Mitochondrial diabetes in children: seek and you will find it

Maternally Inherited Diabetes and Deafness (MIDD) is a rare form of diabetes due to defects in mitochondrial DNA (mtDNA). 3243 A>G is the mutation most frequently associated with this condition, but other mtDNA variants have been linked with a diabetic phenotype suggestive of MIDD. From 1989 to 2009, we clinically diagnosed mitochondrial diabetes in 11 diabetic children. Diagnosis was based on the presence of one or more of the following criteria: 1) maculopathy; 2) hearing impairment; 3) maternal heritability of diabetes/impaired fasting glucose and/or hearing impairment and/or maculopathy in three consecutive generations (or in two generations if 2 or 3 members of a family were affected). We sequenced the mtDNA in the 11 probands, in their mothers and in 80 controls. We identified 33 diabetes-suspected mutations, 1/33 was 3243A>G. Most patients (91%) and their mothers had mutations in complex I and/or IV of the respiratory chain. We measured the activity of these two enzymes and found that they were less active in mutated patients and their mothers than in the healthy control pool. The prevalence of hearing loss (36% vs 75-98%) and macular dystrophy (54% vs 86%) was lower in our mitochondrial diabetic adolescents than reported in adults. Moreover, we found a hitherto unknown association between mitochondrial diabetes and celiac disease. In conclusion, mitochondrial diabetes should be considered a complex syndrome with several phenotypic variants. Moreover, deafness is not an essential component of the disease in children. The whole mtDNA should be screened because the 3243A>G variant is not as frequent in children as in adults. In fact, 91% of our patients were mutated in the complex I and/or IV genes. The enzymatic assay may be a useful tool with which to confirm the pathogenic significance of detected variants.     

Cardiac abnormalities in patients with mitochondrial DNA mutation 3243A>G

Background

Tissues that depend on aerobic energy metabolism suffer most in diseases caused by mutations in mitochondrial DNA (mtDNA). Cardiac abnormalities have been described in many cases, but their frequency and clinical spectrum among patients with mtDNA mutations is unknown.

Methods

Thirty-nine patients with the 3243A>G mtDNA mutation were examined, methods used included clinical evaluation, electrocardiogram, Holter recording and echocardiography. Autopsy reports on 17 deceased subjects were also reviewed. The degree of 3243A>G mutation heteroplasmy was determined using an Apa I restriction fragment analysis. Better hearing level (BEHL_{0.5–4 kHz}) was used as a measure of the clinical severity of disease.

Results

Left ventricular hypertrophy (LVH) was diagnosed in 19 patients (56%) by echocardiography and in six controls (15%) giving an odds ratio of 7.5 (95% confidence interval; 1.74–67). The dimensions of the left ventricle suggested a concentric hypertrophy. Left ventricular systolic or diastolic dysfunction was observed in 11 patients. Holter recording revealed frequent ventricular extrasystoles (>10/h) in five patients. Patients with LVH differed significantly from those without LVH in BEHL_{0.5–4 kHz}, whereas the contribution of age or the degree of the mutant heteroplasmy in skeletal muscle to the risk of LVH was less remarkable.

Conclusions

Structural and functional abnormalities of the heart were common in patients with 3243A>G. The risk of LVH was related to the clinical severity of the phenotype, and to a lesser degree to age, suggesting that patients presenting with any symptoms from the mutation should also be evaluated for cardiac abnormalities.     

Clinical phenotype, prognosis and mitochondrial DNA mutation load in mitochondrial encephalomyopathies

