactive intermediates has been implicated in a number of relatively common disorders, including neurodegenerative diseases, diabetes, and cancer. Understanding the role mitochondrial dysfunction plays in the pathogenesis of common disorders has provided unique insights into a number of diseases and offers hope for potential new therapies.

Pelvic lymphocyst infection associated with maternally inherited diabetes mellitus

A 45-year-old woman with 20-year history of diabetes mellitus was admitted to our hospital because of high fever and abdominal pain. Radical hysterectomy and bilateral pelvic lymphadenectomy had been performed 4 months before admission for invasive cervical cancer. On admission, elastic hard tumors were palpable in the lower abdomen. Laboratory examination showed positive C-reactive protein (CRP), anemia and renal dysfunction. Computed tomography (CT) revealed several lymphocysts in the pelvis. She was diagnosed with infection of pelvic lymphocysts. Since her mother also had diabetes associated with deafness, we examined mitochondrial DNA in leukocytes and detected an A to G transition at the nucleotide position of 3243 (A3243G mutation). She was diagnosed as maternally inherited diabetes mellitus and deafness (MIDD). Puncture of the cysts followed by administration of antibiotics resulted in marked improvement of symptoms and laboratory findings. This is a rare case of pelvic lymphocyst infection in a patient with a mitochondrial disorder. Although the exact mechanism of infection is not clear, MIDD may represent an unusual risk factor for infection, and further investigation is necessary to assess the influence of mitochondrial dysfunction on the immune system. Pelvic lymphocyst infection should be considered in the differential diagnosis of abdominal pain and fever in patients with MIDD after abdominal surgery.

Mitochondrial deafness

The last decade has led to the identification of several mitochondrial DNA mutations associated with hearing loss. Since the only known function of the human mitochondrial chromosome is to participate in the production of chemical energy through oxidative phosphorylation, it was not unexpected that mitochondrial mutations interfering with energy production could cause systemic neuromuscular disorders, which have as one of their features hearing impairment. Surprisingly, however, inherited mitochondrial mutations also have been found to be a cause of non-syndromic hearing loss, and predispose to aminoglycoside induced hearing loss, while acquired mitochondrial mutations have been proposed as one of the causes of presbycusis. After a brief review of mitochondrial genetics, we will outline the different mitochondrial mutations associated with hearing loss, describe the audiological features, and discuss the clinical relevance of diagnosing these mutations. Clinical expression of these mitochondrial mutations is dependent on environmental exposures and nuclear-encoded modifier genes. Preventive and therapeutic strategies will depend on identification and avoidance of the environmental exposures, and the identification of the nuclear-encoded modifier genes. Experimental approaches to identify these modifier genes will be presented.

A novel class of tests for the detection of mitochondrial DNA-mutation involvement in diseases

We develop a novel class of tests to detect mitochondrial DNA (mtDNA)-mutation involvement in complex diseases by the study of affected pedigree members. For a pedigree, affected individuals are first considered and are then connected through their relatives. We construct a reduced pedigree from an original pedigree. Each configuration of a reduced pedigree is given a score, with high scores given to configurations that are consistent with mtDNA-mutation involvement and low scores given to configurations that are not consistent with mtDNA-mutation involvement. For many pedigrees, the weighted sum of scores of the pedigrees is calculated. The tests are formed by comparing the observed score with the expected score under the null hypothesis that only nuclear autosomal mutations are involved. We study the optimality of score functions and weights under the heterogeneity model without phenocopies. We also develop a method to estimate the contribution that mtDNA mutations make if they are involved under a heterogeneity model. Finally, we apply our methods to three data sets: Leber hereditary optic neuropathy, a disease that has been proved to be caused by mtDNA mutations; non-insulin-dependent diabetes mellitus (NIDDM); and hypertension (HTN). We find evidence of mtDNA-mutation involvement in all three diseases. The estimated fraction of patients with NIDDM due to mtDNA-mutation involvement is 22% (95% confidence interval [CI] 6%-38%). The fraction of patients with HTN potentially due to mtDNA-mutation involvement is estimated at 55% (95% CI 45%-65%).

