Dominant NARS1 mutations causing axonal Charcot-Marie-Tooth disease expand NARS1-associated diseases

Yeast models for Charcot-Marie-Tooth disease-causing aminoacyl-tRNA synthetase alleles reveal the cellular basis of disease

Charcot-Marie-Tooth disease (CMT) is a genetically diverse hereditary disorder that affects the motor and sensory nerves, impacting about 1 in 2500 people. It can be inherited through autosomal dominant (AD), autosomal recessive (AR), or X-linked genetic patterns. CMT2, one of the primary subtypes, is characterized by axonal degeneration and commonly presents with muscle weakness, atrophy, foot deformities, and sensory loss. Aminoacyl-tRNA synthetases (aaRSs) play an important role in the genetic underpinnings of CMT2, with more than 60 disease-causing alleles identified across eight different aaRSs, including alanyl-, asparaginyl-, histidyl-, glycyl-, methionyl-, tryptophanyl-, seryl-, and tyrosyl-tRNA synthetases. Mutations in aaRS genes can lead to destabilization of the enzyme, reduced aminoacylation, and aberrant protein complex formation. Yeast as a simple organism provides a robust model system to study the pathogenic effects of aaRS CMT mutations. In this review, we discuss the advantages and limitations of the yeast model systems for CMT2-causative mutations in aaRS.

tRNA synthetase activity is required for stress granule and P-body assembly

In response to stress, translation initiation is suppressed and ribosome runoff via translation elongation drives mRNA assembly into ribonucleoprotein (RNP) granules including stress granules and P-bodies. Defects in translation elongation activate the integrated stress response. If and how stalled ribosomes are removed from mRNAs during translation elongation stress to drive RNP granule assembly is not clear. We demonstrate the integrated stress response is induced upon tRNA synthetase inhibition in part via ribosome collision sensing. However, saturating levels of tRNA synthetase inhibitors do not induce stress granules or P-bodies and prevent RNP granule assembly upon exogenous stress. The loss of tRNA synthetase activity causes persistent ribosome stalls that can be released with puromycin but are not rescued by ribosome-associated quality control pathways. Therefore, tRNA synthetase activity is required for ribosomes to run off mRNAs during stress to scaffold cytoplasmic RNP granules. Our findings suggest ribosome stalls can persist in human cells and uniquely uncouple ribonucleoprotein condensate assembly from the integrated stress response.

Transfer RNA and small molecule therapeutics for aminoacyl-tRNA synthetase diseases

Aminoacyl-tRNA synthetases catalyze the ligation of a specific amino acid to its cognate tRNA. The resulting aminoacyl-tRNAs are indispensable intermediates in protein biosynthesis, facilitating the precise decoding of the genetic code. Pathogenic alleles in the aminoacyl-tRNA synthetases can lead to several dominant and recessive disorders. To date, disease-specific treatments for these conditions are largely unavailable. We review pathogenic human synthetase alleles, the molecular and cellular mechanisms of tRNA synthetase diseases, and emerging approaches to allele-specific treatments, including small molecules and nucleic acid-based therapeutics. Current treatment approaches to rescue defective or dysfunctional tRNA synthetase mutants include supplementation with cognate amino acids and delivery of cognate tRNAs to alleviate bottlenecks in translation. Complementary approaches use inhibitors to target the integrated stress response, which can be dysregulated in tRNA synthetase diseases.

Characterization of a novel heterozygous variant in the histidyl-tRNA synthetase gene associated with Charcot-Marie-Tooth disease type 2W

Heterozygous pathogenic variants in the histidyl-tRNA synthetase (HARS) gene are associated with Charcot-Marie-Tooth (CMT) type 2W disease, classified as an axonal peripheral neuropathy. To date, at least 60 variants causing CMT symptoms have been identified in seven different aminoacyl-tRNA synthetases, with eight being found in the catalytic domain of HARS. The genetic data clearly show a causative role of aminoacyl-tRNA synthetases in CMT; however, the cellular mechanisms leading to pathology can vary widely and are unknown in the case of most identified variants. Here we describe a novel HARS variant, c.412T>C; p.Y138H, identified through a CMT gene panel in a patient with peripheral neuropathy. To determine the effect of p.Y138H we employed a humanized HARS yeast model and recombinant protein biochemistry, which identified a deficiency in protein dimerization and a growth defect which shows mild but significant improvement with histidine supplementation. This raises the potential for a clinical trial of histidine.

The evolving spectrum of complex inherited neuropathies

PURPOSE OF REVIEW

Inherited peripheral neuropathies can be divided into those diseases in which peripheral neuropathy is the sole or main feature of the disease (Charcot-Marie-Tooth disease) and those in which peripheral neuropathy is just one feature of a more complex syndrome. In recent years there has been a substantial expansion in the number of genes associated with complex neuropathy syndromes.

RECENT FINDINGS

This review will focus on emerging themes in this group of diseases, namely the increasing number of diseases due to repeat expansions; the emergence of both recessive and dominant negative alleles in the same gene producing a common phenotype and diseases in which there is selective loss of the allele from haematopoietic stem cells making genetic diagnosis on blood derived DNA problematic.

