Cell Specific Ablation of GALC and the Pathogenesis of Krabbe Disease

Krabbe Disease (KD) is characterized by progressive demyelination in the central and peripheral nervous systems (CNS and PNS). KD affects infants and rapidly progresses to clinical decline and death. The only treatment is Hematopoietic Stem Cell Transplantation (HSCT), which has limited efficacy. KD is caused by mutations in GALC, leading to a loss-of-function in the lysosomal enzyme galactosylceramidase (GALC), responsible for the degradation of galactosylceramide and psychosine. Psychosine accumulates in patients with KD and is toxic to myelinating glia in the CNS (oligodendrocytes) and PNS (Schwann cells). Recent data suggest that psychosine toxicity in KD may also target neurons primarily, causing axonal degeneration independent of demyelination. It is also hypothesized that limitations of HSCT for KD may reflect non-rescued PNS myelination, causing paralysis and autonomic dysfunction leading to sudden death. However, the PNS contribution to KD pathogenesis has not been well defined. Similarly, the primary role of KD on neurons, independent of demyelination, is unknown. To explore the role of these cells in KD, we have generated two complementary mouse models that use conditional mutagenesis. The first is a Galc conditional knockout mouse (cKO), in which exon 9 is flanked by two loxP sites. Preliminary data shows that the fully recombined cKO mouse has a lethal demyelinating phenotype, similar to the well-studied Galc mutant mouse, twitcher. By comparing the fully recombined cKO mouse to Schwann cell-specific or neuron-specific Cre mice, we hope to understand the role of these cells in KD pathogenesis. The second is a transgenic mouse that can selectively express human GALC in a Cre-specific fashion (iGALC). iGALC contains a lox-stop-lox cassette in intron 1, which will allow for Cre-driven recombination and subsequent expression of GALC. We will use the iGALC mice to determine the impact of GALC expression in twitcher Schwann cells or neurons. iGALC founders were produced and we are currently validating the transgene’s function. Experimental mice will be characterized for disease progression by clinical, pathological and molecular measures. Our data will provide information on the interplay between glia and neurons in neurodegeneration, a central emerging question in most neurological diseases. In addition, findings from this study will clarify disease mechanisms of KD and the limitations of HSCT, which will allow us to develop better therapies for KD in the future.

M. Laura Feltri, MD