End-stage diseases of the central nervous system (CNS), in which significant parts of the brain are irreversibly compromised, are notoriously difficult to treat because of limited regenerative capacity in the face of ongoing disease toxicity. CNS injury resulting from slow accumulation uncleared substrates of deficient lysosomal hydrolases – the lysosomal storage diseases (LSDs) – is emblematic of such diseases. Consequently, our dual stem cell approach is designed to intervene before irreversible injury, to protect minimally affected cells. LSDs are attractive targets for this strategy because they are diagnosed before irreversible injury occurs and have a generally slow course, but ultimately lead to profound morbidity or death. Further, the primary genetic defect is known. For preclinical development, there are superb animal models for many LSDs, including some on an immune-deficient background. In recent years, three therapeutic approaches have been used clinically for the LSDs: enzyme replacement therapy (ERT), substrate reduction therapy, and stem cell transplantation (SCT). Of these, only SCT offers the chance for a lifelong effect following a single intervention. Although gene therapy has had recent notable successes, it will not be useful for diseases that affect multiple organ systems and will ultimately fail in many patients, as will enzyme replacement therapy, because of immune neutralization of the ‘foreign’ replaced enzyme.
SCT directly corrects enzyme function of LSD by providing cells with an active form of the missing or defective enzyme. Clinical hematopoietic SCT (HSCT, or bone marrow or umbilical cord blood transplantation) has already been shown to be very effective for several LSDs, especially for treating the pathology of non-CNS organs such as spleen and liver. However, CNS treatment is inadequate or suboptimal in most LSDs. Even in the common LSD mucopolysaccharidosis type I (MPS1), for which HSCT is clinically used and can slow or prevent mental regression, this treatment is ultimately not effective enough to achieve normal brain function. Transplanted MPS1 patients continue to experience slowed neurocognitive development or learning disabilities, despite a relatively good response in peripheral organ systems, and animal models confirm low levels of enzyme in the brain following peripheral SC transplantation. These data suggest that direct transplantation of cells into the brain, such as neural stem cell transplantation (NSCT), is essential for preserving brain function. This approach is supported by several animal studies. Considerable evidence, then, suggests that combination therapy, using HSCT with NSCT, could be the first, systemically-effective therapy for MPS1 and other LSDs.
Though 1) the CNS is a relatively immune-privileged site, 2) undifferentiated stem cells express low levels of MHC molecules, and 3) patients transplanted with fetal-derived tissue can show long-term engraftment without immunosuppression, it is still clear that immune-matching of transplanted cells is ideal to guarantee life-long tolerance of a CNS graft. Further, in young children the lifetime cost of pharmacologic immunosuppression is enormous and is associated with well-known major morbidities. Importantly, we have demonstrated that migratory neural stem cells (NSCs) transplanted directly into the brain have significant ameliorative effects on an LSD that are migration-specific, since fibroblasts, which secrete active enzyme but do not migrate, are ineffective. The source of NSCs (adult SC vs. hESCs) appears not to be important provided the identity and purity of the migratory NSCs are the same. Adult and hESC-derived NSCs show similar engraftment behaviors but different differentiation patterns after transplantation. From these data, an extremely important conclusion can be drawn: For treatment of enzyme deficiencies, the most important characteristic of NSCs is the migratory ability of the cells in setting up long-term residence throughout the brain. Since iPSCs can be derived from a particular donor and migratory NSCs can be derived from these iPSCs, then dual transplants (HSCT, NSCT) from one donor are a solution to allograft rejection, immune neutralization of replaced enzyme, and the significant down-sides of long-term immunosuppression.