Charcot-Marie-Tooth disease type 1A (CMT1A) is an autosomal dominant peripheral neuropathy associated with a large DNA duplication on the short arm of human chromosome 17. The trembler (Tr) mouse serves as a model for CMT1A because of phenotypic similarities and because the Tr locus maps to mouse chromosome 11 in a region of conserved synteny with human chromosome 17. Recently, the peripheral myelin gene Pmp-22 was found to carry a point mutation in Tr mice. We have isolated cDNA and genomic clones for human PMP-22. The gene maps to human chromosome 17p11.2-17p12, is expressed at high levels in peripheral nervous tissue and is duplicated, but not disrupted, in CMT1A patients. Thus, we suggest that a gene dosage effect involving PMP-22 is at least partially responsible for the demyelinating neuropathy seen in CMT1A.
Schwann cells express low levels of myelin proteins in the absence of neurons. When Schwann cells and neurons are cultured together the production of myelin proteins is elevated, and myelin is formed. For peripheral myelin protein 22 (PMP22), the exact amount of protein produced is critical, because peripheral neuropathies result from its underexpression or overexpression. In this study we examined the effect of neurons on Schwann cell PMP22 production in culture and in peripheral nerve using metabolic labeling and pulse-chase studies as well as immunocytochemistry. Most of the newly synthesized PMP22 in Schwann cells is rapidly degraded in the endoplasmic reticulum. Only a small proportion of the total PMP22 acquires complex glycosylation and accumulates in the Golgi compartment. This material is translocated to the Schwann cell membrane in detectable amounts only when axonal contact and myelination occur. Myelination does not, however, alter the rapid turnover of PMP22 in Schwann cells. PMP22 may therefore be a unique myelin protein in that axonal contact promotes its insertion into the Schwann cell membrane and myelin without altering its rapid turnover rate within the cell.
Alterations in peripheral myelin protein 22 (PMP22) gene expression are associated with demyelinating peripheral neuropathies. Overexpression of wild type (wt) PMP22 or inhibition of proteasomal degradation lead to the formation of aggresomes, intracellular ubiquitinated PMP22 aggregates. Aggresome formation has now been observed with two mutant PMP22s, the Tr‐ and TrJ‐PMP22 when the proteasome is inhibited. The formation of these aggresomes required intact microtubules and involved the recruitment of chaperones, including Hsp40, Hsp70, and alphaB‐crystallin. Spontaneously formed ubiquitinated PMP22 aggregates were also observed in Schwann cells of homozygous TrJ mice. Significant upregulation of both the ubiquitin‐proteasomal and lysosomal pathways occurred in affected nerves suggesting that two pathways of PMP22 degradation are present. Thus, the presence of aggresomes appears to be a common finding in neuropathy models of PMP22 overexpression and of some point mutations known to cause neuropathy in mice and humans.
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