De Novo and Bi-allelic Pathogenic Variants in NARS1 Cause Neurodevelopmental Delay Due to Toxic Gain-of-Function and Partial Loss-of-Function Effects

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.

Setting the Stage for Treatment of Aminoacyl-tRNA Synthetase (ARS)1-Deficiencies: Phenotypic Characterization and a Review of Treatment Effects

Aminoacyl-transfer RNA (tRNA) synthetases (ARSs) are key enzymes for protein translation. The number of identified patients with recessive ARS1 deficiencies is rapidly increasing. Initially, only supportive care was available, but in recent years beneficial effects of targeted amino acid supplementation have been described. To allow early treatment and prevention of symptoms, rapid recognition is necessary, as well as insight into the natural history to evaluate treatment effects. We performed a scoping literature search for clinical characteristics and treatment effects of patients with ARS1 deficiencies. Symptoms were matched to Human Phenotype Ontology terms. We identified 438 patients with 20 different ARS1 deficiencies. Overall mortality was 22%. Neurological symptoms were most prevalent across all ARS1 deficiencies (in 87% of patients), including neurodevelopmental disorder (79%), microcephaly (50%) and seizures (46%). Growth issues and ophthalmological symptoms were also prevalent in many ARS1 deficiencies. Two distinct phenotypical clusters were seen: one with multisystemic disease including liver- and lung disease and another with a predominantly neurological phenotype. Supplementation with cognate amino acids was described in 21 patients, with beneficial effects (e.g., improvements in growth, development, liver and lung disease) in the majority. Treatment did not alleviate the most severe phenotypes. Specific symptoms relate to (a cluster of) specific ARS1 deficiencies; the mechanism is not yet understood. Multi-organ involvement should trigger inclusion of ARS1 genes in the diagnostic work-up. Treatment with cognate amino acids is promising, but it remains challenging to distinguish treatment effects from natural history. Synopsis: Treatment with cognate amino acids in ARS1 deficiencies is promising, but it remains challenging to distinguish treatment effects from natural history.

A novel pathogenic DICER1 gene variant is associated with hereditary multinodular goiter in an Argentine family as evidenced by clinical, biochemical and molecular genetic analysis

DICER1 syndrome is an autosomal-dominant disorder that results in malignant or benign tumors. A number of distinct pathogenic germline and somatic variants have been identified as causing multinodular goiter (MNG). The purpose of the present study was to identify and characterize the genetic cause underlying the familial form of MNG through a whole-exome sequencing (WES) analysis in an Argentine family with three affected siblings. Clinical, biochemical and molecular genetics as well as bioinformatics analysis were performed. A novel heterozygous variant in the DICER1 gene was identified in the proband patient by WES. The variant was a single guanine deletion at nucleotide position 2,042 (NM_177438.3:c.2042del) resulting in a frameshift at amino acid 681 with a putative premature stop codon [NP_803187.1:p.Gly681ValfsTer4]. Family segregation analysis showed that his affected sister and his affected brother also were heterozygous for same variant, whereas the father was a healthy heterozygous carrier of the variant and the healthy mother harbor only wild-type alleles in the DICER1 gene. We have also observed that the frameshift variant does not interfere with the pre-mRNA splicing of the exon 13. In addition, two clinically relevant heterozygous variants, not associated with thyroid disease, were also identified in index sibling using the Franklin platform, a frameshift [NP_000234.1:p.Thr55AsnfsTer49] in the MEFV gene (familial mediterranean fever) and a missense [NP_004530.1:p.Ala422Thr] in the NARS1 gene (neurodevelopmental delay and ataxia). In conclusion, in the present study we have identified a novel frameshift variant corresponding to NP_803187.1:p.Gly681ValfsTer4 in the DUF 283 domain of DICER1. The results were in accordance with previous observations confirming the genetic heterogeneity of DICER1 syndrome. Moreover, the identification of this variant in the unaffected father substantiates the hypothesis of incomplete/reduced penetrance.

Aminoacyl-tRNA synthetase defects in neurological diseases

Aminoacyl-tRNA synthetases (aaRSs) are essential enzymes to support protein synthesis in all organisms. Recent studies, empowered by advancements in genome sequencing, have uncovered an increasing number of disease-causing mutations in aaRSs. Monoallelic aaRS mutations typically lead to dominant peripheral neuropathies such as Charcot-Marie-Tooth (CMT) disease, whereas biallelic aaRS mutations often impair the central nervous system (CNS) and cause neurodevelopmental disorders. Here, we review recent progress in the disease onsets, molecular basis, and potential therapies for diseases caused by aaRS mutations, with a focus on biallelic mutations in cytoplasmic aaRSs.

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.

Simultaneous determination of cytosolic aminoacyl-tRNA synthetase activities by LC-MS/MS

In recent years, pathogenic variants in ARS genes, encoding aminoacyl-tRNA synthetases (aaRSs), have been associated with human disease. Patients harbouring pathogenic variants in ARS genes have clinical signs partly unique to certain aaRSs defects, partly overlapping between the different aaRSs defects. Diagnosis relies mostly on genetics and remains challenging, often requiring functional validation of new ARS variants. In this study, we present the development and validation of a method to simultaneously determine aminoacylation activities of all cytosolic aaRSs (encoded by ARS1 genes) in one single cell lysate, improving diagnosis in suspected ARS1 disorders and facilitating functional characterization of ARS1 variants of unknown significance. As proof of concept, we show enzyme activities of five individuals with variants in different ARS1 genes, demonstrating the usability and convenience of the presented method.

