Noninvasive diagnosis of TRIT1-related mitochondrial disorder by measuring i6 A37 and ms2 i6 A37 modifications in tRNAs from blood and urine samples

Evidence for sodium valproate toxicity in mitochondrial diseases: a systematic analysis

Background

We aimed to determine whether sodium valproate (VPA) should be contraindicated in all mitochondrial diseases, due to known VPA-induced severe hepatotoxicity in some mitochondrial diseases.

Methods

We systematically reviewed the published literature for mitochondrial DNA (mtDNA) and common nuclear genotypes of mitochondrial diseases using PubMed, Ovid Embase, Ovid Medline and MitoPhen databases. We extracted patient-level data from peer-reviewed articles, published until July 2022, using the Human Phenotype Ontology to manually code clinical presentations for 156 patients with genetic diagnoses from 90 publications.

Results

There were no fatal adverse drug reactions (ADRs) in the mtDNA disease group (35 patients), and only 1 out of 54 patients with a non-POLG mitochondrial disease developed acute liver failure. There were fatal outcomes in 53/102 (52%) POLG VPA-exposed patients who all harboured recessive mutations.

Conclusions

Our findings confirm the high risk of severe ADRs in any patient with recessive POLG variants irrespective of the phenotype, and therefore recommend that VPA is contraindicated in this group. However, there was limited evidence of toxicity to support a similar recommendation in other genotypes of mitochondrial diseases.

TRIT1 defect leads to a recognizable phenotype of myoclonic epilepsy, speech delay, strabismus, progressive spasticity, and normal lactate levels

TRIT1 defect is a rare, autosomal-recessive disorder of transcription, initially described as a condition with developmental delay, myoclonic seizures, and abnormal mitochondrial function. Currently, only 13 patients have been reported. We reviewed the genetic, clinical, and metabolic aspects of the disease in all known patients, including two novel, unrelated TRIT1 cases with abnormalities in oxidative phosphorylation complexes I and IV in fibroblasts. Taken together the features of all 15 patients, TRIT1 defect could be identified as a potentially recognizable syndrome including myoclonic epilepsy, speech delay, strabismus, progressive spasticity, and variable microcephaly, with normal lactate levels. Half of the patients had oxidative phosphorylation complex measurements and had multiple complex abnormalities.

Extracellular N6 -isopentenyladenosine (i6A) addition induces cotranscriptional i6A incorporation into ribosomal RNAs

N⁶ -isopentenyladenosine (i⁶A), a modified adenosine monomer, is known to induce cell death upon its addition to the culture medium. However, the molecular fate of extracellularly added i⁶A has yet to be identified. Here we show that i⁶A addition to cell culture medium results in i⁶A incorporation into cellular RNA in several cell lines, including the 5-fluorouracil (5-FU)-resistant human oral squamous cell carcinoma cell line FR2-SAS and its parental 5-FU-sensitive cell line SAS. i⁶A was predominantly incorporated into 18S and 28S rRNAs, and i⁶A incorporation into total RNA was mostly suppressed by treating these cell lines with an RNA polymerase I (Pol I) inhibitor. i⁶A was incorporated into RNA even upon inactivation of TRIT1, the only cellular i⁶A-modifying enzyme. These results indicate that upon cellular uptake of i⁶A, it is anabolized to be used for Pol I transcription. Interestingly, at lower i⁶A concentrations, the cytotoxic effect of i⁶A was substantially more pronounced in FR2-SAS cells than in SAS cells. Moreover, in FR2-SAS cells, i⁶A treatment decreased the rate of cellular protein synthesis and increased intracellular protein aggregation, and these effects were more pronounced than in SAS cells. Our work provides insights into the molecular fate of extracellularly applied i⁶A in the context of intracellular nucleic acid anabolism and suggests investigation of i⁶A as a candidate for a chemotherapy agent against 5-FU-resistant cancer cells.

A Case of Combined Oxidative Phosphorylation Deficiency 35 Associated with a Novel Missense Variant of the TRIT1 Gene

Combined oxidative phosphorylation deficiency 35 (COXPD35) is a rare autosomal recessive disorder associated with homozygous or compound heterozygous mutations in the tRNA isopentenyltransferase (TRIT1) gene in chromosome 1p34.2. To date, only 10 types of allelic variants in the TRIT1 gene have been previously reported in 9 patients with COXPD35. Herein, we describe a case with a novel homozygous missense variant in TRIT1. A 6-year, 6-month-old boy presented with global developmental delay, microcephaly, intractable seizures, and failure to thrive. The other main clinical manifestations were intellectual disability, spastic tetraparesis, truncal hypotonia, malnutrition, polyuria and polydipsia, ketotic hypoglycemia, dysmorphic facial features, strabismus, bicuspid aortic valve, and nephrolithiasis. The detailed biochemical, radiological, and metabolic evaluations were unremarkable. Chromosomal analysis confirmed a normal male 46,XY karyotype and the array comparative genomic hybridization analysis revealed no abnormalities. We identified a novel homozygous missense variant of c.246G>C (p.Met82Ile) in the TRIT1 gene, and the variant was confirmed by Sanger sequencing. The present case is the first report describing strabismus, ketotic hypoglycemia, nephrolithiasis, and bicuspid aortic valve in TRIT1-related COXPD35. This study expands the genotype-phenotype spectrum of TRIT1-related COXPD35.