'Trafficking' Abnormality Underlies
Common Form of Type 2 CMT

MDA grantee Robert Baloh at Washington University in St. Louis, with Alan Pestronk, who directs the MDA clinic at that institution, were part of a study team that recently unveiled the mechanism by which mutations in the gene for mitofusin 2 (MFN2) cause Charcot-Marie-Tooth disease (CMT).

Mutations in the MFN2 gene are estimated to cause 19 percent to 33 percent of cases of type 2 CMT, the type that results from damage to the nerve fibers (axons) that run between cell bodies in the spinal cord and the periphery of the body. CMT is characterized by movement and sensory abnormalities, especially in the hands and feet.

Recently, the location of the MFN2 protein has been pinpointed to an outer membrane surrounding mitochondria, miniature “organs” that produce a cell’s energy. (Each mitochondrion has its own membranes.)

In lab experiments using nerve cells, Baloh and colleagues found that mutations in the MFN2 gene cause the mitochondria to form clumps and to cluster at fiber segments near the spinal cord rather than distributing themselves throughout the fiber, as they normally would. They called this a “mitochondrial trafficking abnormality.”

Interestingly, the researchers found that energy production -- the main business of mitochondria -- remained intact.

“Mitochondria are ... localized in [nerve cells] to sites of high energy demand,” the investigators say in their Jan. 10 paper in the Journal of Neuroscience. “Because of the high energy demand at very distant sites from the cell body, the effective trafficking of mitochondria is presumed to be critical for proper [nerve cell] function. ... Given that the peripheral sensory and motor axons are the longest in the body, it is likely that these are among the most sensitive to a mitochondrial trafficking insult.”

They say developing a mouse with MFN2 defects will shed further light on this type of CMT.