Neonatal mitochondrial encephaloneuromyopathy due to a defect of mitochondrial protein synthesis

Selective symmetrical necrotizing encephalopathy secondary to primary mitochondrial disorder in a cat

A 2‐year‐old female cat was referred for progressive neurological signs indicative of involvement of the prosencephalon, cerebellum, and brainstem. Magnetic resonance imaging identified multifocal, bilateral, symmetrical lesions with strong contrast enhancement, affecting multiple areas of the brain. Neuropathology at necropsy showed demyelination, necrotic lesions, spongiosis, and neuropil edema with reactive astrogliosis and neovascularization. Ultrastructural study indicated mitochondrial polymorphism. Genetic investigations outlined 2 polymorphisms within the tRNA‐Leu^(UUR) gene of mitochondrial DNA. Imaging and neuropathological findings were consistent with selective symmetrical necrotizing encephalopathy, for which genetic investigations support mitochondrial pathogenesis.

Msc6p is required for mitochondrial translation initiation in the absence of formylated Met-tRNAfMet

Mitochondrial translation normally requires formylation of the initiator tRNA-met, a reaction catalyzed by the enzyme formyltransferase, Fmt1p and MTFMT in Saccharomyces cerevisiae and human mitochondria, respectively. Yeast fmt1 mutants devoid of Fmt1p, however, can synthesize all mitochondrial gene products by initiating translation with a non-formylated methionyl-tRNA. Yeast synthetic respiratory-deficient fmt1 mutants have uncovered several factors suggested to play a role in translation initiation with non-formylated methionyl-tRNA. Here, we present evidence that Msc6p, a member of the pentatricopeptide repeat (PPR) motif family, is another essential factor for mitochondrial translation in fmt1 mutants. The PPR motif is characteristic of RNA-binding proteins found in chloroplasts and plant and fungal mitochondria, and is generally involved in RNA stability and transport. Moreover, in the present study, we show that the respiratory deficiency of fmt1msc6 double mutants can be rescued by overexpression of the yeast mitochondrial initiation factor mIF-2, encoded by IFM1. The role of Msc6p in translational initiation is further supported by pull-down assays showing that it transiently interacts with mIF-2. Altogether, our data indicate that Msc6p is an important factor in mitochondrial translation with an auxiliary function related to the mIF-2-dependent formation of the initiation complex.

The clinical, biochemical and genetic features associated with RMND1-related mitochondrial disease

Background

Mutations in the RMND1 (Required for Meiotic Nuclear Division protein 1) gene have recently been linked to infantile onset mitochondrial disease characterised by multiple mitochondrial respiratory chain defects.

Methods

We summarised the clinical, biochemical and molecular genetic investigation of an international cohort of affected individuals with RMND1 mutations. In addition, we reviewed all the previously published cases to determine the genotype–phenotype correlates and performed survival analysis to identify prognostic factors.

Results

We identified 14 new cases from 11 pedigrees that harbour recessive RMND1 mutations, including 6 novel variants: c.533C>A, p.(Thr178Lys); c.565C>T, p.(Gln189*); c.631G>A, p.(Val211Met); c.1303C>T, p.(Leu435Phe); c.830+1G>A and c.1317+1G>T. Together with all previously published cases (n=32), we show that congenital sensorineural deafness, hypotonia, developmental delay and lactic acidaemia are common clinical manifestations with disease onset under 2 years. Renal involvement is more prevalent than seizures (66% vs 44%). In addition, median survival time was longer in patients with renal involvement compared with those without renal disease (6 years vs 8 months, p=0.009). The neurological phenotype also appears milder in patients with renal involvement.

Conclusions

The clinical phenotypes and prognosis associated with RMND1 mutations are more heterogeneous than that were initially described. Regular monitoring of kidney function is imperative in the clinical practice in light of nephropathy being present in over 60% of cases. Furthermore, renal replacement therapy should be considered particularly in those patients with mild neurological manifestation as shown in our study that four recipients of kidney transplant demonstrate good clinical outcome to date.

Neurofibromatosis Type 1: A Novel NF1 Mutation Associated with Mitochondrial Complex I Deficiency

Background. Neurofibromatosis type 1 is a multisystemic, progressive disease, with an estimated incidence of 1/3500-2500. Mitochondrial diseases are generally multisystemic and may be present at any age, and the global prevalence is 1/8500. The diagnosis of these disorders is complex because of its clinical and genetic heterogeneity. Case Report. We present a rare case of the association of these two different genetic diseases, in which a heterozygous missense mutation in the NF1 gene was identified which had not yet been described (p.M1149 V). Additionally, the patient is suspected of carrying an unspecified mutation causing respiratory chain complex I deficiency. Clinical presentation included hypotonia, global development delay, reduced growth rate, progressive microcephaly, and numerous café-au-lait spots. Discussion. To the best of our knowledge this is the first report of complex I deficiency in a patient with neurofibromatosis type 1. It is very important to maintain a high index of suspicion for the diagnosis of mitochondrial disorders. In this patient, both the laboratory screening and muscle histology were normal and only the biochemical study of muscle allowed us to confirm the diagnosis.