We studied 42 individuals, including 8 patients with either complete or partial syndrome of mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS), 8 patients with either complete or partial syndrome of myoclonic epilepsy with ragged-red fibers (MERRF) and 26 maternal family members who carried either the A3243G or A8344G mutation of mitochondrial DNA (mtDNA). Clinical manifestations and prognosis were followed up in the patients harboring the A3243G or A8344G mutation. The relationship between clinical features and proportions of mutant mtDNAs in muscle biopsies, blood cells and/or hair follicles was studied. In the 8 regularly followed patients with the A3243G mutation, 4 died within 1 month to 7 years due to status epilepticus and/or recurrent stroke-like episodes. Two patients developed marked mental deterioration and 2 remained stationary. All of the patients harboring the A8344G mutation were stable or deteriorated slightly, except for 1 patient who died due to brain herniation after putaminal hemorrhage. The A3243G and A8344G mtDNA mutations were heteroplasmic in the muscle biopsies, blood cells and hair follicles of both the probands and their maternal family members. The mean proportion of A3243G mutant mtDNA in the muscle biopsies of the patients with MELAS syndrome (68.5 ± 21.3%, range 33–92%) was significantly higher than that of the asymptomatic family members (37.1 ± 12.6%, range 0–51%). The average proportions of A8344G mutant mtDNA in the muscle biopsies (90.1 ± 3.9%, range 89–95%) and hair follicles (93.9 ± 6.4%, range 84–99%) of the patients with MERRF syndrome were also significantly higher than those of the asymptomatic family members (muscle: 40.3 ± 39.5%, range 1–80%; hair follicles: 51.0 ± 44.5%, range 0.1–82%). We concluded that measurement of the proportion of mutant mtDNA in muscle biopsies may provide useful information in the identification of symptomatic patients with mitochondrial encephalomyopathies. For patients with the A3243G mutation, the prognosis was related to status epilepticus and the number of recurrent stroke-like episodes and was much worse than for patients with the A8344G mutation of mtDNA, who had stable or slowly deteriorating clinical courses.     

Abnormal cardiac energetics in patients carrying the A3243G mtDNA mutation measured in vivo using phosphorus MR spectroscopy

Cardiomyopathy is a frequent cause of morbidity and mortality in patients carrying the A3243G transition in the mitochondrial DNA (mtDNA) tRNALeu(UUR) gene, the most common heteroplasmic single mtDNA defect. We used phosphorus magnetic resonance spectroscopy (31P-MRS) to look for evidence of an in vivo bioenergetics defect in patients carrying the A3243G mtDNA mutation with and without echocardiographic signs of left ventricle hypertrophy (LVH). Eight patients, three with LVH, carrying the A3243G mtDNA mutation and 10 healthy subjects underwent one-dimensional chemical shift imaging 31P-MRS. In the patients, mean cardiac phosphocreatine to adenosine triphosphate ratio (PCr/ATP) (1.55 +/- 0.58) was significantly reduced compared to the control group (2.34 +/- 0.14; P < 0.001). Cardiac PCr/ATP was within the normal range only in one case that showed normal echocardiography. Our results point to a central role of bioenergetics deficit in the development of cardiac hypertrophy in patients with the A3243G mtDNA mutation. Impaired cardiac energy metabolism in patients with normal echocardiography suggests that the enhancement of mitochondrial function may be beneficial not only to patients with cardiac hypertrophy but also to those patients carrying the mutation in the absence of signs of cardiac hypertrophy and/or dysfunction but with cardiac bioenergetics deficit.     

Metabolically induced heteroplasmy shifting and l-arginine treatment reduce the energetic defect in a neuronal-like model of MELAS

The m.3243A>G variant in the mitochondrial tRNA(Leu(UUR)) gene is a common mitochondrial DNA (mtDNA) mutation. Phenotypic manifestations depend mainly on the heteroplasmy, i.e. the ratio of mutant to normal mtDNA copies. A high percentage of mutant mtDNA is associated with a severe, life-threatening neurological syndrome known as MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes). MELAS is described as a neurovascular disorder primarily affecting the brain and blood vessels, but the pathophysiology of the disease is poorly understood. We developed a series of cybrid cell lines at two different mutant loads: 70% and 100% in the nuclear background of a neuroblastoma cell line (SH-SY5Y). We investigated the impact of the mutation on the metabolism and mitochondrial respiratory chain activity of the cybrids. The m.3243A>G mitochondrial mutation induced a metabolic switch towards glycolysis in the neuronal cells and produced severe defects in respiratory chain assembly and activity. We used two strategies to compensate for the biochemical defects in the mutant cells: one consisted of lowering the glucose content in the culture medium, and the other involved the addition of l-arginine. The reduction of glucose significantly shifted the 100% mutant cells towards the wild-type, reaching a 90% mutant level and restoring respiratory chain complex assembly. The addition of l-arginine, a nitric oxide (NO) donor, improved complex I activity in the mutant cells in which the defective NO metabolism had led to a relative shortage of NO. Thus, metabolically induced heteroplasmy shifting and l-arginine therapy may constitute promising therapeutic strategies against MELAS.     

