Liver transplantation (LT) has been used to treat a variety of liver diseases. Within hours after birth, neonates can present with disorders of the urea cycle (UCDs), the critical metabolic liver pathway needed to detoxify waste nitrogen from the diet and cellular turnover. The overall incidence of UCDs is estimated to be 1 in 8200 births. An increase in ammonia concentrations is particularly toxic to the central nervous system (CNS), causing brain edema, with multiple episodes affecting survival and often resulting in mental retardation and cerebral palsy. While LT is recommended in neonatal-onset patients with these single-enzyme defects, organ availability is a major limitation and transplantation requires lifelong immunosuppression. In addition, transplant morbidity and mortality are not inconsequential: 1-year survival is about 91.9%, and 5-year survival in children transplanted at less than 5 kg is 74%. Transplantation of genetically corrected embryonic stem cell-derived hepatocytes from the affected patients themselves is a potential way to replace LT for the treatment of metabolic liver disease and will likely not require immunosuppression; importantly, the supply will be limitless, allowing early transplantation before CNS injury. We propose to explore this approach by using a new and robust liver repopulation UCD mouse to advance this therapy: treatment of single-enzyme liver defects with patient-derived and genetically corrected stem cell-induced liver cells.
Unfortunately there is a substantial wait for people in California who need a liver transplant, resulting in many who develop significant disability or die while waiting. While many are adults with chronic disease, some are children with metabolic disorders including urea cycle disorders (UCDs).
UCDs are caused by mutations resulting in enzyme deficiencies responsible for removing waste nitrogen, which as ammonia can cause irreversible brain damage, coma and/or death. Newborns can become catastrophically ill within 36-48 hours after birth. These and other inborn errors represent a substantial cause of brain damage and death among newborns and infants and because many cases remain undiagnosed, or infants with the disorders die without a definitive diagnosis, the exact incidence is unknown and likely underestimated.
Present treatment is dietary for most which is onerous & incomplete; definitive therapy is liver transplantation which is challenging in infants who have greater rates of complications and morbidity. In this proposal we will develop genetically-corrected hepatocyte-like cells from induced pluripotent stem cells from patients with arginase deficiency, a UCD. These will be tested for the ability to correct the disorder in a unique UCD liver repopulation animal model. The advantage of the proposed methodology over current therapy is that genetically-corrected cells will be limitless and will require no major surgery or immunosuppression and its short- & long-term risks.
This Development Candidate Feasibility (DCF) proposal seeks to develop a source of autologous, gene-corrected hepatocytes for treating patients with arginase deficiency, a type of urea cycle disorder (UCD) in which a defective metabolic enzyme in the liver leads to severe damage of the central nervous system. The applicant will initially focus on generating induced pluripotent stem cells (iPSC) from the skin of affected patients, characterizing their defective gene loci, and optimizing the differentiation and expansion of these cells to the hepatic lineage. After choosing the best line(s) to move forward, the applicant will evaluate two different approaches for correcting the enzyme defect using site-specific gene editing techniques. Finally, the applicant proposes to determine whether and to what extent hepatocytes derived from these genetically corrected iPSC can ameliorate metabolic and neurological phenotypes in a relevant preclinical model.
Objective and Milestones
- The Target Product Profile (TPP) lacks metrics to allow development of a candidate for which the feasibility is being assessed. The investigators listed parameters to be tested, but quantitative measures for optimal and minimal activity for each given parameter were not adequately defined.
- While the project milestones are logical and reasonable, they do not sufficiently describe success criteria or go/no-go decision points for proposed components.
Rationale and Significance
- The scientific rationale for the overall approach is compelling. If successfully implemented, this strategy could be transformative not only for the treatment of arginase deficiency and other urea cycle disorders, but also for many additional diseases amenable to treatment by liver transplantation.
- The proposed therapeutic would address an unmet medical need, potentially overcoming several obstacles to liver transplantation as currently practiced including limited organ availability, the need for lifelong immunosuppression and the general risks that accompany surgery.
- Reviewers questioned the extent to which enzyme replacement therapy could reverse the initial neurological injury associated with arginase deficiency.
Feasibility and Design
- The preliminary data are limited but illustrate the technical ability of the investigative team to conduct the proposed experiments, including the targeted gene corrections.
- The research plan reflects a logical, stepwise approach for achieving proof of concept within three years.
- While the preclinical model is scientifically enabling for proof of concept studies, it represents a permissive environment for engraftment and does not mimic a liver where there is no survival based selective advantage for transplanted cells.
- The extent to which the proposed hepatocytes would meet the criteria of a functional, mature cell is unclear. This has been a longstanding hurdle in the stem cell field, and there is little evidence to suggest this approach overcomes it.
- The issue of genomic stability was not adequately addressed in the application, which is an important consideration for safety. Reviewers also noted that introduction of a “suicide gene” for addressing safety concerns might present additional challenges at the regulatory level.
Qualification of the PI (Co-PI, Partner PI, if applicable) and Research Team
- The PI has an outstanding track record in gene therapy and has committed satisfactory effort to this project.
- An excellent research team provides necessary skills in stem cell and gene correction methodologies, but would benefit from deeper expertise in the area of hepatocyte biology.
- The proposed budget is appropriate.
Collaborations, Assets, Resources and Environment
- Institutional support is excellent, and all of the necessary resources and material assets appear to be in place.
- The intellectual property position for this DC was not clearly indicated.
Responsiveness to the RFA
- The proposal is highly responsive, as successful execution would achieve preclinical proof of concept for a human stem cell based development candidate.
- While the gene modification methodologies proposed are not novel, their application for correcting arginase deficiency is. Additional novelty comes from the iPSC approach for yielding greater numbers of cells for dosing.
- The proposed research is not currently represented in CIRM’s translational portfolio.