TRANSGENIC MODELS OF HUMAN LEUKODYSTROPHIES: INSIGHT INTO PATHOMECHANISMS

Some leukodystrophies, such as Pelizaeus-Merzbacher disease (PMD), can be studied very well in homologous mutant mice, whereas other severe myelin disorders, e.g. X-linked Adrenoleukodystrophy (X-ALD) cannot be adequately modeled in corresponding mutant mice. The reason for this discrepancy is not fully understood, but may relate to disease-specific pathomechanisms, in combination with species-specific modifier genes and the overall short life span (2y) of mice.

To better understand the molecular pathology of PMD, we have generated mice that harbor different mutation of the Plp1 gene. Unexpectedly, PMD emerges as a clinical syndrom with a complex overlap of CNS dysmyelination, demyelination, and progressive axon loss, preceded by axonal transport defects. Each aspect is modeled to a different extent in mice with Plp1 point mutations, transgenic Plp1 overexpression, and Plp1 loss of function. At the protein level, an important role is played by cystein residues in extracellular loop regions of PLP/DM20, that engage in abnormal protein cross-links once exposed by local protein misfolding in the oxidative environment of the glial endoplasmic reticulum. These studies were possible with the use of PLP-EGFP fusion proteins expressed in transfected oligodendrocytes and monitored with lime-lapse video microscopy.

For X-ALD, we hypothesized that loss of the mutant ALDP transporter is merely a prediposition of cells to a late-onset peroxisomal dysfunction, that only becomes a clinicallly-relevant problem for oligodendrocytes and adrenal cells. In support of this model, we by-passed experimentally the "at risk" phase of disease and generated a cell-specific peroxisomal dysfunction in mice. By oligodendrocyte-specific ablation of the PEX5 gene, we generated mice that develop an ALD-like disease. Surprisingly, the late-onset progressive loss of myelin is restricted to the subcortical white matter, very similar to human patients, and is associated with axonal loss, gliosis, and even the infiltration of B cells and T cells into brain.