Given the role of POLG in mtDNA replication we looked for evidence of a qualitative or quantitative defect of mtDNA in whole-blood cellular mtDNA because liver tissue was not available from the affected individuals. No mtDNA deletions were detected LDK378 datasheet by long-range PCR and the mtDNA content was no different to age-matched controls (83.9 copies/cell, standard deviation [SD] 58.8; versus 85.8, SD 28.3; Supporting Information Fig. 1A). Following treatment for 10 days with therapeutically relevant doses of VPA (2 and 10 mM) no
significant decrease in mtDNA content was observed (Fig. 3A), nor detectable mtDNA deletions (Supporting Information Fig. 1b) despite the observed cell death. Treatment of control and patient myoblasts with the highest tolerated doses of VPA (50 and 100 mM) still showed no depletion of mtDNA but compromised cell proliferation, with extensive cellular ballooning, vacuolization, and detachment within 3 days of treatment (Supporting
Information Fig. 2). The presence of mtDNA deletions was not investigated in these cells due to the short culture period, making the appearance of deletions highly unlikely. By contrast, EtBr-treated cells grown in parallel showed the expected decrease in mtDNA content after 10 days but no defect of cellular proliferation and no evidence of cell death (Fig. 3B). There was no evidence of apoptosis in any of the cell lines after 10 days Sorafenib of treatment. Multiple mtDNA deletions were not detected in any of the cell pellets, there were no differences in COX activity observed, and β-oxidation metabolites remained within normal limits (Supporting Information Table 2). We therefore extended our studies to postmitotic myotubes, which more closely model mtDNA depletion in vivo.12 MtDNA levels were significantly lower in AHS and Q1236H myotubes than in controls (Fig. 3C). To determine whether mtDNA depletion itself predisposes to further mtDNA loss after VPA exposure, we depleted the myotubes with
didanosine selleck screening library and stavudine, which induce less severe myotube mtDNA depletion than EtBr.12 MtDNA depletion levels in Q1236H myotubes were less than in controls, and similar to the AHS cell lines, but there was no further decrease in mtDNA content with the addition of 10 mM VPA (Fig. 3C). VPA is a branched medium chain fatty acid known to inhibit mitochondrial β-oxidation,16 possibly through the microsomal production of toxic metabolites including 4-ene-VPA,17 or cytosolic and mitochondrial CoA sequestration effects.18 However, we saw no evidence of a β-oxidation defect, making this mechanism unlikely in this context. We also saw no evidence of a secondary mtDNA defect, despite the VPA dose-related growth inhibition and cell death. By contrast, treating identical cell lines with EtBr, didanosine, or stavudine caused profound but recoverable mtDNA depletion without cell death.