Mutation in mitochondrial tRNA(Leu)(UUR) gene in a large pedigree with maternally transmitted type II diabetes mellitus and deafness

Maternally inherited diabetes and deafness (MIDD): a distinct subtype of diabetes associated with a mitochondrial tRNA(Leu)(UUR) gene point mutation

We have recently described a mitochondrial DNA (mtDNA) point mutation at np 3243 in the tRNA(Leu)(UUR) gene in a large Dutch pedigree with maternally inherited diabetes mellitus and deafness (MIDD) illustrating the importance of mitochondrial function in maintenance of a proper glucose homeostasis. In this review we will focus on the prevalence of the mtDNA mutation at np 3243 in diabetic populations, as well as postulate some working models for its pathogenicity.

Germ-line therapy to cure mitochondrial disease: protocol and ethics of in vitro ovum nuclear transplantation

The combination of genuine ethical concerns and fear of learning to use germ-line therapy for human disease must now be confronted. Until now, no established techniques were available to perform this treatment on a human. Through an integration of several fields of science and medicine, we have developed a nine step protocol at the germ-line level for the curative treatment of a genetic disease. Our purpose in this paper is to provide the first method to apply germ-line therapy to treat those not yet born, who are destined to have a life threatening, or a severely debilitating genetic disease. We hope this proposal will initiate the process of a thorough analysis from both the scientific and ethical communities. As such, this proposal can be useful for official groups studying the advantages and disadvantages of germ-line therapy.

Mitochondrial diabetes mellitus--glucose-induced signaling defects and beta-cell loss

Japanese diabetic patients whose mothers were also diabetic were screened, using peripheral leukocytes, for an A to G transition at nucleotide pair 3243 of the mitochondrial gene, a tRNA(Leu)(UUR) mutation. This mutation was identified in four pedigrees from among 300 unrelated patients. Diabetes mellitus cosegretated with the mutation, except in 1 young subject, and was maternally inherited. Long-term follow-up revealed that the underlying disorder in affected members is a progressive impairment of insulin secretion. In accord with this finding, this mutation was found to be highly prevalent in a subset of diabetes mellitus called slowly progressive IDDM; the mutation was identified in 3 of 27 Japanese patients enrolled in the prospective study of islet cell antibody (ICA)-positive, initially non-insulin-dependent diabetic patients, who are very likely to become insulin dependent in several years. The histologic characteristics of slowly progressive IDDM include loss, though incomplete, of pancreatic beta-cells. Mitochondrial gene defects in beta-cells could therefore cause glucose-induced signaling defects as well as beta-cell loss, which explains the wide range of diabetic phenotypes, from NIDDM phenotype to IDDM, in patients with this mitochondrial gene mutation.

NIDDM--genetic marker; glucose transporter, glucokinase, and mitochondria gene

Candidate genes for NIDDM have been screened in Japanese. Mutations in the glucokinase gene were found in apparent late-onset NIDDM patients as well as in MODY patients. Clinical characteristics in the subjects with glucokinase gene mutations are similar to those in Caucasian subjects; diabetes mellitus is generally mild and some patients actually remain as having impaired glucose tolerance. Of great interest is that all affected subjects show blunted insulin secretion response to the glucose challenge, which is most commonly observed in Japanese NIDDM patients. Thus, it is possible that impairment in the regulation of glucokinase gene expression or its enzyme activity is associated with at least some Japanese NIDDM patients, though the prevalence of the mutations in the coding region is relatively low. In contrast, a mitochondrial tRNA(Leu(UUR)) gene mutation at np 3243 appears to be much more common, and diabetes due to this mutation has a progressive nature. Insulin secretory capacity progressively decreases, eventually reaching an insulin-dependent state in most patients. A surprising result is that this gene mutation is often observed in ICA-positive IDDM patients who were initially non-insulin-dependent, so called slowly progressive IDDM patients. These results suggest that the mitochondrial gene mutation may cause beta cell loss in addition to defects in glucose-induced signaling in pancreatic beta cells, which explains that the mitochondrial gene mutation manifests a wide range of diabetic phenotypes, from NIDDM to IDDM.

