Multiple disease-linked myotubularin mutations cause NFL assembly defects in cultured cells and disrupt myotubularin dimerization

Charcot-Marie-Tooth disease (CMT) is an inherited peripheral neuropathy that has been linked to mutations in multiple genes. Mutations in the neurofilament light (NFL) chain gene lead to the CMT2E form whereas mutations in the myotubularin-related protein 2 and 13 (MTMR2 and MTMR13) genes lead to the CMT4B form. These two forms share characteristic pathological hallmarks on nerve biopsies including concentric sheaths ('onion bulbs') and, in at least one case, myelin loops. In addition, MTMR2 protein has been shown to interact physically with both NFL and MTMR13. Here, we present evidence that CMT-linked mutations of MTMR2 can cause NFL aggregation in a cell line devoid of endogenous intermediate filaments, SW13vim(-). Mutations in the protein responsible for X-linked myotubular myopathy (myotubularin, MTM1) also induced NFL abnormalities in these cells. We also show that two MTMR2 mutant proteins, G103E and R283W, are unable to form dimers and undergo phosphorylation in vivo, implicating impaired complex formation in myotubularin-related pathology.