A Novel Mutation in MARS in a Patient with Charcot-Marie-Tooth Disease, Axonal, Type 2U with Congenital Onset

Exploring the evidence for mitochondrial dysfunction and genetic abnormalities in the etiopathogenesis of tropical ataxic neuropathy

Tropical ataxic neuropathy (TAN) is characterised by ataxic polyneuropathy, degeneration of the posterior columns and pyramidal tracts, optic atrophy, and sensorineural hearing loss. It has been attributed to nutritional/toxic etiologies, but evidence for the same has been equivocal. TAN shares common clinical features with inherited neuropathies and mitochondrial disorders, it may be hypothesised that genetic abnormalities may underlie the pathophysiology of TAN. This study aimed to establish evidence for mitochondrial dysfunction by adopting an integrated biochemical and multipronged genetic analysis. Patients (n = 65) with chronic progressive ataxic neuropathy with involvement of visual and/or auditory pathways underwent deep phenotyping, genetic studies including mitochondrial DNA (mtDNA) deletion analysis, mtDNA and clinical exome sequencing (CES), and respiratory chain complex (RCC) assay. The phenotypic characteristics included dysfunction of visual (n = 14), auditory (n = 12) and visual + auditory pathways (n = 29). Reduced RCC activity was present in 13 patients. Mitochondrial DNA deletions were noted in five patients. Sequencing of mtDNA (n = 45) identified a homoplasmic variant (MT-ND6) and a heteroplasmic variant (MT-COI) in one patient each. CES (n = 45) revealed 55 variants in nuclear genes that are associated with neuropathy (n = 27), deafness (n = 7), ataxia (n = 4), and mitochondrial phenotypes (n = 5) in 36 patients. This study provides preliminary evidence that TAN is associated with a spectrum of genetic abnormalities, including those associated with mitochondrial dysfunction, which is in contradistinction from the prevailing hypothesis that TAN is related to dietary toxins. Analysing the functional relevance of these genetic variants may improve the understanding of the pathogenesis of TAN.

[A case of interstitial lung and liver disease caused by MARS1 gene mutation]

The patient is a female infant, 4 months and 9 days old, who was admitted to the hospital due to recurrent fever, cough, and hepatomegaly for over a month. The patient was a healthy full-term infant with a normal birth history. At 2 months and 22 days after birth, she developed recurrent fever, cough, and respiratory distress. Chest imaging revealed diffuse bilateral lung lesions, and fiberoptic bronchoscopy showed interstitial changes in both lungs. These suggested the presence of interstitial lung disease. The patient also presented with hepatomegaly, anemia, hyperlipidemia, hypothyroidism, and malnutrition. Genetic testing indicated compound heterozygous variations in the MARS1 gene. This mutation can cause interstitial lung and liver disease, which is a severe rare disorder that typically manifests in infancy or early childhood. It is inherited in an autosomal recessive manner and characterized by early-onset respiratory insufficiency and liver disease in infants or young children. Since its first reported case in 2013, as of June 2023, only 38 related cases have been reported worldwide. This article reports the multidisciplinary diagnosis and treatment of interstitial lung and liver disease in an infant caused by MARS1 gene mutation.

Idiopathic pulmonary fibrosis with benign SFTPC variant and pathogenic MARS1 mutations: can't see the forest for the trees!

Even in the absence of liver disease, MARS1 screening should be considered in severe lung fibrosis of young individuals. Interpretation of the genetic variants can evolve with improvement of knowledge (databases, bioinformatic tools) over time. https://bit.ly/45OxF5E.

A humanized yeast model reveals dominant-negative properties of neuropathy-associated alanyl-tRNA synthetase mutations

Aminoacyl-tRNA synthetases (ARSs) are essential enzymes that ligate tRNA molecules to cognate amino acids. Heterozygosity for missense variants or small in-frame deletions in six ARS genes causes dominant axonal peripheral neuropathy. These pathogenic variants reduce enzyme activity without significantly decreasing protein levels and reside in genes encoding homo-dimeric enzymes. These observations raise the possibility that neuropathy-associated ARS variants exert a dominant-negative effect, reducing overall ARS activity below a threshold required for peripheral nerve function. To test such variants for dominant-negative properties, we developed a humanized yeast assay to co-express pathogenic human alanyl-tRNA synthetase (AARS1) mutations with wild-type human AARS1. We show that multiple loss-of-function AARS1 mutations impair yeast growth through an interaction with wild-type AARS1, but that reducing this interaction rescues yeast growth. This suggests that neuropathy-associated AARS1 variants exert a dominant-negative effect, which supports a common, loss-of-function mechanism for ARS-mediated dominant peripheral neuropathy.

