The mitochondrial DNA A3243G mutation in Werner's syndrome

OBJECTIVE: The contribution of the A3243G mutation in mitochondria DNA (mtDNA) to diabetes mellitus (DM) in Werner's syndrome (WS) was studied. PATIENTS AND METHOD: DNA samples from peripheral white blood cells (WBCs) originating from 24 Japanese WS patients aged 30-56 were used. For control, 239 subjects aged 15-95 were also used. The mtDNA was amplified using specific primers. After HaeIII digestion, the ratio of the A3243G mutation was compared. RESULTS: The ratio of the A3243G mutation is 0.45+/-0.13% in WS, which is statistically insignificant from those in the control groups at various age. The mutation types of WRN in genomic DNA did not affect the ratio of the A3243G mtDNA mutation. No significant difference was observed concerning to the ratios among the WS patients with and without DM, and also controls. Furthermore, no significant difference was observed in the ratios of A3243G mutation among controls from various age groups. CONCLUSION: The A3243G mutation in mtDNA does not accumulated in WBCs from WS. Mitochondria A3243G mutation may not contribute to the pathogenesis of DM observed in WS.     

早发糖尿病患者线粒体DNA tRNA Leu(UUR)突变的研究

目的 探讨天津地区早发糖尿病(DM)患者线粒体DNA tRNA^leu(UUR) 3243A→G突变的发生率及其相关性。方法 随机选取无亲缘关系、发病年龄≤45岁的DM患者348例，对照组207名，收集相应临床资料，提取外周血基因组DNA，以聚合酶链反应、限制性内切酶片段长度多态性及克隆的方法检测线粒体基因tRNA^tLeu(UUR)3243A→G突变，并进行家系研究。结果 DM组发现2例3243A→G突变，检出率为0．6％，有家族史的DM患者中发生率为1．2％。对照组未发现此突变。3243A→G突变先证者呈典型的线粒体DM表现，其家庭成员的临床表型和基因突变异质性不一致。结论 tRNA^Leu(UUR)3243A→G突变在天津地区发病年龄≤45岁的DM患者中检出率低，在合并其他线粒体病的患者中发生率较高；该突变异质性比例在有丝分裂组织中可能随年龄增加而减小。     

Accumulation of somatic mutation in mitochondrial DNA extracted from peripheral blood cells in diabetic patients

AIMS/HYPOTHESIS: A point mutation of mitochondrial DNA at nucleotide number 3243 A to G is responsible for both the major genetic aetiologies of the MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes) and mitochondrial diabetes. Otherwise, this mutation is also reported to occur as an acquired somatic mutation, possibly due to oxidative stress. Since diabetes can cause severe oxidative stress, we hypothesize that the accumulation of the somatic 3243 A to G mutation in mitochondrial DNA can be accelerated by diabetes. METHODS: DNA was extracted from blood samples of 290 non-diabetic healthy subjects (age 20-60) including 98 newborn infants and from 383 patients with Type II (non-insulin-dependent) diabetes mellitus (age 18-80). The extent of somatic 3243 A to G mutation to total mitochondrial DNA was detected by real-time PCR using the TaqMan Probe. RESULTS: Whereas the level of the 3243 A to G mutation was negligible in the newborn group, it was increased in healthy subjects who were 20 to 29 and 41 to 60 years of age, suggesting that this mutation was somatic. In the diabetic patients the mutation rate increased along with age and the duration of diabetes. In the middle-aged group (age 41-60), the 3243 A to G mutation accumulates fourfold higher in the diabetic patients than the healthy subjects. Moreover, multiple regression analysis showed that the most critical factor associated with this mutation in diabetic patients was the duration of diabetes. CONCLUSION/INTERPRETATION: Diabetes accelerates the accumulation of the somatic 3243 A to G mutation in mitochondrial DNA, which can accelerate the ageing process. This somatic mutation could possibly be a new marker for estimating the duration of diabetes.     

