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

Abstract

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.

Keywords: GCN2; aminoacyl‐tRNA synthetase; genetic disorders; tRNA therapeutics.

Fig. 1

Aminoacylation of tRNA by aminoacyl‐tRNA synthetases. Aminoacylation involves the activation of the cognate amino acid by an aminoacyl‐tRNA synthetase (aaRS). The aaRS binds ATP and the amino acid to form an aminoacyl adenylate intermediate, releasing pyrophosphate. Subsequently, the cognate tRNA is bound, and the transesterification reaction is catalyzed releasing aminoacylated tRNA and AMP. The aminoacylated tRNA is used for peptide synthesis at the ribosome. tRNA synthetase and tRNA structures are derived from the human WARS structure in complex with tryptophanyl‐tRNA (PDB: 2AKE), Graphics of structures were generated using VMD vm [109].

Fig. 2

Schematic representation of a cell showing the transcription and sorting of tRNA synthetases. All tRNA synthetases are nuclear‐encoded and are subsequentially sorted to the cytoplasm and the mitochondria. Seventeen of the tRNA synthetases have dual copies, such that one gene encodes for a protein that functions in the cytoplasm, and another for a protein that functions in the mitochondria. Two of the tRNA synthetases, KARS and GARS, are bifunctional and encoded in a single gene providing transcripts for functions in both the cytoplasm and mitochondria (dashed outline). EPARS is a dual‐synthetase fusion protein in the cytosol. QARS has dedicated aminoacylation activity in the cytoplasm; however, mitochondrial glutaminyl‐tRNA^Gln is synthesized indirectly. Nine cytoplasmic tRNA synthetases form a multi‐synthetase complex (MSC) along with three non‐synthetase proteins. Class I tRNA synthetases are colored blue and class II tRNA synthetases are colored green.

Fig. 3

Cellular consequences of pathogenic aaRS alleles and current treatment approaches. Gain‐of‐function phenotypes often arise from non‐specific substrate recognition, leading to mistranslation, or altered protein–protein interactions. Global mistranslation leads to protein aggregation, activating the heat shock and unfolded protein response. Loss‐of‐function in tRNA synthetases can be caused by for example reduced aminoacylation activity or tRNA binding, loss of structural integrity or dimerization. This results in the accumulation of uncharged tRNA and factors such as GCN2 and ATF4 are activated as part of the integrated stress response. Cognate amino acids and tRNAs supplementation can rescue mutant tRNA synthetases. ISR inhibitors, such as GCN2 inhibitors can attenuate the ISR. tRNA synthetase and tRNA structures are derived from the human WARS structure in complex with tryptophanyl‐tRNA, PDB code 2AKE, graphics of structures were generated using vmd [109].