Insights into the Pathogenesis and Treatment of Krabbe Disease

Krabbe disease (globoid cell leukodystrophy, GLD) is an inherited disease caused by a deficiency in the lysosomal enzyme galactocerebrosidase (GALC). The major galactosylated lipid degraded by GALC is galactosylceramide. However, GALC is also responsible for the degradation of galactosylsphingosine (psychosine), a highly cytotoxic glycolipid. It has been hypothesized that GALC-deficiency leads to psychosine accumulation that preferentially kills oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system. Krabbe disease has traditionally been considered a white matter disease characterized by the loss and disorganization of myelin, infiltration of multinucleated monocytes/macrophages (globoid cells) and lymphocytes, and dysregulation of pro-inflammatory cytokines and chemokines. However, new studies have revealed unexpected neuronal deficiencies. Infantile Krabbe disease is believed to be the most common and aggressive form. However, juvenile and adult onset forms have been described. Children affected with infantile Krabbe disease present with motor dysfunction, cognitive decline, intractable seizures, and premature death between two to five years of age. Murine, canine, and primate models of GALC deficiency have been described and have played an important role in our understanding of this invariably fatal disease. Although there is no cure for Krabbe disease, hematopoietic stem cell transplantation can slow the progression of disease. Recent pre-clinical data indicate that simulataneously targeting multiple pathogenic mechanisms greatly increases efficacy in the murine model of Krabbe disease. A better understanding of the underlying pathogenesis will identify new therapeutic targets that may further increase efficacy.

Significant correction of pathology in brains of twitcher mice following injection of genetically modified mouse neural progenitor cells

Krabbe disease or globoid cell leukodystrophy is an autosomal recessive disorder resulting from mutations in the galactocerebrosidase (GALC) gene. These mutations lead to deficient GALC activity, storage of substrates of the enzyme, including psychosine, death to oligodendrocytes, decreased myelination, production of globoid cells and eventually death to the individual. While most affected individuals are infants, late-onset forms are also recognized. In addition to human patients, several animal models have been well characterized, including the twitcher mouse. A spontaneously transformed progenitor cell line was isolated from an astrocyte-enriched fraction of normal mice, partially characterized and transduced with a retrovirus-containing mouse GALC cDNA to produce increased GALC activity (20-30-fold above baseline). These cells, called MAR-52, were injected into the brains of newborn affected twitcher mice. While there was only a modest increase in lifespan and body weight, there was clear evidence for the correction of the astrocytic gliosis, normal appearing oligodendrocytes and evidence for remyelination. We demonstrate that the exogenously supplied neural progenitor cells can donate GALC enzyme to oligodendrocytes in the brains of affected mice resulting in normal myelination in the area of donor cells. At this time, hematopoietic stem cell transplantation provides the best outcome in affected mice and is the only treatment available for human patients, but it does not result in a cure even when performed in asymptomatic newborns. Complete correction probably will require a combined approach to effectively treat patients with Krabbe disease. With developments in the isolation and characterization of stem cells, this approach may improve the outcome for individuals diagnosed in the future.

Single‐dose intracerebroventricular administration of galactocerebrosidase improves survival in a mouse model of globoid cell leukodystrophy

Globoid cell leukodystrophy (GLD), also known as Krabbe disease, is a devastating, degenerative neurological disorder. It is inherited as an autosomal recessive trait caused by loss-of-function mutations in the galactocerebrosidase (GALC) gene. Previously, we have shown that peripheral injection of recombinant GALC, administered every other day, results in a substantial improvement in early clinical phenotype in the twitcher mouse model of GLD. While we did detect active enzyme in the brain following peripheral administration, most of the administered enzyme was localized to the periphery. Given the substantial central nervous system (CNS) involvement in this disease, we were interested in determining whether or not a single-dose administration of the recombinant enzyme directly to the CNS, which could potentially be achieved clinically, would result in any substantial improvement. Following intracerebroventricular (i.c.v.) administration of GALC we noted a significant, 16.5%, reduction in the GALC substrate psychosine, the abnormal accumulation of which is believed to play a pivotal role in the CNS pathology observed in this disease. Moreover, recombinant GALC was found not only in periventricular regions but also at sites distant to the injection such as the cerebral cortex and cerebellum. Most importantly, animals receiving a single i.c.v. dose of the enzyme at postnatal day 20 survived up to 51 days, which compares favorably to the control twitcher animals, which normally only live to postnatal day 40/42. These results indicate that even a single i.c.v. administration of the recombinant enzyme can have significant clinical impact and suggests that other lysosomal storage disorders with significant CNS involvement may similarly benefit.

Enzyme replacement therapy results in substantial improvements in early clinical phenotype in a mouse model of globoid cell leukodystrophy

Globoid cell leukodystrophy (GLD) or Krabbe disease is a devastating, degenerative neurological disorder caused by mutations in the galactosylceramidase (GALC) gene that severely affect enzyme activity. Currently, treatment options for this disorder are very limited. Enzyme replacement therapy (ERT) has been shown to be effective in lysosomal storage disorders with predominantly peripheral manifestations such as type I Gaucher's and Fabry's disease. Little however is known about the possible benefit of ERT in GLD, which has a substantial central nervous system component. In this study, we examined the effect of peripheral GALC injections in the twitcher mouse model of the disease. Although we were unable to block the precipitous decline that normally occurs just before death, we did observe significant early improvements in motor performance, a substantial attenuation in the initial failure to thrive, and an increase in life span. Immunohistochemical and activity analyses demonstrated GALC uptake in multiple tissues, including the brain. This was associated with a decrease in the abnormal accumulation of the GALC substrate psychosine, which is thought to play a pivotal role in disease pathology. These results indicate that peripheral ERT is likely to be beneficial in GLD.