Children with lethal inherited degenerative diseases of the brain, the lysosomal storage diseases or LSDs, will be among the first to benefit from novel approaches based on stem cell therapy (SCT). This belief is based on a number of medical and experimental observations including:
1) These diseases cause profound mental retardation or lead to death in most affected children;
2) SCT has already been shown to work in the milder forms of LSD that do not affect the brain;
3) The clinical safety of neurotherapy by stem cell transplantation is already being established in the LSDs;
4) Many of the pertinent regulatory hurdles have already been overcome; and
5) Experimental work has clearly shown that SCT can be used to treat the brain in the severe forms of LSD.
This combination of medical and experimental data strongly suggest that the LSDs are uniquely poised to benefit from novel SCT and that this therapy is likely to be successful, a prediction that can less reliably be made for SCT applied to other brain and spinal cord diseases and injuries. Additionally, results of clinical trials of novel stem cell neurotherapies for the LSDs will be directly pertinent to proposed stem cell neurotherapies for some other neurological diseases, including Alzheimer’s disease, Parkinson's disease, multiple sclerosis, and stroke, as specific targets for these and other brain and spinal cord diseases and injuries are better identified.
Our approach is designed to address two major bottlenecks to translating experimental results into clinical practice: that adequately repairing a damaged brain is likely to be extremely difficult or impossible and that, as in all cell, tissue, or organ transplantation, immune rejection of the transplanted material is almost guaranteed. We not only address these two important bottlenecks but also set the stage for efficient translation of our approach into clinical practice because almost all of it is based on techniques that are already in clinical practice or in clinical trials.
Statement of Benefit to California:
We are focusing on a class of childhood brain diseases that causes the child's brain to degenerate and results in severe mental retardation or death. These diseases also affect other organs of the body. Many of these organs can already be successfully treated with stem cell therapy. Our team proposes to take these lessons of success and apply them to treating the brain as well. Because of this established stem cell success, this new stem cell therapy, we propose, has a high probability of success. This will not only provide a potential cure for the children that are treated with this new stem cell therapy, but will also benefit California by 1) reducing the State's burden for caring for these children and 2) providing a successful model of stem cell therapy of the brain that will both bolster public confidence in CIRM's mission to move stem cell therapies into the clinic, and lay the groundwork for using this type of therapy with other brain diseases such as Alzheimer's disease, Parkinson's disease, stroke, and multiple sclerosis.
This proposal addresses two bottlenecks in the stem cell-based treatment of lysosomal storage diseases (LSDs), a group of inherited metabolic disorders that result in the accumulation of unprocessed macromolecules in lysosomes, causing cellular dysfunction and severe clinical complications. The applicant proposes that the major bottlenecks in the development of effective therapies for LSDs are: 1) the ineffectiveness of hematopoietic stem cell transplant (HSCT) for disease manifestations in the central nervous system (CNS); and 2) the need for life-long immunosuppression in patients receiving neural stem cell (NSC) transplants to treat CNS manifestations. To address these issues the applicant proposes to treat both peripheral and CNS pathologies in a mouse model of a LSD (Hurler’s disease) by using HSCT, supplemented by NSC transplants into the brain. The NSCs will be derived from induced pluripotent stem cells (iPSCs) derived from mesenchymal stromal cells (MSCs) cultured from the same bone marrow donor used for HSCT. This strategy guarantees immune-matching and could eliminate the need for immunosuppression. These experiments will be carried out in a novel, long-lived, immune-deficient mouse model of Hurler’s disease generated by the applicant.
The reviewers agreed that this proposal has the potential for high impact. Allogeneic HSCT from healthy donors is currently used to treat patients with LSDs such as Hurler’s disease. But a major problem with this approach is that correction of the enzyme deficiency mainly improves peripheral manifestations of these diseases without affecting CNS manifestations, presumably in part because of limitations of HSC trafficking across the blood/brain barrier. For this reason patients generally also receive enzyme replacement therapy, also with limited effectiveness. This proposal intends to address this bottleneck by supplementing HSCT with injection of NSCs into the CNS. However, one reviewer felt that the justification for using stem cell to address this bottleneck is debatable, noting that microglia could provide an effective vehicle for enzyme replacement in the CNS and these cells enter and migrate freely within it. Therefore the need for neural stem cells, whose capacity for functional integration has not been fully characterized, is unclear. Reviewers agreed that the second bottleneck is potentially important and that the strategy of using immune-matched (to the donor marrow used in BMT), iPS-derived cells for CNS transplantation could have widespread applicability for tissue-specific transplantation.
The reviewers raised a number of questions about the research plan and its feasibility. They described the plan as complex and cited a number of significant challenges, including the creation of iPSCs from stromal cells and their differentiation into a pure NSC population with the capacity to migrate widely and engraft. One reviewer felt that, given the goal of prolonging survival of transplanted NSCs, the proposed examination of the CNS is inadequate. The emphasis is on behavioral and biochemical analyses but monitoring the persistence and integration of neurons and glia derived from the transplant might provide a better readout, and should be done with animal studies. This reviewer also noted that while the derivation of a mouse model for Hurler’s disease bred into an immunodeficient background should aid NSC engraftment it may not be a suitable test bed for evaluating potential benefits of immune-matching transplanted HSCs and NSCs. The reviewer suggested parallel experiments with mismatched transplanted populations to demonstrate the effectiveness of the tolerogenic strategy. Another reviewer raised a number of concerns about the experimental design. The applicant proposes to use a nonmyeloablative conditioning regimen prior to HSCT but provides no specifics. The applicant is concerned about engraftment of human stem cells in the mouse model and mentions the possibility of supplementing conditioning with cyclophosphamide and fludarabine, but presents no data that fludarabine will work in this model. Finally, this reviewer noted that human T cells will follow successful HSCT and have the potential for causing graft-versus-host disease, a possibility that isn’t discussed in the proposal.
The reviewers noted that the applicant has considerable experience in NSC research and transplantation into animal models. The assembled research team contributes expertise in iPSC line derivation and neurodegenerative disease as well as clinical experience with HSCT. One reviewer noted that the choice of an international collaborator for immunology was not fully justified, and that the lack of a specialist neuroscientist for the analysis of data arising from NSC transplantation was a weakness. Another reviewer remarked that the budget seems very large and lacked adequate justification.
Overall, while the reviewers appreciated the novel approach and potential impact of this proposal, they raised a number of concerns about the research plan and its feasibility.