Mitochondrial DNA depletion in children

The first girl of an unrelated couple was noted to have failure to thrive since age 3 months, generalized hypotonia and weakness, hepatomegaly, hypoglycemia, and lactic acidosis at 4 months. She was found to have severe mitochondrial DNA (mtDNA) depletion and respiratory chain complex IV deficiency in both skeletal muscle and liver but without other common mtDNA mutations. Her younger brother developed vomiting at age 3 weeks and was diagnosed as having pyloric stenosis. His skeletal muscle and liver also showed severe mtDNA depletion. He developed generalized weakness and hypotonia, hepatomegaly, and lactic acidosis at age 3 months. Both siblings died of hepatic failure and hemorrhagic complication before 6 months of age. The brother also had chemical pancreatitis, which had not been reported before in mtDNA depletion in children. Severe mtDNA depletion may present with nonspecific symptoms such as vomiting, failure to thrive, and developmental delay; multiorgan involvement such as hepatomegaly, pancreatitis, and myopathy occurs later. Mitochondrial DNA depletion should be considered in the differential diagnosis in children with developmental delay or failure to thrive of unknown etiology.     

A novel homozygous RRM2B missense mutation in association with severe mtDNA depletion

This report describes two brothers, both deceased in infancy, with severe depletion of mitochondrial DNA (mtDNA) in muscle tissue. Both had feeding difficulties, failure to thrive, severe muscular hypotonia and lactic acidosis. One of the boys developed a renal proximal tubulopathy. A novel homozygous c.686 G→T missense mutation in the RRM2B gene, encoding the p53-inducible ribonucleotide reductase subunit (p53R2), was identified. This is the third report on mutations in RRM2B associated with severe mtDNA depletion, which further highlights the importance of de novo synthesis of deoxyribonucleotides (dNTPs) for mtDNA maintenance.     

Defects of the mitochondrial respiratory chain complexes in three pediatric cases with hypotonia and cardiac involvement.

Three children displaying hypotonia, cardiac involvement and defects of the mitochondrial respiratory chain complexes are reported. The first case showed severe neonatal hypotonia, failure to thrive, hepatomegaly, dilation of the right cardiac cavities, profound lactic acidosis and amino aciduria. The boy died at the age of 7 weeks. In the second case hypotonia, severe cardiomyopathy, cyclic neutropenia, lactic acidosis and 3-methylglutaconic aciduria occurred. The boy died at the age of 27 months. The third case presented at the age of 16 months as an acute hypokinetic hypertrophic cardiomyopathy with transient hypotonia and mild lactic acidosis. Spontaneous clinical remission occurred. In all cases muscle biopsy was performed. Morphological studies failed to show ragged-red fibers but there was lipid storage myopathy and decreased cytochrome c oxidase activity. Biochemical studies confirmed the cytochrome c oxidase deficiency in muscle in all cases. It was associated with complex I III deficiency in case 1 and with severe deficits of all respiratory chain complexes in case 2. Post-mortem studies in case 1 indicated that complex IV was reduced in the liver but not in the heart and quantitative analysis of mtDNA revealed a depletion in muscle. Cases 1 and 2 shared some clinical features with fatal infantile myopathy associated with cytochrome c oxidase deficiency, while case 3 displayed a very unusual clinical presentation. The histochemical enzyme reaction of cytochrome c oxidase is useful for the diagnosis of mitochondrial myopathy because ragged-red fibers may be lacking. Finally, biochemical measurement of the different mitochondrial respiratory chain complexes is required because multiple defects are frequent and occasionally related to mtDNA depletion.     

Mitochondrial DNA Depletion Syndromes: Review and Updates of Genetic Basis, Manifestations, and Therapeutic Options

