Artemis is a chemical in all cells in the body that is essential for the normal development of the immune system and repairing damaged DNA. Artemis deficiency (AD) causes Severe Combined Immunodeficiency (SCID-A), a “bubble baby” syndrome associated with increased sensitivity to radiation and chemotherapy. SCID-A is hard to treat with a bone marrow stem cell (SC) transplant from another person due to rejection, reactions from the graft, and toxicity from high dose chemotherapy. Gene corrected (GC) patient’s SC will minimize the risks and cure SCID-A. Our objectives are to 1) Maximize engraftment of GC SC by opening marrow space without using high dose chemotherapy; 2) Assess harmful effects after gene correction of mouse and human SC by developing safety testing suitable for clinical trial use; and 3) Demonstrate that GC human SC from SCID-A patients correct the defective immunity in animal and cell models. Using an AD mouse model we will open marrow spaces by using a genetically engineered drug which targets SC (yr 1) ± an agent which blocks marrow SC attachment (yr 2), and find the minimal effective dose of chemotherapy (year 3). We will test for toxicity using several approaches (yrs 1-3). AD SC will be corrected with our lentiviral vector (AProArt) using clinical trial conditions and cultured on special cells that support SC growth into immune cells. Immunodeficient mice will be injected with GC human SC and human SC differentiation into immune cells evaluated (yrs 2-3).
Artemis deficient Severe Combined Immunodeficiency Disease (SCID-A) results in T-B-NK+ SCID with increased sensitivity to alkylator chemotherapy, and accounts for ~10% of all SCID patients. Athabascan-speaking Native Americans have a very high incidence of SCID-A (2/5000 births) and affected children from other states are sent to [redacted] for curative treatment. California has been among the leading states in instituting newborn screening for SCID and [redacted] is one of the main referring hospitals for these newly-diagnosed babies. In the first year of newborn screening, 8 babies with SCID were born in CA. Currently, the only cure for this otherwise fatal disease is an allogeneic hematopoietic stem cell (HSC) transplant in which high dose alkylator chemotherapy is often necessary to overcome graft rejection and open sufficient bone marrow niches to reconstitute both T and B cell immunity. Successful gene correction of SCID-A will eliminate the need for high dose alkylator chemotherapy and significantly reduce the mortality of HSC transplantation and cost of lengthy hospitalization and long term care for the late effects due to alkylator use in these newborn babies. The approaches that are developed in this project for successful gene therapy without using high dose chemotherapy will benefit all children in California (and elsewhere) with a variety of genetic diseases who may benefit from curative cellular therapy.
The proposed Development Candidate (DC), gene-corrected autologous hematopoietic stem cells (HSC), targets a specific form of severe combined immunodeficiency (SCID) mostly found in Athabascan-speaking Native Americans. It is caused by lack of the Artemis (A) enzyme essential for DNA repair, resulting in patients who lack T cells and B cells, have increased sensitivity to radiation and chemotherapy and therefore show strong adverse responses toward standard conditioning regimens used prior to allogeneic HSC transplantation, the only current treatment option. Autologous HSC transplants would eliminate the need for standard conditioning, although mild conditioning may still be required to open marrow niches. Proposed studies include the evaluation of new, milder approaches in a model system to optimize HSC engraftment through opening marrow niches; production of a lentiviral gene therapy vector, with initial proof –of-concept testing of corrected mouse HSC in a SCID-A mouse model. The applicant will assess whether the gene therapy vector causes adverse events due to insertional mutagenesis. Finally, human SCID-A HSC will be gene-corrected and evaluated for their ability to form T cells and B cells in vitro and in immune deficient mice, to demonstrate disease-modifying activity.
Objective and Milestones
- The objective and aims to achieve a development candidate and test in a model system potential conditioning regimens for the target patient population are generally focused, complete, logical and achievable in three years.
- The target product profile (TPP) is scientifically and clinically reasonable.
- The applicant did not justify the proposed minimum chimerism level for gene-corrected HSC; reviewers were concerned it may not be sufficient to significantly impact patient outcomes.
Rationale and Significance
- The rationale for selecting SCID-A as a disease target is sound, since current allogeneic HSC transplant options for SCID-A patients result in poor outcomes.
- SCID-A is a very rare disease, thus the potential impact on overall population health is small. If successfully developed, the DC will however have a significant impact in the targeted patient population.
- Future clinical trials may experience difficulties in recruiting sufficient numbers of patients due to the very small size of the patient population.
- Because of the sensitivity of SCID-A patients to standard conditioning regimens, evaluating alternative, gentler conditioning approaches is justified. However, this rationale is weakened by the applicant’s description of several long-term survivors who received no conditioning for their HSC transplant.
- Proposed strategies for improving HSC engraftment constitute novel aspects of this proposal.
- Successful improvements of conditioning regimens could have wider applicability.
Research Project Feasibility and Design
- The preliminary data support the expertise and contribution of the PI and collaborators to SCID-A research, including the development of a model that mimics the human disease and the development of improved vectors.
- The research plan is well designed and addresses all activities necessary to achieve a DC in three years.
- Some reviewers criticized the proposed enrichment of HSC being based on two markers, requiring more complicated cell sorting technology than a commercial system that uses one marker, and is available for use under an approved Investigational New Drug (IND) application. Other reviewers were not concerned, as they did not consider this enrichment procedure to be the primary goal for the proposed studies.
- The specific model used to assess disease-modifying activity of gene-corrected human SCID-A HSC, while permitting an assessment of T cell and B cell differentiation capacity, will not mimic the proposed conditioning regimens.
Qualifications of the Principal Investigator (PI) and Research Team
- The PI and team members are well qualified to perform the activities listed in the proposal. Some reviewers were unclear as to whether the PI, Co-PI or key collaborators had product development experience, although it was commented that at least one collaborator could provide important guidance on the vector.
- The PI has assembled an appropriate multidisciplinary team to achieve the project's aims.
- The budget is sizeable and includes a substantial request for equipment that one would expect to be in place in a well-established laboratory. For instance, the budget includes costs for a new cell sorter that will only be partially utilized for activities associated with this proposal. Otherwise the budget is appropriate for the research necessary to achieve the project aims.
Collaborations, Assets, Resources and Environment
- The facilities at the applicant institution are excellent, as is the institutional support.
- Inclusion of two specific collaborators is a strength.
- The use of the proposed vector preparation facility is a strength.
Responsiveness to the RFA
- Human stem cells are necessary for the proposed research and the specific patient population has been identified.
- Reviewers noted that CIRM currently funds two proposals that target genetic correction of HSC, albeit not in this patient population that has challenges to facilitating engraftment.
- A motion was made to move this application into Tier 1, Recommended for Funding. This application targets a pediatric population, a patient population that was considered to be underrepresented in CIRM’s portfolio. Some reviewers reiterated concerns regarding the very small size of the patient population in terms of overall impact and feasibility of a future clinical trial, but others made the argument that an improved treatment for SCID-A patients would have a tremendous impact on that population. The motion carried.