Energy metabolism and mitochondrial defects in X-linked Charcot-Marie-Tooth (CMTX6) iPSC-derived motor neurons with the p.R158H PDK3 mutation

Charcot-Marie-Tooth (CMT) is a group of inherited diseases clinically and genetically heterogenous, characterised by length dependent degeneration of axons of the peripheral nervous system. A missense mutation (p.R158H) in the pyruvate dehydrogenase kinase 3 gene (PDK3) has been identified as the genetic cause for an X-linked form of CMT (CMTX6) in two unrelated families. PDK3 is one of four PDK isoenzymes that regulate the activity of the pyruvate dehydrogenase complex (PDC). The balance between kinases (PDKs) and phosphatases (PDPs) determines the extend of oxidative decarboxylation of pyruvate to generate acetyl CoA, critically linking glycolysis and the energy producing Krebs cycle. We had shown the p.R158H mutation causes hyperactivity of PDK3 and CMTX6 fibroblasts show hyperphosphorylation of PDC, leading to reduced PDC activity and ATP production. In this manuscript we have generated induced pluripotent stem cells (iPSCs) by re-programming CMTX6 fibroblasts (iPSC(CMTX6)). We also have engineered an isogenic control (iPSC(isogenic)) and demonstrated that genetic correction of the p.R158H mutation reverses the CMTX6 phenotype. Patient-derived motor neurons (MN(CMTX6)) show increased phosphorylation of the PDC, energy metabolism defects and mitochondrial abnormalities, including reduced velocity of trafficking mitochondria in the affected axons. Treatment of the MN(CMTX6) with a PDK inhibitor reverses PDC hyperphosphorylation and the associated functional deficits founds in the patient motor neurons, demonstrating that the MN(CMTX6) and MN(isogenic) motor neurons provide an excellent neuronal system for compound screening approaches to identify drugs for the treatment of CMTX6.