Mitochondrial DNA sequence variation in human evolution and disease

Germ-line and somatic mtDNA mutations are hypothesized to act together to shape our history and our health. Germ-line mtDNA mutations, both ancient and recent, have been associated with a variety of degenerative diseases. Mildly to moderately deleterious germ-line mutations, like neutral polymorphisms, have become established in the distant past through genetic drift but now may predispose certain individuals to late-onset degenerative diseases. As an example, a homoplasmic, Caucasian, tRNA(Gln) mutation at nucleotide pair (np) 4336 has been observed in 5% of Alzheimer disease and Parkinson disease patients and may contribute to the multifactorial etiology of these diseases. Moderately to severely deleterious germ-line mutations, on the other hand, appear repeatedly but are eliminated by selection. Hence, all extant mutations of this class are recent and associated with more devastating diseases of young adults and children. Representative of these mutations is a heteroplasmic mutation in MTND6 at np 14459 whose clinical presentations range from adult-onset blindness to pediatric dystonia and basal ganglial degeneration. To the inherited mutations are added somatic mtDNA mutations which accumulate in random arrays within stable tissues. These mutations provide a molecular clock that measures our age and may cause a progressive decline in tissue energy output that could precipitate the onset of degenerative diseases in individuals harboring inherited deleterious mutations.

Insulin resistance as the major cause of impaired glucose tolerance: a self-fulfilling prophesy?

Non-insulin-dependent diabetes (NIDDM) is a heterogeneous state involving various degrees of beta-cell dysfunction and insulin resistance, although the relative importance of these two factors is controversial. Several single gene disorders of carbohydrate metabolism have their main pathophysiological defect largely restricted to the beta-cell or to insulin-sensitive tissues. We have noted that with insulin resistance the fasting plasma glucose is often normal and severe hyperglycaemia occurs after a glucose load. By contrast, in glucokinase-deficient diabetes, which is characterised by reduced insulin secretion, the reverse is the case. Supportive evidence showing that beta-cell dysfunction and insulin resistance may have different effects on fasting and post-prandial glucose concentrations comes from studies of identical twins of NIDDM patients, hemi-pancreatectomised normal subjects, and insulin-resistant Asian subjects. NIDDM is usually preceded by a period of less severe hyperglycemia, referred to as impaired glucose tolerance (IGT). Studies of the IGT phase usually conclude that insulin resistance is the major abnormality and is thus the primary defect in NIDDM. However, the definition of IGT is based on the 2 h plasma glucose after an oral glucose-tolerance test without consideration of the fasting glucose concentrations (provided these concentrations are non-diabetic). Our observations suggest that this definition of IGT may result in the over-representation of insulin-resistant individuals and the under-representation of subjects with beta-cell dysfunction in any cross sectional study of IGT. The belief that the prediabetic state of IGT is dominated by insulin resistance may be a self-fulfilling prophesy because of the excessive emphasis put on the post-prandial rather than the fasting state.

Bilateral sensorineural hearing loss in members of a maternal lineage with mitochondrial point mutation

Pure-tone audiometry was carried out on members of a recently described maternal lineage with sensorineural deafness, harbouring a novel mitochondrial mutation in the gene for tRNA-ser(UCN). This revealed a characteristic pattern of symmetrical bilateral sensorineural hearing losses in each affected individual, predominantly affecting the high-frequencies, but with considerable variability between individuals. No clear correlation was observed between age and severity, but most subjects reported progressive worsening of their condition. Some members of the lineage were found to be heteroplasmic for the tRNA-ser(UCN) mutation. However, the severity of hearing loss was poorly correlated with the representation of the mutant mtDNA, indicating that other, as yet unidentified factors must be involved in the aetiology of this disorder.

Pancreatic beta-cell secretory defect associated with mitochondrial point mutation of the tRNA(LEU(UUR)) gene: a study in seven families with mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS)

Recent evidence suggests possible linkage between diabetes mellitus and mitochondrial gene mutation. We surveyed mitochondrial tRNA(LEU(UUR)) (3243) mutation in 7 mitochondrial encephalomyopathy, lactic acidosis and stroke-like episode (MELAS) families and identified 24 mutated subjects (7 MELAS probands and 17 non-MELAS relatives) as well as 11 non-mutant family members. An OGTT in the 24 mutant relatives revealed 14 diabetic subjects, 3 with impaired glucose tolerance and 7 with normal glucose tolerance and all non-mutant family members as having normal glucose tolerance. Insulinogenic index was significantly reduced in the mutant diabetic subjects and those with impaired and normal glucose tolerance in comparison with the normal control subjects and the non-mutant members. Urinary 24-h C-peptide immunoreactivity excretion was markedly reduced in the mutant diabetic subjects and those with normal and impaired glucose tolerance, compared with the control subjects and the non-mutant family members. Plasma C-peptide immunoreactivity 6 min after glucagon injection was markedly reduced in the mutant diabetic subjects and those with normal and impaired glucose tolerance compared with the control subjects and the non-mutant family members. Si, an index of insulin sensitivity of the four mutant subjects was within normal range. Islet cell antibodies were negative in sera of eight mutated diabetic subjects, 2 and 6 with impaired and normal glucose tolerance, respectively. Diabetic retinopathy and nephropathy were demonstrated in 7 (50%) and 12 (85.7%) of 14 mutant diabetic subjects, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Mitochondrial DNA mutations in diseases of energy metabolism