Diabetes-associated mitochondrial DNA mutation A3243G impairs cellular metabolic pathways necessary for beta cell function

Aims/hypothesis Mitochondrial DNA (mtDNA) mutations cause several diseases, including mitochondrial inherited diabetes and deafness (MIDD), typically associated with the mtDNA A3243G point mutation on tRNALeu gene. The common hypothesis to explain the link between the genotype and the phenotype is that the mutation might impair mitochondrial metabolism expressly required for beta cell functions. However, this assumption has not yet been tested.Methods We used clonal osteosarcoma cytosolic hybrid cells (namely cybrids) harbouring mitochondria derived from MIDD patients and containing either exclusively wild-type or mutated (A3243G) mtDNA. According to the importance of mitochondrial metabolism in beta cells, we studied the impact of the mutation on key parameters by comparing stimulation of these cybrids by the main insulin secretagogue glucose and the mitochondrial substrate pyruvate.Results Compared with control mtDNA from the same patient, the A3243G mutation markedly modified metabolic pathways leading to a high glycolytic rate (2.8-fold increase), increased lactate production (2.5-fold), and reduced glucose oxidation (−83%). We also observed impaired NADH responses (−56%), negligible mitochondrial membrane potential, and reduced, only transient ATP generation. Moreover, cybrid cells carrying patient-derived mutant mtDNA exhibited deranged cell calcium handling with increased cytosolic loads (1.4-fold higher), and elevated reactive oxygen species (2.6-fold increase) under glucose deprivation.Conclusions/interpretation The present study demonstrates that the mtDNA A3243G mutation impairs crucial metabolic events required for proper cell functions, such as coupling of glucose recognition to insulin secretion.     

Gastric dysmotility associated with accumulation of mitochondrial A3243G mutation in the stomach

OBJECTIVE: We analyzed the accumulation of a mitochondrial A-to-G mutation at nucleotide position 3243 (A3243G) in the stomach and gastric motility in patients with gastric symptoms, post-prandial nausea/vomiting and epigastralgia. METHODS: Detection and quantification of A3243G mutation in mtDNA in the gastric mucosa, oral mucosa, leukocyte, and skeletal muscle were performed. Gastric motility was evaluated by gastric myoelectrical activity on electrogastrography (EGG), and gastric emptying was evaluated by measurement of plasma paracetamol concentration before and after meals. PATIENTS OR MATERIALS: Four patients with A3243G mutation in the leukocyte mtDNA and gastric symptoms were examined. RESULTS: The A3243G mutation was detected at higher percentages in the gastric body (69-94% for mutation; mean, 83%) than in the angle portion (37-82%; mean, 52%), the antrum (40-84%; mean, 57%) or leukocytes (28-52%; mean, 39%), and at slightly higher percentages than in the skeletal muscles (45-87%; mean, 70%) or oral mucosae (52-86%; mean, 69%) in the four patients examined. Abnormal EGGs were observed in the three patients examined. The pre-prandial myoelectrical activities were low in these patients (49% in patient 1, 54% in patient 2, 63% in patient 3; normal >70%). The plasma concentrations of paracetamol were low (3.6 microg/ml in patient 1, 2.4 microg/ml in patient 2, <2.0 microg/ml in patient 3; normal, 7-12 microg/ml). CONCLUSION: Accumulation of mitochondrial A3243G mutation in the stomach is a contributory factor in gastric dysmotility and gastric symptoms in patients with the mutation in their leukocytes.     

New insights in the molecular pathogenesis of the maternally inherited diabetes and deafness syndrome.

The 3243A>G mutation in mitochondrial DNA (mtDNA) is a genetic variant that is associated with a high risk of developing diabetes during life. Enhanced aging of pancreatic beta-cells, a reduced capacity of these cells to synthesize large amounts of insulin,and a resetting of the ATP/ADP-regulated K-channel seem to be the pathogenic factors involved.     

Heteroplasmic ratio of the A3243G mitochondrial DNA mutation in single pancreatic beta cells

AIMS/HYPOTHESIS: To examine whether there is a high content of mutated mitochondrial DNA in individual pancreatic beta cells from a patient with the A3243G mitochondrial DNA mutation. METHODS: Tissues were available from a patient with diabetes and the A3243G mutation including pancreatic tissue. We quantified the amount of mutated mitochondrial DNA in tissue homogenates and single pancreatic beta cells using hot last cycle PCR. RESULTS: The percentage ratio of mutated to wild-type mtDNA was high in tissues such as muscle and brain (>60%), but surprisingly low in both pancreatic islets and in individual beta cells from these islets. The islets were smaller in the patient than in control subjects in keeping with a decreased beta-cell mass. CONCLUSIONS/INTERPRETATION: These observations suggest that either the beta cells show increased sensitivity to the effects mtDNA mutations on respiratory chain function, and/or cells with a high mutant load are preferentially removed leading to a progressive decrease in the islet beta-cell mass.     

