Over‐Expression of the DM‐20 Myelin Proteolipid Causes Central Nervous System Demyelination in Transgenic Mice

We have created transgenic mice bearing varying copy numbers of a transgene coding for normal DM-20, the alternatively spliced quantitatively minor isoform of myelin proteolipid protein. Demyelination of the CNS occurs as a consequence of 70 copies of this transgene. Overt symptoms begin at approximately 3 months with a wobbling gait. Occasional seizures lasting a few seconds begin at 3-4 months. These symptoms progress in severity with age. Death occurs by 8-10 months. Myelination in 2-month-old animals, before the onset of any overt symptoms, appears morphologically normal at the electron microscopic level. However, the myelin in these 2-month-old animals has a reduced amount of the major myelin proteolipid protein and about three times as much DM-20 as normal animals. In 7-month-old animals that appear to be undergoing demyelination in the CNS, both the major myelin proteolipid protein and DM-20 are greatly reduced relative to the 2-month-old animal. Mice with 17 copies of the transgene also have a reduced amount of the major myelin proteolipid protein but appear to be otherwise normal and have normal life spans (> 2 yr). Mice with low copy numbers of the transgene (2-4 copies) appear to be unaffected and have normal life spans.