Molecular insights into HSD10 disease: impact of SDR5C1 mutations on the human mitochondrial RNase P complex

Abstract

SDR5C1 is an amino and fatty acid dehydrogenase/reductase, moonlighting as a component of human mitochondrial RNase P, which is the enzyme removing 5′-extensions of tRNAs, an early and crucial step in tRNA maturation. Moreover, a subcomplex of mitochondrial RNase P catalyzes the N1-methylation of purines at position 9, a modification found in most mitochondrial tRNAs and thought to stabilize their structure. Missense mutations in SDR5C1 cause a disease characterized by progressive neurodegeneration and cardiomyopathy, called HSD10 disease. We have investigated the effect of selected mutations on SDR5C1's functions. We show that pathogenic mutations impair SDR5C1-dependent dehydrogenation, tRNA processing and methylation. Some mutations disrupt the homotetramerization of SDR5C1 and/or impair its interaction with TRMT10C, the methyltransferase subunit of the mitochondrial RNase P complex. We propose that the structural and functional alterations of SDR5C1 impair mitochondrial RNA processing and modification, leading to the mitochondrial dysfunction observed in HSD10 patients.

A novelHSD17B10mutation impairing the activities of the mitochondrial RNase P complex causes X-linked intractable epilepsy and neurodevelopmental regression

“…2B). The binary complex with wild-type SDR5C1 displayed a k obs of methyl transfer of 1.02 § 0.09 min ¡1 , similar to reported values, 34 (Fig. 2C).…”

“…2D). Thus, the p.K212E mutation of SDR5C1 impairs the processing activity of mitochondrial RNase P. Notably, the rate of 5 0 -processing of mttRNA Leu (UUR) by the wild-type complex (k obs D 0.8 § 0.1 min ¡1 ) was slower compared to the reported value for mttRNA Ile (k obs D 4.63 § 0.61 min ¡1 ), 34 possibly reflecting a sequence-dependent tRNA effect and differences in assay conditions. However, the decrease in the 5 0 -processing activity by the p.K212E mutation is comparable to that seen with the p. P210S mutation in SDR5C1 (4.5-fold vs. 8.0-fold).…”

“…2A). Notably, while the mutational defect is modest, it is similar to that of the p.P210S mutation, 34 which is separated from the p.K212E mutation by 2 residues in the enzyme sequence and is associated with the infantile form of HSD10 disease. 7 We next determined whether the p.K212E mutation decreases the methyl transferase activity of TRMT10C, which requires the presence of SDR5C1 to be active.…”

“…We chose to determine the mutational effect by comparing known quantities of purified wildtype and mutant enzyme, each expressed and purified from an E. coli over-expression strain. 19,34 The assay used acetoacetylCoA (3-ketoacyl-CoA) as the substrate and monitored its reduction to 3-hydroxyacetyl-CoA accompanied by oxidation of NADH to NAD C . While the wild-type enzyme had an activity of 3.18 § 0.18 units/mg, similar to other reported values, 34 the mutant showed an almost 4-fold decrease of the activity to 0.84 § 0.12 units/mg (Fig.…”

“…7 We next determined whether the p.K212E mutation decreases the methyl transferase activity of TRMT10C, which requires the presence of SDR5C1 to be active. 19,34 We assayed a reconstituted and recombinant form of the TRMT10C-SDR5C sub-complex with AdoMet as the methyl donor and the N 1 of G9 in the unmodified transcript of mt-tRNA Leu (UUR) as the acceptor (Fig. 2B).…”