Loss of FEZ1, a gene deleted in Jacobsen syndrome, causes locomotion defects and early mortality by impairing motor neuron development.

FEZ1-mediated axonal transport plays important roles in central nervous system development but its involvement in the peripheral nervous system is not well-characterised. FEZ1 is deleted in Jacobsen syndrome (JS), an 11q terminal deletion developmental disorder. JS patients display impaired psychomotor skills, including gross and fine motor delay, suggesting that FEZ1 deletion may be responsible for these phenotypes, given its association with the development of motor-related circuits. Supporting this hypothesis, our data shows that FEZ1 is selectively expressed in the rat brain and spinal cord. Its levels progressively increase over the developmental course of human motor neurons derived from embryonic stem cells. Deletion of FEZ1 strongly impaired axon and dendrite development, and significantly delayed the transport of synaptic proteins into developing neurites. Concurring with these observations, Drosophila unc-76 mutants showed severe locomotion impairments, accompanied by a strong reduction of synaptic boutons at neuromuscular junctions. These abnormalities were ameliorated by pharmacological activation of UNC-51/ATG1, a FEZ1-activating kinase, with rapamycin and metformin. Collectively, the results highlight a role for FEZ1 in motor neuron development and implicate its deletion as an underlying cause of motor impairments in JS patients.