Mitochondrial DNA (mtDNA) depletion syndromes (MDS) are a genetically and clinically heterogeneous group of autosomal recessive disorders that are characterized by a severe reduction in mtDNA content leading to impaired energy production in affected tissues and organs. MDS are due to defects in mtDNA maintenance caused by mutations in nuclear genes that function in either mitochondrial nucleotide synthesis (TK2, SUCLA2, SUCLG1, RRM2B, DGUOK, and TYMP) or mtDNA replication (POLG and C10orf2). MDS are phenotypically heterogeneous and usually classified as myopathic, encephalomyopathic, hepatocerebral or neurogastrointestinal. Myopathic MDS, caused by mutations in TK2, usually present before the age of 2 years with hypotonia and muscle weakness. Encephalomyopathic MDS, caused by mutations in SUCLA2, SUCLG1, or RRM2B, typically present during infancy with hypotonia and pronounced neurological features. Hepatocerebral MDS, caused by mutations in DGUOK, MPV17, POLG, or C10orf2, commonly have an early-onset liver dysfunction and neurological involvement. Finally, TYMP mutations have been associated with mitochondrial neurogastrointestinal encephalopathy (MNGIE) disease that typically presents before the age of 20 years with progressive gastrointestinal dysmotility and peripheral neuropathy. Overall, MDS are severe disorders with poor prognosis in the majority of affected individuals. No efficacious therapy is available for any of these disorders. Affected individuals should have a comprehensive evaluation to assess the degree of involvement of different systems. Treatment is directed mainly toward providing symptomatic management. Nutritional modulation and cofactor supplementation may be beneficial. Liver transplantation remains controversial. Finally, stem cell transplantation in MNGIE disease shows promising results.     

Mitochondrial DNA 8993T>G mutation in a child with ornithine transcarbamylase deficiency and leigh syndrome: an unexpected association

MC, female, is the third child of a nonconsanguineous Portuguese couple, born after an uneventful pregnancy and delivery. A positive family history of ornithine transcarbamylase deficiency, associated with the IVS8+1 G>A mutation in the ornithine transcarbamylase gene, prompted prenatal diagnosis with identification of the same mutation in the proband. During an episode of Klebsiella pneumoniae sepsis at 1.5 months of age, lactic acidosis and moderate hyperammonemia were noticed. After a short asymptomatic period, progressive neurologic symptoms, with normal ammonemia, persistent hyperlactacidemia, and typical lesions in brain computed tomography (CT) scan led to a diagnosis of Leigh syndrome. Mitochondrial respiratory chain complex V was reduced in the liver. The mtDNA 8993T>G mutation was identified in the liver, muscle, and blood (82%-87% heteroplasmy). She died at 6 months of age. This case represents a benign phenotype of ornithine transcarbamylase deficiency, associated with a severe mitochondrial respiratory chain disorder due to an mtDNA pathogenic mutation.     

Infantile presentation of the mtDNA A3243G tRNA(Leu (UUR)) mutation

Mitochondrial DNA (mtDNA) disorders are clinically very heterogeneous, ranging from single organ involvement to severe multisystem disease. One of the most frequently observed mtDNA mutations is the A-to-G transition at position 3243 of the tRNA(Leu (UUR)) gene. This mutation is often related to MELAS syndrome. However, not all patients with the A3243G mutation share the same clinical disease expression and, on the contrary, patients clinically exhibiting MELAS syndrome may have other mtDNA mutations. Here we describe two patients with a very early infantile presentation of disease associated with the A3243G mutation. Patient 1 presented with hypotonia, feeding difficulties and failure to thrive (FTT) at the age of 3 months. Laboratory investigations showed persistent hyperlactic acidemia, elevated lactate/pyruvate ratios and elevated alanine concentrations in blood. Developmental delay was progressive and he developed cardiomyopathy and seizures. Death occurred at the age of 3.5 years. Patient 2 was born prematurely and had persistent, severe lactic acidosis from birth on. Moderate biventricular hypertrophy was seen on ultrasound studies of the heart and, suffering from progressive lactic acidosis, he died at the age of 13 days. Because of the rarity of this very early presentation, we searched the literature for other infantile cases associated with the A3243G mutation and found 8 additional ones. In infants presenting with lactic acidosis/hyperlactic acidemia, failure to thrive, hypotonia, seizures and/or cardiomyopathy, mtDNA mutational analysis, also for the disease entities, usually only observed in juveniles or adults is warranted.     

Mitochondrial myopathy of childhood associated with mitochondrial DNA depletion and a homozygous mutation (T77M) in the TK2 gene

