BACKGROUND: ALS is a rapidly fatal, rare neurodegenerative disease typified by the loss of motor neurons in the motor cortex and spinal cord. Twin studies estimate the heritability of sporadic ALS (SALS) to be 61%, yet known ALS-causing alleles only account for 17% of SALS cases. MFN2 is a mitochondrial protein. Mitochondrial function is imperative to neuronal health and is implicated in ALS .

DESIGN/METHODS: Sequencing data for 140 ALS patients was analyzed by VAAST/Phevor to find genes burdened with deleterious variants. MFN2 was a top hit. We screened ALSdb and identified fifteen more variants in MFN2. All 21 variants were expressed in Mfn2 knockout (KO) mouse embryonic fibroblasts (MEFs) to determine rescue efficiency of mitochondrial morphology defects. Rescue efficiency was determined by blinded analysis binning mitochondrial morphology into different categories and software assistance.

RESULTS: All 21 variants in MFN2 were defective in rescuing mitochondrial morphology defects compared to wild-type MFN2 or MFN2 containing common variants (used as controls). Mutant-MFN2 expressing MEFs showed increases in cells with fragmented/intermediate mitochondrial morphology. Algorithms showed mutants had decreased mitochondrial count compared to wildtype.

CONCLUSIONS: The twenty-one mutations in MFN2 in ALS patients rendered the protein insufficient to rescue Mfn2 KOs. A proportion of non-mendelian ALS may be caused by the presence of multiple risk alleles. In addition to mendelian alleles, our study defines risk alleles, so defined because of partial rescue, in MFN2 and how these variants reduce function. MFN2 is known to cause the neuropathy Charcot-Marie-Tooth Type 2A (CMT2A). We hypothesize that CMT2A and ALS are on a disease spectrum and that MFN2 is a novel modifier of ALS.