Loss of Mitochondrial RNA Binding Protein PTCD1 Leads to Cardiomyopathy

Abstract

In eukaryotic cells mitochondria produce energy at the electron transport chain (ETC). ETC assembly requires regulated mitochondrial gene expression, which is predominantly controlled post-transcriptionally by RNA-binding proteins, including the pentatricopeptide repeat (PPR) domain proteins. The PPR domain 1 (PTCD1) protein is involved in RNA metabolism and stability, and Ptcd1 haploinsufficiency in mice causes an increase in transcription, accumulation of precursor transcripts, and decreased protein synthesis. Furthermore, whole-exome analysis has identified a PTCD1 mutation in a patient with cardiomyopathy. To further characterise the molecular role of PTCD1, we investigated a homozygous heart and skeletal muscle-specific knockout in mice. Echocardiography revealed that PTCD1 loss led to dilated cardiomyopathy. Specifically, Ptcd–/– mice (n = 5) showed a significant decrease in fractional shortening (15.7 ± 1.53%) compared with wild-type Ptcd1+/+ mice (n = 5) (24.9 ± 0.91%), and a significant increase in ventricle diameter. Immunoblotting revealed a decrease in ETC subunit abundance, as well as altered expression of various nuclear-encoded ribosomal proteins and proteases, in Ptcd1–/– mice (n = 3) compared with Ptcd1+/+ (n = 3). Northern blotting revealed a compensatory increase in transcription, evident as an increase in mature mRNA and tRNA expression in Ptcd1–/– (n = 3) compared with Ptcd1+/+ mice (n = 3). Interestingly, abundance of the 16S rRNA was decreased, indicating that PTCD1 is required for its stability, and therefore mitoribosome assembly, which could be contributing to the decrease in mitochondrial protein translation observed. These results suggest that PTCD1 is essential for assembly of the mitoribosome and translation, which is necessary for ETC assembly and, ultimately, heart function.