Recurrent myoglobinuria in a sporadic patient with a novel mitochondrial DNA tRNA(Ile) mutation

The differential diagnosis of myoglobinuria includes multiple etiologies, such as infection, inflammation, trauma, endocrinopathies, drugs toxicity, and primary metabolic disorders. Metabolic myopathies can be due to inherited disorders of glycogen metabolism or to defects of fatty acid oxidation. Primary respiratory chain dysfunction is a rare cause of myoglobinuria, but it has been described in sporadic cases with mutations in genes encoding cytochrome b or cytochrome c oxidase (COX) subunits and in four cases with tRNA mutations. We describe a 39-year-old woman with myalgia and exercise-related recurrent myoglobinuria, who harbored a novel mitochondrial DNA mutation at nucleotide 4281 (m.4281A>G) in the tRNA-isoleucine gene. Her muscle biopsy revealed ragged-red and COX-deficient fibers. No deletions or duplication were detected by Southern blot analysis. The m.4281A>G mutation was present in the patient's muscle with a mutation load of 46% and was detected in trace amounts in urine and cheek mucosa. Single-fiber analysis revealed significantly higher levels of the mutation in COX-deficient (65%) than in normal fibers (45%). This novel mutation has to be added to the molecular causes of recurrent myoglobinuria.     

Frequencies of myohistological mitochondrial changes in patients with mitochondrial DNA deletions and the common m.3243A>G point mutation

Frequencies of typical myohistological changes such as ragged red fibers (RRF) and cytochrome c oxidase (COX)‐deficient fibers have been suggested to be dependent on underlying mitochondrial DNA (mtDNA) defect. However, there are no systematic studies comparing frequencies of myohistological changes and underlying genotypes. The histopathological changes were analysed in 29 patients with genetically confirmed mitochondrial myopathies. Genotypes included multiple mtDNA deletions due to POLG1 mutations (n = 11), single mtDNA deletion (n = 10) and mtDNA point mutation m.3243A>G (n = 8). Histochemical reactions, including Gomori‐trichome, COX/SDH (succinate dehydrogenase) and SDH as well as immunohistological reaction with COX‐antibody against subunit I (COI) were carried out in muscle biopsy sections of all patients. The COX‐deficient fibers were observed most frequently in all three patient groups. The frequencies of myopathological changes were not significantly different in the different genotypes in all three histochemical stains. However, there was a tendency to lower means and variations in patients with point mutation. Only COI‐negative fibers were histochemically negative for COX activity in all patient groups. Frequency of COI‐negative fibers was significantly lower in patients with mtDNA point mutation than in patients with deletions. This suggests that impact of point mutation on protein synthesis is less than that of deletions.     

Distinct segregation of the pathogenic m.5667G>A mitochondrial tRNAAsn mutation in extraocular and skeletal muscle in chronic progressive external ophthalmoplegia

Chronic progressive external ophthalmoplegia (CPEO) is a frequent clinical manifestation of disorders caused by pathogenic mitochondrial DNA mutations. However, for diagnostic purposes skeletal muscle tissue is used, since extraocular muscle tissue is usually not available for work-up. In the present study we aimed to identify causative factors that are responsible for extraocular muscle to be primarily affected in CPEO. We performed comparative histochemical and molecular genetic analyses of extraocular muscle and skeletal muscle single fibers in a case of isolated CPEO caused by the heteroplasmic m.5667G>A mutation in the mitochondrial tRNA^Asn gene (MT-TN). Histochemical analyses revealed higher proportion of cytochrome c oxidase deficient fibers in extraocular muscle (41%) compared to skeletal muscle (10%). However, genetic analyses of single fibers revealed no significant difference either in the mutation loads between extraocular muscle and skeletal muscle cytochrome c oxidase deficient single fibers (extraocular muscle 86% ± 4.6%; skeletal muscle 87.8 %± 5.7%, p = 0.246) nor in the mutation threshold (extraocular muscle 74% ± 3%; skeletal muscle 74% ± 4%). We hypothesize that higher proportion of cytochrome c oxidase deficient fibers in extraocular muscle compared to skeletal muscle might be due to facilitated segregation of the m.5667G>A mutation into extraocular muscle, which may explain the preferential ocular manifestation and clinically isolated CPEO.     

