Abstract
Stargardt disease or Juvenile Macular Degeneration is a rare genetic disorder caused by a mutation in the ABCA4 gene that results in degeneration of the macula and loss of central vision. The mutation in the ABCA4 gene causes a build-up of lipofuscin, which is a substance that is left over from the breakdown and absorption of damaged blood cells. This build-up of lipofuscin causes the death of photoreceptor cells and the subsequent degeneration of the macula. Due to the nature of Stargardt’s disease, there are currently no cures or treatment options. However, in recent years, there has been an increase in research and exploration of utilizing stem cell therapy as a potential cure and treatment for Stargardt disease. Growing research in the field of ophthalmology has found that the use of stem cells can potentially replace the loss of photoreceptor cells, slow the progression of the degeneration of vision, and be a potential new treatment option for Stargardt disease.
Introduction
What is Stargardt disease?
Stargardt disease or Juvenile Macular Degeneration is caused by a rare genetic mutation in the ATP-binding cassette, sub-family A (ABC1), member 4 (ABCA4) gene, and can be characterized by loss of central vision, color blindness, and difficulty adjusting to changes in lighting. 1 The ABCA4 gene encodes a transmembrane protein, Rim, that is responsible for transporting N-retinylidine-phosphatidylethanol-amine (N-RPE) to the retinal pigment epithelium (RPE) and is expressed in the outer sections of photoreceptor cells in the foveal and parafoveal areas. 2 When mutations occur in the ABCA4 gene, Rim is unable to transport N-RPE and this leads to a build-up of lipofuscin in the RPE. Accumulation of lipofuscin causes the death of photoreceptor cells and the macular degeneration associated with Stargardt’s disease. 3 Although Stargardt disease is the most prevalent inherited macular dystrophy, its prevalence in the United States “is estimated to be 10–12.5 per 100,000 individuals.” 4 Stargardt disease has a heterozygous autosomal recessive mode of inheritance of the ABCA4 gene. 1 The main issue involving Juvenile Macular Degeneration is a fundamental problem that warrants further study in the current field of ophthalmology: there is no known therapeutic option that can rapidly, effectively, and dramatically alter the prognosis of this rare genetic disease that is used in standard clinical practice. 5 Stargardt’s disease causes progressive vision loss over time, so researchers and clinicians are trying to find an effective option for treating this ophthalmologic disease. Hence, the relevance of analyzing stem cell therapy for Stargardt disease is essential to the modern field of medicine in the 21st century, as there is still no well-defined treatment for this disease. It has not yet been thoroughly investigated whether stem cells specifically provide a full-scale treatment for Stargardt disease rather than just a simple alleviation of symptoms, which makes this analysis unique and important when trying to advance the field of ophthalmology forward by helping treat those who suffer from this degenerative disease.
Stargardt disease can begin to show symptoms in childhood and early adulthood. The loss of vision can progress exponentially until about 20/200 vision and then symptoms will begin to plateau. 6 Stargardt disease is typically diagnosed by an ophthalmologist. The ophthalmologist will dilate the eye to observe the retina and find the accumulation of lipofuscin in a person suffering from Stargardt disease. The ophthalmologist will then use Fluorescein Angiography to complete the diagnosis. Fluorescein Angiography is performed by injecting fluorescein into a vein to obtain photographs of the retina. A retina with Stargardt disease will show darkened tissue as evidence of photoreceptor death and macular degeneration (Figure 1). Patients diagnosed with Stargardt disease are advised to maintain a low vitamin A diet to avoid further lipofuscin build-up and limit UV exposure to avoid further retinal damage. As of today, there are no FDA-approved treatments for Stargardt disease. 6 However, researchers are exploring the options of retinal stem cell therapy.
Images showing the difference between a healthy macula versus a macula with Stargardt disease.
What is stem cell therapy?
Stem cells are cells that can differentiate into specialized cells. They can replace damaged or lost cells. Stem Cell Therapy is performed by using stem cells to specifically target damaged or lost cells that are affected by the disease of interest. 7 This is performed by differentiating the cells into the cell type that is being studied and then using it to restore function in the diseased organ. Another approach is by transplanting stem cells so they secrete trophic factors by using a paracrine effect. This will allow the tissue of the diseased organ to self-restore lost cells. Lastly, the implanted cells can restore function by fusing with the existing cells of the diseased organ. 8 Researchers are exploring the option of using stem cell therapy for Stargardt disease to replace the loss of photoreceptor cells and slow the progression of macular degeneration. The exploration of stem cell therapies offers hope for treating Stargardt disease. Four promising approaches are under investigation, each with unique benefits and challenges.