A variety of degenerative diseases involving deficiencies in mitochondrial bioenergetics have been associated with mitochondrial DNA (mtDNA) mutations. Maternally inherited mtDNA nucleotide substitutions range from neutral polymorphisms to lethal mutations. Neutral polymorphisms are ancient, having accumulated along mtDNA lineages, and thus correlate with ethnic and geographic origin. Mildly deleterious base substitutions have also occurred along mtDNA lineages and have been associated with familial deafness and some cases of Alzheimer's Disease and Parkinson's Disease. Moderately deleterious nucleotide substitutions are more recent and cause maternally-inherited diseases such as Leber's Hereditary Optic Neuropathy (LHON) and Myoclonic Epilepsy and Ragged-Red Fiber Disease (MERRF). Severe nucleotide substitutions are generally new mutations that cause pediatric diseases such as Leigh's Syndrome and dystonia. MtDNA rearrangements also cause a variety of phenotypes. The milder rearrangements generally involve duplications and can cause maternally-inherited adult-onset diabetes and deafness. More severe rearrangements frequently involving detections have been associated with adult-onset Chronic Progressive External Ophthalmoplegia (CPEO) and Kearns-Sayre Syndrome (KSS) or the lethal childhood disorder, Pearson's Marrow/Pancreas Syndrome. Defects in nuclear-cytoplasmic interaction have also been observed, and include an autosomal dominant mutation causing multiple muscle mtDNA deletions and a genetically complex disease resulting in the tissue depletion of mtDNAs. MtDNA nucleotide substitution and rearrangement mutations also accumulate with age in quiescent tissues. These somatic mutations appear to degrade cellular bioenergetic capacity, exacerbate inherited mitochondrial defects and contribute to tissue senescence. Thus, bioenergetic defects resulting from mtDNA mutations may be a common cause of human degenerative disease.

Molecular basis of mitochondrial DNA disease

Mitochondrial ATP production via oxidative phosphorylation (OXPHOS) is essential for normal function and maintenance of human organ systems. Since OXPHOS biogenesis depends on both nuclear- and mitochondrial-encoded gene products, mutations in both genomes can result in impaired electron transport and ATP synthesis, thus causing tissue dysfunction and, ultimately, human disease. Over 30 mitochondrial DNA (mtDNA) point mutations and over 100 mtDNA rearrangements have now been identified as etiological factors in human disease. Because of the unique characteristics of mtDNA genetics, genotype/phenotype associations are often complex and disease expression can be influenced by a number of factors, including the presence of nuclear modifying or susceptibility alleles. Accordingly, these mutations result in an extraordinarily broad spectrum of clinical phenotypes ranging from systemic, lethal pediatric disease to late-onset, tissue-specific neurodegenerative disorders. In spite of its complexity, an understanding of the molecular basis of mitochondrial DNA disease will be essential as the first step toward rationale and permanent curative therapy.

Mitochondrial encephalomyopathies: clinical and molecular analysis

The classification of mitochondrial encephalomyopathies relied upon clinical, biochemical, and histological features until the discovery of mitochondrial DNA defects in 1988. Since then, an outburst of molecular genetic information has aided our understanding of the pathogenesis and the classification of these heterogeneous disorders. Novel concepts of maternal inheritance, mitochondrial DNA (mtDNA) heteroplasmy, tissue distribution, and threshold have explained many of the clinical characteristics. The discovery of point mutations, large-scale mtDNA deletions, duplications, and autosomally inherited disorders with multiple mtDNA deletions have revealed new genetic phenomena. Despite our rapidly expanding understanding of the molecular genetic defects, many questions remain to be explored to fill the gap in our knowledge of the relationship between genotype and clinical phenotype.

Mitochondrial DNA diseases: histological and cellular studies

Large-scale deletions and tRNA point mutations in mitochondrial DNA (mtDNA) are associated with a variety of different mitochondrial encephalomyopathies. Skeletal muscle in these patients shows a typical pathology, characterized by the focal accumulation of large numbers of morphologically and biochemically abnormal mitochondrial (ragged-red fibers). Both mtDNA deletions and tRNA point mutations impair mitochondrial translation and produce deficiencies in oxidative phosphorylation. However, mutant and wild-type mtDNAs co-exist (mtDNA heteroplasmy) and the translation defect is not expressed until the ratio of mutant: wild-type mtDNAs exceeds a specific threshold. Below the threshold the phenotype can be rescued by intramitochondrial genetic complementation. The mosaic expression of the skeletal muscle pathology is thus determined by both the cellular and organellar distribution of mtDNA mutants.

