Role of tRNA modifications in Galloway-Mowat syndrome – tRNAmodGAMOS

Transfer RNAs (tRNAs) are the most abundant and conserved RNA species playing a crucial role in protein translation. Their biogenesis involves several steps of processing and modifications, including intron splicing in a restricted subset of tRNA-encoding genes. In addition, hypomodified or misprocessed tRNAs can serve as a major source of tRNA-derived small RNAs (tsRNAs) that have diverse biological functions in cellular physiology and pathophysiology. Intron splicing is performed by the tRNA splicing endonuclease complex, TSEN, comprising TSEN15, TSEN2, TSEN34 and TSEN54 subunits.

Galloway-Mowat syndrome (GAMOS) is a rare autosomal recessive disorder associating steroid-resistant nephrotic syndrome (SRNS), microcephaly and brain anomalies. The renal disease is due to structural defects of podocytes, post-mitotic cells that play a critical role in plasma filtration. GAMOS is clinically and genetically heterogeneous with, to date, mutations identified in 11 genes encoding ubiquitously expressed proteins with diverse functions. Strikingly, most of the genes encode proteins involved in RNA metabolism, such as uridylate-rich small nuclear RNA processing for WDR73, and more importantly in tRNA metabolism, such as the m7G modification for WDR4, and the biosynthesis of the universal t6A modification of tRNAs decoding ANN codons, including the initiator tRNAMet, for the KEOPS complex components (the catalytic subunit OSGEP, LAGE3, TP53RK, TPRKB, GON7) and the YRDC enzyme. The identification of mutations in these genes raises the fascinating question of common molecular pathways that could be involved in kidney and brain development and function, in line with growing evidence that podocytes and neurons, two post-mitotic cells highly differentiated and polarized, share a large number of features. Intriguingly, we recently identified two novel variants in TSEN54 in a patient with GAMOS. One of these variants is intronic and unexpectedly leads to isolated SRNS when bi-allelic. Of note, recessive mutations have been identified in all the genes encoding TSEN subunits in patients affected with pontocerebellar hypoplasia, a rare neurodevelopmental disorder often associated with microcephaly without kidney damage. These data emphasize the emerging consensus that tRNA modifications contribute to GAMOS pathogenesis.

Our collaborative tRNAmodGAMOS project aims at investigating the extent of tRNA metabolism dysfunction in GAMOS linked to mutations in OSGEP and TSEN54, and in isolated SRNS due to the TSEN54 intronic variant. OSGEP and TSEN54 encode subunit of the KEOPS and the TSEN complex, respectively, both of which are highly conserved among species and ubiquitously expressed, with crucial roles in tRNA metabolism. With this in mind, our proposal integrates the scientific and technical expertise of three complementary research partners, combining relevant disease models and research tools (induced pluripotent stem cell-derived 2D and 3D models, Ribo-seq, transcriptomics/proteomics) to achieve the following objectives: i) explore the role of the TSEN54 intronic variant at the mRNA/protein levels in podocyte and neural progenitors/neuron dysfunction, ii) characterize in depth the defects in tRNAs by assessing their splicing, modificome and expression profiles, and evaluate tsRNA levels, iii) evaluate translation defects to find out whether there is global or specific translation alteration, and iv) characterize the cellular processes altered in iPSC-derived GAMOS and SRNS models.

This project will be instrumental in unveiling novel RNA pathogenic features that may be GAMOS-specific, SRNS-specific, or common to other neurological and glomerular diseases. This project may also shed light on unexpected cell type-specific roles of tRNAs and their derivatives and provide a better understanding of the translation process in general.