Charcot-Marie-Tooth disease (CMT) is the most commonly inherited neurological disorder with a reported prevalence of 1 in 2500 people world-wide. It is found in all races and ethnic groups. CMT is slowly progressive and CMT patients suffer from degeneration of the peripheral nerves that control sensory information of the foot/leg and hand/arm. The nerve degeneration causes the subsequent degeneration of the muscles in the extremities. CMT is divided in two major types, CMT1 and CMT2. CMT1 is a demyelinating neuropathy, and due to mutations in genes important in myelin formation, whereas CMT2 is axonal. Mutations in the neuronal intermediate filament gene, NEFL have been shown to be the primary cause of CMT2. NEFL encodes the neurofilament light (NFL) protein that we have previously shown to be a necessary component for the assembly of neuronal intermediate filaments. Neuronal intermediate filaments form the intermediate filament network in neurons and are the predominant cytoskeletal structure in the axon. Neurofilamentous aggregates both in the neuronal cell bodies and axons are seen in patients with mutations in NEFL, as well as in other neurodegenerative diseases. We have studied the NEFL mutations in transfected cells and found that in both neuronal and non-neuronal cells, mutant NFL resulted in misassembly of the intermediate filament network. This effect was dominant in agreement with the dominant nature of the NEFL mutations associated with CMT2E. In neuronal cells, we found that the mutant proteins caused defects in axonal transport leading to degeneration of neurites. Our studies showed a perfect correlation between pathogenic mutant NFL and the misassembly of filaments in the cultured cells. We are characterizing these mutations in more detail in transgenic animals. We hypothesize that inhibitors of neurofilament misassembly will lead to therapies for CMT. Our cell and animal models will be useful for screening of potential therapeutic agents against the disease.
We are also studying a family of cytoskeletal linker proteins called plakins. One of these plakins, BPAG1 was originally described as a component of the hemidesmosome in the epithelia, where it links the intermediate filaments to the extracellular matrix. Interestingly, the mutant mouse dystonia musculorum (dt), which suffers from a severe hereditary sensory neuropathy is due to mutations in the BPAG1 gene. Focal axonal swellings filled with neurofilaments, mitochondria and membrane bound dense bodies are hallmarks of the pathology of these mice and we are studying the neuronal form of this molecule. A closely related plakin called MACF1 (Microtubule actin crosslinking factor) is also highly expressed in the nervous system. MACF1-/- mice are embryonic lethal, and we have generated a neuron-specific knock-out of MACF1, which shows defects in neuronal migration. We are further characterizing this neuron-specific knock-out and also determining whether MACF1 and BPAG1 have related functions in various tissues.