A point mutation of mitochondrial ATPase 6 gene in Leigh syndrome

Variability of Clinical Phenotypes Caused by Isolated Defects of Mitochondrial ATP Synthase

Disorders of ATP synthase, the key enzyme in mitochondrial energy supply, belong to the most severe metabolic diseases, manifesting as early-onset mitochondrial encephalo-cardiomyopathies. Since ATP synthase subunits are encoded by both mitochondrial and nuclear DNA, pathogenic variants can be found in either genome. In addition, the biogenesis of ATP synthase requires several assembly factors, some of which are also hotspots for pathogenic variants. While variants of MT-ATP6 and TMEM70 represent the most common cases of mitochondrial and nuclear DNA mutations respectively, the advent of next-generation sequencing has revealed new pathogenic variants in a number of structural genes and TMEM70, sometimes with truly peculiar genetics. Here we present a systematic review of the reported cases and discuss biochemical mechanisms, through which they are affecting ATP synthase. We explore how the knowledge of pathophysiology can improve our understanding of enzyme biogenesis and function. Keywords: Mitochondrial diseases o ATP synthase o Nuclear DNA o Mitochondrial DNA o TMEM70.

Reproductive options in mitochondrial disease

Mitochondrial diseases require customized approaches for reproductive counseling, addressing differences in recurrence risks and reproductive options. The majority of mitochondrial diseases is caused by mutations in nuclear genes and segregate in a Mendelian way. Prenatal diagnosis (PND) or preimplantation genetic testing (PGT) are available to prevent the birth of another severely affected child. In at least 15%-25% of cases, mitochondrial diseases are caused by mitochondrial DNA (mtDNA) mutations, which can occur de novo (25%) or be maternally inherited. For de novo mtDNA mutations, the recurrence risk is low and PND can be offered for reassurance. For maternally inherited, heteroplasmic mtDNA mutations, the recurrence risk is often unpredictable, due to the mitochondrial bottleneck. PND for mtDNA mutations is technically possible, but often not applicable given limitations in predicting the phenotype. Another option for preventing the transmission of mtDNA diseases is PGT. Embryos with mutant load below the expression threshold are being transferred. Oocyte donation is another safe option to prevent the transmission of mtDNA disease to a future child for couples who reject PGT. Recently, mitochondrial replacement therapy (MRT) became available for clinical application as an alternative to prevent the transmission of heteroplasmic and homoplasmic mtDNA mutations.

Mitochondrial diseases: expanding the diagnosis in the era of genetic testing

Mitochondrial diseases are clinically and genetically heterogeneous. These diseases were initially described a little over three decades ago. Limited diagnostic tools created disease descriptions based on clinical, biochemical analytes, neuroimaging, and muscle biopsy findings. This diagnostic mechanism continued to evolve detection of inherited oxidative phosphorylation disorders and expanded discovery of mitochondrial physiology over the next two decades. Limited genetic testing hampered the definitive diagnostic identification and breadth of diseases. Over the last decade, the development and incorporation of massive parallel sequencing has identified approximately 300 genes involved in mitochondrial disease. Gene testing has enlarged our understanding of how genetic defects lead to cellular dysfunction and disease. These findings have expanded the understanding of how mechanisms of mitochondrial physiology can induce dysfunction and disease, but the complete collection of disease-causing gene variants remains incomplete. This article reviews the developments in disease gene discovery and the incorporation of gene findings with mitochondrial physiology. This understanding is critical to the development of targeted therapies.

A case report on a novel MT-ATP6 gene variation in atypical mitochondrial Leigh syndrome associated with bilateral basal ganglia calcifications

Leigh Syndrome (LS) is a rare, hereditary progressive neurodegenerative disorder of infancy or early childhood associated with a highly variable clinical presentation even among siblings. Further, genetic heterogeneity makes its diagnosis complicated. Its causative genetic variations are notified in some of the mitochondrial and nuclear genes. Here, we report an atypical case of LS in a 9-year-old boy associated with a novel variation in MT-ATP6 gene. The atypical findings were Bilateral Basal Ganglia Calcification (BGC) and late survival age in the patient. Analyses of the Whole Mitochondrial Genome Sequencing (WMGS) results of the recruited patient and his mother at different read coverage, first at 100× and later repeated at 500×, revealed a novel disease-associated variation in the already known disease-associated MT-ATP6 gene. In conclusion, the present study indicates amalgamation of both neuro-imaging and Next Generation Sequencing (NGS) Technologies aiding the proper diagnosis of LS in atypical cases.