(1) Primary Retinal Cells and Retinal Progenitor Cells:
These cells, either harvested from the patient or sourced from donors, are transplanted to replace damaged photoreceptors and RPE cells. This method has shown potential for vision restoration but faces limitations due to ethical concerns and the difficulty of obtaining sufficient healthy cells. 9
(2) Human Embryonic Stem Cells (hESCs):
Derived from the inner cell mass of the blastocyst, hESCs are pluripotent and can differentiate into various cell types, including RPE cells. Preclinical studies in animal models have demonstrated their effectiveness in integrating with retinal tissue, showing promise for future human applications despite ethical challenges. 9
(3) Induced Pluripotent Stem Cells (iPSCs):
iPSCs are adult somatic cells reprogrammed to an embryonic stem cell-like state. 9 These cells can be patient-specific, reducing immune rejection risks and ethical concerns. 8 However, their use is complicated by the need to correct genetic mutations associated with Stargardt disease before transplantation.
(4) Mesenchymal Stem Cells (MSCs):
MSCs, sourced from bone marrow, adipose tissue, or umbilical cords, have gained attention due to their ability to migrate to injury sites and differentiate into functional cells.10,11 Their immunomodulatory properties further enhance their potential, although their ability to fully restore vision remains under study. 12
Along with the various subtypes of stem cells being used, method of delivery is another factor to consider in treatments. Popular choices for delivery include subretinal, suprachoroidal, and intravitreal injections (Figure 2).13,14 These include injections to the space between RPE and photoreceptors of the retina, injections to the subchoroidal space, and injections to the vitreous space of the eye, respectively. Of the three delivery methods, the subretinal is optimal in the prevention of immune rejections, with retinal detachment and retinal perforation as the most common adverse reactions. The reduced immune response is due to RPEs secreting cytokines to suppress T-cell function. 13 RPE cells are also capable of converting intraocular T cells to regulatory T cells by releasing cytotoxic T lymphocyte-associated antigens. The intravitreal method has proved to be the easiest and safest method. Benefits of the subchoroidal and intravitreal approaches include maintaining safety of the eye anatomy and allowing for a safe, easy route to transplantation. 13
Demonstration of the injection sites for different delivery methods.
Research and analysis
Within the scope of analyzing macular degeneration regarding stem cell transplants, the methodology used to select critical studies analyzing this important phenomenon involved selecting relevant high-impact case studies from the last two decades. Research into stem cell therapy has grown rapidly over the years, and at the turn of the 21st century, there has been heavy emphasis on incorporating stem cell research into different therapy options. Therefore, only case studies and critical sources from the last two decades will be incorporated into this analysis of stem cell treatment for Stargardt disease. The scope of this analysis is focused on stem cell therapies for ophthalmologic illnesses: particularly Stargardt disease. It is imperative that all case studies incorporated regarding stem cell research involve therapeutic approaches to treating degenerative illnesses. As stated previously there are four major approaches pertaining to stem cell therapy. Current stem cell therapy clinical trials are predominantly using the following approaches.
Primary retinal cells and retinal progenitor cells.
Application:
In Stargardt disease, a genetic condition that leads to the degeneration of photoreceptors (specifically cones and rods), the transplantation of fully differentiated photoreceptors and RPE cells is a logical approach. By replacing damaged or lost photoreceptor cells, the visual cycle can be restored, potentially improving, or even reversing vision loss.
Challenges:
Obtaining retinal cells for transplantation is challenging. Sourcing cells from a patient’s own eye is limited by the availability of healthy tissue. Donor cells present ethical and immunological challenges, including the risk of rejection. Moreover, while clinical trials have shown some success in improving vision, the method is not yet widely accessible due to these complications.
2. hESCs.