Impact of genetic background on development of hyperinsulinemia and diabetes in insulin receptor/insulin receptor substrate-1 double heterozygous mice

Type 2 diabetes is a complex disease in which genetic and environmental factors interact to produce alterations in insulin action and insulin secretion, leading to hyperglycemia. To evaluate the influence of genetic background on development of diabetes in a genetically susceptible host, we generated mice that are double heterozygous (DH) for knockout of the insulin receptor and insulin receptor substrate-1 on three genetic backgrounds (C57BL/6 [B6], 129Sv, and DBA). Although DH mice on all backgrounds showed insulin resistance, their phenotypes were dramatically different. B6 DH mice exhibited marked hyperinsulinemia and massive islet hyperplasia and developed early hyperglycemia, with 85% overtly diabetic by 6 months. By contrast, 129Sv DH mice showed mild hyperinsulinemia and minimal islet hyperplasia, and < 2% developed diabetes. DBA mice had slower development of hyperglycemia, intermediate insulin levels, and evidence of islet degeneration, with 64% developing diabetes. Thus, mice carrying the same genetic defects on different backgrounds exhibited the full spectrum of abnormalities observed in humans with type 2 diabetes, which allowed for identification of potential loci that promote development of the diabetic phenotype.

The effect of diabetes on sensorineural hearing loss

OBJECTIVE

To identify whether patients with diabetes have a higher incidence of sensorineural hearing loss than the general population and examine whether control of diabetes is related to severity of hearing loss.

STUDY DESIGN

Retrospective database review; complete data mining of electronic medical record from 1989 to present.

SETTING

Tertiary referral center.

PATIENTS

Electronic medical records from 53461 nondiabetic age-matched patients and 12575 diabetic patients were reviewed.

MAIN OUTCOME MEASURES

Presence or absence of diabetes and/or sensorineural hearing loss, serum creatinine, pure tone hearing (dB), speech discrimination (%), serum cholesterol, and triglycerides.

RESULTS

Sensorineural hearing loss was more common in the diabetic patients than in age0matched nondiabetic patients from the same institutions. Poor control of diabetes, as measured by increasing serum creatinine, but not apparent in hemoglobin A1C laboratory data, correlated with worsening hearing in patients with diabetes who had sensorineural hearing loss.

CONCLUSIONS

Sensorineural hearing loss was more common in patients with diabetes than in the control nondiabetic patients, and severity of hearing loss seemed to correlate with progression of disease as reflected in serum creatinine. This may have been due to microangiopathic disease in the inner ear.

A review of cochlear implantation in mitochondrial sensorineural hearing loss

OBJECTIVE

Mitochondrial sensorineural hearing loss (SNHL) may be nonsyndromic (occurring in isolation), associated with the A1555G mutation in the MTRNR1 gene. Mitochondrial SNHL may also be syndromic, associated with the A3243G point mutation in the MTTL1 gene. In syndromic cases-mitochondrial encephalopathy, lactic acidosis, and strokelike episodes (MELAS), maternally inherited diabetes and deafness, Kearns-Sayre syndrome, and chronic progressive external ophthalmoplegia-the SNHL compounds already existing disabilities. The genetic basis for mitochondrial SNHL and postulated sites of pathologic changes are discussed.

DATA SOURCES

Sources used were relevant clinical and basic science publications.

STUDY SELECTION

A search of the entire databases of Medline and Web of Science, using various subject headings and free-text terms, was used to identify patients with mitochondrial disease having cochlear implants.

DATA EXTRACTION

The data from publications were critically reviewed and tabulated to assess implantation outcomes.

DATA SYNTHESIS

The data were not amenable to formal meta-analysis or valid data summarization, other than descriptive statistics.

CONCLUSIONS

There is an increasing awareness of the prevalence of mitochondrial SNHL and its progressive nature. High-risk candidates warrant genetic testing and family screening. Correlating the data for mitochondrial SNHL as a treatable entity is important, and the authors present an overview of these patients successfully rehabilitated by cochlear implantation.

The search for type 2 diabetes genes

Type 2 diabetes (T2DM) is a serious disease with severe complications. Around one in 10 people alive today suffer from type 2 diabetes or are destined to develop it before they die. Inheritance plays an important role in the cause of type 2 diabetes. A considerable amount of research is devoted to defining the genes involved in the aetiology of this widespread disease. This information is crucial if we are to improve our methods of preventing and treating diabetes. Over the last 25 years there have been considerable advances in our understanding of the genetics of diabetes. Important discoveries have been made in dissecting the genes involved in rare monogenic forms of type 2 diabetes which has become a paradigm for genetic studies of type 2 diabetes. This review focuses on the main approaches currently adopted and our current understanding of the genes involved in susceptibility to type 2 diabetes.

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.

Cardiac autonomic nervous dysfunction in diabetic patients with a mitochondrial DNA mutation: assessment by heart rate variability

OBJECTIVE

To elucidate the degree and characteristics of cardiac autonomic nervous dysfunction in diabetic patients associated with a mitochondrial DNA mutation at base pair 3243.