A novel mitochondrial tRNAPhe mutation causes MERRF syndrome

A woman with typical features of myoclonic epilepsy with ragged red fibers (MERRF) had a novel heteroplasmic mutation (G611A) in the mitochondrial DNA tRNA phenylalanine gene. The mutation was heteroplasmic (91%) in muscle but undetectable in accessible tissues from the patient and her maternal relatives. Single-fiber PCR analysis showed that the proportion of mutant genomes was higher in cytochrome c oxidase (COX)-negative ragged red fibers (RRFs) than in COX-positive non-RRFs. This report shows that typical MERRF syndrome is not always associated with tRNA lysine mutations.     

Clinical and genetic features in a MELAS child with a 3271T>C mutation

A mitochondrial DNA 3271T>C point mutation was reported to be the second most common mutation (following the mutation 3243A>G) in mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) in Japan. This mutation has rarely been reported in other countries. We present an 11-year-old Taiwanese girl with MELAS, who harbored the 3271T>C mutation and had manifested short stature, epilepsia partialis continua, and recurrent basal ganglia infarctions since age 6 years, and rapid intellectual regression, dysarthria, and unsteady gait since age 10 years. The proportion of 3271T>C mutant genomes in various tissues, including urinary sediments, hair follicles, blood leukocytes, and buccal mucosa cells from the patient and her mother, was analyzed by polymerase chain reaction-restriction fragment length polymorphism analysis and quantitative real-time polymerase chain reaction. The proportion of mutant load in the patient's muscles was near 100%. Except for muscle, the highest mutation load was detected in urinary sediments of the patient by both methods. This is the first report involving mutant load analysis with quantitative real-time polymerase chain reaction in the 3271T>C mutation. The results suggest that urinary sediments may be an alternative tissue of choice which can be obtained noninvasively in the diagnosis of mitochondrial DNA 3271T>C mutations.     

A novel mitochondrial tRNA(Leu(UUR)) mutation in a patient with features of MERRF and Kearns-Sayre syndrome

In a patient with clinical features of both myoclonus epilepsy ragged-red fibers (MERRF) and Kearns-Sayre syndrome (KSS), we identified a novel guanine-to-adenine mitochondrial DNA (mtDNA) mutation at nucleotide 3255 (G3255A) of the tRNA(Leu(UUR)) gene. Approximately 5% of the skeletal muscle fibers had excessive mitochondria by succinate dehydrogenase histochemistry while a smaller proportion showed cytochrome c oxidase (COX) deficiency. In skeletal muscle, activities of mitochondrial respiratory chain complexes I, I + III, II + III, and IV were reduced. The G3255A transition was heteroplasmic in all tissues tested: muscle (53%), urine sediment (67%), peripheral leukocytes (22%), and cultured skin fibroblasts (< 2%). The mutation was absent in 50 control DNA samples. Single-fiber analysis revealed a higher proportion of mutation in COX-deficient RRF (94% +/- 5, n = 25) compared to COX-positive non-RRF (18% +/- 9, n = 21). The identification of yet another tRNA(Leu(UUR)) mutation reinforces the concept that this gene is a hot-spot for pathogenic mtDNA mutations.     

Maternally inherited hearing loss in a large kindred with a novel T7511C mutation in the mitochondrial DNA tRNA(Ser(UCN)) gene.

Thirty-six of 43 maternally related members of a large African American family experienced hearing loss. A muscle biopsy specimen from the proband showed cytochrome c oxidase (COX)-deficient fibers but no ragged-red fibers; biochemical analysis showed marked reduction of COX activity. A novel T7511C point mutation in the tRNA(Ser(UCN)) gene was present in almost homoplasmic levels (>95%) in the blood of 18 of 20 family members, and was also found in lower abundance in the other two. Single-fiber PCR showed that the mutational load was greater in COX-deficient muscle fibers. The tRNA(ser(UCN)) gene may be a "hot spot" for mutations associated with maternally transmitted hearing loss.     

