Case report and functional study link MFN2 R334K variant to lethal neonatal disorder and mitochondrial defects

Pathogenic variants in the mitochondrial fusion protein Mitofusin2 typically cause axonal Charcot-Marie-Tooth disease type 2A (CMT2A), a progressively degenerative peripheral neuropathy. Here, we present three siblings with a lethal disorder of neonatal onset who carried a novel homozygous MFN2 variant R334K, which was predicted to be likely pathogenic. Given the severe clinical presentation, which is atypical of MFN2 variants, further functional investigations were warranted to confirm the pathogenicity of the R334K variant. Characterization of patient fibroblasts revealed severe disruptions in all MFN2-related functions assayed, including mitochondrial fragmentation, altered mito-ER contacts, decreased mtDNA copy number and size, increased lipid droplet abundance, and reduced mitochondrial respiration. We also observed reduced Complex I activity, which is noted in cells lacking MFN2, but is not typical of pathogenic MFN2 variants that cause CMT2A. Notably, re-expression of MFN2 R334K in MFN2 knockout cells was unable to rescue mitochondrial fragmentation and the Complex I deficiencies, confirming that the R334K variant is causative of these cellular phenotypes. Furthermore, disease modelling in Drosophila melanogaster demonstrated functional deficiencies of the R334K variant in vivo. Together, these findings confirm that the MFN2 R334K is a novel pathogenic variant causing severe fatal neonatal disease. Finally, we show that pharmacological activation of the integrated stress response rescues the defects caused by MFN2 R334K, offering a potential therapeutic avenue for MFN2 pathology.