Mitochondria as a source of reactive oxygen and nitrogen species: from molecular mechanisms to human health

Mitochondrially generated reactive oxygen species are involved in a myriad of signaling and damaging pathways in different tissues. In addition, mitochondria are an important target of reactive oxygen and nitrogen species. Here, we discuss basic mechanisms of mitochondrial oxidant generation and removal and the main factors affecting mitochondrial redox balance. We also discuss the interaction between mitochondrial reactive oxygen and nitrogen species, and the involvement of these oxidants in mitochondrial diseases, cancer, neurological, and cardiovascular disorders.

Early onset (childhood) monogenic neuropathies

Hereditary neuropathies (HN) with onset in childhood are categorized according to clinical presentation, pathogenic mechanism based on electrophysiology, genetic transmission and, in selected cases, pathological findings. Especially relevant to pediatrics are the items "secondary" versus "primary" neuropathy, "syndromic versus nonsyndromic," and "period of life." Different combinations of these parameters frequently point toward specific monogenic disorders. Ruling out a neuropathy secondary to a generalized metabolic disorder remains the first concern in pediatrics. As a rule, metabolic diseases include additional, orienting symptoms or signs, and their biochemical diagnosis is based on logical algorithms. Primary, motor sensory are the most frequent HN and are dominated by demyelinating autosomal dominant (AD) forms (CMT1). Other forms include demyelinating autosomal recessive (AR) forms, axonal AD/AR forms, and forms with "intermediate" electrophysiological phenotype. Peripheral motor neuron disorders are dominated by AR SMN-linked spinal muscular atrophies. (Distal) hereditary motor neuropathies represent <10% of HN but exhibit large clinical and genetic heterogeneity. Sensory/dysautonomic HN involves five classic subtypes, each one related to specific genes. However, genetic heterogeneity is larger than initially suspected. Syndromic HN distinguish "purely neurological syndromes", which are multisystemic, such as spinocerebellar atrophies +, spastic paraplegias +, etc. Peripheral neuropathy is possibly the presenting feature, including in childhood. Autosomal recessive forms, on average, start more frequently in childhood. "Multiorgan syndromes", on the other hand, are more specific to Pediatrics. AR forms, which are clearly degenerative, prompt the investigation of a large set of pleiotropic genes. Other syndromes expressed in the perinatal period are mainly developmental disorders, and can sometimes be related to specific transcription factors. Systematic malformative workup and ethical considerations are necessary. Altogether, >40 genes with various biological functions have been found to be responsible for primary HN. Many are responsible for various phenotypes, including some without the polyneuropathic trait, and some for various types of transmission.

Hereditary motor-sensory, motor, and sensory neuropathies in childhood

Hereditary neuropathies (HN) are categorized according to clinical presentation, pathogenic mechanism based on electrophysiology, genetic transmission, age of occurrence, and, in selected cases, pathological findings. The combination of these parameters frequently orients towards specific genetic disorders. Ruling out a neuropathy secondary to a generalized metabolic disorder remains the first pediatric concern. Primary, motor-sensory are the most frequent HN and are dominated by demyelinating AD forms (CMT1). Others are demyelinating AR forms, axonal AD/AR forms, and forms with "intermediate" electrophysiological phenotype. Pure motor HN represent<10% of HN but exhibit large clinical and genetic heterogeneity. Sensory/dysautonomic HN cover five classical subtypes, each one related to specific genes. However, genetic heterogeneity is largly greater than initially suspected. Syndromic HN distinguish: "purely neurological syndromes", which are multisystemic, usually AD disorders, such as spinocerebellar atrophies +, spastic paraplegias +, etc. Peripheral Neuropathy may be the presenting feature, including in childhood. Clearly degenerative, AR forms prompt to investigate a large set of pleiotropic genes. Other syndromes, expressed in the perinatal period and comprising malformative features, are mainly developmental disorders, sometimes related to specific transcription factors. Altogether, >40 genes with various biological functions have been found responsible for HN. Many are responsible for various phenotypes, including some without the polyneuropathic trait: for the pediatric neurologist, phenotype/genotype correlations constitute a permanent bidirectional exercise.

Infantile encephaloneuromyopathy and defective mitochondrial translation are due to a homozygous RMND1 mutation

Defects of mitochondrial protein synthesis are clinically and genetically heterogeneous. We previously described a male infant who was born to consanguineous parents and who presented with severe congenital encephalopathy, peripheral neuropathy, myopathy, and lactic acidosis associated with deficiencies of multiple mitochondrial respiratory-chain enzymes and defective mitochondrial translation. In this work, we have characterized four additional affected family members, performed homozygosity mapping, and identified a homozygous splicing mutation in the splice donor site of exon 2 (c.504+1G>A) of RMND1 (required for meiotic nuclear division-1) in the affected individuals. Fibroblasts from affected individuals expressed two aberrant transcripts and had decreased wild-type mRNA and deficiencies of mitochondrial respiratory-chain enzymes. The RMND1 mutation caused haploinsufficiency that was rescued by overexpression of the wild-type transcript in mutant fibroblasts; this overexpression increased the levels and activities of mitochondrial respiratory-chain proteins. Knockdown of RMND1 via shRNA recapitulated the biochemical defect of the mutant fibroblasts, further supporting a loss-of-function pathomechanism in this disease. RMND1 belongs to the sif2 family, an evolutionary conserved group of proteins that share the DUF155 domain, have unknown function, and have never been associated with human disease. We documented that the protein localizes to mitochondria in mammalian and yeast cells. Further studies are necessary for understanding the function of this protein in mitochondrial protein translation.