Stem cells may hold the key to fixing a mutated gene that causes blindness

Stem cell research is being used in South Africa to develop "disease in the dish" models that fix a gene mutation that results in night blindness, tunnel vision and eventually blindness.

If the research pans out, the mutations can be fixed in a process called gene editing, which takes place in stem cells derived from the patient's skin. These stem cells are turned into the tissue compromised in disease, which in this case is the retina of the eye.

It is the first step to understanding how mutations cause disease and whether repairing the defect in the cell may reverse the disease process in a dish model.

Although these stem cell transplants have taken place in some parts of the world, they are still in the clinical trail phase in North America. The research is among the most advanced developments in stem cell treatment.

Night blindness, tunnel vision and eventual visual impairment are symptoms of a disorder called retinitis pigmentosa – one of several inherited eye diseases, or retinal degenerative disorders.

Retinal degenerative disorders occur in one in every 3 500 people across the globe. But these disorders are both genetically and clinically diverse. This is because they are associated with mutations in more than 280 genes. The disorders can be passed down from generation to generation through one or both parents and can impact on the children differently depending on their sex. At least 50% of cases are sporadic.

It may occur with no other clinical findings or it may manifest as part of another disease, such as Usher syndrome, where it is combined with hearing loss. This is linked to the neurosensory or developmental abnormalities. It can also occur as a result of other systemic diseases such as some forms of diabetes.

Understanding the gene

Our research looks at the mechanisms of one gene which causes night blindness and peripheral vision loss. Understanding the mechanism is the first step to devise any treatment for the disease.

The gene, which we identified in 2004 (PRPF8), is involved in the normal functioning of the machinery involved in translating proteins in our bodies. It is found throughout the body but in people who have a mutation it leads to compromised vision.

Ideally, to find the way to treat this disease, mutations that cause night blindness and peripheral vision loss, we should work on a patient's retina. But this tissue is not available from living subjects.

This is where stem cell research comes in. Stem cell research has been instrumental in "disease in a dish" modelling to study a disease of interest or to create an environment to test treatments for the disease. Whether or not a disease can be treated depends on the understanding of the basic biology of the disease. Disease modelling allows scientists to explore how a disease works in the laboratory rather than directly on a patient.

A disease in a dish

A disease model represents the abnormal human biology in a particular disease. Although mice have been used for disease modelling, researchers are moving toward using cells in a dish because mice have ethical limitations and can't fully mirror human diseases.

There are three different types of stem cells: adult stem cells, multipotent mesenchymal stem cells and pluripotent stem cells.

Induced pluripotent stem cells are adult stem cells that are reprogrammed in the laboratory to behave like embryonic stem cells. These cells can be differentiated into many tissue types, which can, theoretically, then be used to treat various diseases.

By making the induced pluripotent stem cells into cellular models for the diseases, it allows researchers to study the effects of certain treatments on the tissue, and this could include either gene editing or gene therapy studies in vitro.

One part of our study uses stem cell technology to harvest easily accessible skin tissue from patients or their unaffected siblings and then grow these into fibroblast cell lines which can be reprogrammed into induced pluripotent stem cells.

These reprogrammed stem cells are very similar to embryonic tissue which then have the potential to be differentiated into photoreceptor and retinal pigment epithelium cells.

Once we have the photoreceptor and retinal pigment epithelium cells we will perform human genome-wide gene expression. This is the process where genetic instructions are used to make a gene product (transcriptome) analysis. This analysis will shed light on the downstream effects of the mutation, and possibly lead to an understanding of why retinal cells manifest the disease rather than other cells, tissues or organs in the body.

The model must reproduce aspects of the disease outside the body. This is quite useful for diseases in which it is difficult to obtain the diseased tissue from the patient such as retinal diseases.

This is because retinal tissue or eye tissue cannot be obtained while the patient is alive. If a person dies there is a limited time frame to collect the tissue. Using the disease in a dish model eliminates these drawbacks.

The link between stem cells and gene correction

If the mutation is corrected in a disease in the dish model, via technology called gene editing, it means that the corrected gene, without its mutation, can theoretically be transplanted into the patient.

This technology provides the promise for treating genetic diseases by transplanting the patient's own genetically corrected human induced pluripotent stem cells.

The potential benefits of stem cell based gene therapy and gene correction are:

• eliminating the need for immunosuppression;

• the ability to generate an unlimited supply of patient-derived transplantable cells; and

• the ability to gene edit the disease causing genetic mutations.

In South Africa researchers are working on gene editing and stem cell technology. However, this is not at the clinical trial or therapeutics phase.

Although clinical trials have started in some countries, this type of research is still in its infancy as there are many obstacles before the use of stem cell based gene therapy arrives in the clinic.

These challenges include determining the safety and efficacy of this type of treatment as long term results are currently unknown.

The patient-specific retinal tissue in a petri dish gives us the potential of doing sophisticated gene editing work. Although this can lead to reversing a cellular disease in a dish, it is hoped to provide insights into our longer term objective of disease-based therapeutics in humans.

Stacey Moses, PhD Student, Division of Human Genetics, University of Cape Town and Raj Ramesar, Professor and Head of the Division of Human Genetics, University of Cape Town.

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