RPE maintains the function of light-sensing photoreceptor neurons. Death of RPE leads to degeneration of the light-sensing neurons and vision loss, which affects the ability to perform daily tasks and may lead to depression. About 20% of Americans between the ages of 65 and 75 years can expect to experience RPE loss, an incidence that will double in this decade. As the current study attempts to show, transplantation of iPS-derived RPE cells has the potential to restore lost vision in humans and preclinical models of RPE loss. We tested this ability in the Rpe65rd12/Rpe65rd12 mouse by using ERG readings and found that vision improved after injection with iPS-derived RPE. Our studies provide evidence for the preclinical feasibility of bringing autologous stem cell transplantation into a phase I/II clinical trial for patients with RPE diseases.
Because of the well-known risks of tumorigenesis associated with reprogramming, our current study extended our previous RPE differentiation protocol (10
). The risk of tumorigenesis associated with iPS differentiation was attributed to a portion of the iPS cells remaining undifferentiated at the end of the reprogramming protocol. Our extended protocol may have contributed to the lack of tumor formation in the present study. These results support the safety of iPS cells for autologous transplantation. We also confirmed the integration of iPS-derived RPE by host retinas. After transplantation, histology revealed RPE was able to adapt to host tissues; no detectable disruption of albino host cells was observed.
Electrophysiological testing provided evidence that iPS-derived RPE grafts can support neuronal function in Rpe65rd12/Rpe65rd12
mice. Previously, we demonstrated the recovery of visual function in this preclinical model after fetal RPE (16
) and ES cell transplantation (10
). To ensure these iPS-derived RPE cells were not rejected, we used SCID Rpe65rd12/Rpe65rd12
mice, which typically die at 7 or 8 months of age, as recipients (14
). After transplantation, ERGs were conducted up to 6 months, covering 75–85% of the lifetime of the SCID mice. To minimize all other sources of variability, mice used in the study were all littermates; to test the therapy efficacy of iPS-derived RPE, transplantation and ERGs were performed concomitantly for all of the mice in the study, and we compared the ERG results between transplanted and fellow eyes of each mouse independently.
Confounding effects of the surgery- induced rescue described in the RCS rat (17
) were avoided as much as possible by the study of mitomycin-C–treated stem cell grafts as control groups. In previous studies of iPS-derived RPE in the RCS rat, iPS transplantation was shown to produce a rescue effect on optokinetic behavior but not ERG function (6
). The RCS rat has a defective MerTK receptor tyrosine kinase that halts phagocytosis within RPE cells and leads lipofuscin to accumulate in the subretinal space, finally leading to the death of photoreceptors before 3 months of age (17
). Rescue via transplantation with either a human immortal RPE cell line (ARPE-19) (19
) or native RPE cells (4
) has been achieved in the RCS rat model. Remarkably, “saline injections” produced a similar efficacy lasting up to a period of 6 months in RCS rats (19
). Improvement may have been due to the effects of the saline in washing outer segment debris from the subretinal space.
Our present study attempted to differentiate between the effects of the surgical injury–induced rescue from cell transplantation by studying control groups grafted with mitomycin-C–treated undifferentiated ES cells. The ERGs from mice in the control groups showed no statistically significant differences between transplanted and control fellow eyes, whereas the ERGs from mice transplanted with iPS-derived RPE cells showed statistically significant improvement in transplanted but not control fellow eyes. The relatively small absolute (but significant) improvement in the ERG reflects the small size of the transplanted area.