Application:
hESCs, being pluripotent, offer the potential to differentiate into any cell type, including RPE cells and photoreceptors. hESC-derived RPE cells have been successfully integrated into the retinas of animal models with degenerative diseases such as Stargardt disease. The goal is to transplant these healthy RPE cells into human patients, restoring the function of diseased RPE cells, which are responsible for supporting photoreceptor function in the retina.
Challenges:
While there has been success in animal models, ethical concerns remain a significant barrier to the use of hESCs in human trials. Since these cells come from embryos, their use is highly regulated and often controversial. In addition, long-term studies are needed to assess the safety and efficacy of hESC-based therapies in humans.
3. iPSCs.
Application:
iPSCs provide a significant advantage in treating Stargardt disease by allowing the creation of patient-specific stem cells that can be reprogrammed into RPE cells or photoreceptors. These cells can be used for personalized treatments, reducing the risk of immune rejection. iPSCs can also be used to study the disease at a cellular level, as researchers can generate retinal cells from a patient’s own tissue in a laboratory and observe how the disease progresses.
Challenges:
Since iPSCs are generated from the patient’s own somatic cells, they may still carry genetic mutation causing Stargardt disease, which could limit their therapeutic effectiveness. As a result, gene-editing techniques are being explored to correct these mutations before transplantation. However, the gene-editing process adds complexity to the treatment, and ensuring that the iPSCs are free from disease-causing mutations before transplantation remains a challenge.
4. MSCs.
Application:
MSCs, derived from sources such as bone marrow or adipose tissue, have gained interest due to their Homing ability, which allows them to migrate to sites of injury or degeneration in the retina. In Stargardt disease, MSCs can be used to deliver therapeutic effects by differentiating into support cells for the retina, potentially reducing inflammation and aiding in tissue repair. In addition, MSCs possess immunomodulatory properties, which help reduce the chances of transplant rejection by suppressing immune responses.
Challenges:
MSC-based therapy for Stargardt disease is still in its experimental stages. While promising for its ability to support retinal repair and reduce immune responses, more research is needed to determine the optimal methods for using MSCs in retinal diseases like Stargardt. There are also concerns about how well MSCs can differentiate into specific retinal cell types needed to restore vision.
Each of these approaches offers unique advantages and limitations in addressing Stargardt disease, with some focusing on replacing damaged cells and others on supporting tissue regeneration. However, the ethical, technical, and biological challenges associated with each method still need to be addressed for these therapies to become viable treatments. There have been numerous clinical trials that explore these four different stem cell treatments, and as more research funding is being allocated toward helping find beneficial treatments for Stargardt disease, more clinical trials will inevitably provide more results as well.
Analyzing modern clinical trials and research regarding ophthalmic diseases, Lund and his associates 15 injected four different human-derived cells into animal models to evaluate the safety and efficacy of this method in treating retinal diseases and studied the effects over a 100-day period. The cells that were used were derived from umbilical cords, placenta, mesenchyme, and dermal fibroblasts. The dermal fibroblasts were used as the control of the study. The researchers used electroretinogram responses, spatial acuity, and luminance thresholds to determine the long-lasting effects of the different cells. Of the four different cells, it was concluded that the umbilical cord-derived cells showed the most promise as they demonstrated photoreceptor rescue and population increase without karyotypic changes. The MSCs showed photoreceptor rescue only to small, localized areas. Lastly, the placenta-derived cells showed little to no changes compared to the control group. Overall, this highly relevant clinical study demonstrates that umbilical cord-derived cells show the most promise in future stem cell research for cures of retinal diseases. 15
Lu et al. 16 used hESCs to treat macular degeneration in animal models. Rats were treated with hESC-RPE transplants into the subretinal space. The study showed no teratoma, tumor formation, or immunization concerns. After 60 days, it was found that the cells sustained normal functioning as well as visual and photoreceptor function without any major abnormalities. Ultimately, the researchers concluded that these studies were safe and helped offer insight into the use of stem cell therapy for macular degeneration treatment. 16 This study was the first of many that showed that when hESC-RPE transplants were implanted into the subretinal space, the animal was able to regain the visual and photoreceptor functions that were previously damaged. This study is promising and shows that through the manipulation and implantation of one specific gene, there was the possibility for returned sustainable normal functioning of the eyes.
