iPSC-derived neuronal models of PANK2-associated neurodegeneration reveal mitochondrial dysfunction contributing to early disease.

Mutations in PANK2 lead to neurodegeneration with brain iron accumulation. PANK2 has a role in the biosynthesis of coenzyme A (CoA) from dietary vitamin B5, but the neuropathological mechanism and reasons for iron accumulation remain unknown. In this study, atypical patient-derived fibroblasts were reprogrammed into induced pluripotent stem cells (iPSCs) and subsequently differentiated into cortical neuronal cells for studying disease mechanisms in human neurons. We observed no changes in PANK2 expression between control and patient cells, but a reduction in protein levels was apparent in patient cells. CoA homeostasis and cellular iron handling were normal, mitochondrial function was affected; displaying activated NADH-related and inhibited FADH-related respiration, resulting in increased mitochondrial membrane potential. This led to increased reactive oxygen species generation and lipid peroxidation in patient-derived neurons. These data suggest that mitochondrial deficiency is an early feature of the disease process and can be explained by altered NADH/FADH substrate supply to oxidative phosphorylation. Intriguingly, iron chelation appeared to exacerbate the mitochondrial phenotype in both control and patient neuronal cells. This raises caution for the use iron chelation therapy in general when iron accumulation is absent.

Autosomal-recessive cerebellar ataxia caused by a novel ADCK3 mutation that elongates the protein: clinical, genetic and biochemical characterisation.

BACKGROUND: The autosomal-recessive cerebellar ataxias (ARCA) are a clinically and genetically heterogeneous group of neurodegenerative disorders. The large number of ARCA genes leads to delay and difficulties obtaining an exact diagnosis in many patients and families. Ubiquinone (CoQ10) deficiency is one of the potentially treatable causes of ARCAs as some patients respond to CoQ10 supplementation. The AarF domain containing kinase 3 gene (ADCK3) is one of several genes associated with CoQ10 deficiency. ADCK3 encodes a mitochondrial protein which functions as an electron-transfer membrane protein complex in the mitochondrial respiratory chain (MRC). METHODS: We report two siblings from a consanguineous Pakistani family who presented with cerebellar ataxia and severe myoclonus from adolescence. Whole exome sequencing and biochemical assessment of fibroblasts were performed in the index patient. RESULTS: A novel homozygous frameshift mutation in ADCK3 (p.Ser616Leufs*114), was identified in both siblings. This frameshift mutation results in the loss of the stop codon, extending the coding protein by 81 amino acids. Significant CoQ10 deficiency and reduced MRC enzyme activities in the index patient's fibroblasts suggested that the mutant protein may reduce the efficiency of mitochondrial electron transfer. CoQ10 supplementation was initiated following these genetic and biochemical analyses. She gained substantial improvement in myoclonic movements, ataxic gait and dysarthric speech after treatment. CONCLUSION: This study highlights the importance of diagnosing ADCK3 mutations and the potential benefit of treatment for patients. The identification of this new mutation broadens the phenotypic spectrum associated with ADCK3 mutations and provides further understanding of their pathogenic mechanism.