SUMMARY

In this review we provide a practical approach to investigating and diagnosing patients with peripheral neuropathy as part of a complex syndrome and provide an updated table of the genes associated with this group of diseases.

Experience of the first adult-focussed undiagnosed disease program in Australia (AHA-UDP): solving rare and puzzling genetic disorders is ageless

BACKGROUND

Significant recent efforts have facilitated increased access to clinical genetics assessment and genomic sequencing for children with rare diseases in many centres, but there remains a service gap for adults. The Austin Health Adult Undiagnosed Disease Program (AHA-UDP) was designed to complement existing UDP programs that focus on paediatric rare diseases and address an area of unmet diagnostic need for adults with undiagnosed rare conditions in Victoria, Australia. It was conducted at a large Victorian hospital to demonstrate the benefits of bringing genomic techniques currently used predominantly in a research setting into hospital clinical practice, and identify the benefits of enrolling adults with undiagnosed rare diseases into a UDP program. The main objectives were to identify the causal mutation for a variety of diseases of individuals and families enrolled, and to discover novel disease genes.

METHODS

Unsolved patients in whom standard genomic diagnostic techniques such as targeted gene panel, exome-wide next generation sequencing, and/or chromosomal microarray, had already been performed were recruited. Genome sequencing and enhanced genomic analysis from the research setting were applied to aid novel gene discovery.

RESULTS

In total, 16/50 (32%) families/cases were solved. One or more candidate variants of uncertain significance were detected in 18/50 (36%) families. No candidate variants were identified in 16/50 (32%) families. Two novel disease genes (TOP3B, PRKACB) and two novel genotype-phenotype correlations (NARS, and KMT2C genes) were identified. Three out of eight patients with suspected mosaic tuberous sclerosis complex had their diagnosis confirmed which provided reproductive options for two patients. The utility of confirming diagnoses for patients with mosaic conditions (using high read depth sequencing and ddPCR) was not specifically envisaged at the onset of the project, but the flexibility to offer recruitment and analyses on an as-needed basis proved to be a strength of the AHA-UDP.

CONCLUSION

AHA-UDP demonstrates the utility of a UDP approach applying genome sequencing approaches in diagnosing adults with rare diseases who have had uninformative conventional genetic analysis, informing clinical management, recurrence risk, and recommendations for relatives.

Early-onset dysphagia and severe neurodevelopmental disorder as early signs in a patient with two novel variants in NARS1: a case report and brief review of the literature

Aminoacyl-tRNA synthetases (ARSs) aminoacylate tRNA molecules with their cognate amino acid, enabling information transmission and providing substrates for protein biosynthesis. They also take part in nontranslational functions, mediated by the presence of other proteins domains. Mutations in ARS genes have been described as responsive to numerous factors, including neurological, autoimmune, and oncological. Variants of the ARS genes, both in heterozygosity and homozygosity, have been reported to be responsible for different pathological pictures in humankind. We present the case of a patient referred in infancy for failure to thrive and acquired microcephaly (head circumference: -5 SD). During follow-up we highlighted: dysphagia (which became increasingly severe until it became incompatible with oral feeding, with gastrostomy implantation, resulting in resolution of feeding difficulties), strabismus, hypotonia. NCV (Nerve Conduction Velocity) showed four limbs neuropathy, neurophysiological examination performed at 2 years of age mainly sensory and demyelinating. Exome sequencing (ES) was performed, detecting two novel compound heterozygous variants in the NARS1 gene (OMIM *108410): NM_004539:c.[662 A > G]; [1155dup], p.[(Asn221Ser)]; [(Arg386Thrfs*19)], inherited from mother and father respectively. In this article, we would like to focus on the presence of progressive dysphagia and severe neurodevelopmental disorder, associated with two novel variants in the NARS1 gene.

A previously unreported NARS1 variant causes dominant distal hereditary motor neuropathy in a French family

BACKGROUND AND AIMS

Pathogenic variants in the NARS1 gene, which encodes for the asparaginyl-tRNA synthetase1 (NARS1) enzyme, were associated with complex central and peripheral nervous system phenotypes. Recently, Charcot-Marie-Tooth (CMT) disease has been linked to heterozygous pathogenic variants in NARS1 in nine patients. Here, we report two brothers and their mother from a French family with distal hereditary motor neuropathy (dHMN) carrying a previously unreported NARS1 variant.

METHODS

The NARS1 variant (c.1555G>C; p.(Gly519Arg)) was identified through whole-genome sequencing (WGS) performed on the family members. Clinical findings, nerve conduction studies (NCS), needle electromyography (EMG), and functional assays in yeast complementation assays are reported here.

RESULTS

The family members showed symptoms of dHMN, including distal weakness and osteoarticular deformities. They also exhibited brisk reflexes suggestive of upper motor neuron involvement. All patients were able to walk independently at the last follow-up. NCS and EMG confirmed pure motor neuropathy. Functional assays in yeast confirmed a loss-of-function effect of the variant on NARS1 activity.

INTERPRETATION

Our findings expand the clinical spectrum of NARS1-associated neuropathies, highlighting the association of NARS1 mutations with dHMN. The benign disease course observed in our patients suggests a slowly progressive phenotype. Further reports could contribute to a more comprehensive understanding of the spectrum of NARS1-associated neuropathies.