Vascular involvement in benign infantile mitochondrial myopathy caused by reversible cytochrome c oxidase deficiency

A novel homoplasmic mt-tRNAGlu m.14701C>T variant presenting with a partially reversible infantile respiratory chain deficiency

BACKGROUND

Reversible infantile respiratory chain deficiency (RIRCD) is a rare mitochondrial disorder associated with variable penetrance and partial to full remission of symptoms.

OBJECTIVE

To describe features of maternally related individuals with a novel variant associated with RIRCD.

MATERIALS AND METHODS

Nine maternally related individuals aged 23 months to 64 years are described through physical examinations, muscle biopsies, histochemical and biochemical analyses, genome sequencing, and cerebral imaging.

RESULTS

A homoplasmic mitochondrial transfer ribonucleic acid for glutamic acid (mt-tRNAGlu) m.14701C>T variant was identified in eight tested individuals out of nine maternally related individuals. Two individuals presented with hypotonia, muscle weakness, feeding difficulties and lactic acidosis at age 3-4 months, and improvement around age 15-23 months with mild residual symptoms at last examination. One individual with less severe symptoms had unknown age at onset and improved around age 4-5 years. Five individuals developed lipoma on the upper back, and one adult individual developed ataxia, while one was unaffected.

CONCLUSIONS

We have identified a novel homoplasmic mt-tRNAGlu m.14701C>T variant presenting with phenotypic and paraclinical features associated with RIRCD as well as ataxia and lipomas, which to our knowledge are new features associated to RIRCD.

Reversible infantile mitochondrial diseases

Mitochondrial diseases are usually severe and progressive conditions; however, there are rare forms that show remarkable spontaneous recoveries. Two homoplasmic mitochondrial tRNA mutations (m.14674T>C/G in mt-tRNA(Glu)) have been reported to cause severe infantile mitochondrial myopathy in the first months of life. If these patients survive the first year of life by extensive life-sustaining measures they usually recover and develop normally. Another mitochondrial disease due to deficiency of the 5-methylaminomethyl-2-thiouridylate methyltransferase (TRMU) causes severe liver failure in infancy, but similar to the reversible mitochondrial myopathy, within the first year of life these infants may also recover completely. Partial recovery has been noted in some other rare forms of mitochondrial disease due to deficiency of mitochondrial tRNA synthetases and mitochondrial tRNA modifying enzymes. Here we summarize the clinical presentation of these unique reversible mitochondrial diseases and discuss potential molecular mechanisms behind the reversibility. Understanding these mechanisms may provide the key to treatments of potential broader relevance in mitochondrial disease, where for the majority of the patients no effective treatment is currently available.

Endothelial ultrastructural alterations of intramuscular capillaries in infantile mitochondrial cytopathies: "mitochondrial angiopathy"

Electron microscopy (EM) is a reliable method for diagnosing mitochondrial diseases in striated muscle biopsy in infancy. Ultrastructural alterations in mitochondria of myofibers are well documented, but there are few studies of endothelial involvement in intramuscular capillaries. Quadriceps femoris biopsies of five representative infants and toddlers, ages neonate to 3.5 years, were performed because of clinical and laboratory data consistent with mitochondrial disease without mitochondrial DNA (mtDNA) mutations and likely with nuclear DNA mutations. Pathological studies included histochemistry, EM, respiratory chain enzymatic assay and mtDNA sequencing and deletion/duplication analysis. EM demonstrated frequent and severe alterations of mitochondria in capillary endothelium. The most constant changes included: either too few or fragmented cristae; stacked and whorled cristae; paracrystallin structures that often were large and spheroid with stress fractures; closely apposed membranes of granular endoplasmic reticulum surrounding mitochondria with loss of the normal intervening layer of cytoplasm; long narrow, thin looped microvilli extending into the lumen; and thick microvilli containing large, abnormal mitochondria. We conclude that mitochondrial cytopathies in early life exhibit more severe ultrastructural alterations in the endothelium than in myofibers and that paracrystallin body structure differs, perhaps due to less rigid surrounding structures. This distribution may explain the frequent lack of prominent histochemical and biochemical abnormalities in muscle biopsies of young patients. Endothelial changes do not distinguish the genetic defects. Vascular involvement in brain contributes to cerebral lesions and neuronal death by impairment of molecular and nutrient transport and ischemia; endothelium in muscle may reflect similar changes.

Reversible infantile respiratory chain deficiency is a unique, genetically heterogenous mitochondrial disease

OBJECTIVES

Homoplasmic maternally inherited, m.14674T>C or m. 14674T>G mt-tRNA(Glu) mutations have recently been identified in reversible infantile cytochrome c oxidase deficiency (or 'benign COX deficiency'). This study sought other genetic defects that may give rise to similar presentations.

PATIENTS

Eight patients from seven families with clinicopathological features of infantile reversible cytochrome c oxidase deficiency were investigated.

METHODS

The study reviewed the diagnostic features and performed molecular genetic analyses of mitochondrial DNA and nuclear encoded candidate genes.