Identification and characterization of the novel m.8305C>T MTTK and m.4440G>A MTTM gene mutations causing mitochondrial myopathies

We report on two novel mtDNA mutations in patients affected with mitochondrial myopathy. The first patient, a 44-year-old woman, had bilateral eyelid ptosis and the m.8305C>T mutation in the MTTK gene. The second patient, a 56-year-old man, had four-limb muscle weakness and the MTTM gene m.4440G>A mutation. Muscle biopsies in both patients showed ragged red fibers and numerous COX-negative fibers as well as a combined defect of complex I, III and IV activities. The two mutations were heteroplasmic and detected only in muscle tissue, with a higher mutation load in COX-negative fibers. Additionally, both mutations occurred in highly conserved mt-tRNA sites, and were not found by an in silico search in 30,589 human mtDNA sequences. Our report further expands the mutational and phenotypic spectrum of diseases associated with mutations in mitochondrial tRNA genes and reinforces the notion that mutations in mitochondrial tRNAs represent hot spots for mitochondrial myopathies in adults.     

A novel mutation in the mitochondrial tRNA(Phe) gene associated with mitochondrial myopathy

We report a novel heteroplasmic T-->C mutation at nt position 582 within the mitochondrial tRNA(Phe) gene of a 70-year-old woman with mitochondrial myopathy. No other family members were affected, suggesting that our patient was a sporadic case. The muscle showed frequent ragged red fibers and 43% cytochrome c oxidase deficient fibers. The mutation alters a conserved base pairing in the aminoacyl acceptor stem. The mutation load was 70% in muscle homogenate and varied from 0 to 95% in individual muscle fiber segments. Cytochrome c oxidase-negative fibers showed significantly higher levels of mutated mtDNA (>75%) than Cytochrome c oxidase-positive fibers (<55%). This mutation adds to the previously described four pathogenic mutations in the tRNA(Phe) gene.     

Identification of the novel mutation m.5658T>C in the mitochondrial tRNA(Asn) gene in a patient with myopathy,bilateral ptosis and ophthalmoparesis

We report a heteroplasmic novel mutation m.5658T>C in the mt-tRNA^Asn gene in a patient who initially presented myopathy, bilateral ptosis and ophthalmoparesis and several years later developed a non-nephrotic proteinuria. The muscle biopsy showed cytochrome c oxidase (COX) negative and ragged red fibers and in the kidney biopsy that was taken in order to identify the causes of non-nephrotic proteinuria, a focal segmental glomerulosclerosis was observed. Using laser capture microdissection we isolated COX negative fibers and COX positive fibers from the muscle of the patient and determined that there was a clear increase in the mutation load in the COX negative muscle fibers. However, the low degree of mutation load found in the renal biopsy of the patient does not allow us to conclude that the m.5658T>C mutation is responsible for focal glomerulosclerosis. Additionally, we hypothesize that the mutated m.5658T nucleotide might be structurally relevant, as it is one of the fifteen nucleotides conserved in all the species analyzed and is situated contiguously to the discriminator base in the 3′end of the mt-tRNA, where the tRNase Z cleaves the 3′ trailer sequence during mt-tRNA maturation.     

Cardiomyopathy is common in patients with the mitochondrial DNA m.3243A>G mutation and correlates with mutation load

Although neuromuscular clinical features often dominate the clinical presentation of mitochondrial disease due to the m.3243A>G mitochondrial DNA (mtDNA) mutation, many patients develop cardiac failure, which is often overlooked until it reaches an advanced stage. We set out to determine whether cardiac complications are sufficiently common to warrant prospective screening in all mutation carriers. Routine clinical echocardiography and 3 Tesla cardiac MRI were performed on ten m.3243A>G mutation carriers and compared to age and gender matched controls, with contemporaneous quadriceps muscle biopsies to measure respiratory chain activity and mtDNA mutation levels. Despite normal echocardiography, all ten m.3243A>G mutation carriers had evidence of abnormal cardiac function on MRI. The degree of cardiac dysfunction correlated with the percentage level of mutant mtDNA in skeletal muscle. Sub-clinical cardiac dysfunction was a universal finding in this study, adding weight to the importance of screening for cardiac complications in patients with m.3243A>G. The early detection of cardiac dysfunction with MRI opens up opportunities to prevent heart failure in these patients through early intervention.     