Schwartz et al. 17 used hESC-RPE in nine patients with Stargardt disease and nine patients with age-related macular degeneration with the goal of studying medium to long-term efficacy and safety over a 22-month period. Stability or improvement of visual acuity was found in 17 of the 18 eyes compared to the control eyes of each patient. Vision was assessed using the best-corrected visual acuity. Only one eye showed continued degeneration. No severe complications or adverse reactions were found in any of the participants in relation to the transplanted cells. However, vitreoretinal surgery and immunosuppression adversity were found. Overall, this study shows promise as further proof that stem cells may be a treatment for Stargardt disease. 17
A study done by Oner et al. 10 described the usage of suprachoroidal adipose tissue-derived MSCs in trials for the treatment of Stargardt and Age-Related Macular Degeneration. Using four patients with each disease, the researchers implanted stem cells into the most degenerated eye of each patient using the suprachoroidal transplantation route. Each of the patients was required to undergo several follow-ups over a 6-month period where they were studied using best-corrected visual acuity tests, multifocal electroretinography, fundus autofluorescence, etc. to evaluate the safety and efficacy of the trial. Overall, it was found that all patients demonstrated visual acuity and field improvements with no severe complications. This study demonstrates the possibility of using suprachoroidal implantation of adipose tissue-derived MSCs as a treatment for Stargardt disease. 10
In a study done by Weiss and Levy in 2021, it was found that the use of bone marrow-derived stem cells is a possible source of treatment for Stargardt disease. 11 The patients were all given bone marrow-derived stem cells (BMSCs) through five different intervention pathways: retrobulbar, subtenon, intravenous, intravitreal, and intraocular. The changes in vision were measured using Snellen and LogMAR scores. Using 34 eyes from 17 participants, they found that 21 eyes showed improvement, 8 showed no difference, and 5 had continued to degenerate throughout the 1-year-long experiment. It was found that patients with 20/200 vision or better either remained stable or experienced improved vision more than those with worse vision, therefore, it was concluded that earlier intervention is necessary. These results were found to be statistically significant with a p-value of 0.0004. It was hypothesized that the use of BMSCs facilitated mitochondria to increase ATP levels and decrease A2E levels, which improved the functionality of the mutated ABCA4 gene that led to vision improvement. Overall, it was found from this clinically relevant study that there were no significant differences between the different intervention pathways, and that bone marrow-derived stem cell therapy shows a promising future in the search for a cure for Stargardt disease. 11
In a study done by Li et al. in 2021, it was found that the use of human embryonic stem cell-derived retinal pigment epithelial cells has the potential to be used as a treatment for Stargardt disease. 18 In this study, the investigators injected hESC-RPE cells using the vitrectomy and subretinal transplant route in one of two eyes of each of the seven trial participants. The other eye of each participant was used as a control. The participants of this study included two males and five females between the ages of 19 and 27. The patients were required to do a 12-month follow-up. Many examinations such as optical coherence tomography, fundus autofluorescence, and best-corrected visual acuity were used to evaluate the efficacy and safety of the transplantation. Within the 4-month postoperation, three patients showed visual improvement. Three patients were found to have lower visual acuity with only one having a significant difference. It was found that none of the patients exhibited any adverse reactions or complications because of the immediate transplantation. Of the seven patients, only two experienced high intraocular pressure that was treated with eye drops two months postoperation. The patients were followed over a 5-year period to examine the long-term safety and efficacy of this transplantation method. This study was found to have a p-value of 0.01, which indicates there was a significant difference between the experimental and control groups. 18
Overall, when comparing the clinical trials that incorporate hESCs and MSCs as stem cell therapies for the treatment of Stargardt disease, there is a clear indication that hESCs provide more clear beneficial outcomes compared to control groups. Meanwhile, more research is required to determine the best methods for using MSCs in retinal diseases such as Stargardt. In addition, human embryonic stem cells are more likely to help limit macular degeneration, as numerous trials have shown how hESC implantation into the subretinal space allowed animals to regain the visual and photoreceptor functions that were previously damaged. There are also concerns about how well MSCs can differentiate into specific retinal cell types needed to restore vision. However, one benefit to the use of MSCs is that these stem cells are derived from bone marrow or adipose tissue, and have a Homing ability, which allows them to migrate to sites of injury or degeneration. Human embryonic stem cells on the other hand come from embryos, making their use highly regulated and often controversial. Therefore, even though clinical trials have demonstrated that hESCs do have a more profound impact on treating Stargardt disease compared to MSCs, the ethical and regulatory nature surrounding hESCs make MSCs another possible alternative, although more research is required to learn more about their effects when utilizing them for ophthalmic treatments.