Microcompartmentation of energy metabolism at the outer mitochondrial membrane: role in diabetes mellitus and other diseases

Complexes made up of the kinases, hexokinase and glycerol kinase, together with the outer mitochondrial membrane voltage-dependent anion channel (VDAC) protein, porin, and the inner mitochondrial membrane protein, the adenine nucleotide translocator, are involved in tumorigenesis, diabetes mellitus, and central nervous system function. Identification of these two mitochondrial membrane proteins, along with an 18 kD protein, as components of the peripheral benzodiazepine receptor, provides independent confirmation of the interaction of porin and the adenine nucleotide translocator to form functional contact sites between the inner and outer mitochondrial membranes. We suggest that these are dynamic structures, with channel conductances altered by the presence of ATP, and that ligand-mediated conformational changes in the porin-adenine nucleotide translocator complexes may be a general mechanism in signal transduction.

Mitochondrial diabetes mellitus: prevalence and clinical characterization of diabetes due to mitochondrial tRNA(Leu(UUR)) gene mutation in Japanese patients

Mutations in the mitochondrial gene were recently identified in a large pedigree of diabetes mellitus and deafness. As the mitochondrial gene is materially inherited, Japanese diabetic patients whose mothers were also diabetic were screened, using peripheral leucocytes, for an A to G transition at nucleotide pair 3243 of the mitochondrial gene, a tRNA(Leu(UUR)) mutation. This mutation was identified in four pedigrees from among 300 unrelated patients who were screened. Diabetes co-segregated with the mutation, except in one young subject, and was maternally inherited. The apparent onset of disease occurred between 11 and 68 years of age. Some of the affected members developed hearing impairment and congestive heart failure due to cardiomyopathy, though generally long after the onset of diabetes, and these patients had therefore not been diagnosed as having a specific form of diabetes. The duration of sulphonyl-urea treatment was not more than 8 years in these pedigrees and affected members were prone to progression to insulin-requiring diabetes. Thus, these patients were secondary sulphonylurea failures. Long-term follow-up revealed that the underlying disorder in affected members is a progressive impairment of insulin secretion. Some were initially diagnosed as having IDDM based on an apparent acute onset in youth and the clinical severity of their diabetes. Others were regarded as having MODY with an aggressive course. The mitochondrial gene mutation or diabetes is not transmitted to all offspring of the affected mothers.(ABSTRACT TRUNCATED AT 250 WORDS)

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

BACKGROUND

Several families have been described in which a mutation of mitochondrial DNA, the substitution of guanine for adenine (A-->G) at position 3243 of leucine transfer RNA, is associated with diabetes mellitus and deafness. The prevalence, clinical features, and pathophysiology of diabetes with this mutation are largely undefined.

METHODS

We studied 55 patients with insulin-dependent diabetes mellitus (IDDM) and a family history of diabetes (group 1), 85 patients with IDDM and no family history of diabetes (group 2), 100 patients with non-insulin-dependent diabetes mellitus (NIDDM) and a family history of diabetes (group 3), and 5 patients with diabetes and deafness (group 4) for the mutation. We also studied the prevalence and characteristics of diabetes in 39 patients with a syndrome consisting of mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes who were known to have the mutation and 127 of their relatives (group 5).

RESULTS

We identified 16 unrelated patients with diabetes associated with the A-->G mutation: 3 patients from group 1 (6 percent), 2 patients from group 3 (2 percent), 3 patients from group 4 (60 percent), and 8 patients from group 5 (21 percent). We also identified 16 additional subjects who had diabetes and the mutation among 42 relatives of the patients with diabetes and the mutation in groups 1, 2, 3, and 4 and 20 affected subjects among the 127 relatives of the patients in group 5. Diabetes cosegregated with the mutation in a fashion consistent with maternal transmission, was frequently (in 61 percent of cases) associated with sensory hearing loss, and was generally accompanied by impaired insulin secretion.

CONCLUSIONS

Diabetes mellitus associated with the A-->G mutation at position 3243 of mitochondrial leucine transfer RNA represents a subtype of diabetes found in both patients with IDDM and patients with NIDDM in Japan.

Mitochondrial gene defects in patients with NIDDM

Non-insulin-dependent diabetes mellitus (NIDDM) has a strong genetic component and maternal factors have recently been implicated in disease inheritance. The mitochondrial myopathies are a group of diseases which often show maternal inheritance as a result of mtDNA defects; some patients have impaired glucose tolerance. Occasional families with maternally inherited diabetes and deafness associated with a deletion or point mutation of mtDNA have been reported. To assess the importance of mitochondrial gene defects in NIDDM, 150 unrelated diabetic subjects from Wales, UK and 68 unrelated patients with diabetes and at least one affected sibling from England, UK were studied. Southern blot analysis did not show any large mtDNA deletions or duplications. One patient had a mutation in the mitochondrial tRNAleu(UUR) gene at bp 3243. This mutation is commonly associated with the syndrome of mitochondrial encephalomyopathy, lactic acidosis and stroke like episodes (MELAS). Study of this patient and his siblings showed a distinct form of late-onset diabetes associated with nerve deafness but no clinical features of the MELAS syndrome. No diabetic subject was shown to have the mtDNA mutation at position 8344 (tRNA(lys)) which has previously been described in the syndrome of mitochondrial encephalomyopathy and red-ragged fibres (MERRF). The role of other mitochondrial gene defects in diabetes and the pathophysiological basis of glucose intolerance in patients with the MELAS mutation requires further elucidation.

