Close relationship between motor impairments and loss of functional motoneurons in a Charcot-Marie-Tooth type 1A model.

Charcot-Marie-Tooth disease type 1A is the most frequent hereditary neuropathy affecting the peripheral nervous system. A partial duplication of chromosome 17 (17p11.2) involving the PMP22 gene is responsible for dysmyelination-demyelination processes leading to motor and sensory impairments. Murine models of this disease are now widely used to investigate the mechanisms occurring at the behavioural and physiological levels. In this study, adult transgenic mice (6 months old) having integrated 7 copies of the human PMP22 gene were used to compare the motor performance, evaluated by using a complex locomotor test (the rotarod test), with both the number of functional motoneurons innervating the soleus muscle and the level of myelination in the sciatic nerve. Two levels of motor deficits were detected and led us to divide the population into two subgroups. In both impaired groups, the level of motor deficit was strongly correlated with the number of functional motoneurons evaluated by retrograde labeling from the muscle, but not with the number of myelinated fibers or the thickness of the myelin sheath (g-ratio). It therefore appears that the number of motor units may be a key element in motor impairments observed in Charcot-Marie-Tooth disease type 1A disease. These findings may have implications for therapeutic procedures, which should focus on the survival of the motoneuronal pool and/or the maintenance of functional neuro-muscular connexions to reduce motor impairments in humans.

PMP22 overexpression causes dysmyelination in mice.

Charcot-Marie-Tooth (CMT) disease is the most frequent hereditary peripheral neuropathy in humans. Its prevalence is about one in 2500. A subform, CMT1A, is transmitted as an autosomal dominant trait. An estimated 75% of patients are affected. This disorder has been shown to be associated with the duplication of a 1.5 Mb region of the short arm of chromosome 17, in which the PMP22 gene has been mapped. We have constructed a murine model of CMT1A by inserting into the murine genome a human YAC containing peripheral myelin protein 22 (PMP22) and its flanking controlling elements. We describe the behaviour of the C22 line (seven copies of YAC, 2.1 times PMP22 overexpression) during the myelination process. Electron microscopy, morphometry, electrophysiology, nerve conduction and expression of specific markers (e.g. Krox20) in normal and pathological Schwann cells demonstrated that PMP22 overexpression leads to a defect in the myelination of axons. The largest axons are the most affected. Only a few demyelination/remyelination processes were observed. Moreover, PMP22 overexpression probably enhances collagen synthesis by fibroblasts, before myelination, demonstrating that structures other than Schwann cells are affected by PMP22 overexpression. Classically, CMT1A was thought to be induced by a demyelination process following a phase of normal myelination, yet our data suggest that dysmyelination should be considered as a major factor for the disease.

Behavioural profiling of a murine Charcot-Marie-Tooth disease type 1A model.

Different features of motor behaviour were studied on a transgenic mouse model of Charcot-Marie-Tooth's disease (CMT). Mutants with 4 or 7 copies of the human PMP22 gene leading to a phenotype significantly close to CMT's disease type 1A were compared with control animals. The aim of the study was to validate this transgenic model and to characterise the impairments occurring in the various lines. Three main types of analysis were performed in 2-month-old mice without any peculiar visible deficit: (i) a study of standardised clinical tests (SHIRPA protocol) demonstrated that only a few motor deficits were expressed; (ii) a measurement of general spontaneous activity by means of a commercial video-tracking system was performed and revealed that the main spontaneous activities were identical in the three lines with, however, some slight localised modifications; and, (iii) by contrast, the three lines respond very differently to the footprints, grip strength, splay test and rotarod test. Even in lines with a significantly limited copy number of the transgene, we observed and quantified impairments. In conclusion, mutants of CMT1A seem to be a very pertinent model of this human pathology and will certainly be useful for therapeutic procedures and for theoretical studies on this disease.