BACKGROUND: The mitochondrial DNA depletion syndrome is an autosomal recessive disorder of infancy or childhood characterized by decreased mitochondrial DNA copy number in affected tissues. Mutations in 2 genes involved in deoxyribonucleotide metabolism, the deoxyguanosine kinase gene (DGK) and the thymidine kinase 2 gene (TK2), have been related to this syndrome. OBJECTIVE: To describe 3 siblings with the myopathic form of mitochondrial DNA depletion syndrome and a homozygous mutation in the TK2 gene. PATIENTS AND METHODS: These children developed normally until 12 to 16 months of age, when they started showing difficulty walking, which rapidly progressed to severe limb weakness. They died of respiratory failure between the ages of 23 and 40 months. Histochemical and biochemical studies of respiratory chain complexes were performed in muscle biopsy specimens. The whole coding region of the TK2 gene was sequenced. RESULTS: Muscle biopsy showed ragged-red cytochrome-c oxidase-negative fibers. All affected siblings had markedly decreased activities of respiratory chain complexes. Southern blot analysis showed severe reduction of the mitochondrial DNA-nuclear DNA ratio in muscle biopsy specimens from all patients, indicating 80% to 90% mitochondrial DNA depletion. Sequencing of the TK2 gene showed a homozygous C-->T transition at nucleotide 228 in exon 5, which changes a threonine to a methionine at position 77 (T77M). CONCLUSIONS: These results document the importance of screening the TK2 gene in patients with myopathic mitochondrial DNA depletion syndrome and confirm that exon 5 is a "hot spot" for TK2 mutations.     

A novel tRNA(Val) mitochondrial DNA mutation causing MELAS

Mitochondrial encephalopathy, lactic acidosis and stroke-like episodes (MELAS) is the most common mitochondrial disease due to mitochondrial DNA (mtDNA) mutations. At least 15 distinct mtDNA mutations have been associated with MELAS, and about 80% of the cases are caused by the A3243G tRNA(Leu(UUR)) gene mutation. We report here a novel tRNA(Val) mutation in a 37-year-old woman with manifestations of MELAS, and compare her clinicopathological phenotype with other rare cases associated tRNA(Val) mutations.     

Defective mitochondrial ATPase due to rare mtDNA m.8969G>A mutation—causing lactic acidosis,intellectual disability,and poor growth

Mutations in mitochondrial ATP synthase 6 (MT-ATP6) are a frequent cause of NARP (neurogenic muscle weakness, ataxia, and retinitis pigmentosa) or Leigh syndromes, especially a point mutation at nucleotide position 8993. M.8969G>A is a rare MT-ATP6 mutation, previously reported only in three individuals, causing multisystem disorders with mitochondrial myopathy, lactic acidosis, and sideroblastic anemia or IgA nephropathy. We present two siblings with the m.8969G>A mutation and a novel, substantially milder phenotype with lactic acidosis, poor growth, and intellectual disability. Our findings expand the phenotypic spectrum and show that mtDNA mutations should be taken account also with milder, stable phenotypes.     

Early-onset encephalomyopathy associated with tissue-specific mitochondrial DNA depletion: A morphological,biochemical and molecular-genetic study

A male infant, born from consanguineous parents, suffered from birth with a progressive neuromuscular disorder characterized by psychomotor delay, hypotonia, muscle weakness and wasting, deep-tendon areflexia and spastic posture. High levels of lactic acid in blood and cerebrospinal fluid suggested a mitochondrial respiratory chain defect. Muscle biopsy revealed raggedred and cytochromec oxidase-negative fibres, lipid accumulation and dystrophic changes. Multiple defects of respiratory complexes were detected in muscle homogenate, but cultured fibroblasts, myoblasts and myotubes were normal. Southern blot analysis showed markedly reduced levels of mitochondrial DNA (mtDNA) in muscle, while lymphocytes, fibroblasts and muscle precursor cells were normal. Neither depletion of mtDNA nor abnormalities of the respiratory complexes were observed in innervated muscle fibres cultured for as long as 4 months. No mutations were observed in two candidate nuclear genes,mtTFA andmtSSB, retro-transcribed, amplified and sequenced from the proband's mRNA. Sequence analysis of the mtDNA D-loop and of the origin of replication of the mtDNA light strand failed to identify potentially pathogenic mutations of these replicative elements in the proband's muscle mtDNA. Our findings indicate that mtDNA depletion is due to a nuclear encoded gene and suggest that the abnormality underlying defective mtDNA propagation must occur after muscle differentiation in vivo.     

An atypical French form of pyruvate carboxylase deficiency

A further case of pyruvate carboxylase deficiency, French type, with a particular clinical presentation and evolution is described. The initial neonatal symptoms started with respiratory distress, severe metabolic acidosis and a tendency to hypoglycemia. However, the clinical course was not rapidly deteriorating. At the age of 6 months he presented acute neurological symptoms, respiratory difficulty, lactic acidosis and hyperammonemia. Amino and organic acid abnormalities strongly suggested pyruvate carboxylase deficiency, which was confirmed by enzymatic studies in cultured fibroblasts and liver necropsy. Progressive deterioration and bronchopneumonia with cardiac failure and renal insufficiency led to death. Anatomic-pathologic studies revealed periventricular cysts and diffuse hypomyelination. Prenatal diagnosis of a further sibling was performed. The neonatal clinical presentation, biochemical abnormalities, and the presence of periventricular cysts suggested a French phenotype. However, the clinical course was less severe, suggesting a residual enzymatic activity and a possible milder mutation.     