温州地区2型糖尿病患者线粒体DNA 3243、3316位点的突变筛查

为了解浙江省温州地区2型糖尿病病人中线粒体DNA tRNALeu (UUR)基因A3243G及NADH 脱氢酶亚单位1 (ND1)基因G3316A位点突变的发生频率, 并探讨突变与2型糖尿病主要临床指标出现的相关性。对随机收集的无血缘关系的244例温州地区2型糖尿病患者进行研究, 同时选择156例无 DM 家族史的糖耐量正常者作为对照组, 用聚合酶链反应及限制性片段长度多态性分析技术进行点突变筛选, 筛选到的异质性突变样本经T-A克隆后再作测序和变性高效液相色谱(DHPLC)确证。结果在244例的2型糖尿病患者中检出A3243G突变1例(0.410%), 156例对照者中未检出该突变, 突变发生率在两组间差异无统计学意义(P>0.05); 2型糖尿病患者中检出G3316A突变4例(1.639%), 156例对照者中检出突变2例(1. 282%), 突变发生率在两组间差异无统计学意义(P>0.05)。结果表明线粒 体tRNALeu (UUR) 基因A3243G突变在浙江温州2型糖尿病人群中发生频率低, 不是温州人群中2型糖尿病的常见病因。线粒体ND1基因G3316A突变在糖尿病人群中的发生频率也较低, 且在正常人群中也有出现, 可能仅为人群中线粒体DNA的基因多态性。     

Maternally transmitted diabetes mellitus associated with the mitochondrial tRNA(Leu(UUR)) A3243G mutation in a four-generation Han Chinese family

We report here the characterization of a four-generation Han Chinese family with maternally transmitted diabetes mellitus. Six (two males/four females) of eight matrilineal relatives in this family exhibited diabetes. The age of onset in diabetes varies from 15 years to 33 years, with an average of 26 years. Two of affected matrilineal relatives also exhibited hearing impairment. Molecular analysis of mitochondrial DNA (mtDNA) showed the presence of heteroplasmic tRNA(Lue(UUR)) A3243G mutation, ranging from 35% to 58% of mutations in blood cells of matrilineal relatives. The levels of heteroplasmic A3243G mutation seem to be correlated with the severity and age-at-onset of diabetes in this family. Sequence analysis of the complete mitochondrial genome in this pedigree revealed the presence of the A3243G mutation and 38 other variants belonging to the Eastern Asian haplogroup M7C. However, none of other mtDNA variants are evolutionarily conserved and implicated to have significantly functional consequence. Thus, the A3243G mutation is the sole pathogenic mtDNA mutation associated with diabetes in this Chinese family.     

Pathogenic mitochondrial DNA mutations are common in the general population

Mitochondrial DNA (mtDNA) mutations are a major cause of genetic disease, but their prevalence in the general population is not known. We determined the frequency of ten mitochondrial point mutations in 3168 neonatal-cord-blood samples from sequential live births, analyzing matched maternal-blood samples to estimate the de novo mutation rate. mtDNA mutations were detected in 15 offspring (0.54%, 95% CI = 0.30–0.89%). Of these live births, 0.00107% (95% CI = 0.00087–0.0127) harbored a mutation not detected in the mother's blood, providing an estimate of the de novo mutation rate. The most common mutation was m.3243A→G. m.14484T→C was only found on sub-branches of mtDNA haplogroup J. In conclusion, at least one in 200 healthy humans harbors a pathogenic mtDNA mutation that potentially causes disease in the offspring of female carriers. The exclusive detection of m.14484T→C on haplogroup J implicates the background mtDNA haplotype in mutagenesis. These findings emphasize the importance of developing new approaches to prevent transmission.     

