Characterization of an Immunodeficient Mucopolysaccharidosis Type I (MPS-I) Mouse Model Suitable for Preclinical Testing of Human Stem Cell and Gene Therapy.

Mucopolysaccharidosis type I (MPS-I; Hurler syndrome) is an inborn metabolic disorder due to lack of the lysosomal glycosaminoglycan (GAG)-degrading enzyme alpha-L-iduronidase (IDUA). The resulting GAG accumulation causes progressive multi-system dysfunction and death in the first decade. We recently identified structural abnormalities in the accumulated GAGs that lead to defective heparin-binding cytokine signaling in MPS-I (Pan C et al, Blood 2005 in press: DOI 10.1182/blood-2005-02-0657), which may constitute a mechanism responsible for the diverse clinical manifestations of MPS-I and related mucopolysaccharidoses. Allogeneic hematopoietic stem cell transplantation (HSCT), if performed early in the disease, ameliorates many clinical features and extends life. However, HSCT is not available to all patients, and also does not adequately correct some of the most devastating features of MPS-I including mental retardation and skeletal deformities. Therefore, novel cellular, enzyme replacement and gene therapy approaches need to be developed for MPS-I. Such therapeutic strategies will best be tested in an immunodeficient host which is less likely to develop immune reactions to transplanted human or gene-corrected cells or their secreted IDUA enzyme. We have bred and characterized a homozygous immunodeficient MPS-I mouse (developed as a heterozygous MPS-I mouse on a NOD/SCID background by Jackson Labs) that is well suited for examining the efficacy of such therapeutic approaches. The phenotype of homozygous NOD/SCID/MPS-I mice closely mimicked the clinical features of human MPS-I including coarsening of facial features, skeletal deformities and diverse behavioral abnormalities. IDUA enzyme was completely undetectable in the liver, spleen, heart, lung, kidney and brain of homozygous animals (P<0.001) and was reduced to approximately 50% in heterozygous animals. Homozygous animals developed marked GAG accumulation (3 to 25 fold) in the same organs (P=0.03 to P<0.0001). Neuropathological examination showed accumulation of GM3 gangliosides in the cerebral peduncles, cerebellum and ventral brainstem of homozygous mice. Importantly, measurement of urinary GAG excretion (5-fold higher in homozygous animals; P<0.003) provided a non-invasive and reliable method that can be used to serially follow the biochemical efficacy of therapeutic interventions. We also identified and validated using rigorous biostatistical methods, a highly reproducible method for evaluating sensorimotor function and learning in this mouse model. This Rotarod Test revealed marked abnormalities in sensorimotor integration involving the cerebellum, niagro-striatal and proprioceptive pathways and motor cortex, as well as in learning. We believe that NOD/SCID/MPS-I mice will provide an extremely suitable animal model for assessing the systemic as well as neurological effects of human stem cell transplantation and gene therapeutic approaches, using the above techniques to measure efficacy. We have started using this model to assess the effect of intra-cerebroventricular implantation of human stem cells on the biochemical, pathological and behavioral abnormalities identified. Similar strategies may be invaluable for developing and studying animal models of other MPSs and related diseases.