Dominant aminoacyl-tRNA synthetase disorders: lessons learned from in vivo disease models

Aminoacyl-tRNA synthetases (ARSs) play an essential role in protein synthesis, being responsible for ligating tRNA molecules to their corresponding amino acids in a reaction known as 'tRNA aminoacylation'. Separate ARSs carry out the aminoacylation reaction in the cytosol and in mitochondria, and mutations in almost all ARS genes cause pathophysiology most evident in the nervous system. Dominant mutations in multiple cytosolic ARSs have been linked to forms of peripheral neuropathy including Charcot-Marie-Tooth disease, distal hereditary motor neuropathy, and spinal muscular atrophy. This review provides an overview of approaches that have been employed to model each of these diseases in vivo, followed by a discussion of the existing animal models of dominant ARS disorders and key mechanistic insights that they have provided. In summary, ARS disease models have demonstrated that loss of canonical ARS function alone cannot fully account for the observed disease phenotypes, and that pathogenic ARS variants cause developmental defects within the peripheral nervous system, despite a typically later onset of disease in humans. In addition, aberrant interactions between mutant ARSs and other proteins have been shown to contribute to the disease phenotypes. These findings provide a strong foundation for future research into this group of diseases, providing methodological guidance for studies on ARS disorders that currently lack in vivo models, as well as identifying candidate therapeutic targets.

Clinicopathological features in two families with MARS-related Charcot-Marie-Tooth disease

Mutations in MARS gene cause dominant Charcot-Marie-Tooth disease (CMT) 2U. The aim of this study is to investigate phenotypic heterogeneities and peripheral neuropathology of MARS-related CMT patients. We identified a heterozygous p. R199Q mutation and an already reported heterozygous p. P800T mutation of MARS gene in two unrelated families using targeted next-generation sequencing. The first pedigree comprised three patients over three generations and the second pedigree comprised two patients over two generations. In addition of an asymptomatic carrier in the second pedigree, all patients presented with childhood-onset length dependent sensorimotor neuropathy with pes cavus. Nerve conduction studies revealed slowing of motor nerve conduction velocities (MNCV) of the median nerve indicating intermediate neuropathy in the patient with the p. R199Q mutation, and normal MNCV with reduced compound muscle action potential indicating axonal neuropathy in the patient with the p. P800T mutation. Magnetic resonance imaging detected a pattern of nerve changes similar to those in demyelinating polyneuropathies in intermediate type (p. R199Q mutation) patients compared with normal in the axonal type (p. P800T mutation) patients. Additionally, sural nerve biopsy revealed loss of myelinated axons with onion bulb formation in both mutations. By electron microscopy, a marked decrease of myelinated and unmyelinated fiber, neurofilaments aggregate with degenerating mitochondria and microtubule loss in axons were frequently found. Denervated Schwann cell complexes and few collagen pockets indicated involvement of unmyelinated Schwann cells. Therefore, the investigated MARS mutations cause not only the known axonal type but also intermediate type neuropathy with involvement of both axons and Schwann cells. Those findings are useful for the differential diagnosis of CMT patients with unknown MARS variants.

Four pedigrees with aminoacyl-tRNA synthetase abnormalities

Aminoacyl tRNA synthetases (ARSs) are highly conserved enzymes that link amino acids to their cognate tRNAs. Thirty-seven ARSs are known and their deficiencies cause various genetic disorders. Variants in some ARSs are associated with the autosomal dominant inherited form of axonal neuropathy, including Charcot-Marie-Tooth (CMT) disease. Variants of genes encoding ARSs often cause disorders in an autosomal recessive fashion. The clinical features of cytosolic ARS deficiencies are more variable, including systemic features. Deficiencies of ARSs localized in the mitochondria are often associated with neurological disorders including Leigh and early-onset epileptic syndromes. Whole exome sequencing (WES) is an efficient way to identify the genes causing various symptoms in patients. We identified 4 pedigrees with novel compound heterozygous variants in ARS genes (WARS1, MARS1, AARS2, and PARS2) by WES. Some unique manifestations were noted. The number of patients with ARSs has been increasing since the application of WES. Our findings broaden the known genetic and clinical spectrum associated with ARS variants.

A bi-allelic loss-of-function SARS1 variant in children with neurodevelopmental delay, deafness, cardiomyopathy, and decompensation during fever

Aminoacyl-tRNA synthetases (aaRS) are ubiquitously expressed enzymes responsible for ligating amino acids to their cognate tRNA molecules through an aminoacylation reaction. The resulting aminoacyl-tRNA is delivered to ribosome elongation factors to participate in protein synthesis. Seryl-tRNA synthetase (SARS1) is one of the cytosolic aaRSs and catalyzes serine attachment to tRNA^Ser . SARS1 deficiency has already been associated with moderate intellectual disability, ataxia, muscle weakness, and seizure in one family. We describe here a new clinical presentation including developmental delay, central deafness, cardiomyopathy, and metabolic decompensation during fever leading to death, in a consanguineous Turkish family, with biallelic variants (c.638G>T, p.(Arg213Leu)) in SARS1. This missense variant was shown to lead to protein instability, resulting in reduced protein level and enzymatic activity. Our results describe a new clinical entity and expand the clinical and mutational spectrum of SARS1 and aaRS deficiencies.

