The MFN2 Q367H variant from a patient with late-onset distal myopathy reveals a novel pathomechanism connected to mtDNA-mediated inflammation

Abstract

BackgroundMFN2encodes a multifunctional mitochondrial protein best known for its role mitochondrial fusion. While pathogenic variants inMFN2typically cause Charcot-Marie-Tooth disease subtype 2A, an axonal peripheral neuropathy, exome sequencing identified an uncharacterizedMFN2variant, Q367H, in a patient diagnosed with late-onset distal myopathy without peripheral neuropathy. Although impaired mitochondrial fusion can cause mtDNA-mediated inflammation via TLR9 activation of NF-kB, which is linked to myopathy in a mouse model of MFN1 deficiency, this pathway has not yet been functionally linked toMFN2pathology.MethodsTo investigate if the Q367H MFN2 variant contributes to the patient phenotype, we applied several biochemical and molecular biology techniques to characterize patient fibroblasts and transdifferentiated myoblasts for several functions mediated by MFN2. We also examined TLR9 and cGAS-STING mtDNA-mediated inflammatory pathways.FindingsPatient fibroblasts showed changes consistent with impairment of several MFN2 functions. When grown in standard glucose media, patient fibroblasts had reduced oxidative phosphorylation and elevated levels of lipid droplets. When grown in galactose media, patient fibroblasts had fragmented mitochondria, reduced mito-ER contact sites, and enlarged mtDNA nucleoids. Notably, under both media conditions, mtDNA was present outside of the mitochondrial network, where it co-localized with early endosomes. We also observed activation of both TLR9/NF-kB and cGAS-STING inflammation in fibroblasts. Moreover, the inflammatory signaling was increased 3-10 fold in transdifferentiated patient myoblasts, which also exhibited reduced mito-ER contacts and altered mtDNA nucleoids.InterpretationWe report a patient with myopathy, but without the typical peripheral neuropathy associated withMFN2disease variants. As elevated inflammation can cause myopathy, linking the Q367H MFN2 variant with elevated TLR9 and cGAS/STING signaling, which is amplified in transdifferentiated myoblasts, provides novel insight into the patient’s phenotype. Thus, we establish a potential novel pathomechanism connecting MFN2 dysfunction to mtDNA-mediated inflammation.