Krabbe disease: genetic aspects and progress toward therapy.

Krabbe disease or globoid cell leukodystrophy is a disorder involving the white matter of the peripheral and central nervous systems. Mutations in the gene for the lysosomal enzyme galactocerebrosidase (GALC) result in low enzymatic activity and decreased ability to degrade galactolipids found almost exclusively in myelin. The pathological changes observed, including the presence of globoid cells and decreased myelin, appear to result from the toxic nature of psychosine and accumulation of galactosylceramide that cannot be degraded due to the GALC deficiency. Over 60 mutations have been identified in this gene. The great majority are disease-causing; however, a few are considered polymorphisms. While most patients present with symptoms within the first 6 months of life, others present later in life including adulthood. Even patients with the same genotype can have very different clinical presentations and course. The reason for this is not known. Treatment at this time is limited to hematopoietic stem cell transplantation that appears to slow the progression of the disease and improve the magnetic resonance images. Studies using stem cells and viral vectors to transduce transplantable cells are under way in model systems. In culture, oligodendrocytes from the twitcher mouse model can assume a normal appearance after differentiation if GALC activity is provided via viral transduction or uptake from donor cells. Therefore continued myelination and/or remyelination in patients will require supplying GALC activity by transplanted cells or viral vectors to still functional endogenous oligodendrocytes or transplantation of normal oligodendrocytes or stem cells that can differentiate into oligodendrocytes. Using the animal models these options can be explored.

Retrovirus-mediated gene transfer and galactocerebrosidase uptake into twitcher glial cells results in appropriate localization and phenotype correction.

Galactocerebrosidase (GALC) is deficient in all tissues from human patients and animal models with globoid cell leukodystrophy (GLD) or Krabbe disease. The deficiency results in decreased lysosomal catabolism of certain galactolipids including galactosylceramide and psychosine that are synthesized maximally during myelination. According to current theories, the accumulation of psychosine in humans and animals with GLD induces oligodendrocyte degeneration and myelination ceases. Transduction of oligodendrocytes from twitcher mice with a retroviral vector containing the GALC cDNA can correct the enzyme deficiency in these cells. Our data show that twitcher astrocytes and oligodendrocytes can internalize exogenous GALC, as well as donate the enzyme to the mutant glial cells. Antibodies against human GALC localized the GALC antigen in retrovirally transduced cells and cells receiving enzyme via cell to cell secretion and uptake to the lysosomal fraction. In fact immunocytochemical studies in transduced oligodendrocytes revealed that the GALC colocalizes in vesicles lysosomal-associated membrane protein-2 (LAMP2) (+). Moreover, labeling cells with anti-GALC and a marker for oligodendrocytes demonstrated that, upon differentiation, transduced, twitcher oligodendrocytes attained the normal branched process configuration, while untransduced cells show only abnormal morphology. Phenotype correction in mutant oligodendrocytes has also been observed after enzyme transfer. These studies indicate that GALC activity supplied to cultured oligodendrocytes from twitcher mice by different methods can correct the pathological phenotype of these cells.