Exploring the Landscape of Pre- and Post-Synaptic Pediatric Disorders with Epilepsy: A Narrative Review on Molecular Mechanisms Involved

Neurodevelopmental disorders (NDDs) are a group of conditions affecting brain development, with variable degrees of severity and heterogeneous clinical features. They include intellectual disability (ID), autism spectrum disorder (ASD), attention-deficit/hyperactivity disorder (ADHD), often coexisting with epilepsy, extra-neurological comorbidities, and multisystemic involvement. In recent years, next-generation sequencing (NGS) technologies allowed the identification of several gene pathogenic variants etiologically related to these disorders in a large cohort of affected children. These genes encode proteins involved in synaptic homeostasis, such as SNARE proteins, implicated in calcium-triggered pre-synaptic release of neurotransmitters, or channel subunit proteins, such as post-synaptic ionotropic glutamate receptors involved in the brain's fast excitatory neurotransmission. In this narrative review, we dissected emerged molecular mechanisms related to NDDs and epilepsy due to defects in pre- and post-synaptic transmission. We focused on the most recently discovered SNAREopathies and AMPA-related synaptopathies.

Expanding the genetic spectrum of hereditary motor sensory neuropathies in Pakistan

BACKGROUND

Hereditary motor and sensory neuropathy (HMSN) refers to a group of inherited progressive peripheral neuropathies characterized by reduced nerve conduction velocity with chronic segmental demyelination and/or axonal degeneration. HMSN is highly clinically and genetically heterogeneous with multiple inheritance patterns and phenotypic overlap with other inherited neuropathies and neurodegenerative diseases. Due to this high complexity and genetic heterogeneity, this study aimed to elucidate the genetic causes of HMSN in Pakistani families using Whole Exome Sequencing (WES) for variant identification and Sanger sequencing for validation and segregation analysis, facilitating accurate clinical diagnosis.

METHODS

Families from Khyber Pakhtunkhwa with at least two members showing HMSN symptoms, who had not previously undergone genetic analysis, were included. Referrals for genetic investigations were based on clinical features suggestive of HMSN by local neurologists. WES was performed on affected individuals from each family, with Sanger sequencing used to validate and analyze the segregation of identified variants among family members. Clinical data including age of onset were assessed for variability among affected individuals, and the success rate of genetic diagnosis was compared with existing literature using proportional differences and Cohen's h.

RESULTS

WES identified homozygous pathogenic variants in GDAP1 (c.310 + 4 A > G, p.?), SETX (c.5948_5949del, p.(Asn1984Profs*30), IGHMBP2 (c.1591 C > A, p.(Pro531Thr) and NARS1 (c.1633 C > T, p.(Arg545Cys) as causative for HMSN in five out of nine families, consistent with an autosomal recessive inheritance pattern. Additionally, in families with HMSN, a SETX variant was found to cause cerebellar ataxia, while a NARS1 variant was linked to intellectual disability. Based on American College of Medical Genetics and Genomics criteria, the GDAP1 variant is classified as a variant of uncertain significance, while variants in SETX and IGHMBP2 are classified as pathogenic, and the NARS1 variant is classified as likely pathogenic. The age of onset ranged from 1 to 15 years (Mean = 5.13, SD = 3.61), and a genetic diagnosis was achieved in 55.56% of families with HMSN, with small effect sizes compared to previous studies.

CONCLUSIONS

This study expands the molecular genetic spectrum of HMSN and HMSN plus type neuropathies in Pakistan and facilitates accurate diagnosis, genetic counseling, and clinical management for affected families.

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.

Identification of diagnostic candidates in Mendelian disorders using an RNA sequencing-centric approach

BACKGROUND

RNA sequencing (RNA-seq) is increasingly being used as a complementary tool to DNA sequencing in diagnostics where DNA analysis has been uninformative. RNA-seq enables the identification of aberrant splicing and aberrant gene expression, improving the interpretation of variants of unknown significance (VUSs), and provides the opportunity to scan the transcriptome for aberrant splicing and expression in relevant genes that may be the cause of a patient's phenotype. This work aims to investigate the feasibility of generating new diagnostic candidates in patients without a previously reported VUS using an RNA-seq-centric approach.

METHODS

We systematically assessed the transcriptomic profiles of 86 patients with suspected Mendelian disorders, 38 of whom had no candidate sequence variant, using RNA from blood samples. Each VUS was visually inspected to search for splicing abnormalities. Once aberrant splicing was identified in cases with VUS, multiple open-source alternative splicing tools were used to investigate if they would identify what was observed in IGV. Expression outliers were detected using OUTRIDER. Diagnoses in cases without a VUS were explored using two separate strategies.

