Effects of PMP22 duplication and deletions on the axonal cytoskeleton

Axonal loss in Charcot-Marie-Tooth type ZA (CMT1A) is an important feature correlated with the functional disability in affected individuals. It is no t known, however, how the most common genetic defect in Schwann cells (PMP2 2 duplication) causes the CMT1A phenotype and results in axonal loss. In th is study, sural nerve segments from individuals with PMP22 duplications or deletions, causing the reciprocal disorder hereditary neuropathy with press ure palsies (HNPP), were grafted into the cut ends of the sciatic nerve of nude mice. The xenografts and host segments were studied at 2, 4, 6, 8, 12, and 16 weeks after grafting and compared with the controls from healthy vo lunteers. Within the CMT1A xenografts, the nude mice axons in the proximal part of the graft showed a significant increase in axonal area with an incr ease in the neurofilament and membranous organelle (mitochondria) density, compared with distal graft and distal host segments. A preferential distal axonal loss, associated with a perpetual axonal atrophy, degeneration, and axonal sprouting was observed over time, with increasing intensity at 8 to IG weeks. These alterations were seen to a lesser extent in HNPP xenografts and were not observed in controls. In addition, the onset of regeneration- associated myelination was delayed, more significantly in HNPP xenografts t han those of CMT1A. Our findings indicate that the PMP22 duplication in Sch wann cells results in an impairment in the normal axonal cytoskeletal organ ization, resulting in distal axonal degeneration and fiber loss, and the af fect of PMP22 deletion on axonal cytoskeleton is less deleterious.