Mitochondrial complex I deficiency caused by a deleterious NDUFA11 mutation

Complex I deficiency is the most common respiratory chain defect, clinically manifesting by severe neonatal lactic acidosis, Leigh's disease, or various combinations of cardiac, hepatic, and renal disorders. Using homozygosity mapping, we identified a splice-site mutation in the NDUFA11 gene in six patients from three unrelated families. The patients presented with encephalocardiomyopathy or fatal infantile lactic acidemia. The mutation is predicted to abolish the first transmembrane domain of the gene product, thereby destabilizing the enzymatic complex. Mutation analysis of the NDUFA11 is warranted in isolated complex I deficiency presenting with infantile lactic acidemia or encephalocardiomyopathy.     

Neonatal mitochondrial encephaloneuromyopathy due to a defect of mitochondrial protein synthesis

Mitochondrial diseases are clinically and genetically heterogeneous disorders due to primary mutations in mitochondrial DNA (mtDNA) or nuclear DNA (nDNA). We studied a male infant with severe congenital encephalopathy, peripheral neuropathy, and myopathy. The patient's lactic acidosis and biochemical defects of respiratory chain complexes I, III, and IV in muscle indicated that he had a mitochondrial disorder while parental consanguinity suggested autosomal recessive inheritance. Cultured fibroblasts from the patient showed a generalized defect of mitochondrial protein synthesis. Fusion of cells from the patient with 143B206 rho(0) cells devoid of mtDNA restored cytochrome c oxidase activity confirming the nDNA origin of the disease. Our studies indicate that the patient has a novel autosomal recessive defect of mitochondrial protein synthesis.     

Functional analysis of a novel POLγA mutation associated with a severe perinatal mitochondrial encephalomyopathy

Mutations in the mitochondrial DNA polymerase gamma catalytic subunit (POLγA) compromise the stability of mitochondrial DNA (mtDNA) by leading to mutations, deletions and depletions in mtDNA. Patients with mutations in POLγA often differ remarkably in disease severity and age of onset. In this work we have studied the functional consequence of POLγA mutations in a patient with an uncommon and a very severe disease phenotype characterized by prenatal onset with intrauterine growth restriction, lactic acidosis from birth, encephalopathy, hepatopathy, myopathy, and early death. Muscle biopsy identified scattered COX-deficient muscle fibers, respiratory chain dysfunction and mtDNA depletion. We identified a novel POLγA mutation (p.His1134Tyr) in trans with the previously identified p.Thr251Ile/Pro587Leu double mutant. Biochemical characterization of the purified recombinant POLγA variants showed that the p.His1134Tyr mutation caused severe polymerase dysfunction. The p.Thr251Ile/Pro587Leu mutation caused reduced polymerase function in conditions of low dNTP concentration that mimic postmitotic tissues. Critically, when p.His1134Tyr and p.Thr251Ile/Pro587Leu were combined under these conditions, mtDNA replication was severely diminished and featured prominent stalling. Our data provide a molecular explanation for the patient´s mtDNA depletion and clinical features, particularly in tissues such as brain and muscle that have low dNTP concentration.     

Selective muscle fiber loss and molecular compensation in mitochondrial myopathy due to TK2 deficiency

A 12-year-old patient with mitochondrial DNA (mtDNA) depletion syndrome due to TK2 gene mutations has been evaluated serially over the last 10 years. We observed progressive muscle atrophy with selective loss of type 2 muscle fibers and, despite severe depletion of mtDNA, normal activities of respiratory chain (RC) complexes and levels of COX II mitochondrial protein in the remaining muscle fibers. These results indicate that compensatory mechanisms account for the slow progression of the disease. Identification of factors that ameliorate mtDNA depletion may reveal new therapeutic targets for these devastating disorders.     

