Ultrastructural lesions of axonal mitochondria in patients with childhood-onset Charcot-Marie-Tooth disease due to MFN2 mutations

We present neuropathological findings based on sural nerve biopsy in six children with mutations of the mitofusin 2 gene (MFN2). All six children had severe axonal neuropathies (mild or severe hereditary motor and sensory neuropathy, HMSN), with onset in early childhood All had a marked decrease in the density of mainly large myelinated fiberes. Although neurophysiological findings were suggestive,e of axonal degeneration, some onion bulbs were present in each case. Unequivocal mitochondrial changes were apparent only on longitudinal sections. Many axonal mitochondria appeared smaller than normal and round or spherical instead of tubular. These mitochondria were abnormally, aggregated, accumulating primarily at the axon periphery. This peripheral distribution was clearest in residual large myelinated fibers. The inner and outer mitochondrial membranes were irregular and the cristae were quite often disrupted These changes were observed in both myelinated and unmyelinated fibers. Mitofusin 2 is a large mitochondrial transmembrane GTPase, with two coiled coil domains and tow transmembrane spans. It is targeted to the outer mitochondrial membrane, where it interacts with mitofusin I to regulate the mitochondrial network architecture by stimulating mitochondrial fusion. The mitochondrial changes we observed could thus result from abnormal mitochondrial fusion and fission. Neuropathologic abnormalities can be sufficiently characteristic to suggest the genetic basis of some hereditary neuropathies such as those associated with mutations in MPZ, GJB1, GDAP1, MTMR2. SH3TC2, PRX, FGD4 and LMNA. This may also be true of MFN2-related neuropathies.