ELAC2 Mutations Cause a Mitochondrial RNA Processing Defect Associated with Hypertrophic Cardiomyopathy

Abstract

The human mitochondrial genome encodes RNA components of its own translational machinery to produce the 13 mitochondrial-encoded subunits of the respiratory chain. Nuclear-encoded gene products are essential for all processes within the organelle, including RNA processing. Transcription of the mitochondrial genome generates large polycistronic transcripts punctuated by the 22 mitochondrial (mt) tRNAs that are conventionally cleaved by the RNase P-complex and the RNase Z activity of ELAC2 at 5' and 3' ends, respectively. We report the identification of mutations in ELAC2 in five individuals with infantile hypertrophic cardiomyopathy and complex I deficiency. We observed accumulated mtRNA precursors in affected individuals muscle and fibroblasts. Although mature mt-tRNA, mt-mRNA, and mt-rRNA levels were not decreased in fibroblasts, the processing defect was associated with impaired mitochondrial translation. Complementation experiments in mutant cell lines restored RNA processing and a yeast model provided additional evidence for the disease-causal role of defective ELAC2, thereby linking mtRNA processing to human disease.

Fatal neonatal encephalopathy and lactic acidosis caused by a homozygous loss-of-function variant in COQ9

“…In order to characterize the metabolite in human cells, we performed different UPLC-ESI-MS/MS analysis techniques such as full scan, precursor ion scan or multiple reaction monitoring mode. In accordance with the studies in mice we specifically detected 6-demethoxy ubiquinone 10 in COQ9 fibroblasts (Figure 2c and d).As 6-demethoxy ubiquinone 10 is hydroxylated by COQ7 to produce 6-hydroxy ubiquinone 10 , the accumulation of this intermediate can be explained by the reduced amount of COQ7. 9 As expected, the expression of COQ9 normalized COQ7 and 6-demethoxy ubiquinone 10 levels (Figure 2d).…”

“…Expression of COQ9 was confirmed by western blotting (Figure 1c). 10 The expression of COQ9 in control cells with normal or reduced CoQ 10 levels due to COQ2 variants had no influence on complex II/III activity (Figure 2a). However, after expression of wild-type cDNA in patient fibroblasts carrying the COQ9 variant, we observed a significant fourfold increase in complex II/III activity.…”

“…In addition, lentiviral expression of wild-type COQ9 cDNA in fibroblasts of a patient with a disease-causing homozygous variant in COQ2 showed no effect on the activity of complex II/III, demonstrating the specificity of our complementation strategy in the COQ9 patient. (b) Total amount of CoQ 10 was measured before and after lentiviral transduction as well as after treatment with CoQ 10 . Both patient cell lines showed significantly reduced amounts of CoQ 10 .…”

“…Coenzyme Q 10 (CoQ 10 ) is a lipid-soluble, naturally occurring substance, which functions as an essential cofactor of the mitochondrial respiratory chain. 1 It acts as an electron carrier from complex I and complex II to complex III and thereby goes through a redox cycle between the oxidized (ubiquinone) and the reduced form (ubiquinol).…”

“…However, biosynthesis is the major source of CoQ 10 , which involves the coordinated function of at least 13 genes (PDSS1, PDSS2, COQ2, COQ3, COQ4, COQ5, COQ6, COQ7, ADCK3, ADCK4, COQ9, COQ10A and COQ10B). To date, defects in eight of these genes were identified as a cause of primary CoQ 10 deficiency disorders (PDSS1, PDSS2, COQ2, COQ4, COQ6, ADCK3, ADCK4 and COQ9), a clinically heterogeneous group of diseases, which frequently manifests in childhood.…”