RESEARCH DESIGN AND METHODS

We investigated heart rate variability using 24-h Holter monitoring in 10 diabetic patients with the mutation compared with 55 ordinary diabetic patients and 45 nondiabetic control subjects.

RESULTS

Age and sex were similar in the three groups. Between patients with the mutation and ordinary diabetic patients, the duration of diabetes and blood glycemic levels were not different. In the time domain analysis of heart rate variability, patients with the mutation and ordinary diabetic patients had significantly smaller SDNN index and pNN50 than control subjects. Compared with ordinary diabetic patients, patients with the mutation had smaller SDNN index (P < 0.02), but rMSSD and pNN50 were not different. In the frequency domain analysis, total, low frequency (LF), and high frequency (HF) spectra were significantly smaller in patients with the mutation and ordinary diabetic patients than in control subjects. Compared with ordinary diabetic patients, patients with the mutation had smaller total and LF spectra (P < 0.02). However, HF spectra were not significantly different. Notably, the LF/HF spectra ratio was lower in patients with the mutation than in ordinary diabetic patients and control subjects (P < 0.05), but this ratio was similar in ordinary diabetic patients and control subjects.

CONCLUSIONS

Our results suggest that diabetic patients with the mitochondrial DNA mutation have more severely impaired cardiac autonomic nervous function with sympathovagal imbalance, as compared with ordinary diabetic patients.

Switch to anaerobic glucose metabolism with NADH accumulation in the beta-cell model of mitochondrial diabetes. Characteristics of betaHC9 cells deficient in mitochondrial DNA transcription

To elucidate the mechanism underlying diabetes caused by mitochondrial gene mutations, we created a model by applying 0.4 microg/ml ethidium bromide (EtBr) to the murine pancreatic beta cell line betaHC9; in this model, transcription of mitochondrial DNA, but not that of nuclear DNA, was suppressed in association with impairment of glucose-stimulated insulin release (Hayakawa, T., Noda, M., Yasuda, K., Yorifuji, H., Taniguchi, S., Miwa, I., Sakura, H., Terauchi, Y., Hayashi, J.-I., Sharp, G. W. G., Kanazawa, Y., Akanuma, Y., Yazaki, Y., and Kadowaki, T. (1998) J. Biol. Chem. 273, 20300-20307). To elucidate fully the metabolism-secretion coupling in these cells, we measured glucose oxidation, utilization, and lactate production. We also evaluated NADH autofluorescence in betaHC9 cells using two-photon excitation laser microscopy. In addition, we recorded the membrane potential and determined the ATP and ADP contents of the cells. The results indicated 22.2 mm glucose oxidation to be severely decreased by EtBr treatment compared with control cells (by 63% on day 4 and by 78% on day 6; both p < 0.01). By contrast, glucose utilization was only marginally decreased. Lactate production under 22.2 mm glucose was increased by 2.9- and 3.5-fold by EtBr treatment on days 4 and 6, respectively (both p < 0.01). Cellular NADH at 2.8 mm glucose was increased by 35 and 43% by EtBr on days 4 and 6 (both p < 0.01). These data suggest that reduced expression of the mitochondrial electron transport system causes NADH accumulation in beta cells, thereby halting the tricarboxylic acid cycle on one hand, and on the other hand facilitating anaerobic glucose metabolism. Glucose-induced insulin secretion was lost rapidly along with the EtBr treatment with concomitant losses of membrane potential depolarization and the [Ca(2+)](i) increase, whereas glibenclamide-induced changes persisted. This is the first report to demonstrate the connection between metabolic alteration of electron transport system and that of tricarboxylic acid cycle and its impact on insulin secretion.

A practical approach to the diagnosis and management of MELAS: case report and review

BACKGROUND

Mitochondrial encephalomyopathy, lactic acidosis with stroke-like episodes (MELAS) is a mitochondrial disorder and an important diagnostic consideration in the young patient with nonhemorrhagic stroke. Its presentation is varied and diagnosis is based on early recognition of the clinical features and correct interpretation of laboratory and radiologic studies.

SUMMARY

In this article, we report a patient with MELAS and review the clinical, laboratory, and neuroradiologic features of the condition. In the young patient with multiple stroke-like episodes in different vascular territories and neuroradiologic features of transient abnormalities in varying regions, laboratory testing for MELAS must be performed. The presence of ragged red fibers in skeletal muscle and biochemical demonstration of defects in mitochondrial respiratory enzymes strongly support the diagnosis. Molecular genetic testing for abnormalities in mitochondrial DNA will confirm the diagnosis. The importance of a thorough assessment of family history is also emphasized. The basic principles of mitochondrial genetics and the common point mutations and rearrangements of mitochondrial DNA associated with MELAS are reviewed. Although treatment options are limited, several therapeutic agents have been studied.