Discussion
Currently, there is no FDA-approved treatment for Stargardt disease. However, researchers have been studying a plethora of potential approaches to solve this major health condition using stem cell therapy, gene therapy, or creating a synthetic vitamin A supplement.
Stem cell therapy currently revolves around the four major approaches (Table 1) that were discussed previously. But even these approaches have their potential challenges. The use of cell therapy is still a developing concept that requires future research, which will allow for more clinical trials to take place. As of now, these approaches have been seen to be beneficial and useful as a potential treatment and cure option. With stem cell therapy, it has been demonstrated several times by different researchers that not only can it slow the progression of the disease but has the potential to restore lost vision with little to no complications. Visual improvement was seen in the studies done by Weiss and Levy, 11 Li et al., 18 and Oner et al. 10 with other researchers showing slowed progression of the disease with their approaches. Future applications may involve deciding which approach exhibits the greatest level of visual field improvements with the least risk of severe complications.
Represents a comparison of retinal cell transplantation approaches.
Text in bold represents the key feautures associated with retinal cell transplantation.
Stem cell therapy focuses on replacing damaged or lost retinal cells. This approach involves using stem cells such as hESCs or iPSCs to generate healthy RPE cells or photoreceptors, which are then transplanted into the retina. Stem cell therapy offers the potential to restore lost vision even in advanced stages of the disease by replenishing the damaged cells with new, functional ones. In addition, iPSCs can be derived from the patient’s own cells, minimizing the risk of immune rejection. However, challenges arise from the risk of these cells carrying the same genetic mutation as the patient, as well as the complexity of ensuring they integrate and function properly in the retina. Moreover, stem cell therapy does not directly address the genetic cause of the disease, meaning it may not prevent future degeneration of newly transplanted cells unless combined with other treatments.
When performing a comprehensive analysis of these four different stem cell treatments, it is clear that they have different modes of action when trying to limit macular degeneration. For instance, MSCs help modulate immune response, which can reduce the transplant rejection risk, and have the ability to translocate to the target tissue, whereas iPSCs utilize transcription factors to resemble an embryonic-like state and can be personalized from the affected individual’s own cells. These different modes of action inherently have different benefits and outcomes, and some may be better suited for implementation over others depending on the context of the patient’s history of present illness or past medical conditions, and even the patient’s own preferences. For example, a patient who has a suppressed immune system may be at a lower risk of having a transplant rejection, and may not require the use of MSCs as much as another patient who has been known to have strong immune responses. It is important to note that not all stem cell therapies have been analyzed with the amount of research, as there is a much better understanding of how hESCs work compared to MSCs. However, a better understanding of each type of stem cell treatment will arise as more clinical studies are performed over time.
Gene therapy aims to correct the underlying genetic mutation that causes Stargardt disease, a different mode of action compared to typical stem cell treatments. Stargardt disease arises primarily due to mutations in the ABCA4 gene, which leads to the accumulation of toxic by-products in the retina, damaging photoreceptors. By delivering a functional copy of the ABCA4 gene to retinal cells using viral vectors, gene therapy seeks to restore normal protein function and halt disease progression. However, similar to how stem cell therapy aims to replace broken or damaged cells with healthy cells, gene therapy targets broken or damaged genes with healthy ones. 23 Oftentimes, this is done by using adeno-associated virus (AAD), a non-enveloped virus, which works to transfer DNA to the cells of interest. 24 However, the gene associated with Stargardt disease, ABCA4, is too large to be transferred by this virus, so researchers are exploring the possibility of breaking down and reconnecting the gene parts within the cells. 25 Sun et al. approached this problem by using PEG-ECO/pGRK1-ABCA4-S/MAR nanoparticles to express the ABCA4 gene in photoreceptors. 26 While this study showed promise for slowing the progression of lipofuscin and visual acuity improvement in mice, more research needs to be conducted on human subjects to test its efficacy and safety.