Poor correlations in the levels of pathogenic mitochondrial DNA mutations in polar bodies versus oocytes and blastomeres in humans

Because the mtDNA amount remains stable in the early embryo until uterine implantation, early human development is completely dependent on the mtDNA pool of the mature oocyte. Both quantitative and qualitative mtDNA defects therefore may negatively impact oocyte competence or early embryonic development. However, nothing is known about segregation of mutant and wild-type mtDNA molecules during human meiosis. To investigate this point, we compared the mutant levels in 51 first polar bodies (PBs) and their counterpart (oocytes, blastomeres, or whole embryos), at risk of having (1) the "MELAS" m.3243A>G mutation in MT-TL1 (n = 30), (2) the "MERRF" m.8344A>G mutation in MT-TK (n = 15), and (3) the m.9185T>G mutation located in MT-ATP6 (n = 6). Seven out of 51 of the PBs were mutation free and had homoplasmic wild-type counterparts. In the heteroplasmic PBs, measurement of the mutant load was a rough estimate of the counterpart mutation level (R(2) = 0.52), and high mutant-load differentials between the two populations were occasionally observed (ranging from -34% to +34%). The mutant-load differentials between the PB and its counterpart were higher in highly mutated PBs, suggestive of a selection process acting against highly mutated cells during gametogenesis or early embryonic development. Finally, individual discrepancies in mutant loads between PBs and their counterparts make PB-based preconception diagnosis unreliable for the prevention of mtDNA disorder transmission. Such differences were not observed in animal models, and they emphasize the need to conduct thorough studies on mtDNA segregation in humans.     

A novel mutation in the mitochondrial 16S rRNA gene in a patient with MELAS syndrome, diabetes mellitus, hyperthyroidism and cardiomyopathy

Using RNase protection analysis, we found a novel C to G mutation at nucleotide position 3093 of mitochondrial DNA (mtDNA) in a previously reported 35-year-old woman exhibiting clinical features of mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) syndrome together with diabetes mellitus, hyperthyroidism and cardiomyopathy. The patient also had an A3243G mutation in the tRNA(Leu(UUR)) gene and a 260-base pair duplication in the D-loop of mtDNA. The fibroblasts of the patient were cultured and used for the construction of cybrids using cytoplasmic transfer of the patient's mtDNA to the mtDNA-less rho(0) cells. RNA isolated from the cybrids was subjected to RNase protection analysis, and a C3093G transversion at the 16S rRNA gene and a MELAS-associated A3243G mutation of mtDNA were detected. The novel C3093G mutation together with the A3243G transition were found in muscle biopsies, hair follicles and blood cells of this patient and also in her skin fibroblasts and cybrids. The proportion of the C3093G mutant mtDNA in muscle biopsies of the patient was 51%. In contrast, the mutation was not detected in three sons of the proband. To characterize the impact of the mtDNA mutation-associated defects on mitochondrial function, we determined the respiratory enzyme activities of the primary culture of fibroblasts established from the proband, her mother and her three sons. The proportions of mtDNA with the C3093G transversion and the A3243G transition in the fibroblasts of the proband were 45 and 58%, respectively. However, the fibroblasts of the proband's mother and children harbored lower levels of mtDNA with the A3243G mutation but did not contain the C3093G mutation. The complex I activity in the proband's fibroblasts was decreased to 47% of the control but those of the fibroblasts of the mother and three sons of the proband were not significantly changed. These findings suggest that the C3093G transversion together with the A3243G transition of mtDNA impaired the respiratory function of mitochondria and caused the atypical MELAS syndrome associated with diabetes mellitus, hyperthyroidism and cardiomyopathy in this patient.     