CONCLUSIONS

The diagnosis of MELAS should be considered in the young patient with stroke, especially when accompanied by other clinical features such as seizures, encephalopathy, and muscle weakness. Laboratory evaluation can provide an accurate diagnosis, especially when the appropriate mitochondrial DNA studies are performed. Genetic counseling should be provided to patients with MELAS associated with mitochondrial DNA point mutations. Better understanding of the molecular basis of the condition may result in the development of effective treatment strategies.

Age of onset possibly associated with the degree of heteroplasmy in two male siblings with diabetes mellitus having an A to G transition at 3243 of mitochondrial DNA

We describe two male siblings with diabetes mellitus caused by mitochondrial 3243 mutation. The level of heteroplasmy in peripheral blood leucocytes was determined by a last-cycle hot PCR method. The younger brother, who had 39% heteroplasmy, developed diabetes at age of 25, and demonstrated a lean body habitus and blunted insulin secretion. The elder brother, who had 22% heteroplasmy, was diagnosed at the age of 42. The younger brother showed a higher increment of serum lactate after exercise. In these siblings the level of heteroplasmy in their peripheral blood leucocytes appeared to be associated with age of onset of diabetes.

The prevalence of the mitochondrial DNA 16189 variant in non-diabetic Korean adults and its association with higher fasting glucose and body mass index

AIMS

To evaluate the prevalence of the 16189 variant of mitochondrial DNA in Korean adults and its association with insulin resistance.

METHODS

We investigated 160 non-diabetic subjects from a community-based diabetes survey conducted in Yonchon County, Korea in 1993. We extracted the DNA from peripheral blood and examined the 16189 variant by polymerase chain reaction and restrictive enzyme digestion. We compared body mass index (BMI), blood pressure, fasting plasma glucose, 2-h plasma glucose after 75 g glucose load, fasting insulin, cholesterol, and homeostasis model assessment of insulin resistance and beta-cell function between the subjects with 16189 variant and wild type.

RESULTS

The prevalence of the 16189 variant in Korean adults was 28.8% (46 of 160). Subjects with the 16189 variant had higher fasting glucose and BMI than those with wild type, but fasting insulin, homeostasis model assessment of insulin resistance and beta-cell function, cholesterol, and blood pressure were not different between two groups.

CONCLUSION

Our results provide evidence for an association of a frequent mitochondrial polymorphism with higher fasting glucose and the risk factors of diabetes mellitus.

Genetic approaches to the molecular understanding of type 2 diabetes

The appreciation that individual susceptibility to type 2 diabetes (T2D) and related components of the dysmetabolic syndrome has a strong inherited component provides a coherent framework within which to develop a molecular understanding of the pathogenesis of T2D. This review focuses on the main approaches currently adopted by researchers seeking to identify the inherited basis of T2D and the present state of our knowledge. One central theme that emerges is that progress in defining the genetic basis of the common, multifactorial forms of T2D is hindered by etiological heterogeneity: T2D is likely to represent the final common pathway of diverse interacting primary disturbances. Such heterogeneity equally compromises efforts to understand the basis for T2D by use of other approaches, such as cellular biochemistry and classical physiology. Analyses that seek to ally sophisticated physiological characterization with measures of genomic variation are likely to provide powerful tools for redressing the loss of power associated with such heterogeneity.

Perspective: the search for genes for type 2 diabetes in the post-genome era

Twin and family studies have demonstrated a strong genetic component to type 2 diabetes mellitus (T2DM), but mapping the susceptibility genes that account for this risk has proved difficult. At least seven single gene defects are known to cause T2DM, often with early onset and insulin deficiency, but these causes account for 5% or less of all T2DM. A large number of candidate genes have been evaluated for typical T2DM, but few have been confirmed in multiple studies, and among these, the effect on individual risk is modest. A large number of genome-wide scans have been published in the last few years, and at least four regions show evidence in multiple studies. However, only NIDDM1 has been mapped to a single gene, and that gene (calpain 10) appears to have a major role only in selected populations. Work is ongoing in many laboratories and multiple populations to map additional regions, but T2DM and other complex diseases have proved recalcitrant to current methodology. In addition to the ongoing progress in completing the genome sequence and in developing a comprehensive map of single nucleotide polymorphisms, new statistical models will be needed to incorporate the multiple loci with modest effect and the known environmental interactions that characterize the susceptibility to T2DM.

