Masayo Takahashi, an ophthalmologist at the RIKEN Center for Developmental Biology (CDB) in Kobe, Japan, has pioneered the use of induced pluripotent stem cells (iPSCs) to treat patients with degenerative retinal diseases.
Takahashi isolated skin cells from her patients, and then had them reprogrammed into iPSCs in the laboratory through a combination of genetic engineering and cell culture techniques. These iPSCs have many similarities with embryonic stem cells, including pluripotency, which is the potential to differentiate into any adult cell type.
Once induced pluripotent stem cell lines were established from her patient’s skin cells, they had their genomes sequenced for safety purposes, and then differentiated into retinal pigmented epithelial (RPE) cells. RPE cells lie beneath the neural retina and support the photoreceptors that respond to light. When the RPE cells die off, the photoreceptors also begin to die.
Takahashi watched the transplantation of the RPE cells that she had grown in the laboratory into the back of a woman’s damaged retina. This transplant would constitute the first test of the therapeutic potential of iPSCs in people. Takahashi described the transplant as “like a sacred hour.”
Takahashi has collaborated with Shinya Yamanaka, the discoverer of iPSC technology. She devised ways to convert the iPS cells into sheets of RPE cells. She then tested the resulting cells in mice and monkeys, jumped the various regulatory loops, recruited patients for her clinical trial, and practiced growing cells from those patients. Finally, she was ready to try the transplants in people with a common condition called age-related macular degeneration, in which wayward blood vessels destroy photoreceptors and vision. The transplants are meant to cover the retina, patch up the epithelial layer and support the remaining photoreceptors. Watching the procedure, “I could feel the tension of the surgeon,” Takahashi said.
This transplant surgery occurred approximately a year ago. Some new data on this patient is available.
As of 6 months after the transplant, the procedure appears to be safe. The one-year safety report should appear soon. Prior to the transplant, the patient was a series of 18 anti-vascular endothelial growth factor (anti-VEGF) ocular injections for both eyes to cope with the constant recurrence of the disease. However, data presented by Dr. Takahashi showed that the patient had subretinal fibrotic tissue removed during the transplant surgery in order to make room for the RPE cells. Once the RPE cells were implanted, the patient experienced no recurrence of neovascularization at the 6-month point. This is significant because she has not had any other anti-VEGF injections since the transplant. Her visual acuity was stabilized and there have been no safety related concerns to date.
I must grant that this is only one patient, but so far, these results look, at least hopeful. Hopefully other patients will be treated in this trial, and hopefully, they will experience the same success that the first patient is enjoying. We also hope and pray that the first patient will continue to experience relief from her retinal degeneration.
As to the treatment of the second patient of this trial, Takahashi has hit a snag. Some mutations were detected in the iPS cell-derived RPE cells prepared for the second patient. No one knows if these mutations make these cells dangerous to implant. Regulatory guidelines, at this point, are also no help. Apparently, the cells have three single-nucleotide change and three copy-number changes that are present in the RPE cells that were not detectable in the patient’s original skin fibroblasts. The copy-number changes were, in all cases, single-gene deletions. One of the single-nucleotide changes is listed in a database of cancer somatic mutations, but only linked to a single cancer. Further evaluation of these mutations shows that they were not in “driver genes for tumor formation,” according to Dr. Takahashi.
Tumorigenicity tests in laboratory animals has established that the RPE cells are safe. Remember that the presence of a mutation does not necessarily mean that these RPE cells can be tumorigenic.
However, Takahashi has still decided to not transplant these cells into the second patients. Part of the reason is caution, but the other reason is compliance with new Japanese law on Regenerative Medicine, which became effective after iPS trial was begun. This law, however, does not specify how safe a cell line has to be before it can be transplanted into a patient.
RIKEN’s decision to halt the trial is probably a good idea. After all, this is the first trial with iPSCs and it is important to get it right. Even though the RPE cells were widely thought to be safe to use, Takahashi decided not to implant another patient with RPEs derived from their own cells. Instead, they decided to use RPEs made from donated iPSC lines. Therefore, Takahashi is in discussions government officials to determine how this change of focus for the trial affects their compliance with Japanese law.
Frankly, this might be a very savvy move on Takahashi’s part. As Peter Karagiannis, a spokesperson for the Center for iPS Cell Research and Application, noted: “As of now, autologous would not be a feasible way of providing wide-level clinical therapy. At the experimental level it’s fine, but if it’s going to be mass-produced or industrialized, it has to be allogeneic.”
Therefore, the RIKEN institute is moving forward with allogeneic iPSC-derived RPEs. RIKEN will work in collaboration with the Center for iPS Cell Research and Application (CiRA) in Kyoto, Japan, which has several well characterized, partially-matched lines whose safety profiles have been established by strict, rigorous safety testing methods. However, immunological rejection remains a concern, even if these cells are transplanted into an isolated tissue like the eye where to immune system typically is not allowed. The simple fact is that no one knows if the cells will be rejected until they are used in the trial.
An additional concern is that CiRA has not typed its cells for minor histocompatibility antigens, which can cause T cell–mediated transplant rejection.
Nevertheless, Takahashi and her team deserve a good deal of credit for their work and vigilance.