Detection of eight frequently encountered point mutations in mitochondria in Chinese patients suggestive of mitochondrial encephalomyopathies

To evaluate eight frequently encountered mitochondrial DNA (mtDNA) point mutations (A3243G, T8993G/C, A8344G, A1555G, G11778A, G3460A and T14484C) in Chinese, we recruited 1559 sporadic patients suspected of mitochondrial diseases and 206 family members. In suspected patients, 158 cases were detected with one of these eight mtDNA mutations (10.1%). A3243G was the most common mtDNA mutation both in suspected patients (9.4%) and in the relatives (34.2%). In addition, the ratios of A3243G (mutant/wild-type) and A8344G were significantly correlated with the patients’ age of examination. Moreover, in 76 unrelated probands, the ratio of A3243G was correlated well with their seizures and myopathies.     

Mutations in mitochondrial DNA accumulate differentially in three different human tissues during ageing.

In 60 human tissue samples (encompassing skeletal muscle, heart and kidney) obtained from subjects aged from under 1 to 90 years, we used quantitative PCR procedures to quantify mitochondrial DNA (mtDNA) molecules carrying the 4977 bp deletion (mtDNA4977) and 3243 A-->G base substitution. In addition, the prevalence of multiple mtDNA deletions was assessed in a semi-quantitative manner. For all three tissues, the correlations between the accumulation of the particular mtDNA mutations and age of the subject are highly significant. However, differential extents of accumulation of the two specific mutations in the various tissues were observed. Thus, the mean abundance (percentage of mutant mtDNA out of total mtDNA) of mtDNA4977in a subset of age-matched adults is substantially higher in skeletal muscle than in heart and kidney. However, the mean abundance of the 3243 A-->G mutation in skeletal muscle was found to be lower than that in heart and kidney. Visualisation of arrays of PCR products arising from multiple mtDNA deletions in DNA extracted from adult skeletal muscle, was readily made after 30 cycles of PCR. By contrast, in DNA extracted from adult heart or kidney, amplification for 35 cycles of PCR was required to detect multiple mtDNA deletions. Although such multiple deletions are less abundant in heart and kidney than in skeletal muscle, in all tissue extracts there are unique patterns of bands, even from different tissues of the same subject. The differential accumulation of mtDNA4977, other mtDNA deletions and the 3243 A-->G mutation in the three tissues analysed presumably reflects different metabolic and senescence characteristics of these various tissues.     

Intracellular heteroplasmy for disease-associated point mutations in mtDNA: implications for disease expression and evidence for mitotic segregation of heteroplasmic units of mtDNA

Studies in vitro have shown that a respiratorydeficient phenotype is expressed by cells when the proportion of mtDNA with a disease-associated mutation exceeds a threshold level, but analysis of tissues from patients with mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes (MELAS) have failed to show a consistent relationship between the degree of heteroplasmy and biochemical expression of the defect. One possible explanation for this phenomenon is that there is variation of heteroplasmy between individual cells that is not adequately reflected by the mean heteroplasmy for a tissue. We have confirmed this by study of fibroblast clones from subjects heteroplasmic for the MELAS 3243 (A G) mtDNA mutation. Similar observations were made with fibroblast clones derived from two subjects heteroplasmic for the 11778 (GA) mtDNA mutation of Leber's hereditary optic neuropathy. For the MELAS 3243 mutation, the distribution of mutant mtDNA between different cells was not randomly distributed about the mean, suggesting that selection against cells with high proportions of mutant mtDNA had occurred. To explore the way in which heteroplasmic mtDNA segregates in mitosis we followed the distribution of heteroplasmy between clones over approximately 15 generations. There was either no change or a decrease in the variance of intercellular heteroplasmy for the MELAS 3243 mutation, which is most consistent with segregation of heteroplasmic units of multiple mtDNA molecules in mitosis. After mitochondria from one of the MELAS 3243 fibroblast cultures were transferred to a mitochondrial DNA-free (⁰) cell line derived from osteosarcoma cells by cytoplast fusion, the mean level and intercellular distribution of heteroplasmy was unchanged. We interpret this as evidence that somatic segregation (rather than nuclear background or cell differentiation state) is the primary determinant of the level of heteroplasmy.     