Variability of the pancreatic islet beta cell/liver (GLUT 2) glucose transporter gene in NIDDM patients

The purpose of these experiments was to test the hypothesis that impaired glucose-stimulated insulin secretion in NIDDM is due to mutations in the islet beta cell/liver glucose transporter (GLUT 2) gene. Using oligonucleotide primers flanking each of the 11 exons, the structural portion of the gene was studied by PCR-SSCP analysis. DNA from African-American females (n = 48), who had gestational diabetes but developed overt NIDDM after delivery, was studied. Each SSCP variant was sequenced directly from genomic DNA. Two amino acid substitutions from the previously reported sequence were found, one in exon 3 and the other in exon 4B. Four additional silent mutations in the coding region, and six intron mutations outside the splice junction consensus sequences, were also identified. The mutation GTC x ATC in exon 4B substituted Val197 to Ile197. This amino acid substitution was found in only one NIDDM patient in a single allele, and was not found in 52 control subjects. This residue exists in the fifth membrane spanning domain, and Val at this position is conserved in mouse and rat GLUT 2, and human GLUT 1 to GLUT 4. The other codon change in exon 3, ACT x ATT, substituted Thr110 to Ile110 in the second membrane spanning domain. To determine the frequency of this non-conservative amino acid substitution, a PCR-LCR assay was developed. This assay was simple and highly specific for detection of this single nucleotide substitution. The allelic frequency of the ATT (Ile110) in NIDDM patients (39.6%, n = 48) and that in controls (47.1%, n = 52) did not differ (p = 0.32, Fisher's exact test).(ABSTRACT TRUNCATED AT 250 WORDS)

Mitochondrial myopathy with progressive decrease in mitochondrial tRNA(Leu)(UUR) mutant genomes

A female patient with mitochondrial myopathy had a mitochondrial DNA mutation at nucleotide pair 3243, commonly seen in patients with mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes (MELAS), but unlike MELAS patients, she had no central nervous system symptoms. Muscle weakness, which was most severe when she was 7 years old, improved gradually with age. Comparison of two muscle biopsies obtained at an interval of 12.5 years (7 and 20 years of age, respectively), revealed that the number of ragged-red fibers was markedly decreased and histochemical cytochrome c oxidase activity increased in parallel with the decrease in population of mutant genomes.

Genetic disorders that masquerade as multiple sclerosis

There are many genetic disorders that have signs and symptoms suggestive of multiple sclerosis and that may easily be overlooked in the evaluation of both adult and pediatric multiple sclerosis patients. The recognition of a genetic disorder as the cause of a patient's "multiple sclerosis" phenotype has important implications not only for the patient, but often also for others in the patient's family who may be at risk for the same disease. We present here a review of single gene disorders that can masquerade as multiple sclerosis. For each disorder, the major clinical and biochemical characteristics are discussed, together with the appropriate testing to screen for and confirm the diagnosis. In addition, guidelines are presented for when to suspect an underlying genetic condition in a patient with a diagnosis of definite or probable multiple sclerosis. The great variety of genetic disorders that can masquerade as multiple sclerosis and the many implications of a genetic diagnosis underscore the importance of recognizing genocopies of multiple sclerosis.

Mitochondrial DNA alterations and genetic diseases: a review

We review the main features of human mitochondrial function and structure, and in particular mitochondrial transcription, translation, and replication cycles. Furthermore, some pecularities such as mitochondria's high polymorphism, the existence of mitochondrial pseudogenes, and the various considerations to take into account when studying mitochondrial diseases will also be mentioned. Mitochondrial syndromes mostly affecting the nervous system have, during the past few years, been associated with mitochondrial DNA (mt DNA) alterations such as deletions, duplications, mutations and depletions. We suggest a possible classification of mitochondrial diseases according to the kind of mt DNA mutations: structural mitochondrial gene mutation as in LHON (Leber's Hereditary Optic Neuropathy) and NARP (Neurogenic muscle weakness, Ataxia and Retinitis Pigmentosa) as well as some cases of Leigh's syndrome; transfer RNA and ribosomal RNA mitochondrial gene mutation as in MELAS (Mitochondrial Encephalomyopathy, Lactic Acidosis and Strokelike Episodes) or MERRF (Myoclonic Epilepsy with Ragged Red Fibers) or deafness with aminoglycoside; structural with transfer RNA mitochondrial gene mutations as observed in large-scale deletions or duplications in Kearns-Sayre syndrome, Pearson's syndrome, diabetes mellitus with deafness, and CPEO (Chronic Progressive External Ophtalmoplegia). Depletions of the mt DNA may also be classified in this category. Even though mutations are generally maternally inherited, most of the deletions are sporadic. However, multiple deletions or depletions may be transmitted in a mendelan trait which suggests that nuclear gene products play a primary role in these processes. The relationship between a mutation and a particular phenotype is far from being fully understood. Gene dosage and energic threshold, which are tissue-specific, appear to be the best indicators. However, the recessive or dominant behavior of both the wild type or the mutated genome appears to play a significant role, which can be verified with in vitro studies.

Mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes (MELAS): current concepts

Mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes (MELAS) syndrome is one of many mitochondrially inherited multisystem diseases. The features of 110 reported mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes patients are reviewed to define the clinical spectrum of this disease. The clinical disorder, in addition to emerging concepts of genetic etiology, is promoting our understanding of mitochondrial functions. New knowledge may lead to more rational therapies. Finally, the recent revolution in the study of mitochondrial diseases may further our understanding of other degenerative disorders and even aging.

A novel mitochondrial point mutation in a maternal pedigree with sensorineural deafness

We have detected a novel mitochondrial mutation in a maternal pedigree, at least 13 of whose members have sensorineural hearing loss of varying severity, but who exhibit no other pathological features. The mutation, at np 7445, converts the 3' terminal T residue of tRNA-ser(UCN) to a C, and also brings about a silent alteration to the COI stop codon. The mutation destroys an XbaI site, within which a second mutation, at np 7444, has previously been reported in association with Leber's hereditary optic neuropathy. Predominantly mutant mtDNA was found in all 13 family members surveyed, whether or not they are overtly affected by deafness, and some individuals appeared homoplasmic, within the limits of detection. The novel mutation was not found in over 600 normal controls, nor in any of 27 other maternally unrelated individuals with deafness Other mutations found in mitochondrial disorders were also absent from this pedigree.

Two novel pathogenic mitochondrial DNA mutations affecting organelle number and protein synthesis. Is the tRNA(Leu(UUR)) gene an etiologic hot spot?

We identified two patients with pathogenic single nucleotide changes in two different mitochondrial tRNA genes: the first mutation in the tRNA(Asn) gene, and the ninth known mutation in the tRNA(Leu(UUR)) gene. The mutation in tRNA(Asn) was associated with isolated ophthalmoplegia, whereas the mutation in tRNA(Leu(UUR)) caused a neurological syndrome resembling MERRF (myoclonus epilepsy and ragged-red fibers) plus optic neuropathy, retinopathy, and diabetes. Both mutations were heteroplasmic, with higher percentages of mutant mtDNA in affected tissues, and undetectable levels in maternal relatives. Analysis of single muscle fibers indicated that morphological and biochemical alterations appeared only when the proportions of mutant mtDNA exceeded 90% of the total cellular mtDNA pool. The high incidence of mutations in the tRNA(Leu(UUR)) gene suggests that this region is an "etiologic hot spot" in mitochondrial disease.

Rad: a member of the Ras family overexpressed in muscle of type II diabetic humans

To identify the gene or genes associated with insulin resistance in Type II (non-insulin-dependent) diabetes mellitus, subtraction libraries were prepared from skeletal muscle of normal and diabetic humans and screened with subtracted probes. Only one clone out of 4000 was selectively overexpressed in Type II diabetic muscle as compared to muscle of non-diabetic or Type I diabetic individuals. This clone encoded a new 29-kilodalton member of the Ras-guanosine triphosphatase superfamily and was termed Rad (Ras associated with diabetes). Messenger ribonucleic acid of Rad was expressed primarily in skeletal and cardiac muscle and was increased an average of 8.6-fold in the muscle of Type II diabetics as compared to normal individuals.

Mitochondrial ribosomal RNA gene mutation in a patient with sporadic aminoglycoside ototoxicity

PURPOSE

Aminoglycoside-induced deafness has been described in a number of Chinese pedigrees. In nearly all of these families, affected individuals were related through the maternal side. Because mitochondrial DNA is transmitted exclusively through mothers, it had been speculated that a mutation in the mitochondrial DNA might predispose these maternally related family members to aminoglycoside ototoxicity. Recently, we analyzed three such families with multiple cases of ototoxic deafness and identified a pathogenic mutation in the mitochondrial 12S ribosomal RNA gene at nucleotide position 1555. The purpose of the current study is to analyze individuals with no family history of deafness, who had severe hearing loss after aminoglycoside exposure, for presence or absence of this particular mitochondrial DNA mutation.

