Activation of autophagy, observed in liver tissues from patients with Wilson disease and from Atp7b-deficient animals, protects hepatocytes from copper-induced apoptosis

While investigating the genetic causes of neurodegenerative diseases associated with clinical and neuropathologic features of adult-onset neuronal ceroid lipofuscinosis, we identified one extended family with a hexanucleotide repeat expansion within the C9orf72 gene. Mutations in C9orf72 are known to have incomplete penetrance, but disease modifiers have not been identified. In the studied pedigree, we found that the neuropsychiatric phenotype was present only when C9orf72 expansion was co-inherited with a rare allele carrying in cis configuration two heterozygous missense mutations encoding for G253A/L425M variant of the calcium-dependent mitochondrial ATP-Mg/Pi carrier SLC25A24. To characterize the pathogenicity, we studied SLC25A24 expression in affected skin fibroblasts and transiently transfected CAD5 cells. We found that the content of SLC25A24 is decreased in affected fibroblasts that also showed altered structure of mitochondria and decreased content of mtDNA. Characterization of heterologously expressed mutated SLC25A24 that was purified and reconstituted into liposomes revealed altered transport properties with 3.8-fold decrease of Vmax. Subsequent studies in C. elegans showed that mutations of SLC25A24 have an effect on locomotion in vivo, probably by reducing the release or the synthesis of acetylcholine. Our results support the role of SLC25A24 in neuronal homeostasis and demonstrate that mitochondrial dysfunction can be one of the modifying factors of disease penetrance in families with the C9orf72 hexanucleotide repeat expansion.