Zhao T1, Goedhart C1, Sam P2, Lingrell S3, Lamont RE1, Bernier FP1, Sinasac DS1, Parboosingh JS1, Care4Rare Canada, Vance J3, Claypool S2, Innes AM1, Shutt TE1 1Alberta Children's Hospital Research Institute, Department of Medical Genetics, Cumming School of Medicine, University of Calgary, Calgary, Alberta, Canada; 2Dept of Medicine and Group on Molecular and Cell Biology of Lipids, University of Alberta, Edmonton, Alberta, Canada. 3Department of Physiology, The Johns Hopkins University School of Medicine, Baltimore, MD, USA
Exome sequencing of two sisters with congenital cataracts, short stature and white matter changes identified compound heterozygous mutations in the PISD gene as leading candidates. PISD encodes the phosphatidylserine decarboxylase enzyme that converts phosphatidylserine (PS) to phosphatidylethanolamine (PE) in the inner mitochondrial membrane. Although not typical of mitochondrial disease presentation, the phenotypes of these patients resemble recently described “mitochondrial chaperonopathies”, prompting further investigation into whether PISD is a novel mitochondrial disease gene. To determine whether the candidate PISD mutations lead to mitochondrial dysfunction and are causative for the phenotypes, patient fibroblasts were examined. As evidence for mitochondrial dysfunction, we observed decreased maximal oxygen consumption rates, and increased sensitivity to 2-deoxyglucose. Patient fibroblasts also exhibited a more fragmented mitochondrial network, similar to previous reports in cells depleted of PISD. Additionally, decreased conversion of PS to PE, and depletion of total cellular PE levels are consistent with impaired PISD enzyme activity. Moreover, treatment with lyso-PE, which can replenish the mitochondrial pool of PE, restored mitochondrial morphology in patient fibroblasts. Functional characterization of the PISD mutations demonstrates that one mutation causes an alternative splice product with a premature truncation of the protein. Meanwhile, the second mutation prevents autocatalytic self-processing of the PISD protein required for its decarboxylase activity. We posit that accumulation of this unprocessed protein may impair mitochondrial protein homeostasis and contribute to the “mitochondrial chaperonopathy” phenotype in the patients. Collectively, these findings provide strong evidence that PISD is a novel mitochondrial disease gene.