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Isolated defects of the mitochondrial respiratory complex II (succinate dehydrogenase, SDH) are rare, accounting for approximately 2% of all respiratory chain deficiency diagnoses. Here, we report clinical and molecular investigations of three family members with a heterozygous mutation in the large flavoprotein subunit SDHA previously described to cause complex II deficiency. The index patient presented with bilateral optic atrophy and ocular movement disorder, a progressive polyneuropathy, psychiatric involvement, and cardiomyopathy. Two of his children presented with cardiomyopathy and methylglutaconic aciduria in early childhood. The daughter deceased at the age of 7 months due to cardiac insufficiency. The 30-year old son presents with cardiomyopathy and developed bilateral optic atrophy in adulthood. Of the four nuclear encoded proteins composing complex II (SDHA, SDHB, SDHC, SDHD) and currently known assembly factors SDHAF1 and SDHAF2 mainly recessively inherited mutations have been described in SDHA, SDHB, SDHD, and SDHAF1 to be causative for mitochondrial disease phenotypes. This is the second report presenting autosomal dominant inheritance of a SDHA mutation.© 2016 Wiley Periodicals, Inc., (© 2016 Wiley Periodicals, Inc.)

We report a 10-year-old girl presenting with severe neonatal hypertrophic cardiomyopathy (HCM), feeding difficulties, mildly abnormal facial features, and progressive skeletal muscle symptoms but with normal cognitive development. Targeted oligonucleotide-selective sequencing of 101 cardiomyopathy genes revealed the genetic diagnosis, and the mutation was verified by Sanger sequencing in the patient and her parents. To offer insights into the potential mechanism of patient mutation, protein structural analysis was performed using the resolved structure of human activated HRAS protein with bound GTP analogue (PDB id 5P21) in Discovery Studio 4.5 (Dassault Systèmes Biovia, San Diego, CA). The patient with hypertrophic cardiomyopathy and normal cognitive development was diagnosed with an HRAS mutation c.173C>T (p.T58I), a milder variant of Costello syndrome affecting a highly conserved amino acid, threonine 58. Our analysis suggests that the p.G12 mutations slow GTP hydrolysis rendering HRAS unresponsive to GTPase activating proteins, and resulting in permanently active state. The p.T58I mutation likely affects binding of guanidine-nucleotide-exchange factors, thereby promoting the active state but also allowing for slow inactivation. Patients with the HRAS mutation c.173C>T (p.T58I) might go undiagnosed because of the milder phenotype compared with other mutations causing Costello syndrome. We expand the clinical and molecular picture of the rare HRAS mutation by reporting the first case in Europe and the fourth case in the literature. Our protein structure analysis offers insights into the mechanism of the mildly activating p.T58I mutation. © 2016 Wiley Periodicals, Inc., (© 2016 Wiley Periodicals, Inc.)

Objective: We report novel defects of mitochondrial translation elongation factor Ts (EFTs), with high carrier frequency in Finland and expand the manifestations of this disease group from infantile cardiomyopathy to juvenile neuropathy/encephalopathy disorders., Methods: DNA analysis, whole-exome analysis, protein biochemistry, and protein modeling., Results: We used whole-exome sequencing to find the genetic cause of infantile-onset mitochondrial cardiomyopathy, progressing to juvenile-onset Leigh syndrome, neuropathy, and optic atrophy in 2 siblings. We found novel compound heterozygous mutations, c.944G>A [p.C315Y] and c.856C>T [p.Q286X], in the TSFM gene encoding mitochondrial EFTs. The same p.Q286X variant was found as compound heterozygous with a splice site change in a patient from a second family, with juvenile-onset optic atrophy, peripheral neuropathy, and ataxia. Our molecular modeling predicted the coding-region mutations to cause protein instability, which was experimentally confirmed in cultured patient cells, with mitochondrial translation defect and lacking EFTs. Only a single TSFM mutation has been previously described in different populations, leading to an infantile fatal multisystem disorder with cardiomyopathy. Sequence data from 35,000 Finnish population controls indicated that the heterozygous carrier frequency of p.Q286X change was exceptionally high in Finland, 1:80, but no homozygotes were found in the population, in our mitochondrial disease patient collection, or in an intrauterine fetal death material, suggesting early developmental lethality of the homozygotes., Conclusions: We show that in addition to early-onset cardiomyopathy, TSFM mutations should be considered in childhood and juvenile encephalopathies with optic and/or peripheral neuropathy, ataxia, or Leigh disease., (© 2014 American Academy of Neurology.)

