Molecular Genetic Study of Dysmyelinating Diseases

Abstract

Myelin is an important structure for facilitating the conduction of impulses along the axons both in the central nervous system (CNS) and peripheral nervous system. Myelin basic protein (MBP) is a major protein in the CNS myelin. The MBP gene has been cloned and well characterized. Two mutant mice shiverer and myelin deficient (mid) are autosomal recessive mutants that show severe symptoms such as intentional tremor. They have been found to have a mutation in the MBP gene that results in poor myelination in the CNS. It was found that rearrangement within the MBP gene results in low expression of the gene. In shiverer, MBP gene is partially deleted and MBP mRNA is essentially absent. In mld, the content of MBP in the CNS is greatly reduced, but MBP is clearly detected. Molecular genetic study revealed that MBP gene is duplicated tandemly and a large portion of the duplication is inverted upstream of the intact copy. In the present study, to elucidate the mechanism of repression of MBP gene expression, we performed RNase protection studies. And antisense RNA complementary to exons 3 and 7, which correspond to the inverted segment, was detected. This abnormal transcript was shown to localize in nucleus, and to form an RNA: RNA duplex with sense RNA. These findings suggested that inhibition of transport from the nucleus or selective degradation of the duplex is responsible to for the reduced expression of the MBP gene in the mld mutant. The mechanism of gene rearrangement at the mid locus was also characterized. Cosmid clones encompassing whole MBP gene loci from control and mld genomic library were isolated. The recombination points indicated that the duplication and inversion observed in mld occurred due to nonhomologous recombination.