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Post-transcriptional tRNA modifications are numerous and require a large set of highly conserved enzymes in humans and other organisms. In yeast, the loss of many modifications is tolerated under unstressed conditions; one exception is the N 6 -threonyl-carbamoyl-adenosine (t 6 A) modification, loss of which causes a severe growth phenotype. Here we aimed at a molecular diagnosis in a brother and sister from a consanguineous family who presented with global developmental delay, failure to thrive and a renal defect manifesting in proteinuria and hypomagnesemia. Using exome sequencing, the patients were found to be homozygous for the c.974G&gt;A (p.(Arg325Gln)) variant of the KAE1 gene. KAE1 is a constituent of the KEOPS complex, a five-subunit complex that catalyzes the second biosynthetic step of t 6 A in the cytosol. The yeast KAE1 allele carrying the equivalent mutation did not rescue the t 6 A deficiency of the kae1Δ yeast strain as efficiently as the WT allele; furthermore, t 6 A levels quantified by LC-MS/MS were lower in the kae1Δ strain which was complemented by the mutation than in the kae1Δ strain, which was complemented by the WT allele. We conclude that homozygosity for c.974G&gt;A (p.(Arg325Gln)) in KAE1 likely exerts its pathogenic effect by perturbing t 6 A synthesis, thereby interfering with global protein production. This is the first report of t 6 A biosynthesis defect in human. KAE1 joins the growing list of cytoplasmic tRNA modification enzymes, all associated with severe neurological disorders.

tRNA N6-adenosine threonylcarbamoyltransferase defect due to KAE1/TCS3 (OSGEP) mutation manifest by neurodegeneration and renal tubulopathy