Yeast as a Tool to Understand the Significance of Human Disease-Associated Gene Variants

At present, the great challenge in human genetics is to provide significance to the growing amount of human disease-associated gene variants identified by next generation DNA sequencing technologies. Increasing evidences suggest that model organisms are of pivotal importance to addressing this issue. Due to its genetic tractability, the yeast Saccharomyces cerevisiae represents a valuable model organism for understanding human genetic variability. In the present review, we show how S. cerevisiae has been used to study variants of genes involved in different diseases and in different pathways, highlighting the versatility of this model organism.

Associations between Neurological Diseases and Mutations in the Human Glycyl-tRNA Synthetase

Aminoacyl-RNA synthetases (aaRSs) are among the key enzymes of protein biosynthesis. They are responsible for conducting the first step in the protein biosynthesis, namely attaching amino acids to the corresponding tRNA molecules both in cytoplasm and mitochondria. More and more research demonstrates that mutations in the genes encoding aaRSs lead to the development of various neurodegenerative diseases, such as incurable Charcot–Marie–Tooth disease (CMT) and distal spinal muscular atrophy. Some mutations result in the loss of tRNA aminoacylation activity, while other mutants retain their classical enzyme activity. In the latter case, disease manifestations are associated with additional neuron-specific functions of aaRSs. At present, seven aaRSs (GlyRS, TyrRS, AlaRS, HisRS, TrpRS, MetRS, and LysRS) are known to be involved in the CMT etiology with glycyl-tRNA synthetase (GlyRS) being the most studied of them.

Protein instability associated with AARS1 and MARS1 mutations causes Trichothiodystrophy

Trichothiodystrophy (TTD) is a rare hereditary neurodevelopmental disorder defined by sulfur-deficient brittle hair and nails and scaly skin, but with otherwise remarkably variable clinical features. The photosensitive TTD (PS-TTD) forms exhibits in addition to progressive neuropathy and other features of segmental accelerated aging and is associated with impaired genome maintenance and transcription. New factors involved in various steps of gene expression have been identified for the different non-photosensitive forms of TTD (NPS-TTD), which do not appear to show features of premature aging. Here, we identify alanyl-tRNA synthetase 1 and methionyl-tRNA synthetase 1 variants as new gene defects that cause NPS-TTD. These variants result in the instability of the respective gene products alanyl- and methionyl-tRNA synthetase. These findings extend our previous observations that TTD mutations affect the stability of the corresponding proteins and emphasize this phenomenon as a common feature of TTD. Functional studies in skin fibroblasts from affected individuals demonstrate that these new variants also impact on the rate of tRNA charging, which is the first step in protein translation. The extension of reduced abundance of TTD factors to translation as well as transcription redefines TTD as a syndrome in which proteins involved in gene expression are unstable.

Alveolar proteinosis of genetic origins

Pulmonary alveolar proteinosis (PAP) is a rare form of chronic interstitial lung disease, characterised by the intra-alveolar accumulation of lipoproteinaceous material. Numerous conditions can lead to its development. Whereas the autoimmune type is the main cause in adults, genetic defects account for a large part of cases in infants and children. Even if associated extra-respiratory signs may guide the clinician during diagnostic work-up, next-generation sequencing panels represent an efficient diagnostic tool. Exome sequencing also allowed the discovery of new variants and genes involved in PAP. The aim of this article is to summarise our current knowledge of genetic causes of PAP.

Aminoacyl-tRNA synthetases in Charcot-Marie-Tooth disease: A gain or a loss?

Charcot‐Marie‐Tooth disease (CMT) is one of the most common inherited neurodegenerative disorders with an increasing number of CMT‐associated variants identified as causative factors, however, there has been no effective therapy for CMT to date. Aminoacyl‐tRNA synthetases (aaRS) are essential enzymes in translation by charging amino acids onto their cognate tRNAs during protein synthesis. Dominant monoallelic variants of aaRSs have been largely implicated in CMT. Some aaRSs variants affect enzymatic activity, demonstrating a loss‐of‐function property. In contrast, loss of aminoacylation activity is neither necessary nor sufficient for some aaRSs variants to cause CMT. Instead, accumulating evidence from CMT patient samples, animal genetic studies or protein conformational analysis has pinpointed toxic gain‐of‐function of aaRSs variants in CMT, suggesting complicated mechanisms underlying the pathogenesis of CMT. In this review, we summarize the latest advances in studies on CMT‐linked aaRSs, with a particular focus on their functions. The current challenges, future direction and the promising candidates for potential treatment of CMT are also discussed.