A whole mitochondrial genome screening in a MELAS patient: a novel mitochondrial tRNA(Val) mutation

Mitochondrial encephalopathy, lactic acidosis and strokelike episodes (MELAS) syndrome is a mitochondrial disorder characterized by a wide variety of clinical presentations and a multisystemic organ involvement. In this study, we report a Tunisian girl with clinical features of MELAS syndrome who was negative for the common m.3243A>G mutation, but also for the reported mitochondrial DNA (mtDNA) mutations and deletions. Screening of the entire mtDNA genome showed several known mitochondrial variants besides to a novel transition m.1640A>G affecting a wobble adenine in the anticodon stem region of the tRNA(Val). This nucleotide was conserved and it was absent in 150 controls suggesting its pathogenicity. In addition, no mutations were found in the nuclear polymerase gamma-1 gene (POLG1). These results suggest further investigation nuclear genes encoding proteins responsible for stability and structural components of the mtDNA or to the oxidative phosphorylation machinery to explain the phenotypic variability in the studied family.     

Heteroplasmy levels of mtDNA1555A>G mutation is positively associated with diverse phenotypes and mutation transmission in a Chinese family

The mtDNA 1555A>G mutation was considered to be one of the most common causes of aminoglycoside-induced and non-syndromic hearing loss. However, this mutation was always found in homoplasmy with high phenotypic heterogeneity. Recently this mutation in heteroplasmy has been reported in several studies. In the present study, we have collected a large Chinese family harboring heteroplasmic mtDNA 1555A>G mutation with diverse clinical phenotypes. To investigate the relationship between the mutation load and the severity of hearing loss under Eastern Asian background, we performed clinical, molecular, genetic and phylogenic analysis. This pedigree was characterized by coexistence of eight subjects with homoplasmic mutation and ten subjects with various degrees of heteroplasmy, and the results suggested that there was a strong correlation between the mutation load and the severity/age-onset of hearing loss (r=0.758, p<0.001). We noticed that the mutation level of offspring was associated with their mothers' in this pedigree, which indicated that maybe exist a regular pattern during the process of the heteroplasmic transmission. In addition, analysis of the complete mtDNA genome of this family revealed that it belonged to Eastern Asian haplogroup B4C1. In addition, a rare homoplasmic mtDNA 9128T>C variant was identified, it located at a strictly conserved site of mtDNA ATP6 gene.     

The investigation and diagnosis of pathogenic mitochondrial DNA mutations in human urothelial cells

Patients with mitochondrial DNA disease are amongst the most challenging to diagnose and manage given the striking phenotypic and genetic heterogeneity, which characterise these conditions. Recently, we and others have demonstrated the m.3243A>G mutation, one of the most common mitochondrial DNA pathogenic mutations, is present at clinically relevant levels in urinary epithelium, thus providing a practical, non-invasive test for diagnosis and mutation screening. In this study we further evaluate the use of these cells in detecting the m.3243A>G mutation, other mtDNA tRNA gene point mutations including the m.8344A>G mutation and single large-scale mtDNA deletions. We observe a robust relationship between m.3243A>G levels in urothelial cells and clinically affected tissues that does not change with time. Conversely, single large-scale mtDNA deletions can be detected in urothelial cells, with higher levels present in younger patients with more severe disease, but generally mtDNA deletion levels are not representative of those seen in a clinically affected tissue. Our results have implications for the diagnosis, management and counselling of families with mtDNA disease.     