A novel mutation in the tRNAIle gene (MTTI) affecting the variable loop in a patient with chronic progressive external ophthalmoplegia (CPEO)

We describe a 62-year-old woman with chronic progressive external ophthalmoplegia (CPEO), multiple lipomas, diabetes mellitus, and a novel mitochondrial DNA (mtDNA) mutation at nucleotide 4302 (4302A>G) of the tRNA^Ile gene (MTTI). This is the first mutation at position 44 in the variable loop (V loop) of any mitochondrial tRNA.The muscle biopsy revealed 10% ragged-red/ragged-blue fibers and 25% cytochrome c oxidase (COX)-deficient fibers.No deletions or duplications were detected by Southern blot analysis. The 4302A>G transition was present only in the patient’s muscle and single-fiber analysis revealed significantly higher levels of the mutation in COX-deficient than in normal fibers. Like tRNA^Leu(UUR), tRNA^Ile appears to be a “hot spot” for mtDNA mutations causing CPEO.     

A new mutation in the mitochondrial tRNA(Ala) gene in a patient with ophthalmoplegia and dysphagia

We describe a new mutation in the tRNA(Ala) gene, a T-->C transition at nucleotide position 5628, in a 62-year-old woman with late onset chronic progressive external ophthalmoplegia, dysphagia and mild proximal myopathy. The mutation is heteroplasmic and disrupts a highly conserved A-U base pair within the anticodon stem of the tRNA(Ala). Cytochrome c oxidase-negative fibers harbor a significantly higher level of mutated mtDNA than cytochrome c oxidase-positive fibers. This is the first mutation in the tRNA(Ala) gene which satisfies accepted criteria for pathogenicity.     

线粒体DNA MT-TL1基因A3243G突变所致的临床表型系统综述和研究进展

线粒体DNA MT-TL1基因m.3243A>G(A3243G)突变是最常见的致病性线粒体基因突变,也是临床表现最为复杂的突变之一.临床表型的累及范围包括脑和神经、心脏、骨骼肌、内分泌、胃肠道、皮肤等多个器官和系统,严重程度从无症状到致命性的猝死综合征.而且最新研究发现携带m.3243A>G突变的孕妇在孕产期并发症方面...     

Novel chloride channel gene mutations in two unrelated Chinese families with myotonia congenita

Myotonia congenita (MC) is a genetic disease characterized by mutations in the muscle chloride channel gene (CLCN1). To date, approximately 130 different mutations on the CLCN1 gene have been identified. However, most of the studies have focused on Caucasians, and reports on CLCN1 mutations in Chinese population are rare. This study investigated the mutation of CLCN1 in two Chinese families with MC. Direct sequencing of the CLCN1 gene revealed a heterozygous mutation (892G>A, resulting in A298T) in one family and a compound heterozygous mutations (782A>G, resulting in Y261C; 1679T>C, resulting in M560T) in the other family, None of the 100 normal controls had these mutations. Our findings add more to the available information on the CLCN1 mutation spectrum, and provide a valuable reference for studying the mutation types and inheritance pattern of CLCN1 in the Chinese population.     

Two pathogenic mutations in the mitochondrial DNA tRNA Leu(UUR) gene (T3258C and A3280G) resulting in variable clinical phenotypes

We studied two patients with ragged-red fibers and combined defects of the mitochondrial respiratory chain in their muscle biopsy. One had mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes, and harbored a T3258C transition in the tRNA(Leu(UUR)) gene. The other showed myopathy plus cardiomyopathy and had an A3280G mutation in the same gene. Both mutations were heteroplasmic, abundant in muscle of the patients, less abundant in blood, and still less abundant in blood from their maternal relatives. In both patients, single muscle fiber analysis revealed greater abundance of mutant genomes in ragged-red fibers than in normal fibers, supporting the pathogenicity of both mutations.     