MATERIALS AND METHODS

Blood was obtained from 36 Chinese individuals who became deaf after aminoglycoside exposure and had no family history of deafness. The DNA of these individuals was extracted, amplified by the polymerase chain reaction, and analyzed for the mitochondrial ribosomal RNA gene mutation by allele-specific oligonucleotide hybridization and Southern blot analysis.

RESULTS

In one of these 36 sporadic cases, we identified the nucleotide 1555 A-->G mutation in the mitochondrial genome.

CONCLUSION

This finding implies that a small proportion of individuals at risk for aminoglycoside ototoxicity harbor the specific mitochondrial DNA mutation identified in the familial cases. In these individuals, a genetic susceptibility to the ototoxic effects of aminoglycosides can be diagnosed, and deafness can be prevented in maternal relatives by avoiding the use of these antibiotics.

Mitochondrial cytopathies

Defects of the mitochondrial respiratory chain and mutations of mitochondrial DNA have now been associated with a wide range of human diseases. The precise pathogenetic mechanisms by which these biochemical abnormalities induce tissue dysfunction are not understood. The identification of a mutation in the proline anticodon and in the 12S RNA genes of mitochondrial DNA are interesting new additions to the catalogue of pathogenetic mutations of this genome. The recent demonstration of nuclear complementation of mitochondrial DNA depletion provides the opportunity to identify nuclear genes involved in mitochondrial DNA replication. The possible role for mitochondrial deficiencies in certain neurodegenerative diseases and in the ageing process have given additional momentum to research in this area. Treatment for the mitochondrial 'cytopathies' remains disappointing and improvement in this area awaits a better understanding of their aetiology.

Wolfram syndrome: a mitochondrial-mediated disorder?

Mitochondrial DNA mutations cause several human diseases, (eg, Leber's hereditary optic neuropathy). Wolfram syndrome (characterised by diabetes insipidus, diabetes mellitus, optic atrophy, and deafness) also has, in some cases, a mitochondrial origin. The disease, often familial, has been well documented as an autosomal recessive disorder, and most of the clinical phenotypes are consistent with an ATP supply defect that is often seen in mitochondrial-mediated disorders. We propose a dual genome defect model for Wolfram syndrome in which nuclear genetic defects or mitochondrial genetic defects can independently lead to the disease. This model suggests that besides a mitochondrial gene defect alone, a nuclear gene defect, which interferes with the normal function of mitochondria (probably with a normal mitochondrial genome), can also be the underlying explanation for the pleiotropic features of Wolfram syndrome. This hypothesis explains how an autosomal recessive disorder can result in mitochondrial dysfunction, and has a general application in the identification of candidate genes for the various important phenotypes (eg, deafness and diabetes mellitus) seen in mitochondrial disorders.

Mitochondrial gene mutation in islet-cell-antibody-positive patients who were initially non-insulin-dependent diabetics

Autoimmunity is thought to lead to islet-cell-antibody (ICA) formation in diabetes mellitus. However, we found a mitochondrial gene mutation at nucleotide pair 3243 in 3 of 27 Japanese ICA-positive, initially non-insulin-dependent diabetic patients. All 3 progressed to insulin-dependency within 13-31 months, whereas 5 of the other 24 are non-insulin-dependent after 54-90 months. ICA, at least in these 3 patients, may follow gradual beta-cell destruction due to mitochondrial gene mutation, although the possibility of beta-cells with the mutation being susceptible to autoimmune destruction cannot be excluded.

Mitochondrial ribosomal RNA mutation associated with both antibiotic-induced and non-syndromic deafness

Maternally transmitted non-syndromic deafness was described recently both in pedigrees with susceptibility to aminoglycoside ototoxicity and in a large Arab-Israeli pedigree. Because of the known action of aminoglycosides on bacterial ribosomes, we analysed the sequence of the mitochondrial rRNA genes of three unrelated patients with familial aminoglycoside-induced deafness. We also sequenced the complete mitochondrial genome of the Arab-Israeli pedigree. All four families shared a nucleotide 1555 A to G substitution in the 12S rRNA gene, a site implicated in aminoglycoside activity. Our study offers the first description of a mitochondrial rRNA mutation leading to disease, the first cases of non-syndromic deafness caused by a mitochondrial DNA mutation and the first molecular genetic study of antibiotic-induced ototoxicity.