Acute mitochondrial myopathy with respiratory insufficiency and motor axonal polyneuropathy

Background: Mitochondrial myopathies (MMs) are mainly presented with chronic muscle weakness and accompanied with other syndromes. MM with acute respiratory insufficiency is rare. Aims: To reveal the clinical, pathological and molecular characteristics of a life-threatening MM. Methods: Muscle biopsy and enzyme staining were performed in skeletal muscles. Mitochondrial DNA (mtDNA) sequencing was analyzed and heteroplasmy were quantified by pyrosequencing. Results: All three patients had tachycardia, acute lactic acidosis, dyspnea and sudden severe muscle weakness. Two patients had calf edema and abdominal pain, and one had a heart attack. Electromyography in two patients showed dramatically decreased axonal amplitudes of motor nerves. Muscle biopsies showed ragged red fibers and dramatic mitochondrial abnormality. A mtDNA m.3243A>G mutation was identified in Patient 1 (mutation load: 29% in blood and 73% in muscle) and Patient 3 (79% in blood and 89% in muscle). A mtDNA m.8344A>G mutation was found in Patient 2 (mutation load 80.4% in blood). Conclusion: MM characterized by lactic acidosis, respiratory failure and acute motor axonal neuropathy is life threatening.     

Hepatocerebral mitochondrial DNA depletion syndrome caused by deoxyguanosine kinase (DGUOK) mutations

BACKGROUND: Autosomal recessive mutations in deoxyguanosine kinase (DGUOK) have been identified in the hepatocerebral form of mitochondrial DNA (mtDNA) depletion syndrome. OBJECTIVES: To describe the clinical spectrum of DGUOK-related mtDNA depletion syndrome in 6 children and to summarize the literature. RESULTS: We identified pathogenic mutations in DGUOK in 6 children with the hepatocerebral form of mtDNA depletion syndrome. We describe the clinical, neuroradiologic, histologic, and genetic features in these children. All children showed severe hepatopathy, while involvement of other organs (skeletal muscle and brain) was variable. We identified 5 novel mutations (1 of them in 2 children) and 2 previously described mutations. Three different mutations affected the initial methionine, suggesting a mutational hot spot. One of our patients underwent liver transplantation; pathologic findings revealed (in addition to diffuse hepatopathy) a hepatocellular carcinoma, implying a possible link between mtDNA depletion syndrome and tumorigenesis. CONCLUSION: We studied 12 children with infantile hepatoencephalopathies and mtDNA depletion syndrome and found pathogenic DGUOK mutations in 6, suggesting that this gene defect is a frequent but not an exclusive cause of the hepatic form of mtDNA depletion syndrome.     

Genotypes and clinical phenotypes in children with cytochrome-c oxidase deficiency

Cytochrome c oxidase (COX) deficiency has been associated with a wide spectrum of clinical features and may be caused by mutations in different genes of both the mitochondrial and the nuclear DNA. In an attempt to correlate the clinical phenotype with the genotype in 16 childhood cases, mtDNA was analysed for deletion, depletion, and mutations in the three genes encoding COX subunits and the 22 tRNA genes. Furthermore, nuclear DNA was analysed for mutations in the SURF1, SCO2, COX10, and COX17 genes and cases with mtDNA depletion were analysed for mutations in the TK2 gene. SURF1-mutations were identified in three out of four cases with Leigh syndrome while a mutation in the mitochondrial tRNA (trp) gene was identified in the fourth. One case with mtDNA depletion had mutations in the TK2 gene. In two cases with leukoencephalopathy, one case with encephalopathy, five cases with fatal infantile myopathy and cardiomyopathy, two cases with benign infantile myopathy, and one case with mtDNA depletion, no mutations were identified. We conclude that COX deficiency in childhood should be suspected in a wide range of clinical settings and although an increasing number of genetic defects have been identified, the underlying mutations remain unclear in the majority of the cases.     

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.     

Functional respiratory chain studies in mitochondrial cytopathies. Support for mitochondrial DNA heteroplasmy in myoclonus epilepsy and ragged red fibers (MERRF) syndrome

Summary Mitochondrial respiratory chain function was investigated with polarographic and enzymatic studies, and correlated with immunoblot studies using a battery of probes against respiratory chain holocomplexes in a series of patients with myoclonus epilepsy and ragged red fibers (MERRF) syndrome. State III respiration rates in intact skeletal muscle mitochondria were normal in two cases, suggested site I deficiency in one case and a midrespiratory defect in another. Immunological studies of complex I showed reduced levels of several subunits with the apparent absence of two bands (which at 45 and 42 kDa, coincide with the predicted electrophoretic mobility of the ND5 gene product) in one case. Complex I, III and IV composition was normal in the other three cases indicating no major disruption of complex assembly. A differing severity of skeletal muscle respiratory chain impairment in a group of unrelated patients with severe cerebral clinical involvement is best explained by uneven tissue distribution between brain and muscle of a heteroplasmic mtDNA mutation. The relationship between MERRF and mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS) encephalopathies is reappraised by extension of this hypothesis.