One key advantage of gene therapy however is that it targets the root cause of Stargardt disease, potentially offering a long-term solution by correcting the genetic defect before further damage occurs. However, gene therapy is limited by its ability to work only in the initial stages of the disease, when there are still viable retinal cells to treat. Furthermore, delivering large genes such as ABCA4 is technically challenging, and there are risks associated with the viral delivery systems used.
It has also been seen that vitamin A, a fat-soluble molecule plays a crucial role in converting photons into electric signals during the visual cycle. 27 A2E and ATR are dimers of vitamin A and have been found in large enough quantities to be toxic to RPE in lipofuscin of patients with retinal diseases such as Stargardt’s disease. 28 Researchers have explored potentially creating synthetic vitamin A that can be processed by patients with retinal diseases to decrease lipofuscin build-up. 29 Studies done by Kaufman et al. 27 and Issa et al. 28 used deuterium at the C20 position of vitamin A which showed slower progression of A2E formation than wild-type vitamin A. This showed promise of slowing the progression of diseases such as Stargardt. However, there was no discussion of improved vision in either study.
Objectively, all of the studies showed promising results for future applications, but they all came with their own risks: retinal detachment, intraocular pressure, and immunosuppression. Even though the creation of synthetic vitamin A is being explored currently, there are only two major potential mainstream solutions. Gene therapy and stem cell therapy are two promising approaches for treating Stargardt disease, but they differ significantly in their mechanisms, applications, and challenges. In summary, while gene therapy targets the genetic cause of Stargardt disease to prevent further damage, stem cell therapy focuses on replacing damaged cells to restore lost function. Both approaches hold promise, but they differ in how they tackle the disease and the specific challenges they face.
Regarding implementing ocular stem cell transplants on a large scale, modern research gives supporting evidence that this goal is not too far from being reached. Largely due to massive funding from large-scale research institutions, as well as investment capital being directed toward groundbreaking scientific research and stem cell treatments, more and more stem cell transplants have been occurring over the last few decades. Huge strides in advancing not only the knowledge behind different stem cell transplants but also data on clinical trials and overall effects, have amassed largely in the last 25 years alone. Hence, there is great scalability in using stem cell therapy as a treatment for helping many individuals who suffer from macular degeneration. However, stem cell transplants have shown to be safe for the patient (or host body), if proper immunosuppressive techniques, drugs, and protocols are followed. Therefore, as with all transplants and treatment therapies, clinical research always helps in learning the best ways to navigate proper stem cell transplants and the successful incorporation of these new cells into the recipient’s body. If this treatment for Stargardt disease is incorporated on a large scale, it is important to learn about the clinical implications of utilizing these stem cells within a transplant, such as by analyzing which drugs work best in patients, as well as minimizing any drug interactions with other medications used for individuals with other eye problems. This important analysis is essential in making sure patients are safe when undergoing this form of treatment for macular degeneration.
The innovative technique of incorporating stem cells as a treatment to help counteract the prognosis of Stargardt disease is unique yet promising, as thousands of people every year suffer macular degeneration from this disease. As of now, there is no defined standard that has been consistently able to treat Stargardt disease in affected patients. Transplanting stem cells into affected individuals would be immensely beneficial, limiting the development of macular degeneration and stopping worsening symptoms. Using stem cell transplants as an effective ocular treatment for Stargardt disease would be a groundbreaking therapeutic advancement in the field of ophthalmology, as it can drastically alter the prognosis for individuals suffering from this ocular disease which does not currently have a reliable, effective, and dependent methodology of treatment.
Conclusion
Stargardt’s disease is characterized by the loss of central vision because of build-up of lipofuscin in the macula due to mutations in the ABCA4 gene. Recently, researchers have been exploring the option of using stem cell therapy to replace damaged and lost cells in the macula to potentially restore vision and prevent further degeneration. Many of the studies researching the use of stem cell therapy to treat Stargardt’s disease showed promising results. While the results show hope for a cure for Stargardt’s disease, further animal and human studies need to occur to determine which stem cell therapy approach will result in the best long-term efficacy and least amount of risk to the patient. Overall, while there is currently no FDA-approved treatment for Stargardt’s disease, stem cell therapy has presented benefits to patients across several studies and shows promise to help those who suffer from Stargardt’s disease.