RESULTS

RNA-seq allowed us to assess 71% of VUSs, detecting aberrant splicing in 14/48 patients with a VUS. We identified four new diagnoses by detecting novel aberrant splicing events in patients with no candidate sequence variants from prior DNA testing (n = 32) or where the candidate VUS did not affect splicing (n = 23). An additional diagnosis was made through the detection of skewed X-inactivation.

CONCLUSION

This work demonstrates the utility of an RNA-centric approach in identifying novel diagnoses in patients without candidate VUSs. It underscores the utility of blood-based RNA analysis in improving diagnostic yields and highlights optimal approaches for such analyses.

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.

Allelic heterogeneity and abnormal vesicle recycling in PLAA-related neurodevelopmental disorders

The human PLAA gene encodes Phospholipase-A2-Activating-Protein (PLAA) involved in trafficking of membrane proteins. Through its PUL domain (PLAP, Ufd3p, and Lub1p), PLAA interacts with p97/VCP modulating synaptic vesicles recycling. Although few families carrying biallelic PLAA variants were reported with progressive neurodegeneration, consequences of monoallelic PLAA variants have not been elucidated. Using exome or genome sequencing we identified PLAA de-novo missense variants, affecting conserved residues within the PUL domain, in children affected with neurodevelopmental disorders (NDDs), including psychomotor regression, intellectual disability (ID) and autism spectrum disorders (ASDs). Computational and in-vitro studies of the identified variants revealed abnormal chain arrangements at C-terminal and reduced PLAA-p97/VCP interaction, respectively. These findings expand both allelic and phenotypic heterogeneity associated to PLAA-related neurological disorders, highlighting perturbed vesicle recycling as a potential disease mechanism in NDDs due to genetic defects of PLAA.

Clinical and molecular heterogeneity of VPS13D-related neurodevelopmental and movement disorders

BACKGROUND

The VPS13 family of proteins has been implicated in lipid transport and trafficking between endoplasmic reticulum and organelles, to maintain homeostasis of subcellular membranes. Recently, pathogenic variants in each human VPS13S gene, have been linked to distinct human neurodevelopmental or neurodegenerative disorders. Within the VPS13 family of genes, VPS13D is known to be implicated in mitochondria homeostasis and function.

METHODS

We investigated a Pakistani sibship affected with neurodevelopmental impairment and severe hyperkinetic (choreoathetoid) movements. Whole exome sequencing (WES) and Sanger sequencing were performed to identify potential candidate variants segregating in the family. We described clinical phenotypes and natural history of the disease during a 3-year clinical follow-up and summarized literature data related to previously identified patients with VPS13D-related neurological disorders.

RESULTS

We identified by WES an homozygous non-synonymous variant in VPS13D (c.5723 T > C; p.Ile1908Thr) as the potential underlying cause of the disease in our family. Two young siblings developed an early-onset neurological impairment characterized by global developmental delay, with impaired speech and motor milestones, associated to hyperkinetic movement disorders as well as progressive and non-progressive neurological abnormalities.

CONCLUSION

In this study we delineated the heterogeneity of VPS13D-related clinical phenotypes and described a novel VPS13D homozygous variant associated with severe neurological impairment. Further studies will be pivotal to understand the exact VPS13D function and its impact on mitochondria homeostasis, brain development and regulation of movements, to further clarify genotype-phenotype correlations and provide crucial prognostic information and potential therapeutic implications.

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

Pathogenic variants in six aminoacyl-tRNA synthetase (ARS) genes are implicated in neurological disorders, most notably inherited peripheral neuropathies. ARSs are enzymes that charge tRNA molecules with cognate amino acids. Pathogenic variants in asparaginyl-tRNA synthetase (NARS1) cause a neurological phenotype combining developmental delay, ataxia and demyelinating peripheral neuropathy. NARS1 has not yet been linked to axonal Charcot-Marie-Tooth disease. Exome sequencing of patients with inherited peripheral neuropathies revealed three previously unreported heterozygous NARS1 variants in three families. Clinical and electrophysiological details were assessed. We further characterized all three variants in a yeast complementation model and used a knock-in mouse model to study variant p.Ser461Phe. All three variants (p.Met236del, p.Cys342Tyr and p.Ser461Phe) co-segregate with the sensorimotor axonal neuropathy phenotype. Yeast complementation assays show that none of the three NARS1 variants support wild-type yeast growth when tested in isolation (i.e. in the absence of a wild-type copy of NARS1), consistent with a loss-of-function effect. Similarly, the homozygous knock-in mouse model (p.Ser461Phe/Ser472Phe in mouse) also demonstrated loss-of-function characteristics. We present three previously unreported NARS1 variants segregating with a sensorimotor neuropathy phenotype in three families. Functional studies in yeast and mouse support variant pathogenicity. Thus, NARS1 is the seventh ARS implicated in dominant axonal Charcot-Marie-Tooth disease, further stressing that all dimeric ARSs should be evaluated for Charcot-Marie-Tooth disease.

Delayed Graft Function in Pediatric Kidney Transplant: Risk Factors and Outcomes

OBJECTIVES

We aimed to identify risk factors and outcomes of delayed graft function in pediatric kidney transplant.

MATERIALS AND METHODS

This retrospective study included all kidney transplant recipients ≤19 years old followed up in our department for a period of 34 years, from January 1989 to December 2022.