RESULTS

Patients presented with subacute onset of profound hypotonia, feeding difficulties and lactic acidosis within the first months of life. Although recovery was remarkable, a mild myopathy persisted into adulthood. Histopathological findings in muscle included increased lipid and/or glycogen content, ragged-red and COX negative fibres. Biochemical studies suggested more generalised abnormalities than pure COX deficiency. Clinical improvement was reflected by normalisation of lactic acidosis and histopathological abnormalities. The m.14674T>C mt-tRNA(Glu) mutation was identified in four families, but none had the m. 14674T>G mutation. Furthermore, in two families pathogenic mutations were also found in the nuclear TRMU gene which has not previously been associated with this phenotype. In one family, the genetic aetiology still remains unknown.

CONCLUSIONS

Benign COX deficiency is better described as 'reversible infantile respiratory chain deficiency'. It is genetically heterogeneous, and patients not carrying the m.14674T>C or T>G mt-tRNA(Glu) mutations may have mutations in the TRMU gene. Diagnosing this disorder at the molecular level is a significant advance for paediatric neurologists and intensive care paediatricians, enabling them to select children with an excellent prognosis for continuing respiratory support from those with severe mitochondrial presentation in infancy.

Molecular basis of infantile reversible cytochrome c oxidase deficiency myopathy

Childhood-onset mitochondrial encephalomyopathies are usually severe, relentlessly progressive conditions that have a fatal outcome. However, a puzzling infantile disorder, long known as ‘benign cytochrome c oxidase deficiency myopathy’ is an exception because it shows spontaneous recovery if infants survive the first months of life. Current investigations cannot distinguish those with a good prognosis from those with terminal disease, making it very difficult to decide when to continue intensive supportive care. Here we define the principal molecular basis of the disorder by identifying a maternally inherited, homoplasmic m.14674T>C mt-tRNA^Glu mutation in 17 patients from 12 families. Our results provide functional evidence for the pathogenicity of the mutation and show that tissue-specific mechanisms downstream of tRNA^Glu may explain the spontaneous recovery. This study provides the rationale for a simple genetic test to identify infants with mitochondrial myopathy and good prognosis.

Reversible multiorgan system involvement in a neonate with complex IV deficiency

Mitochondrial respiratory chain deficiencies can present as fulminant liver failure or disease, and the prognosis when associated with severe neonatal lactic acidosis is frequently guarded. We report the case of a neonate who presented with acute liver failure and fulminant lactic acidosis with profound complex IV deficiency documented in muscle and liver biopsies. The neonate subsequently experienced clinical resolution by 3 months of age, and was observed to have reversibility of the biochemical deficiency noted in muscle. This case illustrates that resolution of this severe neonatal phenotype does occur, of importance for accurate prognostic and genetic counseling for such affected neonates.

Pathology of mitochondrial encephalomyopathies

Muscle biopsy provides the best tissue to confirm a mitochondrial cytopathy. Histochemical features often correlate with specific syndromes and facilitate the selection of biochemical and genetic studies. Ragged-red fibres nearly always indicate a combination defect of respiratory complexes I and IV. Increased punctate lipid within myofibers is a regular feature of Kearns-Sayre and PEO, but not of MELAS and MERRF. Total deficiency of succinate dehydrogenase indicates a severe defect in Complex II; total absence of cytochrome-c-oxidase activity in all myofibres correlates with a severe deficiency of Complex IV or of coenzyme-Q10. The selective loss of cytochrome-c-oxidase activity in scattered myofibers, particularly if accompanied by strong succinate dehydrogenase staining in these same fibres, is good evidence of mitochondrial cytopathy and often of a significant mtDNA mutation, though not specific for Complex IV disorders. Glycogen may be excessive in ragged-red zones. Ultrastructure provides morphological evidence of mitochondrial cytopathy, in axons and endothelial cells as well as myocytes. Abnormal axonal mitochondria may contribute to neurogenic atrophy of muscle, a secondary chronic feature. Quantitative determinations of respiratory chain enzyme complexes, with citrate synthase as an internal control, confirm the histochemical impressions or may be the only evidence of mitochondrial disease. Biological and technical artifacts may yield falsely low enzymatic activities. Genetic studies screen common point mutations in mtDNA. The brain exhibits characteristic histopathological alterations in mitochondrial diseases. Skin biopsy is useful for mitochondrial ultrastructure in smooth erector pili muscles and axons; skin fibroblasts may be grown in culture. Mitochondrial alterations occur in many nonmitochondrial diseases and also may be induced by drugs and toxins.

