A new entity in the NARS2 variant: the first reported case of type 1 diabetes mellitus associated with the phenotype

Compound heterozygous variants of the NARS2 gene in siblings with refractory seizures: two case report and literature review

Background

Biallelic variants in NARS2 that encodes the mitochondrial asparaginyl-tRNA synthetase are associated with a wide spectrum of clinical phenotypes. Herein, we report on two siblings carrying the same compound heterozygous missense variants in NARS2, to improve the understanding of the phenotypic heterogeneity of NARS2 variants.

Case presentation

The two probands, a 3-year-old female (Patient 1) and a 16-month-old male (Patient 2), were clinically suspected of Combined oxidative phosphorylation deficiency 24 (COXPD24). Both presented with neurological manifestations, including refractory epilepsy, developmental delay and motor developmental regression, within the first year of life, accompanied by symmetrical brain lesions identified on magnetic resonance imaging (MRI). To elucidate the underlying genetic etiology, whole-exome sequencing (WES) was performed, followed by Sanger sequencing validation in the patients and their non-consanguineous parents. Genetic analysis revealed that both probands harbored identical compound heterozygous variants in the NARS2 gene: c.1253G>A (p.Arg418His) and c.1163C>T (p.Thr388Met). Notably, the c.1163C>T (p.Thr388Met) variant in NARS2 represents a novel finding, further expanding the genetic spectrum associated with this disorder.

Conclusions

Our findings expand the mutational spectrum of NARS2 and highlight the associated phenotypic heterogeneity, underscoring the critical role of NARS2 in epilepsy and neurodevelopmental processes. For pediatric patients with refractory epilepsy, early genetic testing is essential to improve diagnostic accuracy, refine prognostic stratification, and guide personalized treatment strategies. Additionally, mitochondrial drug cocktail therapy may be beneficial for epilepsy caused by NARS2 mutations.

A homozygous TARS2 variant is a novel cause of syndromic neonatal diabetes

AIMS

Neonatal diabetes is a monogenic condition which can be the presenting feature of complex syndromes. The aim of this study was to identify novel genetic causes of neonatal diabetes with neurological features including developmental delay and epilepsy.

METHODS

We performed genome sequencing in 27 individuals with neonatal diabetes plus epilepsy and/or developmental delay of unknown genetic cause. Replication studies were performed in 123 individuals with diabetes diagnosed aged ≤1 year without a known genetic cause using targeted next-generation sequencing.

RESULTS

Three individuals, all diagnosed with diabetes in the first week of life, shared a rare homozygous missense variant, p.(Arg327Gln), in TARS2. Replication studies identified the same homozygous variant in a fourth individual diagnosed with diabetes at 1 year. One proband had epilepsy, one had development delay and two had both. Biallelic TARS2 variants cause a mitochondrial encephalopathy (COXPD-21) characterised by severe hypotonia, epilepsy and developmental delay. Diabetes is not a known feature of COXPD-21. Current evidence suggests that the p.(Arg327Gln) variant disrupts TARS2's regulation of the mTORC1 pathway which is essential for β-cells.

CONCLUSIONS

Our findings establish the homozygous p.(Arg327Gln) TARS2 variant as a novel cause of syndromic neonatal diabetes and uncover a role for TARS2 in pancreatic β-cells.

Neonatal diabetes mellitus around the world: Update 2024

Neonatal diabetes mellitus (NDM), defined as diabetes with an onset during the first 6 months of life, is a rare form of monogenic diabetes. The initial publications on this condition began appearing in the second half of the 1990s and quite surprisingly, the search for new NDM genes is still ongoing with great vigor. Between 2018 and early 2024, six brand new NDM-genes have been discovered (CNOT1, FICD, ONECUT1, PDIA6, YIPF5, ZNF808) and three genes known to cause different diseases were identified as NDM-genes (EIF2B1, NARS2, KCNMA1). In addition, NDM cases carrying mutations in three other genes known to give rise to diabetes during childhood have been also identified (AGPAT2, BSCL2, PIK3R1). As a consequence, the list of NDM genes now exceeds 40. This genetic heterogeneity translates into many different mechanism(s) of disease that are being investigated with state-of-the-art methodologies, such as induced pluripotent stem cells (iPSC) and human embryonic stem cells (hESC) manipulated with the CRISPR technique of genome editing. This diversity in genetic causes and the pathophysiology of diabetes dictate the need for a variety of therapeutic approaches. The aim of this paper is to provide an overview on recent achievements in all aspects of this area of research.

Novel NARS2 variants in a patient with early-onset status epilepticus: case study and literature review

BACKGROUND

NARS2 as a member of aminoacyl-tRNA synthetases was necessary to covalently join a specific tRNA to its cognate amino acid. Biallelic variants in NARS2 were reported with disorders such as Leigh syndrome, deafness, epilepsy, and severe myopathy.

CASE PRESENTATION

Detailed clinical phenotypes were collected and the NARS2 variants were discovered by whole exome sequencing and verified by Sanger sequencing. Additionally, 3D protein structure visualization was performed by UCSF Chimera. The proband in our study had early-onset status epilepticus with abnormal EEG and MRI results. She also performed global developmental delay (GDD) and myocardial dysfunction. Next-generation sequencing (NGS) and Sanger sequencing revealed compound heterozygous missense variants [NM_024678.6:exon14: c.1352G > A(p.Arg451His); c.707T > C(p.Phe236Ser)] of the NARS2 gene. The proband develops refractory epilepsy with GDD and hyperlactatemia. Unfortunately, she finally died for status seizures two months later.

CONCLUSION

We discovered two novel missense variants of NARS2 in a patient with early-onset status epilepticus and myocardial dysfunction. The NGS enables the patient to be clearly diagnosed as combined oxidative phosphorylation deficiency 24 (COXPD24, OMIM:616,239), and our findings expands the spectrum of gene variants in COXPD24.

Progressive Mitochondrial Encephalopathy Due to the Novel Compound Heterozygous Variants c.182C>T and c.446A>AG in NARS2: A Case Report

Progressive mitochondrial encephalopathy manifesting as developmental delay, regression, epilepsy, myoclonus, dystonia, and spasticity due to a novel compound heterozygous variant in NARS2 has not been reported. The patient is a 3.5-year-old female with normal psychomotor development until she experienced her first generalized status epilepticus at 4.5 months of age. After seizure control, generalized myoclonus and psychomotor regression became evident. She suffered from two other epileptic states and seizure control remained inadequate despite the use of multiple anti-seizure drugs. Neurologic examination revealed generalized hypotonia, discoordination, unstable eye contact, drooling, open mouth, myoclonus, periodic torticollis, and ankle contractions. Cerebral MRI revealed hydrocephalus ex vacuo due to diffuse cortical and subcortical atrophy bilaterally and incomplete myelination. Genetic testing at 12 months of age revealed the compound heterozygous variants chr11: 78204182C>T and chr11: 78282446A>AG in NARS2. Despite anti-seizure drugs, mitochondrial cocktail, and cannabidiol, the disease progressed to intractable seizures and severe tetraspasticity. In summary, this case demonstrates that compound heterozygous variants in NARS2 can phenotypically manifest exclusively in the brain with intractable epilepsy, myoclonus, developmental delay, regression, hypotonia, cerebral atrophy, and hypomyelination, followed by tetraspasticity and dystonia.