RESULTS

We included 113 kidney transplant recipients. Delayed graft function occurred in 17 cases (15%). Posttransplant red blood cell transfusion was strongly associated with delayed graft function (adjusted odds ratio = 23.91; 95% CI, 2.889-197.915). Use of allografts with multiple arteries and cold ischemia time >20 hours were risk factors for delayed graft function (adjusted odds ratio = 52.51 and 49.4; 95% CI, 2.576-1070.407 and 1.833-1334.204, respectively). Sex-matched transplants and living donors were protective factors for delayed graft function (adjusted odds ratio = 0.043 and 0.027; 95% CI, 0.005-0.344 and 0.003-0.247, respectively). Total HLA mismatches <3 played a protective role for delayed graft function (adjusted odds ratio = 0.114; 95% CI, 0.020-0.662), whereas transplant within compatible but different blood types increased the risk of delayed graft function (adjusted odds ratio = 20.54; 95% CI, 1.960- 215.263). No significant correlation was shown between delayed graft function and allograft survival (P = .190). Our study suggested delayed graft function as a key factor in allograft rejection-free survival (adjusted odds ratio = 3.832; 95% CI, 1.186-12.377). Delayed graft function was a negative factor for early graft function; patients with delayed graft function had a lower estimated glomerular filtration rate at discharge (P = .024) and at 3 (P = .034), 6 (P = .019), and 12 months (P = .011) posttransplant.

CONCLUSIONS

Delayed graft function is a major determinant of early graft function and allograft rejection-free survival. Further research is required to establish proper preventive measures.

Neuroimaging in PRUNE1 syndrome: a mini-review of the literature

Prune exopolyphosphatase 1 (PRUNE1) is a short-chain phosphatase that is part of the aspartic acid-histidine-histidine (DHH) family of proteins. PRUNE1 is highly expressed in the central nervous system and is crucially involved in neurodevelopment, cytoskeletal rearrangement, cell migration, and proliferation. Recently, biallelic PRUNE1 variants have been identified in patients with neurodevelopmental disorders, hypotonia, microcephaly, variable cerebral anomalies, and other features. PRUNE1 hypomorphic mutations mainly affect the DHH1 domain, leading to an impactful decrease in enzymatic activity with a loss-of-function mechanism. In this review, we explored both the clinical and radiological spectrum related to PRUNE1 pathogenic variants described to date. Specifically, we focused on neuroradiological findings that, together with clinical phenotypes and genetic data, allow us to best characterize affected children with diagnostic and potential prognostic implications.

Neuroimaging features of WOREE syndrome: a mini-review of the literature

The WWOX gene encodes a 414-amino-acid protein composed of two N-terminal WW domains and a C-terminal short-chain dehydrogenase/reductase (SDR) domain. WWOX protein is highly conserved among species and mainly expressed in the cerebellum, cerebral cortex, brain stem, thyroid, hypophysis, and reproductive organs. It plays a crucial role in the biology of the central nervous system, and it is involved in neuronal development, migration, and proliferation. Biallelic pathogenic variants in WWOX have been associated with an early infantile epileptic encephalopathy known as WOREE syndrome. Both missense and null variants have been described in affected patients, leading to a reduction in protein function and stability. The most severe WOREE phenotypes have been related to biallelic null/null variants, associated with the complete loss of function of the protein. All affected patients showed brain anomalies on magnetic resonance imaging (MRI), suggesting the pivotal role of WWOX protein in brain homeostasis and developmental processes. We provided a literature review, exploring both the clinical and radiological spectrum related to WWOX pathogenic variants, described to date. We focused on neuroradiological findings to better delineate the WOREE phenotype with diagnostic and prognostic implications.

A PAK1 Mutational Hotspot Within the Regulatory CRIPaK Domain is Associated With Severe Neurodevelopmental Disorders in Children

BACKGROUND

P-21-activated kinases (PAKs) are protein serine/threonine kinases, part of the RAS/mitogen-activated protein kinase pathway. PAK1 is highly expressed in the central nervous system and crucially involved in neuronal migration and brain developmental processes. Recently, de novo heterozygous missense variants in PAK1 have been identified as an ultrarare cause of pediatric neurodevelopmental disorders.

METHODS

We report a series of children affected with postnatal macrocephaly, neurodevelopmental impairment, and drug-resistant epilepsy. Repeated electroencephalographic (EEG) and video-EEG evaluations were performed over a two- to 10-year period during follow-up to delineate electroclinical histories. Genetic sequencing studies and computational evaluation of the identified variants were performed in our patient cohort.

RESULTS

We identified by whole-exome sequencing three novel de novo variants in PAK1 (NM_001128620: c.427A>G, p.Met143Val; c.428T>C, p.Met143Thr; c.428T>A, p.Met143Lys) as the underlying cause of the disease in our families. The three variants affected the same highly conserved Met143 residue within the cysteine-rich inhibitor of PAK1 (CRIPaK) domain, which was identified before as a PAK1 inhibitor target. Computational studies suggested a defective autoinhibition presumably due to impaired PAK1 autoregulation as a result of the recurrent substitution.