Prevalence of mitochondrial diabetes in southwestern Finland: a molecular epidemiological study

Mitochondrial diabetes and deafness (MIDD) is a subtype of diabetes mellitus (DM) that most commonly results from the m.3243A > G mutation in mitochondrial DNA (mtDNA). Sensorineural hearing loss is a typical accompanying feature. Previous studies have suggested a prevalence of ~1–1.5 % for MIDD. We studied the molecular epidemiology of MIDD among young (aged 18–45 years) adults in a defined population in southwestern Finland. Of the identified cohort of 1,532 patients with DM, we received blood samples of 299 patients and analyzed them for the m.3243A > G mutation and for mtDNA haplogroups. We found three DM patients (1.0 %) with the m.3243A > G mutation. All the three patients with DM and m.3243A > G also had severe hearing impairment that required use of hearing aid. MtDNA haplogroup U was more prevalent among patients with maternal family history of DM. We conclude that among young adults, ~1 % of all DM is associated with the m.3243A > G mutation. We suggest that all patients with both DM and hearing impairment, at least in this age group, should undergo investigation for this mutation. Furthermore, our results suggest that mtDNA haplogroup U is associated with maternal family history of DM.     

缺血性卒中患者线粒体DNA点突变研究

对56例缺血性卒中患者及38例正常对照者检测粒体（mt）的A3243G和G13513A点突变，提取外周血淋巴细胞总DNA。经热启动聚合酶链反应扩增线粒体相关的DNA片段，ApaI酶切后电泳检测长工片段多肽（RFLP），采用敏感性较高的非放射性银梁单链构象多态性（SSCP）技术分析扩增产物。结果显示，7例患者SSCP出现额外条带，具有显著性差异（Fisher确切概率法，P=0．039）。在2例患者中检出mtRNAA3243G突变，未见G1513A点突变ApaI酶切片段的长度片段多态性显示，A3243G突变率较低（3．57％），认为线粒体DNA突变异致的呼吸链功能障碍与缺血性卒中的发生相关。     

Prevalence and progression of diabetes in mitochondrial disease

Aims/Hypothesis The aims of this study were (1) to determine the prevalence and rate of progression in diabetes secondary to mitochondrial DNA (mtDNA) mutations; and (2) to determine whether percentage heteroplasmy predicts clinical outcome in patients carrying the m.3243A>G mutation. Methods We prospectively assessed 242 patients attending a specialist neuromuscular clinic using a validated mitochondrial disease rating scale. Retrospective clinical data on these patients from up to 25 years of follow-up were also included. Percentage heteroplasmy in blood, urine and muscle was determined for the m.3243A>G group and correlated against clinical features. Results Patients carrying the m.3243A>G mutation formed the largest group of patients with diabetes (31/81 patients). The highest prevalence of diabetes was in the m.12258C>A group (2/2 patients), the lowest in the multiple mtDNA deletions group (3/43 patients). The earliest age of onset was in the m.3243A>G group (37.9 years) with the highest age of presentation in the multiple deletion group (56.3 years). Of patients presenting with m.3243A>G, 12.9% required insulin; an additional 32.3% progressed to insulin requirement over a mean of 4.2 years after presentation. Percentage heteroplasmy in blood, urine or muscle did not predict progression of diabetes or risk of developing complications. Early age of presentation with diabetes did predict poor clinical outcome. Conclusions/Interpretation Although patients carrying the m.3243A>G mutation account for the majority of cases of diabetes secondary to mtDNA mutations, several other genotypes are also associated with the development of diabetes, some with high penetrance. All show a gradual progression to insulin requirement. Percentage heteroplasmy is a poor predictor of severity of diabetes in the m.3243A>G group.     

Novel mitochondrial DNA length variants and genetic instability in a family with diabetes and deafness.

We have identified two locations with novel multiplasmic length variants in the mitochondrial DNA of a family with diabetes and deafness. At nt568 in the D-loop, the 6-bp polycytidine tract was found to be variable in length up to a total of 12 residues. A second region with length variants was found at nt8281 in the intergenic COII-tRNA(Lys) region, which consists of two copies of the 9-bp repeat CCCCCTCTA. Only the second repeat occurs in a heteroplasmic C(9-14)A form with both T residues largely deleted. In addition, the mtDNA contained a number of new homoplasmic point mutations. Both length variants are stably inherited in a maternal way with no major changes in their length distribution. In contrast, during culture of fibroblasts from the proband the average length of the polycytidine tracts is increased at both locations indicating a fibroblast-specific genetic instability. Cybrid cells containing mtDNA from the proband proliferate less efficient than cybrids with wild-type mtDNA in co-culture experiments, suggesting functional consequences of the mtDNA length variants or the additional homoplasmic point mutations. Since oxygen consumption was not severely affected, these mutation seem less detrimental for mitochondrial function than the A3243G diabetogenic mutation and most other pathogenic mtDNA mutations. The contribution of mtDNA length variants to the phenotype of members of this family is discussed.     