Background: The genetic complexity of infantile cardiomyopathies is remarkable, and the importance of mitochondrial translation defects as a causative factor is only starting to be recognised. We investigated the genetic basis for infantile onset recessive hypertrophic cardiomyopathy in two siblings., Methods and Results: Analysis of respiratory chain enzymes revealed a combined deficiency of complexes I and IV in the heart and skeletal muscle. Exome sequencing uncovered a homozygous mutation (L156R) in MRPL44 of both siblings. MRPL44 encodes a protein in the large subunit of the mitochondrial ribosome and is suggested to locate in close proximity to the tunnel exit of the yeast mitochondrial ribosome. We found severely reduced MRPL44 levels in the patient's heart, skeletal muscle and fibroblasts suggesting that the missense mutation affected the protein stability. In patient fibroblasts, decreased MRPL44 affected assembly of the large ribosomal subunit and stability of 16S rRNA leading to complex IV deficiency. Despite this assembly defect, de novo mitochondrial translation was only mildly affected in fibroblasts suggesting that MRPL44 may have a function in the assembly/stability of nascent mitochondrial polypeptides exiting the ribosome. Retroviral expression of wild-type MRPL44 in patient fibroblasts rescued the large ribosome assembly defect and COX deficiency., Conclusions: These findings indicate that mitochondrial ribosomal subunit defects can generate tissue-specific manifestations, such as cardiomyopathy.

Mutations in Pol γ represent a major cause of human mitochondrial diseases, especially those affecting the nervous system in adults and in children. Recessive mutations in Pol γ represent nearly half of those reported to date, and they are nearly uniformly distributed along the length of the POLG1 gene (Human DNA Polymerase gamma Mutation Database); the majority of them are linked to the most severe form of POLG syndrome, Alpers-Huttenlocher syndrome. In this report, we assess the structure-function relationships for recessive disease mutations by reviewing existing biochemical data on site-directed mutagenesis of the human, Drosophila and yeast Pol γs, and their homologs from the family A DNA polymerase group. We do so in the context of a molecular model of Pol γ in complex with primer-template DNA, which we have developed based upon the recently solved crystal structure of the apoenzyme form. We present evidence that recessive mutations cluster within five distinct functional modules in the catalytic core of Pol γ. Our results suggest that cluster prediction can be used as a diagnosis-supporting tool to evaluate the pathogenic role of new Pol γ variants.

Objective: Mitochondrial DNA polymerase γ (POLG1) mutations in children often manifest as Alpers syndrome, whereas in adults, a common manifestation is mitochondrial recessive ataxia syndrome (MIRAS) with severe epilepsy. Because some patients with MIRAS have presented with ataxia or epilepsy already in childhood, we searched for POLG1 mutations in neurologic manifestations in childhood., Methods: We investigated POLG1 in 136 children, all clinically suspected to have mitochondrial disease, with one or more of the following: ataxia, axonal neuropathy, severe epilepsy without known epilepsy syndrome, epileptic encephalopathy, encephalohepatopathy, or neuropathologically verified Alpers syndrome., Results: Seven patients had POLG1 mutations, and all of them had severe encephalopathy with intractable epilepsy. Four patients had died after exposure to sodium valproate. Brain MRI showed parieto-occipital or thalamic hyperintense lesions, white matter abnormality, and atrophy. Muscle histology and mitochondrial biochemistry results were normal in all., Conclusions: POLG1 analysis should belong to the first-line DNA diagnostic tests for children with an encephalitis-like presentation evolving into epileptic encephalopathy with liver involvement (Alpers syndrome), even if brain MRI and morphology, respiratory chain activities, and the amount of mitochondrial DNA in the skeletal muscle are normal. POLG1 analysis should precede valproate therapy in pediatric patients with a typical phenotype. However, POLG1 is not a common cause of isolated epilepsy or ataxia in childhood.