Infantile-onset disorders of mitochondrial replication and protein synthesis

The first living newborn case with 7706G˃A missense mutation: Alpers-Huttenlocher syndrome

Alpers-Huttenlocher syndrome (AHS) is an uncommon autosomal recessive mitochondrial DNA depletion disease. The classic clinical triad of progressive developmental regression, liver degeneration, and seizures helps define the disorder, but a wide range of clinical expressions occur. The most common mutations in childhood have been identified in the cytochrome c oxidase Ⅰ and Ⅳ genes. The 7706G˃A missense mutation in the Cox Ⅱ gene was previously reported in one case after postmortem histological study. Consequently, our patient is the first patient diagnosed with AHS with a 7706G˃A missense mutation in the Cox Ⅱ gene while alive. We proposed that 7706G˃A missense mutation is rare and should be more lethal than other mutations that cause Alpers-Huttenlocher syndrome.

Molecular basis of Leigh syndrome: a current look

Leigh Syndrome (OMIM 256000) is a heterogeneous neurologic disorder due to damage in mitochondrial energy production that usually starts in early childhood. The first description given by Leigh pointed out neurological symptoms in children under 2 years and premature death. Following cases brought some hypothesis to explain the cause due to similarity to other neurological diseases and led to further investigation for metabolic diseases. Biochemical evaluation and specific metabolic profile suggested impairment in energy production (OXPHOS) in mitochondria. As direct approach to involved tissues is not always possible or safe, molecular analysis is a great cost-effective option and, besides biochemical results, is required to confirm the underlying cause of this syndrome face to clinical suspicion. The Next Generation Sequencing (NGS) advance represented a breakthrough in molecular biology allowing simultaneous gene analysis giving short-time results and increasing the variants underlying this syndrome, counting over 75 monogenic causes related so far. NGS provided confirmation of emerging cases and brought up diagnosis in atypical presentations as late-onset cases, which turned Leigh into a heterogeneous syndrome with variable outcomes. This review highlights clinical presentation in both classic and atypical phenotypes, the investigation pathway throughout confirmation emphasizing the underlying genetic heterogeneity and increasing number of genes assigned to this syndrome as well as available treatment.

Mitochondrial DNA depletion syndrome causing liver failure

BACKGROUND

Mitochondrial DNA depletion syndromes are disorders of Mitochondrial DNA maintenance causing varied manifestations, including fulminant liver failure.

CASE CHARACTERISTICS

Two infants, presenting with severe fatal hepatopathy.

OBSERVATION

Raised serum lactate, positive family history (in first case), and absence of other causes of acute liver failure.

OUTCOME

Case 1 with homozygous mutation, c.3286C>T (p.Arg1096Cys) in POLG gene and case 2 with compound heterozygous mutations, novel c.408T>G (p.Tyr136X) and previously reported c.293C>T (p.Pro98Leu), in MPV17 gene.

MESSAGE

Mitochondrial DNA depletion syndrome is a rare cause of severe acute liver failure in children.

A novel SUCLA2 mutation in a Portuguese child associated with "mild" methylmalonic aciduria

Succinyl-coenzyme A synthase is a mitochondrial matrix enzyme that catalyzes the reversible synthesis of succinate and adenosine triphosphate (ATP) from succinyl-coenzyme A and adenosine diphosphate (ADP) in the tricarboxylic acid cycle. This enzyme is made up of α and β subunits encoded by SUCLG1 and SUCLA2, respectively. We present a child with severe muscular hypotonia, dystonia, failure to thrive, sensorineural deafness, and dysmorphism. Metabolic investigations disclosed hyperlactacidemia, moderate urinary excretion of methylmalonic acid, and elevated levels of C4-dicarboxylic carnitine in blood. We identified a novel homozygous p.M329V in SUCLA2. In cultured cells, the p.M329V resulted in a reduced amount of the SUCLA2 protein, impaired production of mitochondrial ATP, and enhanced production of reactive oxygen species, which was partially reduced by using 5-aminoimidazole-4-carboxamide ribonucleotide in the culture medium. Expanding the array of SUCLA2 mutations, we suggested that reactive oxygen species scavengers are likely to impact on disease prognosis.