核黄素反应性脂质沉积性肌病伴感觉共济失调性神经病3例报告

目的研究核黄素反应性脂质沉积性肌病伴感觉共济失调性神经病的临床、电生理、病理和基因改变特点。方法 3例男性患者来自2个家系,其中2例为兄弟,发病年龄41～43岁,主要症状是四肢肌无力,伴随双足麻木和行走不稳。查体发现四肢近端肌力下降、末梢性感觉丧失和Romberg征阳性。3例患者的血尿代谢筛查均提示存在血多种脂酰肉碱水平升高和尿戊二酸水平增高。3例患者均进行了神经电生理、肌肉活检以及电子转移黄素蛋白脱氢酶(ETFDH)基因检查,2例进行腓肠神经活检。结果 3例患者的肌电图分别出现肌源性损害、可疑神经源性损害和无异常。3例患者的四肢感觉神经传导速度显著减慢或不能引出,运动神经传导速度仅在1例出现轻度减慢。3例患者的骨骼肌均可见肌纤维内脂肪滴显著增多,2例有个别破碎红纤维,2例出现小角状肌纤维。2例患者的腓肠神经均可见有髓神经纤维中-重度减少,伴随有髓神经纤维轴索变性和再生。3例患者均携带ETFDH基因的复合杂合突变,其中2兄弟为c.65A>G和c.242T>C,另1例为c.770A>G和c.1450 T>C。结论 ETFDH基因突变导致的核黄素反应性脂质沉积性肌病可以伴随感觉共济失调性神经病。     

Mitochondrial myopathy and rhabdomyolysis associated with a novel nonsense mutation in the gene encoding cytochrome c oxidase subunit I

Mitochondrial DNA (mtDNA) mutations associated with rhabdomyolysis are rare but have been described in sporadic cases with mutations in the cytochrome b and cytochrome c oxidase (COX) genes and in 3 cases with tRNALeu mutation. We report a novel heteroplasmic G6708A nonsense mutation in the mtDNA COI gene encoding COX subunit I in a 30-year-old woman with muscle weakness, pain, fatigue, and one episode of rhabdomyolysis. Histochemical examination of muscle biopsy specimens revealed reduced COX activity in the majority of the muscle fibers (approximately 90%) and frequent ragged red fibers. Biochemical analysis showed a marked and isolated COX deficiency. Analysis of DNA extracted from single fibers revealed higher levels of the mutation in COX-deficient fibers (> 95%) compared with COX-positive fibers (1%-80%). The mutation was not detected in a skin biopsy, cultured myoblasts, or blood leukocytes. Nor was it identified in blood leukocytes from the asymptomatic mother, indicating a de novo mutation that arose after germ layer differentiation. Western blot analysis and immunohistochemical staining revealed that reduced levels of COX subunit I were accompanied by reduced levels of other mtDNA encoded subunits, as well as nuclear DNA encoded subunit IV, supporting the concept that COX subunit I is essential for the assembly of complex IV in the respiratory chain.     

Frequency of polymorphisms in the B-domain of factor V gene in APC-resistant patients

In this study we investigated a group of patients in whom a resistance to APC (activated protein C) was found but no Leiden mutation existed in the presence of missense mutations in the first 1200 bp of the Exon 13 (B-domain) in the factor V (FV) gene. The determination of the APC response was performed using the Immunochrom(R) APC response Test Kit. The mutations were determined by temperature gradient gel electrophoresis and DNA sequencing. In the APC-resistant patients without the FV Leiden, we found 4 silent mutations (2298C>T, 2325T>C, 2379A>G, 2391A>G) and 4 missense mutations (2540A>C, 2663A>G, 2684A>G, 2863A>G), which code for the amino acids N789T (GenBank Accession # AF119360), K830R, H837R, and K897E. In all of the patients and controls, the polymorphisms at nucleotide positions 2391, 2663, 2684, and 2863 appeared to be associated. In the major allele all bases are A (A allele) and in the minor allele are G (G allele). A significantly lower G allele frequency was observable in the patient group than in the control group (0.14 vs. 0.31; p<0.05). The frequency of the 2540C allele, which is associated with the 2379G and the 4070G allele (non-Leiden!), did not differ significantly between the patient and the control groups. We suggest that the G allele, which is not associated with the FV Leiden mutation, as well as the [2379G; 2540C; 4070G] allele have no influence on the APC cofactor function itself, or only subtly as determined in the test systems used.