Increased risk for cardiorespiratory failure associated with the A3302G mutation in the mitochondrial DNA encoded tRNALeu(UUR) gene

Myoclonic epilepsy with ragged red fibers syndrome associated with mitochondrial 3302A>G mutation in the MT‑TL1 gene: A case report

A 37-year-old woman presented with proximal limb weakness, an unstable gait, tiredness and paroxysmal jitters. Neurological examination showed decreased deep tendon reflexes and positive signs indicating damage to the cerebellum. The patient's children reported no symptoms but were found to have the mitochondrial 3302A>G mutation in the mitochondrially encoded tRNA-Leu (UUA/G) 1 gene. The patient presented with increased blood lactic acid and lactic acid dehydrogenase levels, myopathy-related limb muscle electromyographic activities, ragged red fibers (RRFs), cytochrome oxidase-negative muscle fibers and mitochondrial 3302A>G mutation. Inverted lactic acid peaks in the basal ganglia, an atrophied cerebellum and multiple electroencephalographic spike waves were also observed. Therefore, myoclonic epilepsy with RRFs syndrome with the 3302A>G mutation was considered.

Aberrant RNA processing contributes to the pathogenesis of mitochondrial diseases in trans-mitochondrial mouse model carrying mitochondrial tRNALeu(UUR) with a pathogenic A2748G mutation

Mitochondrial tRNAs are indispensable for the intra-mitochondrial translation of genes related to respiratory subunits, and mutations in mitochondrial tRNA genes have been identified in various disease patients. However, the molecular mechanism underlying pathogenesis remains unclear due to the lack of animal models. Here, we established a mouse model, designated 'mito-mice tRNALeu(UUR)2748', that carries a pathogenic A2748G mutation in the tRNALeu(UUR) gene of mitochondrial DNA (mtDNA). The A2748G mutation is orthologous to the human A3302G mutation found in patients with mitochondrial diseases and diabetes. A2748G mtDNA was maternally inherited, equally distributed among tissues in individual mice, and its abundance did not change with age. At the molecular level, A2748G mutation is associated with aberrant processing of precursor mRNA containing tRNALeu(UUR) and mt-ND1, leading to a marked decrease in the steady-levels of ND1 protein and Complex I activity in tissues. Mito-mice tRNALeu(UUR)2748 with ≥50% A2748G mtDNA exhibited age-dependent metabolic defects including hyperglycemia, insulin insensitivity, and hepatic steatosis, resembling symptoms of patients carrying the A3302G mutation. This work demonstrates a valuable mouse model with an inheritable pathological A2748G mutation in mt-tRNALeu(UUR) that shows metabolic syndrome-like phenotypes at high heteroplasmy level. Furthermore, our findings provide molecular basis for understanding A3302G mutation-mediated mitochondrial disorders.

Mitochondrial Heteroplasmy

Genetic polymorphisms, in concert with well-characterized etiology and progression of major pathologies, plays a significant role in aberrant processes afflicting human populations. Mitochondrial heteroplasmy represents a dynamically determined co-expression of inherited polymorphisms and somatic pathology in varying ratios within individual mitochondrial DNA (mtDNA) genomes with repetitive patterns of tissue specificity. The ratios of the MtDNA genomes represent a balance between healthy and pathological cellular outcomes. Mechanistically, cardiomyopathies have profound alterations of normative mitochondrial function. Certain allele imbalances in the nuclear mitochondrial genome are associated with key energy mitochondrial proteins. Mitochondrial heteroplasmy may manifest itself at critical protein expression points, e.g., cytochrome c oxidase (COX). Pathological mtDNA mutations also are associated with the development of congestive heart failure. Interestingly, mitochondrial 'normal vs. abnormal' ratios of various heteroplasmic populations may occur in families. In the translational context of human health and disease, we discuss the need for determining critical foci to probe multiple biological roles of mitochondrial heteroplasmy in cardiomyopathy.