CONCLUSIONS

We delineated the electroclinical phenotypes of PAK1-related neurological disorders and highlight a novel mutational hotspot that may involve defective autoinhibition of the PAK1 protein. The three novel variants affecting the same hotspot residue within the CRIPaK domain highlight potentially impaired PAK1-CRIPaK interaction as a novel disease mechanism. These findings shed light on possible future treatments targeted at the CRIPaK domain, to modulate PAK1 activity and function.

NUP85 as a Neurodevelopmental Gene: From Podocyte to Neuron

Pathogenic gene variants encoding nuclear pore complex (NPC) proteins were previously implicated in the pathogenesis of steroid-resistant nephrotic syndrome (SRNS). The NUP85 gene, encoding nucleoporin, is related to a very rare form of SRNS with limited genotype-phenotype information. We identified an Italian boy affected with an SRNS associated with severe neurodevelopmental impairment characterized by microcephaly, axial hypotonia, lack of achievement of motor milestones, and refractory seizures with an associated hypsarrhythmic pattern on electroencephalography. Brain magnetic resonance imaging (MRI) showed hypoplasia of the corpus callosum and a simplified gyration of the cerebral cortex. Since the age of 3 years, the boy was followed up at our Pediatric Nephrology Department for an SRNS, with a focal segmental glomerulosclerosis at renal biopsy. The boy died 32 months after SRNS onset, and a Whole-Exome Sequencing analysis revealed a novel compound heterozygous variant in NUP85 (NM_024844.5): 611T>A (p.Val204Glu), c.1904T>G (p.Leu635Arg), inherited from the father and mother, respectively. We delineated the clinical phenotypes of NUP85-related disorders, reviewed the affected individuals so far reported in the literature, and overall expanded both the phenotypic and the molecular spectrum associated with this ultra-rare genetic condition. Our study suggests a potential occurrence of severe neurological phenotypes as part of the NUP85-related clinical spectrum and highlights an important involvement of nucleoporin in brain developmental processes and neurological function.

The Cardiofaciocutaneous Syndrome: From Genetics to Prognostic-Therapeutic Implications

Cardiofaciocutaneous (CFC) syndrome is one of the rarest RASopathies characterized by multiple congenital ectodermal, cardiac and craniofacial abnormalities with a mild to severe ocular, gastrointestinal and neurological involvement. It is an autosomal dominant syndrome, with complete penetrance, caused by heterozygous pathogenic variants in the genes BRAF, MAP2K1/MEK1, MAP2K2/MEK2, KRAS or, rarely, YWHAZ, all part of the RAS-MAPK pathway. This pathway is a signal transduction cascade that plays a crucial role in normal cellular processes such as cell growth, proliferation, differentiation, survival, metabolism and migration. CFC syndrome overlaps with Noonan syndrome, Costello syndrome, neurofibromatosis type 1 and Legius syndrome, therefore making the diagnosis challenging. Neurological involvement in CFC is more severe than in other RASopathies. Phenotypic variability in CFC patients is related to the specific gene affected, without a recognized genotype-phenotype correlation for distinct pathogenic variants. Currently, there is no specific treatment for CFC syndrome. Encouraging zebrafish model system studies suggested that, in the future, MEK inhibitors could be a suitable treatment of progressive phenotypes of CFC in children. A multidisciplinary care is necessary for appropriate medical management.

Genomic Analysis of Multiplex Consanguineous Families Reveals Causes of Neurodevelopmental Disorders with Epilepsy

INTRODUCTION

Neurodevelopmental disorders (NDD) are a diverse group of disorders that affect the development of the nervous system. Epilepsy is a common phenotypic aspect of NDD.

METHODS

We recruited eight consanguineous families from Pakistan which segregated recessively inherited NDD with epilepsy. Magnetic Resonance imaging (MRI) and Electroencephalogram (EEG) were completed. Exome sequencing was carried out for selected participants from each family. The exome data were analyzed for exonic and splice-site variants that had allele frequencies of less than 0.01 in public databases.

RESULTS

Clinical investigations determined that developmental delay, intellectual disability and seizures were manifested by most patients in early childhood. EEG findings were abnormal in the participants of four families. MRI revealed demyelination orcerebral atrophic changes in multiple participants. We identified four novel homozygous variants including nonsense andmissense variants in OCLN, ALDH7A1, IQSEC2 and COL3A1, segregating with the phenotypes in the participants of four families. Previously reported homozygous variants of CNTNAP2, TRIT1 and NARS1 were found in individuals from three families. Clinical utility was observed in directing treatment in case of patients with an ALDH7A1 variant which included pyridoxine administration and enabling accurate counseling about the natural history and recurrence risk.

CONCLUSION

Our results add to the clinical and molecular delineation of very rare NDD with epilepsy. The high success rate of exome sequencing is likely attributable to the expectation of homozygous variants in patients of consanguineous families, and in one case, the availability of positional mapping data that greatly aided the variant prioritization.

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.