母系遗传性糖尿病伴耳聋发病机制及诊治

母系遗传性糖尿病伴耳聋(MIDD)是一种线粒体疾病,在糖尿病患者中约占1％.其中tRNALeu(UUR) A3243G突变引起胰岛β细胞胰岛素分泌功能障碍是其致糖尿病的主要原因,但具体机制仍不明确.近年EndoC-βH1细胞系的建立为研究其机制提供了途径.尿道上皮细胞与线粒体疾病表型相关性最好,尿液可作为临床诊断和检测线粒体DNA异质性首选的样本,而MIDD的治疗仍以抗氧化剂和预防糖尿病并发症为主.A3243G突变的临床表型和MIDD突变的组织异质性仍是研究的热点,其研究对MIDD的诊断和治疗具有重要的指导意义.     

Clinical features and heteroplasmy in blood, urine and saliva in 34 Dutch families carrying the m.3243A > G mutation

The m.3243A > G mutation has become known as the MELAS mutation. However, many other clinical phenotypes associated with this mutation have been described, most frequently being maternally inherited diabetes and deafness (MIDD). The m.3243A > G mutation, can be detected in virtually all tissues, however heteroplasmy differs between samples. Recent reports indicate, a preference to perform mutation analysis in urinary epithelial cells (UEC). To test this, and to study a correlation between the mutational load in different tissues with two mitochondrial scoring systems (NMDAS and NPMDS) we investigated 34 families carrying the m.3243A > G mutation. Heteroplasmy was determined in three non-invasively collected samples, namely leucocytes, UEC and buccal mucosa. We included 127 patients, of which 82 carried the m.3243A > G mutation. None of the children (n?=?11) had specific complaints. In adults (n?=?71), a median NMDAS score of 15 (IQR 10-24) was found. The most prevalent symptoms were hearing loss(48%), gastro-intestinal problems(42%), exercise intolerance(38%) and glucose intolerance(37%). Ten patients had neurologic involvement. Buccal mucosa had the best correlation with the NMDAS in all adults (r?=?0.437,p? G mutation causes a wide variety of signs and symptoms, MIDD being far more prevalent than MELAS. Looking at the characteristics of the three non-invasively available tissues for testing heteroplasmy we confirm that UEC are the preferred sample to test.     

Increased risk of stroke in patients with the A12308G polymorphism in mitochondria

Factors which increase the risk of stroke in patients with the A3243G (mitochondrial encephalomyopathy, lactic acidosis, and stroke [MELAS]) mutation in human mitochondrial DNA are unclear. Previous work on lung-cancer cells with an A3243G mutation showed that a mutation in the mitochondrial transfer gene for leucine tRNA(Leu(CUN)) was able to ameliorate the A3243G-induced biochemical phenotype. We analysed the tRNA(Leu(CUN)) gene in 48 unrelated A3243G cases. We showed that a polymorphism, A12308G, in tRNA(Leu(CUN)) increases the risk of developing stroke in patients with the A3243G mutation (relative risk=2.17). This may have implications for genetic counselling.     

Rapd screening mitochondrial DNA mutation by using denaturing high—performance liquid chromatography

AIM: To optimize conditions of DHPLC and analyze the effectiveness of various DNA polymerases on DHPLC resolution, and evaluate the sensitivity of DHPLC in the mutation screening of mitochondrial DNA (mtDNA). METHODS: Two fragments of 16s gene of mitochondrial DNA (one of them F2 is a mutant fragment) and an A3243G mutated fragment were used to analyze the UV detection limit and determine the minimum percentage of mutant PCR products for DHPLC and evaluate effects of DNA polymerases on resolution of DHPLC. Under the optimal conditions, we analyzed the mtDNA mutations from muscle tissues of mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS) and screened blindly for variances in D-loop region of mtDNA from human gastric tumor specimen. RESULTS:Ten A3243G variants were detected in 12 cases of MELAS, no alterations were detected in controls and these results were consistent with the results obtained by analysis of RFLP with ApaI. We also identified 26 D-loop variances in 46 cases of human gastric cancer tissues and 38 alterations in 13 gastric cancer cell lines. The mutation of mtDNA at 80 ng PCR products containing a minimum of 5% mutant sequences could be detected by using DHPLC with UV detector. Moreover, Ampli-Taq Gold polymerase was equally as good as the proofreading DNA polymerase (e.g., Pfu) in eliminating the false positive produced by Taq DNA polymerases. CONCLUSION: DHPLC is a powerful, rapid and sensitive mutation screening method for mtDNA. Proofreading DNA polymerase is more suitable for DHPLC analysis than Taq polymerase.     

