Transfer RNA and human disease

Pathological mutations in tRNA genes and tRNA processing enzymes are numerous and result in very complicated clinical phenotypes. Mitochondrial tRNA (mt-tRNA) genes are hotspots for pathological mutations and over 200 mt-tRNA mutations have been linked to various disease states. Often these mutations prevent tRNA aminoacylation. Disrupting this primary function affects protein synthesis and the expression, folding, and function of oxidative phosphorylation enzymes. Mitochondrial tRNA mutations manifest in a wide panoply of diseases related to cellular energetics, including COX deficiency (cytochrome C oxidase), mitochondrial myopathy, MERRF (Myoclonic Epilepsy with Ragged Red Fibers), and MELAS (mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes). Diseases caused by mt-tRNA mutations can also affect very specific tissue types, as in the case of neurosensory non-syndromic hearing loss and pigmentary retinopathy, diabetes mellitus, and hypertrophic cardiomyopathy. Importantly, mitochondrial heteroplasmy plays a role in disease severity and age of onset as well. Not surprisingly, mutations in enzymes that modify cytoplasmic and mitochondrial tRNAs are also linked to a diverse range of clinical phenotypes. In addition to compromised aminoacylation of the tRNAs, mutated modifying enzymes can also impact tRNA expression and abundance, tRNA modifications, tRNA folding, and even tRNA maturation (e.g., splicing). Some of these pathological mutations in tRNAs and processing enzymes are likely to affect non-canonical tRNA functions, and contribute to the diseases without significantly impacting on translation. This chapter will review recent literature on the relation of mitochondrial and cytoplasmic tRNA, and enzymes that process tRNAs, to human disease. We explore the mechanisms involved in the clinical presentation of these various diseases with an emphasis on neurological disease

The Usher Syndrome Type IIIB Histidyl-tRNA Synthetase Mutation Confers Temperature Sensitivity

Histidyl-tRNA synthetase (HARS) is a highly conserved translation factor that plays an essential role in protein synthesis. HARS has been implicated in the human syndromes Charcot-Marie-Tooth (CMT) Type 2W and Type IIIB Usher (USH3B). The USH3B mutation, which encodes an Y454S substitution in HARS, is inherited in an autosomal recessive fashion and associated with childhood deafness, blindness and episodic hallucinations during acute illness. The biochemical basis of the pathophysiologies linked to USH3B is currently unknown. Here, we present a detailed functional comparison of wild type and Y454S HARS enzymes. Kinetic parameters for enzymes and canonical substrates were determined using both steady state and rapid kinetics. Enzyme stability was examined using differential scanning fluorimetry. Finally, enzyme functionality in primary cell culture was assessed. Our results demonstrate that the Y454S substitution leaves HARS amino acid activation, aminoacylation and tRNA(His) binding functions largely intact compared with WT HARS, and the mutant enzyme dimerizes similarly to wild type. Interestingly, during our investigation it was revealed that the kinetics of amino acid activation differs from the previously characterized bacterial HisRS. Despite the similar kinetics, differential scanning fluorimetry revealed that Y454S is less thermally stable than WT HARS, and cells from Y454S patients grown at elevated temperature demonstrate diminished protein synthesis compared to wild type cells. The thermal sensitivity associated with the Y454S mutation represents offers a biochemical basis for understanding USH3B