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

Protein synthesis is a fundamental process that underpins almost every aspect of cellular functioning. Intriguingly, despite their common function, recessive mutations in aminoacyl-tRNA synthetases (ARSs), the family of enzymes that pair tRNA molecules with amino acids prior to translation on the ribosome, cause a diverse range of multi-system disorders that affect specific groups of tissues. Neurological development is impaired in most ARS-associated disorders. In addition to central nervous system defects, diseases caused by recessive mutations in cytosolic ARSs commonly affect the liver and lungs. Patients with biallelic mutations in mitochondrial ARSs often present with encephalopathies, with variable involvement of peripheral systems. Many of these disorders cause severe disability, and as understanding of their pathogenesis is currently limited, there are no effective treatments available. To address this, accurate in vivo models for most of the recessive ARS diseases are urgently needed. Here, we discuss approaches that have been taken to model recessive ARS diseases in vivo, highlighting some of the challenges that have arisen in this process, as well as key results obtained from these models. Further development and refinement of animal models is essential to facilitate a better understanding of the pathophysiology underlying recessive ARS diseases, and ultimately to enable development and testing of effective therapies.

Clinical genetics of Charcot-Marie-Tooth disease

Recent research in the field of inherited peripheral neuropathies (IPNs) such as Charcot-Marie-Tooth (CMT) disease has helped identify the causative genes provided better understanding of the pathogenesis, and unraveled potential novel therapeutic targets. Several reports have described the epidemiology, clinical characteristics, molecular pathogenesis, and novel causative genes for CMT/IPNs in Japan. Based on the functions of the causative genes identified so far, the following molecular and cellular mechanisms are believed to be involved in the causation of CMTs/IPNs: myelin assembly, cytoskeletal structure, myelin-specific transcription factor, nuclear related, endosomal sorting and cell signaling, proteasome and protein aggregation, mitochondria-related, motor proteins and axonal transport, tRNA synthetases and RNA metabolism, and ion channel-related mechanisms. In this article, we review the epidemiology, genetic diagnosis, and clinicogenetic characteristics of CMT in Japan. In addition, we discuss the newly identified novel causative genes for CMT/IPNs in Japan, namely MME and COA7. Identification of the new causes of CMT will facilitate in-depth characterization of the underlying molecular mechanisms of CMT, leading to the establishment of therapeutic approaches such as drug development and gene therapy.

Precision medicine for developmental and epileptic encephalopathies in Africa-strategies for a resource-limited setting

PURPOSE

Sub-Saharan Africa bears the highest burden of epilepsy worldwide. A presumed proportion is genetic, but this etiology is buried under the burden of infections and perinatal insults in a setting of limited awareness and few options for testing. Children with developmental and epileptic encephalopathies (DEEs) are most severely affected by this diagnostic gap in Africa, because the rate of actionable findings is highest in DEE-associated genes.

METHODS

We tested 234 genetically naive South African children diagnosed with/possible DEE using gene panels, exome sequencing, and chromosomal microarray. Statistical comparison of electroclinical features in children with and children without candidate variants was performed to identify characteristics most likely predictive of a positive genetic finding.

RESULTS

Of the 41 (of 234) children with likely/pathogenic variants, 26 had variants supporting precision therapy. Multivariate regression modeling highlighted neonatal or infantile-onset seizures and movement abnormalities as predictive of a positive genetic finding. We used this, coupled with an emphasis on precision medicine outcomes, to propose the pragmatic "Think-Genetics" strategy for early recognition of a possible genetic etiology.

CONCLUSION

Our findings emphasize the importance of an early genetic diagnosis in DEE. We designed the Think-Genetics strategy for early recognition, appropriate interim management, and genetic testing for DEE in resource-constrained settings.

Clinical and genetic analyses of premature mitochondrial encephalopathy with epilepsia partialis continua caused by novel biallelic NARS2 mutations

Biallelic NARS2 mutations can cause various neurodegenerative diseases, leading to growth retardation, intractable epilepsy, and hearing loss in early infancy and further progressing to spastic paraplegia, neurodegeneration, and even death. NARS2 mutations are associated with mitochondrial dysfunction and cause combined oxidative phosphorylation deficiency 24 (COXPD24). Relatively few cases have been reported worldwide; therefore, the pathogenesis of COXPD24 is poorly understood. We studied two unrelated patients with COXPD24 with similar phenotypes who presented with intractable refractory epilepsia partialis continua, hearing loss, and growth retardation. One patient died from epilepsy. Three novel NARS2 variants (case 1: c.185T > C and c.251 + 2T > G; case 2: c.185T > C and c.509T > G) were detected with whole-exome sequencing. c.251 + 2T > G is located at the donor splicing site in the non-coding sequence of the gene. The minigene experiment further verified that c.251 + 2T > G caused variable splicing abnormalities and produced truncated proteins. Molecular dynamics studies showed that c.185T > C and c.509T > G reduced the binding free energy of the NARS2 protein dimer. The literature review revealed fewer than 30 NARS2 variants. These findings improved our understanding of the disease phenotype and the variation spectrum and revealed the potential pathogenic mechanism of non-coding sequence mutations in COXPD24.

