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It has been generally thought that tRNA modifications are stable and static, and their frequencies are rarely regulated. N6-threonylcarbamoyladenosine (t6A) occurs at position 37 of five mitochondrial (mt-)tRNA species. We show that YRDC and OSGEPL1 are responsible for t6A37 formation, utilizing L-threonine, ATP, and CO2/bicarbonate as substrates. OSGEPL1-knockout cells exhibit respiratory defects and reduced mitochondrial translation. We find low level of t6A37 in mutant mt-tRNA isolated from the MERRF-like patient’s cells, indicating that lack of t6A37 results in pathological consequences. Kinetic measurements of t6A37 formation reveal that the Km value of CO2/bicarbonate is extremely high (31 mM), suggesting that CO2/bicarbonate is a rate-limiting factor for t6A37 formation. Consistent with this, we observe a low frequency of t6A37 in mt-tRNAs isolated from human cells cultured without bicarbonate. These findings indicate that t6A37 is regulated by sensing intracellular CO2/bicarbonate concentration, implying that mitochondrial translation is modulated in a codon-specific manner under physiological conditions.

Duchenne muscular dystrophy (DMD) is a lethal hereditary muscle disease caused by mutations in the gene encoding the muscle protein dystrophin. These mutations result in a shift in the open reading frame leading to loss of the dystrophin protein. Antisense oligonucleotides (ASOs) that induce exon skipping correct this frame shift during pre-mRNA splicing and partially restore dystrophin expression in mouse and dog models. We conducted a phase 1, open-label, dose-escalation clinical trial to determine the safety, pharmacokinetics, and activity of NS-065/NCNP-01, a morpholino ASO that enables skipping of exon 53. Ten patients with DMD (6 to 16 years old), carrying mutations in the dystrophin gene whose reading frame would be restored by exon 53 skipping, were administered NS-065/NCNP-01 at doses of 1.25, 5, or 20 mg/kg weekly for 12 weeks. The primary endpoint was safety; the secondary endpoints were pharmacokinetics and successful exon skipping. No severe adverse drug reactions were observed, and no treatment discontinuation occurred. Muscle biopsy samples were taken before and after treatment and compared by reverse transcription polymerase chain reaction (RT-PCR), immunofluorescence, and Western blotting to assess the amount of exon 53 skipping and dystrophin expression. NS-065/NCNP-01 induced exon 53 skipping in dystrophin-encoding mRNA in a dose-dependent manner and increased the dystrophin/spectrin ratio in 7 of 10 patients. Furthermore, the amount of exon skipping correlated with the maximum drug concentration in plasma () and the area under the concentration-time curve in plasma (AUC ). These results indicate that NS-065/NCNP-01 has a favorable safety profile and promising pharmacokinetics warranting further study in a phase 2 clinical trial.

Duchenne muscular dystrophy (DMD), a rare genetic disorder characterized by progressive muscle weakness, is caused by the absence or a decreased amount of the muscle cytoskeletal protein dystrophin. Currently, several therapeutic approaches to cure DMD are being investigated, which can be categorized into two groups: therapies that aim to restore dystrophin expression, and those that aim to compensate for the lack of dystrophin. Therapies that restore dystrophin expression include read-through therapy, exon skipping, vector-mediated gene therapy, and cell therapy. Of these approaches, the most advanced are the read-through and exon skipping therapies. In 2014, ataluren, a drug that can promote ribosomal read-through of mRNA containing a premature stop codon, was conditionally approved in Europe. In 2016, eteplirsen, a morpholino-based chemical capable of skipping exon 51 in premature mRNA, received conditional approval in the USA. Clinical trials on vector-mediated gene therapy carrying micro- and mini- dystrophin are underway. More innovative therapeutic approaches include CRISPR/Cas9-based genome editing and stem cell-based cell therapies. Here we review the current status of therapeutic approaches for DMD, focusing on therapeutic approaches that can restore dystrophin.

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