线粒体DNA变异与老年2型糖尿病患者易感性的相关性

目的 研究湖北地区老年2型糖尿病(T2DM)患者中线粒体基因突变的发生率及其相关性.方法 采用PCR-RFLP、基因测序技术,对175例老年T2DM患者和200例糖耐量正常的健康老年对照组进行检测.结果 MIND1 3316(G→A)、MTTL1 3243(A→G)、MIND13394( T→C)、MIND14216(T→C) MIND14164(A→G)和MIND2 5178( T→C)变异率分别为3.26％、2.72％、1.71％、4％、34.9％;对照组检出3316(G→A)突变2例(0.99％)、4164 5例(0.99％)、5718(T→C)变异64例(32.3％),未检出3394、4216的点突变;两组间3394(T→C)变异率差别有统计学意义(P＜0.05);且T2DM组5178A基因型血清TC水平低于5178C基因型(P＜0.05),但TG、LDL-C、HDL-C、apoA、apoB、Lp(a)水平两组无统计学意义.结论 3394( T→C)与老年T2DM患者的易感性有一定关联,5178(T→C)变异与湖北地区老年汉族人T2DM的脂代谢相关.     

Induced pluripotent stem cells generated from diabetic patients with mitochondrial DNA A3243G mutation

Aims/hypothesis

The aim of this study was to generate induced pluripotent stem (iPS) cells from patients with mitochondrial DNA (mtDNA) mutation.

Methods

Skin biopsies were obtained from two diabetic patients with mtDNA A3243G mutation. The fibroblasts thus obtained were infected with retroviruses encoding OCT4 (also known as POU5F1), SOX2, c-MYC (also known as MYC) and KLF4. The stem cell characteristics were investigated and the mtDNA mutation frequencies evaluated by Invader assay.

Results

From the two diabetic patients we isolated four and ten putative mitochondrial disease-specific iPS (Mt-iPS) clones, respectively. Mt-iPS cells were cytogenetically normal and positive for alkaline phosphatase activity, with the pluripotent stem cell markers being detectable by immunocytochemistry. The cytosine guanine dinucleotide islands in the promoter regions of OCT4 and NANOG were highly unmethylated, indicating epigenetic reprogramming to pluripotency. Mt-iPS clones were able to differentiate into derivatives of all three germ layers in vitro and in vivo. The Mt-iPS cells exhibited a bimodal degree of mutation heteroplasmy. The mutation frequencies decreased to an undetectable level in six of 14 clones, while the others showed several-fold increases in mutation frequencies (51–87%) compared with those in the original fibroblasts (18–24%). During serial cell culture passage and after differentiation, no recurrence of the mutation or no significant changes in the levels of heteroplasmy were seen.

Conclusions/interpretation

iPS cells were successfully generated from patients with the mtDNA A3243G mutation. Mutation-rich, stable Mt-iPS cells may be a suitable source of cells for human mitochondrial disease modelling in vitro. Mutation-free iPS cells could provide an unlimited, disease-free supply of cells for autologous transplantation therapy.     

Atypical clinical presentations of the A3243G mutation, usually associated with MELAS

Mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) is a clinical syndrome associated with mitochondrial abnormalities. In approximately 80% of patients, the syndrome is associated with the A3243G mutation. However, it has been realized that the A3243G mutation is not uncommon in the general population and is found in many patients with clinical presentations other than MELAS. We present four patients who presented with rhabdomyolysis, muscle fatigue, external ophthalmoplegia and myoclonic jerks respectively. These patients were all found to have the A3243G mutation on muscle biopsy. These patients illustrate the variety of presentations associated with A3243G mutation.     

线粒体糖尿病的分子发病机制

尽管已发现线粒体糖尿病有多个致病点突变,但tRNALLeu(UUR)3243A→G突变仍是目前国际上惟一公认的位点.该基因突变遵循严格的母系遗传且各组织细胞突变型比例存在异质性.该基因突变引起胰岛β细胞胰岛素分泌功能障碍,可能引起外周胰岛素抵抗.胰岛β细胞过早的功能衰退和凋亡,胰岛素合成减少以及ATP/ADP介导的胰岛分泌功能障碍可能是该位点突变致糖尿病发生、发展的主要原因.本文就该突变类型所致糖尿病的分子发病机制作一综述.