Mitochondrial diabetes: pathophysiology, clinical presentation, and genetic analysis

This study provides a compact overview on the most common form of the maternally inherited diabetes and deafness syndrome (MIDD) that associates with an A-G mutation in mitochondrial DNA at position 3243 in the tRNA(Leu,UUR) gene. The pathobiochemistry and pathophysiology is discussed. The mutation leads predominantly to a reduced insulin secretion by beta cells in response to glucose stimulation, however, without marked involvement of autoimmune processes as seen in type 1 diabetes mellitus. The underlying biochemical mechanism leading to beta cell dysfunction is discussed. Furthermore, the clinical presentation of the disease is summarized as are the methods to detect the A3243G mutation, particular in view of the often low levels of heteroplasm of the A3243G mutation.

Mitochondrial diabetes mellitus

This review discusses the current insight by which mutations in mitochondrial DNA (mtDNA) contribute to the development of particular disease states with emphasis on diabetes mellitus. Mitochondria are the power factories of the cells and produce ATP by oxidizing reducing equivalents via the respiratory chain. These reducing equivalents originate mainly from the citric acid cycle that also occurs within the mitochondria. Human mitochondria contain their own genetic material in the form of circular DNA that encodes for only a fraction of the mitochondrial components. The other mitochondrial components are nuclear encoded. Pathogenic mutations in mtDNA can affect the activity of the respiratory chain, thereby leading to the reduced generation of ATP. However, mitochondria not only produce ATP but they also regulate cytosolic concentrations of signaling molecules such as calcium and iron ions. The metabolic processes within mitochondria such as the citric acid cycle determine the concentration of metabolites that can also act as signalling molecules. Furthermore, the respiratory chain and mitochondrion-associated monoamine oxidase are major producers of reactive oxygen radicals. As a result, mutations in mtDNA can deregulate multiple processes within cells and the balance of this deregulation may contribute to the clinical phenotype.

Genome-wide search for type 2 diabetes in Japanese affected sib-pairs confirms susceptibility genes on 3q, 15q, and 20q and identifies two new candidate Loci on 7p and 11p

The genetic background that predisposes the Japanese population to type 2 diabetes is largely unknown. Therefore, we conducted a 10-cM genome-wide scan for type 2 diabetes traits in the 359 affected individuals from 159 families, yielding 224 affected sib-pairs of Japanese origin. Nonparametric multipoint linkage analyses performed in the whole population showed one suggestive linked region on 11p13-p12 (maximum logarithm of odds score [MLS] 3.08, near Pax6) and seven potentially linked regions (MLS >1.17) at 1p36-p32, 2q34, 3q26-q28, 6p23, 7p22-p21, 15q13-q21, and 20q12-q13 (near the gene for hepatocyte nuclear factor-4alpha [HNF-4alpha]). Subset analyses according to maximal BMI and early age at diagnosis added suggestive evidence of linkage with type 2 diabetes at 7p22-p21 (MLS 3.51), 15q13-q21 (MLS 3.91), and 20q12-q13 (MLS 2.32). These results support previous indication for linkage found on chromosome 3q, 15q, and 20q in other populations and identifies two new potential loci on 7p and 11p that may confer genetic risk for type 2 diabetes in the Japanese population.

A case of diabetes, deafness, cardiomyopathy, and central sleep apnea: novel mitochondrial DNA polymorphisms

We describe a case of diabetes mellitus complicated by neurosensory hearing loss, cardiomyopathy, and sleep apnea syndrome. A 48-year-old man who was admitted for treatment of a lacerated tendon of the right shoulder was also found to require preoperative control of diabetes, a condition that had been diagnosed 4 years earlier. The family pedigree suggested maternal inheritance of diabetes. The patient also had neurosensory hearing loss and the central type of sleep apnea syndrome. His myocardium was hypertrophic and the ultrastructural analysis showed morphologically abnormal mitochondria. On the basis of the apparent characteristic manifestations, we speculated that he had a mitochondrial disease. To elucidate the responsible mutation of mitochondrial DNA, we sequenced the patient's entire mitochondrial DNA derived from blood leukocytes and found 40 sequence variants. Three of those, 5466 A/G, 7912 G/A, and 10601 T/C, have not yet been reported. Nine of the 40 variants were accompanied by an amino acid replacement, including 5466 A/G. Although we could not determine the most significant mutation, the variants of mitochondrial DNA may have been associated with this patient's unusually variable clinical manifestations.