Screening of common point-mutations and discovery of new T14727C change in mitochondrial genome of Vietnamese encephalomyopathy patients

Vietnamese patients (106) tentatively diagnosed with encephalomyopathy were screened for the presence of 15 common point mutations in mitochondria using PCR-RFLP. The screened mutations include A3243G, T3271C and T3291C for Mitochondrial Encephalopathy, Lactic Acidosis and Stroke-like episodes (MELAS); A8344G and T8356C for Myoclonus Epilepsy and Rag-Red Fibers (MERRF); G11778A, G3460A and T14484C for Leber's Hereditary Optic Neuropathy (LHON); T8993G/C and T9176G for Leigh syndrome; A1555G for deafness syndrome; G4298A, T10010C, T14728C and T14709C for neuromuscular syndrome. As a result, 6 cases of A3243G (5.7%) and 2 cases of T14727C (3.9%) were found. The 6 cases of A3243G mutation were heteroplasmic at different levels (4.23-80.85%). The T14727C change was discovered for the first time in the MTTE gene encoding for tRNA(Glu) and showed homoplasmy. The T14727C change was probably a mutation because it was further confirmed as vertically inherited from the mother and not the result of isolated polymorphism.

Different laboratory and muscle biopsy findings in a family with an m.8851T>C mutation in the mitochondrial MTATP6 gene

We report the second known family with a very rare, maternally inherited missense m.8851T>C mutation in the mitochondrial MTATP6 gene. A failure to thrive, microcephaly, psychomotor retardation and hypotonia were present in a 3-year-old girl with a high mtDNA mutation load (87-97%). Ataxia and Leigh syndrome were subsequently documented in a neurological examination and brain MRI. A muscle biopsy demonstrated decreased ATP synthase and an accumulation of succinate dehydrogenase products, indicating mitochondrial myopathy. Her 36-year-old mother (68% blood heteroplasmy) developed peripheral neuropathy and muscle weakness at the age of 22 years. Our findings extend the clinical and laboratory phenotype associated with the m.8851T>C mutation.

Mitochondrial DNA haplogroup Y is associated to Leigh syndrome in Chinese population

Although Leigh syndrome (LS) is a well characterized clinical mitochondrial disorder; the exact mutation is not found in all cases and it is not clear whether matrilineal background has contributed to this disease. To address this issue, we extensively studied and compared the haplogroup composition of a sample of 171 Chinese LS patients with that of 1597 controls. Our results show that haplogroup Y may increase the risk of LS in Chinese by 2.867 fold (95% CI=1.135-7.240, P=0.020). Haplogroup B5 has also this trend (1.737 fold, 95% CI=0.961-3.139), but with a borderline P-value (P=0.065). Both haplogroups belong to macro-haplogroup N and share a common reverse mutation on nucleotide position 10398 (A10398G). In fact, the combined haplogroup N with 10398G is also associated with an increased risk for LS (OR=1.882, 95% CI=1.134-3.124, P=0.013).

