Clinical and molecular findings in a family expressing a novel heterozygous variant of the G elongation factor mitochondrial 1 gene

Molecular pathways in mitochondrial disorders due to a defective mitochondrial protein synthesis

Mitochondria play a central role in cellular metabolism producing the necessary ATP through oxidative phosphorylation. As a remnant of their prokaryotic past, mitochondria contain their own genome, which encodes 13 subunits of the oxidative phosphorylation system, as well as the tRNAs and rRNAs necessary for their translation in the organelle. Mitochondrial protein synthesis depends on the import of a vast array of nuclear-encoded proteins including the mitochondrial ribosome protein components, translation factors, aminoacyl-tRNA synthetases or assembly factors among others. Cryo-EM studies have improved our understanding of the composition of the mitochondrial ribosome and the factors required for mitochondrial protein synthesis and the advances in next-generation sequencing techniques have allowed for the identification of a growing number of genes involved in mitochondrial pathologies with a defective translation. These disorders are often multisystemic, affecting those tissues with a higher energy demand, and often present with neurodegenerative phenotypes. In this article, we review the known proteins required for mitochondrial translation, the disorders that derive from a defective mitochondrial protein synthesis and the animal models that have been established for their study.

Whole exome sequencing identifies a novel compound heterozygous GFM1 variant underlying developmental delay, dystonia, polymicrogyria, and severe intellectual disability in a Pakhtun family

Mitochondrial protein synthesis requires three elongation factors including EF-Tu (TUFM; OMIM 602389), EF-Ts (TSFM; OMIM 604723), and EF-G1 (GFM1; OMIM 606639). Pathogenic variants in any of these three members result in defective mitochondrial translation which can impart an oxidative phosphorylation (OXPHOS) deficiency. In this study, we investigated a consanguineous Pakhtun Pakistani family. There were four affected siblings at the time of this study and one affected girl had died in infancy. The index patient had severe intellectual disability, global developmental delay, dystonia, no speech development, feeding difficulties, and nystagmus. MRI brain presented thinning of corpus callosum and polymicrogyria. Whole exome sequencing revealed a novel compound heterozygous variant in GFM1 located on chromosome 3q25.32. Sanger sequencing confirmed recessive segregation of the maternal (NM_001308164.1:c.409G > A; p.Val137Met) and paternal (NM_001308164.1:c.1880G > A; p.Arg627Gln) variants in all the four affected siblings. These variants are classified as "likely-pathogenic" according to the recommendation of ACMG/AMP guideline. GFM1 alterations mostly lead to severe phenotypes and the patients may die in early neonatal life; however, four of the affected siblings had survived till the ages of 10-17 years, without developing any life-threatening conditions. Mostly, in cousin marriages, the pathogenic variants are identical-by-descent, and affected siblings born to such parents are homozygous. Three homozygous variants were shortlisted in the analysis of the WES data, but Sanger sequencing did not confirm their segregation with the disease phenotype. This is the first report from Pakistan expanding pathogenicity of GFM1 gene.

Mitochondrial Protein Translation: Emerging Roles and Clinical Significance in Disease

Mitochondria are one of the most important organelles in cells. Mitochondria are semi-autonomous organelles with their own genetic system, and can independently replicate, transcribe, and translate mitochondrial DNA. Translation initiation, elongation, termination, and recycling of the ribosome are four stages in the process of mitochondrial protein translation. In this process, mitochondrial protein translation factors and translation activators, mitochondrial RNA, and other regulatory factors regulate mitochondrial protein translation. Mitochondrial protein translation abnormalities are associated with a variety of diseases, including cancer, cardiovascular diseases, and nervous system diseases. Mutation or deletion of various mitochondrial protein translation factors and translation activators leads to abnormal mitochondrial protein translation. Mitochondrial tRNAs and mitochondrial ribosomal proteins are essential players during translation and mutations in genes encoding them represent a large fraction of mitochondrial diseases. Moreover, there is crosstalk between mitochondrial protein translation and cytoplasmic translation, and the imbalance between mitochondrial protein translation and cytoplasmic translation can affect some physiological and pathological processes. This review summarizes the regulation of mitochondrial protein translation factors, mitochondrial ribosomal proteins, mitochondrial tRNAs, and mitochondrial aminoacyl-tRNA synthetases (mt-aaRSs) in the mitochondrial protein translation process and its relationship with diseases. The regulation of mitochondrial protein translation and cytoplasmic translation in multiple diseases is also summarized.