Whole-exome sequencing identifies a mutation in the mitochondrial ribosome protein MRPL44 to underlie mitochondrial infantile cardiomyopathy

1Research Programs Unit, Molecular Neurology, Biomedicum-Helsinki, University of Helsinki, Helsinki, Finland.
Journal of Medical Genetics (Impact Factor: 6.34). 01/2013; 50(3). DOI: 10.1136/jmedgenet-2012-101375
Source: PubMed

ABSTRACT 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.

These findings indicate that mitochondrial ribosomal subunit defects can generate tissue-specific manifestations, such as cardiomyopathy.

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    • "Mitochondria have their own ribosomes with at least 80 ribosomal proteins forming the large 39S and small 28S subunits Fig. 2 Types of combined respiratory chain defects (typical results) and their causes J Inherit Metab Dis (Rackham and Filipovska 2014). Mutations have been reported in MRPL3 (Galmiche et al 2011), MRPL12 (Serre et al 2013), MRPL44 (Carroll et al 2013), MRPS16 (Miller et al 2004) and MRPS22 (Saada et al 2007) so far. The 12S and 16S ribosomal RNAs are encoded on the mitochondrial DNA. "
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    ABSTRACT: Inherited disorders of mitochondrial energy metabolism form a large and heterogeneous group of metabolic diseases. More than 250 gene defects have been reported to date and this number continues to grow. Mitochondrial diseases can be grouped into (1) disorders of oxidative phosphorylation (OXPHOS) subunits and their assembly factors, (2) defects of mitochondrial DNA, RNA and protein synthesis, (3) defects in the substrate-generating upstream reactions of OXPHOS, (4) defects in relevant cofactors and (5) defects in mitochondrial homeostasis. Deficiency of more than one respiratory chain enzyme is a common finding. Combined defects are found in 49 % of the known disease-causing genes of mitochondrial energy metabolism and in 57 % of patients with OXPHOS defects identified in our diagnostic centre. Combined defects of complexes I, III, IV and V are typically due to deficiency of mitochondrial DNA replication, RNA metabolism or translation. Defects in cofactors can result in combined defects of various combinations, and defects of mitochondrial homeostasis can result in a generalised decrease of all OXPHOS enzymes. Noteworthy, identification of combined defects can be complicated by different degrees of severity of each affected enzyme. Furthermore, even defects of single respiratory chain enzymes can result in combined defects due to aberrant formation of respiratory chain supercomplexes. Combined OXPHOS defects have a great variety of clinical manifestations in terms of onset, course severity and tissue involvement. They can present as classical encephalomyopathy but also with hepatopathy, nephropathy, haematologic findings and Perrault syndrome in a subset of disorders.
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    • "The responsible genetic defects are located in genes that function at different levels of mitochondrial translation. These include mt-tRNAs and mt-rRNAs [2], nuclear genes encoding mitochondrial ribosomal proteins (MRPL3 [3], MRPL44 [4], MRPS16 [5], MRPS22 [6]), tRNA modifying proteins (MTO1 [7], PUS1 [8], TRMU [9]), a growing list of aminoacyl-tRNA synthetases (AARS2 [10], DARS2 [11], EARS2 [12], FARS2 [13], HARS2 [14], MARS2 [15], RARS2 [16], SARS2 [17], YARS2 [18]), translation elongation factors (GFM1 [19], TUFM [20], TSFM [21]), a gene involved in mitochondrial RNA processing (ELAC2 [22]), and a gene of unknown function (C12orf65 [23]). Due to the large number of genes involved NGS-based genetic analysis provides substantial advantages over sequential testing of candidate genes, which leaves many patients without a molecular diagnosis in routine clinical diagnostics. "
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    ABSTRACT: Defects of mitochondrial oxidative phosphorylation (OXPHOS) are associated with a wide range of clinical phenotypes and time courses. Combined OXPHOS deficiencies are mainly caused by mutations of nuclear genes that are involved in mitochondrial protein translation. Due to their genetic heterogeneity it is almost impossible to diagnose OXPHOS patients on clinical grounds alone. Hence next generation sequencing (NGS) provides a distinct advantage over candidate gene sequencing to discover the underlying genetic defect in a timely manner. One recent example is the identification of mutations in MTFMT that impair mitochondrial protein translation through decreased formylation of Met-tRNAMet. Here we report the results of a combined exome sequencing and candidate gene screening study. We identified nine additional MTFMT patients from eight families who were affected with Leigh encephalopathy or white matter disease, microcephaly, mental retardation, ataxia, and muscular hypotonia. In four patients, the causal mutations were identified by exome sequencing followed by stringent bioinformatic filtering. In one index case, exome sequencing identified a single heterozygous mutation leading to Sanger sequencing which identified a second mutation in the non-covered first exon. High-resolution melting curve-based MTFMT screening in 350 OXPHPOS patients identified pathogenic mutations in another three index cases. Mutations in one of them were not covered by previous exome sequencing. All novel mutations predict a loss-of-function or result in a severe decrease in MTFMT protein in patients’ fibroblasts accompanied by reduced steady-state levels of complex I and IV subunits. Being present in 11 out of 13 index cases the c.626C > T mutation is one of the most frequent disease alleles underlying OXPHOS disorders. We provide detailed clinical descriptions on eleven MTFMT patients and review five previously reported cases.
    Molecular Genetics and Metabolism 01/2013; 111(3). DOI:10.1016/j.ymgme.2013.12.010 · 2.63 Impact Factor
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    Biochimica et Biophysica Acta 08/2013; 1840(4). DOI:10.1016/j.bbagen.2013.08.010 · 4.66 Impact Factor
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