Defects in RNA metabolism in mitochondrial disease

The expression of mitochondrially-encoded genes requires the efficient processing of long precursor RNAs at the 5' and 3' ends of tRNAs, a process which, when disrupted, results in disease. Two such mutations reside within mt-tRNA^Leu(UUR); a m.3243A>G transition, which is the most common cause of MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes), and m.3302A>G which often causes mitochondrial myopathy (MM). We used parallel analysis of RNA ends (PARE) that captures the 5' terminal end of 5'-monophosphorylated mitochondrial RNAs to compare the effects of the m.3243A>G and m.3302A>G mutations on mitochondrial tRNA processing and downstream RNA metabolism. We confirmed previously identified RNA processing defects, identified common internal cleavage sites and new sites unique to the m.3243A>G mutants that do not correspond to transcript ends. These sites occur in regions of predicted RNA secondary structure, or are in close proximity to such regions, and may identify regions of importance to the processing of mtRNAs.

De novo mtDNA point mutations are common and have a low recurrence risk

Background

Severe, disease-causing germline mitochondrial (mt)DNA mutations are maternally inherited or arise de novo. Strategies to prevent transmission are generally available, but depend on recurrence risks, ranging from high/unpredictable for many familial mtDNA point mutations to very low for sporadic, large-scale single mtDNA deletions. Comprehensive data are lacking for de novo mtDNA point mutations, often leading to misconceptions and incorrect counselling regarding recurrence risk and reproductive options. We aim to study the relevance and recurrence risk of apparently de novo mtDNA point mutations.

Methods

Systematic study of prenatal diagnosis (PND) and recurrence of mtDNA point mutations in families with de novo cases, including new and published data. ‘De novo’ based on the absence of the mutation in multiple (postmitotic) maternal tissues is preferred, but mutations absent in maternal blood only were also included.

Results

In our series of 105 index patients (33 children and 72 adults) with (likely) pathogenic mtDNA point mutations, the de novo frequency was 24.6%, the majority being paediatric. PND was performed in subsequent pregnancies of mothers of four de novo cases. A fifth mother opted for preimplantation genetic diagnosis because of a coexisting Mendelian genetic disorder. The mtDNA mutation was absent in all four prenatal samples and all 11 oocytes/embryos tested. A literature survey revealed 137 de novo cases, but PND was only performed for 9 (including 1 unpublished) mothers. In one, recurrence occurred in two subsequent pregnancies, presumably due to germline mosaicism.

Conclusions

De novo mtDNA point mutations are a common cause of mtDNA disease. Recurrence risk is low. This is relevant for genetic counselling, particularly for reproductive options. PND can be offered for reassurance.

Association of nuclear and mitochondrial genes with audiological examinations in Iranian patients with nonaminoglycoside antibiotics-induced hearing loss

Mitochondrial DNA mutations play an important role in causing sensorineural hearing loss. The purpose of this study was to determine the association of the mitochondrial genes RNR1, MT-TL1, and ND1 as well as the nuclear genes GJB2 and GJB6 with audiological examinations in nonfamilial Iranians with cochlear implants, using polymerase chain reaction, DNA sequencing, and RNA secondary structure analysis. We found that there were no novel mutations in the mitochondrial gene 12S rRNA (MT-RNR1) in patients with and without GJB2 mutation (GJB2⁺ and GJB2⁻, respectively), but a total of six polymorphisms were found. No mutations were observed in tRNA^Leu(UUR) (MT-TL1). Furthermore, eight polymorphisms were found in the mitochondrial ND1 gene. Additionally, no mutations were observed in the nuclear GJB6 gene in patients in the GJB2⁻ and GJB2⁺ groups. The speech intelligibility rating and category of auditory perception tests were statistically assessed in patients in the GJB2⁻ and GJB2⁺ groups. The results indicated that there was a significant difference (P<0.05) between the categories of auditory perception score in the GJB2⁻ group compared to that in the GJB2⁺ group. Successful cochlear implantation was observed among individuals with GJB2 mutations (GJB2⁺) and mitochondrial polymorphisms compared to those without GJB2 mutations (GJB2⁻). In conclusion, the outcome of this study suggests that variation in the mitochondrial and nuclear genes may influence the penetrance of deafness. Therefore, further genetic and functional studies are required to help patients in making the best choice for cochlear implants.