Type 2 (non-insulin-dependent) diabetes mellitus associated with a mutation of the glucokinase gene in a Japanese family

Mutations were screened for in the glucokinase gene of 25 Japanese patients with Type 2 (non-insulin-dependent) diabetes mellitus. Each exon was scanned by electrophoresis of enzymatically amplified DNA segments under non-denaturing conditions and variants were sequenced. A variant pattern was detected in exon 5 of one patient. Direct sequencing of this exon revealed a single nucleotide substitution in codon 188 (GCT-->ACT) of one of two alleles resulting in the mutation of Ala188-->Thr, an invariant residue in the sequence of all mammalian glucokinases and hexokinases. This mutation was not found in 40 normal control subjects. The proband had been diagnosed with Type 2 diabetes at the age of 62 years. Four other members of her family have the same mutation and all have Type 2 diabetes or impaired glucose tolerance. The youngest age at diagnosis of Type 2 diabetes in these other members was 13 years, suggesting that her pedigree was maturity-onset diabetes of the young (MODY). All subjects with the Thr188 mutation show a decreased insulin secretory response during oral glucose tolerance testing. Mutations in the glucokinase gene associated with Type 2 diabetes have been previously identified in Caucasian (French and British) subjects. This study indicates that mutations in this gene are also implicated in the development of Type 2 diabetes in Asians. Further studies are required to determine the frequency of mutations in glucokinase among Japanese patients with Type 2 diabetes.

Diabetes mellitus is one of the heterogeneous phenotypic features of a mitochondrial DNA point mutation within the tRNALeu(UUR) gene

A heteroplasmic point mutation (transition A-to-G at nucleotide position 3,243 in the mitochondrial tRNALeu(UUR) gene) is found in a family suffering from a syndrome with diabetes, deafness and cardiomyopathy as the predominant clinical features.

Mutations of the human glucokinase gene and diabetes mellitus

The enzyme glucokinase catalyzes the phosphorylation of glucose and plays a key role in the regulation o f insulin secretion by pancreatic beta cells and glucose disposal in hepatocytes. Recent studies have shown that mutations in the gene encoding this key regulatory enzyme of glycolysis are a common cause of an autosomal dominant form of non-insulin-dependent (type 2) diabetes mellitus that has an onset often during childhood. The association of mutations in the glucokinase gene with impaired pancreatic cell function underscores the importance of glycolysis in the regulation of insulin secretion and suggests that mutations in other genes expressed in the beta-cell that also control rate-limiting steps in glucose metabolism may lead to diabetes.

Patients with lipodystrophic diabetes mellitus of the Seip-Berardinelli type, express normal insulin receptors

Lipodystrophic diabetes mellitus of the Seip-Berardinelli type is a syndrome associated with insulin resistance and recessive inheritance. We have examined whether mutations in the insulin receptor are pathogenetic factors in this syndrome. Fibroblasts from three different patients with Seip-Berardinelli's lipodystrophy were tested for insulin binding, and insulin-stimulated receptor autophosphorylation. In addition, the coding region of both alleles of the iinsulin receptor gene was sequenced. No abnormalities in the number of high affinity insulin binding sites, and insulin-stimulated receptor autophosphorylation were detected. The insulin receptor related insulin-like growth factor I receptor also showed no functional changes. DNA sequence analysis of the amplified exons of the insulin receptor gene showed a silent mutation in patient 1 at codon Ser339, changing AGT to AGC. In patient 2 a heterozygous Met for Val substitution at position 985 was detected, which is a rare polymorphism. In patient 3 no mutations, other than described polymorphisms, were observed. These findings demonstrate that the primary genetic lesion in Seip-Berardinelli's lipodystrophy is outside the insulin receptor gene and that an involvement of the insulin-like growth factor I receptor is also unlikely.

Atypical clinical presentations associated with the MELAS mutation at position 3243 of human mitochondrial DNA

Mitochondrial encephalopathy, lactic acidosis and stroke-like episodes (MELAS) is commonly associated with an A-->G transition at position 3243 of the mitochondrial DNA. To determine the diversity of clinical syndromes associated with this mutation, 91 patients with mitochondrial encephalomyopathies that did not conform to the MELAS phenotype were screened. Twenty one patients with the 3243 mutation, most of whom had progressive external ophthalmoplegia (PEO) were found. Clinical features did not distinguish PEO patients with the 3243 mutation from those with large-scale deletions of mtDNA. However, most cases with single large-scale mtDNA deletions were sporadic, whereas most patients with the 3243 mutation had affected maternal relatives. Histochemical studies of muscle showed that cytochrome c oxidase (COX) deficiency was more severe in patients with PEO than in patients with typical MELAS, even though PEO patients had a lower percentage of mutant genomes in muscle. These data imply that the 3243 mutation is a major cause of familial PEO, and suggests that the threshold number of mtDNAs harboring the 3243 mutation necessary to affect a particular tissue vary in different patients. The proportion of mutant genomes in combination with other, still undefined, tissue-specific modulating factors seem to determine the overall clinical syndrome.