# tRNA and tsRNA: From Heterogeneity to Multifaceted Regulators
## 7 Emerging Stars in Disease and Therapeutic Modulation  
A growing body of evidence has established a significant link between tRNA biology and a range of pathological conditions, including neurodegenerative diseases [178], metabolic disorders [179], and cancer [180], among others. While some diseases involve mutations on tRNA genes, particularly mitochondrial tRNA mutations, which have been extensively summarized in several excellent reviews [41,181], an increasing number of studies have identified the etiology of some diseases, with alterations in RNA pol III, transcription factors, tRNA gene methylation, histone methylation, tRNA modifying enzymes, and aminoacyl-tRNA synthetases [41,182,183]. These factors collectively modulate the landscape of cellular tRNA expression profiles, tRNA modification signatures, and amino acid charging levels, thereby dysregulated tRNA biological functions and contributing to disease pathogenesis. A prominent example of how altered tRNA expression profiles affect biological functions is found in cancer, where the composition of tRNA pools favors the translation of oncogenic factors rich in specific codons, promoting disease progression [27]. Furthermore, modifications in the tRNA epistranscriptome impact anticodon-codon affinity, the recognition of wobble base positions, and tRNA stability [7, 8, 9], all of which influence translation efficiency. For instance, the loss of PUS1 function leads to the absence of \(\mathbf{\Psi}\) modifications at specific sites on tRNA (positions 27 and 28), which is associated with the development of mitochondrial myopathy and sideroblastic anemia (MLASA) [179]. It is hypothesized that these modifications affect tRNA structure formation and stability. Additionally, tRNA charging levels play a crucial role in determining the availability of tRNAs for translation. In the mitochondrial encephalopathy with lactic acidosis and stroke-like episodes (MELAS) syndrome cell model, it was observed that the content of mt-tRNA Leu(UUR) decreased by more than 50% compared to normal cells, and its aminoacylation efficiency was similarly reduced by 50% [184]. When considering both the reduction in mt-tRNA Leu(UUR) levels and the aminoacylation efficiency, the actual level of aminoacylated mt-tRNA Leu(UUR) was estimated to be only 25% to 30% of that in the control group [184], emphasizing the association of abnormal aminoacylation levels with disease. Instead of tRNA, tsRNAs are emerging as novel diagnostic biomarkers and crucial regulators in various diseases. The diversity of expression signatures of tsRNAs in human diseases such as epilepsy [185], acute myeloid leukemia [28], chronic lymphocytic leukemia [186], lung cancer [187], breast cancer [188], and aging [189] enable tsRNAs to serve as promising candidates for diagnostic and therapy applications. Moreover, a recent study found a significant production of tsRNA Glu(CTC) within mitochondria in brains with Alzheimer's disease. The accumulation of tsRNA Glu(CTC) competes with mitochondrial leucine tRNA for binding to leucyl-tRNA synthetase 2 (LaRs2), disrupting the aminoacylation of leucine tRNA and impairing the translation of mitochondrial proteins. This disruption affects both the cristae structure and glutamate synthesis, contributing to accelerated brain aging and the progression of Alzheimer's disease [189]. The clinical application of tRNA in therapeutic strategies for certain diseases has proved to be quite exciting, particularly with the use of suppressor tRNA (Sup-tRNA). Sup-tRNA is a unique type of tRNA capable of recognizing and decoding stop codons (such as UGA, UAA, and UAG) on mRNA, allowing for the insertion of a specific amino acid during protein synthesis rather than terminating translation [190]. This capability makes Sup-tRNA a potentially powerful tool in both basic research and clinical treatment, especially in solving genetic disorders where premature stop codons (nonsense mutations)cause the early termination of protein synthesis, leading to functional deficiencies.