Syndromes associated with mitochondrial DNA depletion

Mitochondrial dysfunction accounts for a large group of inherited metabolic disorders most of which are due to a dysfunctional mitochondrial respiratory chain (MRC) and, consequently, deficient energy production. MRC function depends on the coordinated expression of both nuclear (nDNA) and mitochondrial (mtDNA) genomes. Thus, mitochondrial diseases can be caused by genetic defects in either the mitochondrial or the nuclear genome, or in the cross-talk between the two. This impaired cross-talk gives rise to so-called nuclear-mitochondrial intergenomic communication disorders, which result in loss or instability of the mitochondrial genome and, in turn, impaired maintenance of qualitative and quantitative mtDNA integrity. In children, most MRC disorders are associated with nuclear gene defects rather than alterations in the mtDNA itself.

The mitochondrial DNA depletion syndromes (MDSs) are a clinically heterogeneous group of disorders with an autosomal recessive pattern of transmission that have onset in infancy or early childhood and are characterized by a reduced number of copies of mtDNA in affected tissues and organs. The MDSs can be divided into least four clinical presentations: hepatocerebral, myopathic, encephalomyopathic and neurogastrointestinal. The focus of this review is to offer an overview of these syndromes, listing the clinical phenotypes, together with their relative frequency, mutational spectrum, and possible insights for improving diagnostic strategies.

Mitochondrial disease in childhood: nuclear encoded

Primary mitochondrial disorders are clinically and genetically heterogeneous, caused by an alteration(s) in either mitochondrial DNA or nuclear DNA, and affect the respiratory chain's ability to undergo oxidative phosphorylation, leading to decreased production of adenosine triphosphophate and subsequent energy failure. These disorders may present at any age, but children tend to have an acute onset of disease compared with subacute or slowly progressive presentation in adults. Varying organ involvement also contributes to the phenotypic spectrum seen in these disorders. The childhood presentation of primary mitochondrial disease is mainly due to nuclear DNA mutations, with mitochondrial DNA mutations being less frequent in childhood and more prominent in adulthood disease. The clinician should be aware of the pediatric presentation of mitochondrial disease and have an understanding of the myriad of nuclear genes responsible for these disorders. The nuclear genes can be best understood by utilizing a classification system of location and function within the mitochondria.

Pontocerebellar hypoplasia type 6 caused by mutations in RARS2: definition of the clinical spectrum and molecular findings in five patients

Recessive mutations in the mitochondrial arginyl-transfer RNA synthetase (RARS2) gene have been associated with early onset encephalopathy with signs of oxidative phosphorylation defects classified as pontocerebellar hypoplasia 6. We describe clinical, neuroimaging and molecular features on five patients from three unrelated families who displayed mutations in RARS2. All patients rapidly developed a neonatal or early-infantile epileptic encephalopathy with intractable seizures. The long-term follow-up revealed a virtual absence of psychomotor development, progressive microcephaly, and feeding difficulties. Mitochondrial respiratory chain enzymes in muscle and fibroblasts were normal in two. Blood and CSF lactate was abnormally elevated in all five patients at early stages while appearing only occasionally abnormal with the progression of the disease. Cerebellar vermis hypoplasia with normal aspect of the cerebral and cerebellar hemispheres appeared within the first months of life at brain MRI. In three patients follow-up neuroimaging revealed a progressive pontocerebellar and cerebral cortical atrophy. Molecular investigations of RARS2 disclosed the c.25A>G/p.I9V and the c.1586+3A>T in family A, the c.734G>A/p.R245Q and the c.1406G>A/p.R469H in family B, and the c.721T>A/p.W241R and c.35A>G/p.Q12R in family C. Functional complementation studies in Saccharomyces cerevisiae showed that mutation MSR1-R531H (equivalent to human p.R469H) abolished respiration whereas the MSR1-R306Q strain (corresponding to p.R245Q) displayed a reduced growth on non-fermentable YPG medium. Although mutations functionally disrupted yeast we found a relatively well preserved arginine aminoacylation of mitochondrial tRNA. Clinical and neuroimaging findings are important clues to raise suspicion and to reach diagnostic accuracy for RARS2 mutations considering that biochemical abnormalities may be absent in muscle biopsy.