Leigh syndrome: clinical and neuroimaging follow-up

Leigh syndrome, caused by dysfunction in mitochondrial energy metabolism, is an inherited, heterogeneous, and progressive neurodegenerative disorder of infancy and childhood. From 1983 to August 2006, 14 cases diagnosed with Leigh syndrome were studied in terms of characteristic neuroimaging findings and abnormal mitochondrial configurations under electron microscopy, as well as molecular analysis. Of the 14 cases, 11 presented clinical features before age 1 (79%). All 14 presented with variable symptoms of central nervous system involvement. The three most common symptoms were developmental delay (12/14; 86%), seizures (11/14; 79%), and altered consciousness (8/14; 57%). Extra-central nervous system manifestations were observed in 10 of the 14 cases, the most common symptoms being failure to thrive (5/14; 36%), pericardial effusion and dilated cardiomyopathy (3/14; 21%), and liver function impairment (3/14; 21%). In all 14 cases, neuroimaging revealed abnormal findings over the basal ganglion, brainstem, or both. The putamen was the most common lesion site in the basal ganglia (11/12; 92%). Cranial magnetic resonance imaging was used for follow-up in 6 cases because of changes in clinical features; in all 6 cases the imaging revealed evolution in the brain. Cranial magnetic resonance spectroscopy was performed in 3 cases and in 2 of them revealed lactate peaks during deterioration of the disease course. The prognosis for Leigh syndrome was poor during long-term follow-up. Seven cases were early fatalities, before 1 year and 6 months of age. Follow-up cranial magnetic resonance imaging together with magnetic resonance spectroscopy in cases with clinical evolution is helpful for monitoring this disease.

Mitochondrial ATP synthase disorders: molecular mechanisms and the quest for curative therapeutic approaches

In mammals, the majority of cellular ATP is produced by the mitochondrial F1F(O)-ATP synthase through an elaborate catalytic mechanism. While most subunits of this enzymatic complex are encoded by the nuclear genome, a few essential components are encoded in the mitochondrial genome. The biogenesis of this multi-subunit enzyme is a sophisticated multi-step process that is regulated on levels of transcription, translation and assembly. Defects that result in diminished abundance or functional impairment of the F1F(O)-ATP synthase can cause a variety of severe neuromuscular disorders. Underlying mutations have been identified in both the nuclear and the mitochondrial DNA. The pathogenic mechanisms are only partially understood. Currently, the therapeutic options are extremely limited. Alternative methods of treatment have however been proposed, but still encounter several technical difficulties. The application of novel scientific approaches promises to deepen our understanding of the molecular mechanisms of the ATP synthase, unravel novel therapeutic pathways and improve the unfortunate situation of the patients suffering from such diseases.

Transmission and prenatal diagnosis of the T9176C mitochondrial DNA mutation

A family presented with three affected children with Leigh syndrome, a progressive neurodegenerative disorder. Analysis of the OXPHOS complexes in muscle of two affected patients showed an increase in activity of pyruvate dehydrogenase and a decrease of complex V activity. Mutation analysis revealed the T9176C mutation in the mtATPase 6 gene (OMIM 516060) and the mutation load was above 90% in the patients. Unaffected maternal relatives were tested for carrier-ship and one of them, with a mutation load of 55% in blood, was pregnant with her first child. The possibility of prenatal diagnosis was evaluated. The main problem was the lack of data on genotype-phenotype associations for the T9176C mutation and on variation of the mutation percentage in tissues and in time. Therefore, multiple tissues of affected and unaffected carriers were analysed. Eventually, prenatal diagnosis was offered with understanding by the couple that there could be considerable uncertainty in the interpretation of the results. Prenatal diagnosis was carried out twice on cultured and uncultured chorion villi and amniotic fluid cells. The result was a mutation percentage just below the assumed threshold of expression (90%). The couple decided to continue the pregnancy and an apparently healthy child was born with an as yet unclear prognosis. This is the first prenatal diagnosis for a carrier of the T9176C mutation. Prenatal diagnosis for this mutation is technically reliable, but the prognostic predictions are not straightforward.

Sequence polymorphisms within the human mitochondrial genes MTATP6, MTATP8 and MTND4

By sequencing the control region of mitochondrial DNA, the majority of human DNA samples can be differentiated. A further increase in differentiation probability may be possible, e.g. by extending the sequenced region to coding regions of the mitochondrial genome. Restriction to those positions that do not result in a change of the amino acids guarantees that the information thus obtained does not refer to phenotypically relevant information. In the present study the sequence data of the mitochondrial genes MTATP6, MTATP8 and MTND4 were collected from 109 subjects and analyzed in order to define variable positions suitable for identification purposes. There were 32 variable base positions among 850 bases studied from MTATPase genes and 1,200 bases of the MTND4 gene showed 28 variable positions. "Hot spots" for base exchanges were found in both regions and one position (position 11,719 in the MTND4 gene) seems to be suitable for SNP investigation for forensic purposes.