The presence of concomitant diseases shows a tendency to worsen the clinical and neurophysiological CMT1A phenotype, especially in patients with CMT1A and diabetes mellitus, where higher values in the CMT neuropathy score and clinical motor subscore have been observed.
Consequently, excess PMP22 resulting from its genetic duplication and overexpression has been directly associated with the peripheral neuropathy called Charcot-Marie-Tooth disease type 1A (CMT1A), the most prevalent type of CMT.
Hereditary neuropathy with liability to pressure palsies (HNPP), Charcot-Marie Tooth disease type 1A (CMT1A), Dejerine-Sottas syndrome, and congenital hypomyelinating neuropathy are all associated with defects in PMP22 gene.
Point mutations in the peripheral myelin protein 22 (PMP22) gene rarely cause the hereditary neuropathies Charcot-Marie-Tooth disease type 1A (CMT1A) and hereditary neuropathy with liability to pressure palsies (HNPP), both of which show a demyelinating phenotype.
Molecular and clinical studies of copy number variants involving chromosome 17 began with locus-specific studies of Charcot-Marie-Tooth disease type 1A (CMT1A, OMIM #118220) and hereditary neuropathy with liability to pressure palsies (HNPP, OMIM #162500), which laid the foundation for the paradigm of duplication/deletion and gene-dosage for our understanding of genomic disorders.
Genomic rearrangements involving the peripheral myelin protein gene (PMP22) in human chromosome 17p12 are associated with neuropathy: duplications cause Charcot-Marie-Tooth disease type 1A (CMT1A), whereas deletions lead to hereditary neuropathy with liability to pressure palsies (HNPP).
High-dose ascorbic acid has been shown to have remyelinating potential and to correct the phenotype of a transgenic mouse model of CMT1A by decreasing expression of PMP22.
The transgenic CMT rat with about 1.6-fold PMP22 overexpression exhibits clinical abnormalities, such as reduced nerve conduction velocity and lower grip strength that mimick findings in CMT1A patients.
In this study we showed that microarray MAPH measurement of PMP22 gene dosage correlates well with PMP22 gene dosage determined by capillary MAPH and that copy number was accurately reported in analyses of DNA from 38 individuals, 12 of which were known to have Charcot-Marie-Tooth disease type 1A (CMT1A).
Together, these data indicate that as a result of missorting and inefficient proteasomal degradation, the aggregation of PMP22 and recruitment of autophagosomes and lysosomes are key factors in the subcellular pathogenesis of CMT1A neuropathies.
Aggregates containing ubiquitin and peripheral myelin protein 22 (PMP22) have been observed in the Trembler J mouse model of Charcot-Marie-Tooth disease type 1A demyelinating neuropathy.