Loss of seryl-tRNA synthetase (SARS1) causes complex spastic paraplegia and cellular senescence

BACKGROUND

Aminoacyl-tRNA synthetases (ARS) are key enzymes catalysing the first reactions in protein synthesis, with increasingly recognised pleiotropic roles in tumourgenesis, angiogenesis, immune response and lifespan. Germline mutations in several ARS genes have been associated with both recessive and dominant neurological diseases. Recently, patients affected with microcephaly, intellectual disability and ataxia harbouring biallelic variants in the seryl-tRNA synthetase encoded by seryl-tRNA synthetase 1 (SARS1) were reported.

METHODS

We used exome sequencing to identify the causal variant in a patient affected by complex spastic paraplegia with ataxia, intellectual disability, developmental delay and seizures, but without microcephaly. Complementation and serylation assays using patient's fibroblasts and an Saccharomyces cerevisiae model were performed to examine this variant's pathogenicity.

RESULTS

A de novo splice site deletion in SARS1 was identified in our patient, resulting in a 5-amino acid in-frame insertion near its active site. Complementation assays in S. cerevisiae and serylation assays in both yeast strains and patient fibroblasts proved a loss-of-function, dominant negative effect. Fibroblasts showed an abnormal cell shape, arrested division and increased beta-galactosidase staining along with a senescence-associated secretory phenotype (raised interleukin-6, p21, p16 and p53 levels).

CONCLUSION

We refine the phenotypic spectrum and modes of inheritance of a newly described, ultrarare neurodevelopmental disorder, while unveiling the role of SARS1 as a regulator of cell growth, division and senescence.

Aminoacyl-tRNA synthetases in human health and disease

The Aminoacyl-tRNA Synthetases (aaRSs) are an evolutionarily ancient family of enzymes that catalyze the esterification reaction linking a transfer RNA (tRNA) with its cognate amino acid matching the anticodon triplet of the tRNA. Proper functioning of the aaRSs to create aminoacylated (or “charged”) tRNAs is required for efficient and accurate protein synthesis. Beyond their basic canonical function in protein biosynthesis, aaRSs have a surprisingly diverse array of non-canonical functions that are actively being defined. The human genome contains 37 genes that encode unique aaRS proteins. To date, 56 human genetic diseases caused by damaging variants in aaRS genes have been described: 46 are autosomal recessive biallelic disorders and 10 are autosomal dominant monoallelic disorders. Our appreciation of human diseases caused by damaging genetic variants in the aaRSs has been greatly accelerated by the advent of next-generation sequencing, with 89% of these gene discoveries made since 2010. In addition to these genetic disorders of the aaRSs, anti-synthetase syndrome (ASSD) is a rare autoimmune inflammatory myopathy that involves the production of autoantibodies that disrupt aaRS proteins. This review provides an overview of the basic biology of aaRS proteins and describes the rapidly growing list of human diseases known to be caused by genetic variants or autoimmune targeting that affect both the canonical and non-canonical functions of these essential proteins.

WARS1 and SARS1: Two tRNA synthetases implicated in autosomal recessive microcephaly

Aminoacylation of transfer RNA (tRNA) is a key step in protein biosynthesis, carried out by highly specific aminoacyl-tRNA synthetases (ARSs). ARSs have been implicated in autosomal dominant and autosomal recessive human disorders. Autosomal dominant variants in tryptophanyl-tRNA synthetase 1 (WARS1) are known to cause distal hereditary motor neuropathy and Charcot-Marie-Tooth disease, but a recessively inherited phenotype is yet to be clearly defined. Seryl-tRNA synthetase 1 (SARS1) has rarely been implicated in an autosomal recessive developmental disorder. Here, we report five individuals with biallelic missense variants in WARS1 or SARS1, who presented with an overlapping phenotype of microcephaly, developmental delay, intellectual disability, and brain anomalies. Structural mapping showed that the SARS1 variant is located directly within the enzyme's active site, most likely diminishing activity, while the WARS1 variant is located in the N-terminal domain. We further characterize the identified WARS1 variant by showing that it negatively impacts protein abundance and is unable to rescue the phenotype of a CRISPR/Cas9 wars1 knockout zebrafish model. In summary, we describe two overlapping autosomal recessive syndromes caused by variants in WARS1 and SARS1, present functional insights into the pathogenesis of the WARS1-related syndrome and define an emerging disease spectrum: ARS-related developmental disorders with or without microcephaly.

Fatal systemic disorder caused by biallelic variants in FARSA

Background

Aminoacyl tRNA transferases play an essential role in protein biosynthesis, and variants of these enzymes result in various human diseases. FARSA, which encodes the α subunit of cytosolic phenylalanyl-tRNA synthetase, was recently reported as a suspected causal gene for multiorgan disorder. This study aimed to validate the pathogenicity of variants in the FARSA gene.

Results

Exome sequencing revealed novel compound heterozygous variants in FARSA, P347L and R475Q, from a patient who initially presented neonatal-onset failure to thrive, liver dysfunction, and frequent respiratory infections. His developmental milestones were nearly arrested, and the patient died at 28 months of age as a result of progressive hepatic and respiratory failure. The P347L variant was predicted to disrupt heterodimer interaction and failed to form a functional heterotetramer by structural and biochemical analyses. R475 is located at a highly conserved site and is reported to be involved in phenylalanine activation and transfer to tRNA. The R475Q mutant FARSA were co-purified with FARSB, but the mutant enzyme showed an approximately 36% reduction in activity in our assay relative to the wild-type protein. Additional functional analyses on variants from previous reports (N410K, F256L, R404C, E418D, and F277V) were conducted. The R404C variant from a patient waiting for organ transplantation also failed to form tetramers but the E418D, N410K, F256L, and F277V variants did not affect tetramer formation. In the functional assay, the N410K located at the phenylalanine-binding site exhibited no catalytic activity, whereas other variants (E418D, F256L and F277V) exhibited lower ATPase activity than wild-type FARSA at low phenylalanine concentrations.

