Acute infantile liver failure due to mutations in the TRMU gene

Abstract

Acute liver failure in infancy accompanied by lactic acidemia was previously shown to result from mtDNA depletion. We report on 13 unrelated infants who presented with acute liver failure and lactic acidemia with normal mtDNA content. Four died during the acute episodes, and the survivors never had a recurrence. The longest follow-up period was 14 years. Using homozygosity mapping, we identified mutations in the TRMU gene, which encodes a mitochondria-specific tRNA-modifying enzyme, tRNA 5-methylaminomethyl-2-thiouridylate methyltransferase. Accordingly, the 2-thiouridylation levels of the mitochondrial tRNAs were markedly reduced. Given that sulfur is a TRMU substrate and its availability is limited during the neonatal period, we propose that there is a window of time whereby patients with TRMU mutations are at increased risk of developing liver failure.

Figure 1

Histopathological Findings in Liver Tissue (A) Liver tissue showing marked oncocytic change in the hepatocytes (arrow) and focal ballooning degeneration of hepatocytes (arrowhead) (H&E). (B) Hepatic tissue with markedly disrupted architecture characterized by nodule formation with prominent sinusoidal fibrosis (Masson Trichrome stain).

Figure 2

Analysis of Mitochondrial Translation in the Patients' Fibroblasts The mitochondrial translation products on SDS-PAAG are indicated according to a standard pattern. Assays were performed in the fibroblasts of a control (lane 1) and three patients (lanes 2–4 for patients 2624, 2859, and 1910, respectively). The relative values were normalized to tubulin (panel below the autoradiographs) and are presented as a diagram. Error bars represent the results of two independent experiments.

Figure 3

The Mutations Identified in the TRMU Gene The mutations identified in the TRMU gene of patients with acute liver failure, depicted on a schematic representation of the conserved domains (NCBI conserved domains website). G14 is one of six residues (red arrowheads) that form the P loop motif (SGGXDS), which is an ATP-binding motif commonly found in enzymes responsible for RNA modifications.

Figure 4

Thio-Modification in Mitochondrial tRNAs Analysis of thio-modification at position 2 of the wobble uridine via RNA hybridization of mitochondrial (mt-tRNA-Lys, mt-tRNA-Glu, and mt-tRNA-Gln) and cytoplasmic (cy-tRNA-Lys) tRNAs separated in APM-containing gels (+APM, upper panel). For quantification, the same amount of RNA obtained from patient and control fibroblasts was separated in gels without APM (-APM, middle panel). The retarded diffused zones correspond to the thiolated and nonthiolated versions of each tRNA (Thiolated and Nonthiolated, respectively). The hybridization probes and the numbers of the RNA samples are indicated at the top of the autoradiographs; the numbers correspond to the samples described under the diagram at the bottom. The quantification of the modification is presented at the bottom panel and is expressed as a percentage of the thiolated signal from the thiolated + nonthiolated signals (as presented in the -APM gel at the middle panel), normalized against the control fibroblasts. The deviations are indicated as a result of two to three independent measures (for the control fibroblasts, the deviation was quasi null and is therefore not indicated).

Figure 5

Quantification of Mitochondrial tRNAs by RNA Hybridization RNA was isolated from the fibroblasts of a control (1) and three patients (2–4 for patients 2624, 2859, and 1910, respectively). Relative values normalized to the 5S rRNA signal are presented in the diagram below the autoradiographs (the various tRNAs are indicated on the x axis only by their respective amino acid abbreviation; thus, Leu stands for mitochondrial tRNA-Leu transcript). Average values of two to three independent experiments are presented. The error was never higher than 10%.