Novel Insight into the Potential Pathogenicity of Mitochondrial Dysfunction Resulting from PLP1 Duplication Mutations in Patients with Pelizaeus-Merzbacher Disease.

Among the hypomyelinating leukodystrophies, Pelizaeus-Merzbacher disease (PMD) is a representative disorder. The disease is caused by different types of PLP1 mutations, among which PLP1 duplication accounts for ∼70% of the mutations. Previous studies have shown that PLP1 duplications lead to PLP1 retention in the endoplasmic reticulum (ER); in parallel, recent studies have demonstrated that PLP1 duplication can also lead to mitochondrial dysfunction. As such, the respective roles and interactions of the ER and mitochondria in the pathogenesis of PLP1 duplication are not clear. In both PLP1 patients' and healthy fibroblasts, we measured mitochondrial respiration with a Seahorse XF Extracellular Analyzer and examined the interactions between the ER and mitochondria with super-resolution microscopy (spinning-disc pinhole-based structured illumination microscopy, SD-SIM). For the first time, we demonstrated that PLP1 duplication mutants had closer ER-mitochondrion interfaces mediated through structural and morphological changes in both the ER and mitochondria-associated membranes (MAMs). These changes in both the ER and mitochondria then led to mitochondrial dysfunction, as reported previously. This work highlights the roles of MAMs in bridging PLP1 expression in the ER and pathogenic dysfunction in mitochondria, providing novel insight into the pathogenicity of mitochondrial dysfunction resulting from PLP1 duplication. These findings suggest that interactions between the ER and mitochondria may underlie pathogenic mechanisms of hypomyelinating leukodystrophies diseases at the organelle level.