Mitochondrial recycling and aging of cardiac myocytes: the role of autophagocytosis

The mechanisms of mitochondrial alterations in aged post-mitotic cells, including formation of so-called 'giant' mitochondria, are poorly understood. To test whether these large mitochondria might appear due to imperfect autophagic mitochondrial turnover, we inhibited autophagocytosis in cultured neonatal rat cardiac myocytes with 3-methyladenine. This resulted in abnormal accumulation of mitochondria within myocytes, loss of contractility, and reduced survival time in culture. Unlike normal aging, which is associated with slow accumulation of predominantly large defective mitochondria, pharmacological inhibition of autophagy caused only moderate accumulation of large (senescent-like) mitochondria but dramatically enhanced the numbers of small mitochondria, probably reflecting their normally more rapid turnover. Furthermore, the 3-methyladenine-induced accumulation of large mitochondria was irreversible, while small mitochondria gradually decreased in number after withdrawal of the drug. We, therefore, tentatively conclude that large mitochondria selectively accumulate in aging post-mitotic cells because they are poorly autophagocytosed. Mitochondrial enlargement may result from impaired fission, a possibility supported by depressed DNA synthesis in large mitochondria. Nevertheless, enlarged mitochondria retained immunoreactivity for cytochrome c oxidase subunit 1, implying that mitochondrial genes remain active in defective mitochondria. Our findings suggest that imperfect autophagic recycling of these critical organelles may underlie the progressive mitochondrial damage, which characterizes aging post-mitotic cells.

Methylene blue restores spatial memory retention impaired by an inhibitor of cytochrome oxidase in rats

Cytochrome oxidase is the mitochondrial enzyme that catalyzes the utilization of oxygen for the electron transport chain during cellular respiration. Chronic subcutaneous infusion of sodium azide, an inhibitor of cytochrome oxidase, produced a spatial memory retention deficit in rats in a holeboard maze. Methylene blue, which has been shown to increase oxygen consumption in vitro, was used to restore mitochondrial electron transport in order to facilitate memory consolidation. Administration of 1 mg/kg methylene blue after training, during the memory consolidation period, completely restored the memory retention impaired by the inhibitor of cytochrome oxidase. This suggests that methylene blue may compensate for impaired mitochondrial respiration and improve spatial memory retention. Memory retention deficits found in some neurodegenerative diseases may be improved by drugs targeting impaired mitochondrial respiration.

Defects in mitochondrial respiratory complexes III and IV, and human pathologies

Here, relationships between alterations in tissue-specific content, protein structure, activity, and/or assembly of respiratory complexes III and IV induced by mutations in corresponding genes and various human pathologies are reviewed. Cytochrome bc(1) complex and cytochrome c oxidase (COX) deficiencies have been detected in a heterogeneous group of neuromuscular and non-neuromuscular diseases in childhood and adulthood, presenting a number of clinical phenotypes of variable severity. Such disorders can be caused by mutations located either in mitochondrial genes or in nuclear genes encoding structural subunits of the complexes or corresponding assembly factors/chaperones. Of the defects in mitochondrial DNA genes, mutations in cytochrome b subunit of complex III, and in structural subunits I-III of COX have been described to date. As to defects in nuclear DNA genes, mutations in genes encoding the complexes assembly factors such as the BCS1L protein for complex III; and SURF-1, SCO1, SCO2, and COX10 for complex IV have been identified so far.

Reversible fulminant lactic acidosis and liver failure in an infant with hepatic cytochrome-c oxidase deficiency

Cytochrome-c oxidase (COX) is the most common respiratory chain complex involved in liver failure, either as a single enzyme deficiency or as part of multiple enzyme deficiencies. We describe an infant who presented with fulminant lactic acidosis in the neonatal period. The lactic acidosis resolved spontaneously but liver and pancreatic insufficiency ensued. Isolated cytochrome-c oxidase deficiency was found in liver but not in muscle and fibroblasts. mtDNA rearrangements or depletion were ruled out. By the age of one year, liver and pancreatic functions have normalized completely and neurodevelopment is normal.

The mitochondrial-lysosomal axis theory of aging: accumulation of damaged mitochondria as a result of imperfect autophagocytosis

Cellular manifestations of aging are most pronounced in postmitotic cells, such as neurons and cardiac myocytes. Alterations of these cells, which are responsible for essential functions of brain and heart, are particularly important contributors to the overall aging process. Mitochondria and lysosomes of postmitotic cells suffer the most remarkable age-related alterations of all cellular organelles. Many mitochondria undergo enlargement and structural disorganization, while lysosomes, which are normally responsible for mitochondrial turnover, gradually accumulate an undegradable, polymeric, autofluorescent material called lipofuscin, or age pigment. We believe that these changes occur not only due to continuous oxidative stress (causing oxidation of mitochondrial constituents and autophagocytosed material), but also because of the inherent inability of cells to completely remove oxidatively damaged structures (biological 'garbage'). A possible factor limiting the effectiveness of mitochondial turnover is the enlargement of mitochondria which may reflect their impaired fission. Non-autophagocytosed mitochondria undergo further oxidative damage, resulting in decreasing energy production and increasing generation of reactive oxygen species. Damaged, enlarged and functionally disabled mitochondria gradually displace normal ones, which cannot replicate indefinitely because of limited cell volume. Although lipofuscin-loaded lysosomes continue to receive newly synthesized lysosomal enzymes, the pigment is undegradable. Therefore, advanced lipofuscin accumulation may greatly diminish lysosomal degradative capacity by preventing lysosomal enzymes from targeting to functional autophagosomes, further limiting mitochondrial recycling. This interrelated mitochondrial and lysosomal damage irreversibly leads to functional decay and death of postmitotic cells.