Deletion of a splicing enhancer disrupts PLP1/DM20 ratio and myelin stability

PLP1 and DM20, major myelin proteins, are generated by developmentally regulated alternative splicing. In the post-natal brain, PLP1 is the predominant product. Deletion of a splicing enhancer in PLP1 intron 3 causes a mild form of Pelizaeus-Merzbacher disease and reduces PLP1 specific splicing in vitro (Hobson, G. M., Huang, Z., Sperle, K., Stabley, D. L., Marks, H. G., and Cambi, F., 2002. A PLP splicing abnormality is associated with an unusual presentation of PMD. Ann. Neurol. 52, 477-488). We sought to investigate the pathogenic role of the mutation and to determine the consequences on the developmental regulation of PLP1 alternative splicing and myelin stability and function in vivo. We have generated a knockin mouse that carries deletion of the intronic splicing enhancer and have characterized the PLP1/DM20 ratio by Real Time RT-PCR and Western blot analysis in the developing and mature brain and examined the clinical and pathological phenotype by motor testing and electron microscopy. The deletion impairs the increase in the PLP1/DM20 transcript and protein ratio at the time of myelination and in adulthood and results in a PLP1 hypomorph. Electron microscopy shows abnormal myelin wraps with fragmented myelin whorls, which are progressive with age, suggesting a defect in myelin stability. Phenotypic characterization of the knockin mouse shows a defect in motor coordination. The data indicate that the intronic splicing enhancer is necessary for the developmental increase in PLP1/DM20 ratio and that full PLP1 dosage is necessary for myelin stability and brain function. This knockin mouse represents a useful model to investigate the mechanisms of disease in human disorders in which PLP1 expression is reduced.