Conclusions

Our data demonstrated the pathogenicity of biallelic variants in FARSA and suggested the implication of hypomorphic variants in severe phenotypes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-022-02457-9.

The tRNA regulome in neurodevelopmental and neuropsychiatric disease

Transfer (t)RNAs are 70-90 nucleotide small RNAs highly regulated by 43 different types of epitranscriptomic modifications and requiring aminoacylation ('charging') for mRNA decoding and protein synthesis. Smaller cleavage products of mature tRNAs, or tRNA fragments, have been linked to a broad variety of noncanonical functions, including translational inhibition and modulation of the immune response. Traditionally, knowledge about tRNA regulation in brain is derived from phenotypic exploration of monogenic neurodevelopmental and neurodegenerative diseases associated with rare mutations in tRNA modification genes. More recent studies point to the previously unrecognized potential of the tRNA regulome to affect memory, synaptic plasticity, and affective states. For example, in mature cortical neurons, cytosine methylation sensitivity of the glycine tRNA family (tRNA^Gly) is coupled to glycine biosynthesis and codon-specific alterations in ribosomal translation together with robust changes in cognition and depression-related behaviors. In this Review, we will discuss the emerging knowledge of the neuronal tRNA landscape, with a focus on epitranscriptomic tRNA modifications and downstream molecular pathways affected by alterations in tRNA expression, charging levels, and cleavage while mechanistically linking these pathways to neuropsychiatric disease and provide insight into future areas of study for this field.

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.

Prominent and Regressive Brain Developmental Disorders Associated with Nance-Horan Syndrome

Nance-Horan syndrome (NHS) is a rare X-linked developmental disorder caused mainly by loss of function variants in the NHS gene. NHS is characterized by congenital cataracts, dental anomalies, and distinctive facial features, and a proportion of the affected individuals also present intellectual disability and congenital cardiopathies. Despite identification of at least 40 distinct hemizygous variants leading to NHS, genotype-phenotype correlations remain largely elusive. In this study, we describe a Sicilian family affected with congenital cataracts and dental anomalies and diagnosed with NHS by whole-exome sequencing (WES). The affected boy from this family presented a late regression of cognitive, motor, language, and adaptive skills, as well as broad behavioral anomalies. Furthermore, brain imaging showed corpus callosum anomalies and periventricular leukoencephalopathy. We expand the phenotypic and mutational NHS spectrum and review potential disease mechanisms underlying the central neurological anomalies and the potential neurodevelopmental features associated with NHS.

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.

Biallelic Variants in KIF17 Associated with Microphthalmia and Coloboma Spectrum

Microphthalmia, anophthalmia, and coloboma (MAC) are a group of congenital eye anomalies that can affect one or both eyes. Patients can present one or a combination of these ocular abnormalities in the so called “MAC spectrum”. The KIF17 gene encodes the kinesin-like protein Kif17, a microtubule-based, ATP-dependent, motor protein that is pivotal for outer segment development and disc morphogenesis in different animal models, including mice and zebrafish. In this report, we describe a Sicilian family with two siblings affected with congenital coloboma, microphthalmia, and a mild delay of motor developmental milestones. Genomic DNA from the siblings and their unaffected parents was sequenced with a clinical exome that revealed compound heterozygous variants in the KIF17 gene (NM_020816.4: c.1255C > T (p.Arg419Trp); c.2554C > T (p.Arg852Cys)) segregating with the MAC spectrum phenotype of the two affected siblings. Variants were inherited from the healthy mother and father, are present at a very low-frequency in genomic population databases, and are predicted to be deleterious in silico. Our report indicates the potential co-segregation of these biallelic KIF17 variants with microphthalmia and coloboma, highlighting a potential conserved role of this gene in eye development across different species.

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.

Loss of NARS1 impairs progenitor proliferation in cortical brain organoids and leads to microcephaly

Asparaginyl-tRNA synthetase1 (NARS1) is a member of the ubiquitously expressed cytoplasmic Class IIa family of tRNA synthetases required for protein translation. Here, we identify biallelic missense and frameshift mutations in NARS1 in seven patients from three unrelated families with microcephaly and neurodevelopmental delay. Patient cells show reduced NARS1 protein, impaired NARS1 activity and impaired global protein synthesis. Cortical brain organoid modeling shows reduced proliferation of radial glial cells (RGCs), leading to smaller organoids characteristic of microcephaly. Single-cell analysis reveals altered constituents of both astrocytic and RGC lineages, suggesting a requirement for NARS1 in RGC proliferation. Our findings demonstrate that NARS1 is required to meet protein synthetic needs and to support RGC proliferation in human brain development.