The Mitochondrial tRNALeu(UUR) A3302G Mutation may be Associated With Insulin Resistance in Woman With Polycystic Ovary Syndrome

The aim of this study was to investigate the role of mitochondrial DNA (mtDNA) mutations in polycystic ovary syndrome (PCOS) with insulin resistance (IR), and to explore the possible maternally effects on PCOS. We performed clinical, genetic, and molecular characterization of a Han Chinese family with maternally inherited IR, and we further investigated the possible relationship between mitochondrial genetic background, copy number, and IR. Most strikingly, members from the first and second generation of this family exhibited the type 2 diabetes mellitus (T2DM) with IR, while the member in the third generation of this family manifested the PCOS. Sequence analysis of the complete mitochondrial genome showed the presence of a homoplasmic A3302G in the acceptor arm of transfer RNA(Leu(UUR)) (tRNA(Leu(UUR))) gene. This mutation disrupted the highly conserved base pairing (2T-71A) and resulted a failure in mt-tRNA metabolism. Analysis of the mitochondrial copy number showed that the patients with PCOS and IR had lower copy number than the health controls, suggesting that mitochondrial dysfunction may be involved in the pathogenesis of IR. Taken together, the A3302G mutation was a pathogenic mutation associated with IR in this Chinese family.

MELAS phenotype associated with m.3302A>G mutation in mitochondrial tRNA(Leu(UUR)) gene

The m.3302A>G mutation in the mitochondrial tRNA(Leu(UUR)) gene has been identified in only 12 patients from 6 families, all manifesting adult-onset slowly progressive myopathy with minor central nervous system involvement. An 11-year-old boy presented with progressive proximal-dominant muscle weakness from age 7years. At age 10, he developed recurrent stroke-like episodes. Mitochondrial myopathy, encephalopathy, lactic acidosis, plus stroke-like episodes (MELAS) was diagnosed by clinical symptoms and muscle biopsy findings. Mitochondrial gene analysis revealed a heteroplasmic m.3302A>G mutation. Histological examination showed strongly SDH reactive blood vessels (SSVs), not present in previous cases with myopathies due to the m.3302A>G mutation. These findings broaden the phenotypic spectrum of this mutation.

Mitochondrial tRNA(Leu(UUR)) mutation m.3302A > G presenting as childhood-onset severe myopathy: threshold determination through segregation study

Mitochondrial tRNA(Leu(UUR)) mutation m.3302A > G is associated with respiratory chain complex I deficiency and has been described as a rare cause of mostly adult-onset slowly progressive myopathy. Five families with 11 patients have been described so far; 5 of them died young due to cardiorespiratory failure. Here, we report on a segregation study in a family with an index patient who already presented at the age of 18 months with proximal muscular hypotonia, abnormal fatigability, and lactic acidosis. This early-onset myopathy was rapidly progressive. At 8 years, the patient is wheel-chair bound, requires nocturnal assisted ventilation, and suffers from recurrent respiratory infections. Severe complex I deficiency and nearly homoplasmy for m.3302A > G were found in muscle. We collected blood, hair, buccal swabs and muscle biopsies from asymptomatic adults in this pedigree and determined heteroplasmy levels in these tissues as well as OXPHOS activities in muscle. All participating asymptomatic adults had normal OXPHOS activities. In contrast to earlier reports, we found surprisingly little variation of heteroplasmy levels in different tissues of the same individual. Up to 45% mutation load in muscle and up to 38% mutation load in other tissues were found in non-affected adults. The phenotypic spectrum of tRNA(Leu(UUR)) m.3302A > G mutation seems to be wider than previously described. A threshold of more than 45% heteroplasmy in muscle seems to be necessary to alter complex I activity leading to clinical manifestation. The presented data may be helpful for prognostic considerations and counseling in affected families.