Mitochondrial DNA variations in patients with maternally inherited diabetes and deafness syndrome

Mitochondrial DNA (mtDNA) variants have been implicated in the pathogenesis of diabetes. A mutation in the tRNA leucine gene at position 3243 has been previously reported in mtDNA of maternally inherited diabetes and deafness (MIDD) patients. Because the true prevalence of the mitochondrial origin in diabetes may be underestimated, we searched for potentially diabetogenic anomalies of mtDNA in 9 patients highly suspected of mitochondrial diabetes selected on maternally inheritance and clinical features. In order to detect high levels of mutant DNA, the mtDNA of muscle sample of 2 patients was totally sequenced and the 22 tRNA genes and flanking sequences of 7 patients were analyzed. A new homoplasmic mutation at position 8381 was found in the ATPase 8 gene of mtDNA of a MIDD patient. The prevalence of three homoplasmic variations (G1888A, T4216G, A4917G) was significantly higher in the small group of MIDD patients compared to controls and other subjects groups. This study demonstrated in our patients sample the high frequency of homoplasmic variations, which could play a role by themselves or in combination, in the pathogenesis of diabetes.     

Epidemiology of A3243G, the mutation for mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes: prevalence of the mutation in an adult population.

Mitochondrial diseases are characterized by considerable clinical variability and are most often caused by mutations in mtDNA. Because of the phenotypic variability, epidemiological studies of the frequency of these disorders have been difficult to perform. We studied the prevalence of the mtDNA mutation at nucleotide 3243 in an adult population of 245,201 individuals. This mutation is the most common molecular etiology of MELAS syndrome (mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes), one of the clinical entities among the mitochondrial disorders. Patients with diabetes mellitus, sensorineural hearing impairment, epilepsy, occipital brain infarct, ophthalmoplegia, cerebral white-matter disease, basal-ganglia calcifications, hypertrophic cardiomyopathy, or ataxia were ascertained on the basis of defined clinical criteria and family-history data. A total of 615 patients were identified, and 480 samples were examined for the mutation. The mutation was found in 11 pedigrees, and its frequency was calculated to be >=16. 3/100,000 in the adult population (95% confidence interval 11.3-21. 4/100,000). The mutation had arisen in the population at least nine times, as determined by mtDNA haplotyping. Clinical evaluation of the probands revealed a syndrome that most frequently consisted of hearing impairment, cognitive decline, and short stature. The high prevalence of the common MELAS mutation in the adult population suggests that mitochondrial disorders constitute one of the largest diagnostic categories of neurogenetic diseases.     

Hearing impairment in patients with 3243A→G mtDNA mutation: phenotype and rate of progression

The relationship between the phenotype and the genotype is complex in diseases caused by mutations in mitochondrial DNA (mtDNA). The 3243A-->G mutation in mtDNA frequently leads to sensorineural hearing impairment (HI), a phenotype that can be assessed in severity by audiometry; hence, consecutive audiograms can give an estimate of the rate of HI progression.We examined the audiological phenotype of 38 patients (14 men, 24 women; mean age: 45+/-14 years) who possessed the 3243A-->G mutation and who belonged to a population-based cohort ascertained in the province of Northern Ostrobothnia, Finland. The subjects took part in an otorhinolaryngologic examination, including audiometry. Factors modulating the severity of HI were analyzed, and the rate of HI progression was calculated. The better ear hearing level (BEHL) at frequencies 0.5, 1, 2, and 4 kHz (BEHL0.5-4kHz) was greater than 20 dB suggesting HI in 28 patients (74%). A good correlation (r=0.428, P=0.009) was found between BEHL0.5-4kHz and the degree of the mutant heteroplasmy. BEHL0.5-4kHz was worse in men than in women, and women outnumbered men among patients with normal hearing or mild HI. In addition, 181 consecutive audiograms were reviewed from 24 patients with HI. The rate of HI progression was calculated to be 2.9 dB/year in men and 1.5 dB/year in women, being clearly faster than the rates that have been observed in the corresponding age group in the general population. A high degree of mutant heteroplasmy, male gender, and age were found to increase the severity of HI. Phenotypic difference by gender may thus be a more universal phenomenon in mitochondrial diseases, not only being associated with Leber's hereditary optic neuropathy. This study provides the first estimate of the rate of disease progression among patients with the 3243A-->G mutation.