Human mitochondrial transfer RNAs: role of pathogenic mutation in disease

The human mitochondrial genome encodes 13 proteins. All are subunits of the respiratory chain complexes involved in energy metabolism. These proteins are translated by a set of 22 mitochondrial transfer RNAs (tRNAs) that are required for codon reading. Human mitochondrial tRNA genes are hotspots for pathogenic mutations and have attracted interest over the last two decades with the rapid discovery of point mutations associated with a vast array of neuromuscular disorders and diverse clinical phenotypes. In this review, we use a scoring system to determine the pathogenicity of the mutations and summarize the current knowledge of structure-function relationships of these mutant tRNAs. We also provide readers with an overview of a large variety of mechanisms by which mutations may affect the mitochondrial translation machinery and cause disease.

Molecular dysfunction associated with the human mitochondrial 3302A>G mutation in the MTTL1 (mt-tRNALeu(UUR)) gene

The gene encoding mt-tRNA^Leu(UUR), MT-TL1, is a hotspot for pathogenic mtDNA mutations. Amongst the first to be described was the 3302A>G transition which resulted in a substantial accumulation in patient muscle of RNA19, an unprocessed RNA intermediate including mt-16S rRNA, mt-tRNA^Leu(UUR) and MTND1. We have now been able to further assess the molecular aetiology associated with 3302A>G in transmitochondrial cybrids. Increased steady-state levels of RNA19 was confirmed, although not to the levels previously reported in muscle. This data was consistent with an increase in RNA19 stability. The mutation resulted in decreased mt-tRNA^Leu(UUR) levels, but its stability was unchanged, consistent with a defect in RNA19 processing responsible for low tRNA levels. A partial defect in aminoacylation was also identified, potentially caused by an alteration in tRNA structure. These deficiencies lead to a severe defect in respiration in the transmitochondrial cybrids, consistent with the profound mitochondrial disorder originally associated with this mutation.

Mitochondrial DNA mutations in renal cell carcinomas revealed no general impact on energy metabolism

Previously, renal cell carcinoma tissues were reported to display a marked reduction of components of the respiratory chain. To elucidate a possible relationship between tumourigenesis and alterations of oxidative phosphorylation, we screened for mutations of the mitochondrial DNA (mtDNA) in renal carcinoma tissues and patient-matched normal kidney cortex. Seven of the 15 samples investigated revealed at least one somatic heteroplasmic mutation as determined by denaturating HPLC analysis (DHPLC). No homoplasmic somatic mutations were observed. Actually, half of the mutations presented a level of heteroplasmy below 25%, which could be easily overlooked by automated sequence analysis. The somatic mutations included four known D-loop mutations, four so far unreported mutations in ribosomal genes, one synonymous change in the ND4 gene and four nonsynonymous base changes in the ND2, COI, ND5 and ND4L genes. One renal cell carcinoma tissue showed a somatic A3243G mutation, which is a known frequent cause of MELAS syndrome (mitochondrial encephalomyopathy, lactic acidosis, stroke-like episode) and specific compensatory alterations of enzyme activities of the respiratory chain in the tumour tissue. No difference between histopathology and clinical progression compared to the other tumour tissues was observed. In conclusion, the low abundance as well as the frequently observed low level of heteroplasmy of somatic mtDNA mutations indicates that the decreased aerobic energy capacity in tumour tissue seems to be mediated by a general nuclear regulated mechanism.

Rapid screening of the entire mitochondrial DNA for low-level heteroplasmic mutations

Alterations of the mitochondrial DNA (mtDNA) are implicated in various pathological conditions. In this study, we used denaturing high performance liquid chromatography (DHPLC) as a method to rapidly screen the entire mtDNA for mutations. Overlapping DNA fragments, amplified by one single cycling protocol from frozen pre-formulated PCR mixes, were subjected to DHPLC analysis. Single DHPLC injections of fragments yielded straightforward interpretation of results with a detection limit down to 1% mtDNA heteroplasmy. Furthermore, collection and re-amplification of low degree heteroduplex peak-fractions allowed sequence analysis of mtDNA mutations down to the detection limit of the DHPLC method. In order to demonstrate that the method has diagnostic value, we analyzed and confirmed known mtDNA mutations in patient samples.