The Journey to Cure Genetic Diseases
Many human diseases are based on abnormalities in the genome. For example, sickle-cell anemia is a blood disorder characterized by abnormal sickle-shaped blood cells and is caused by a single base mutation in the beta-globin chain of hemoglobin. Various cancers are also genetic diseases. What led Angelina Jolie to decide to have a preventive double mastectomy? It was a single gene, BRCA1, one of the tumor-suppressing genes. Angelina Jolie has a harmful mutation in the gene, which dramatically increases the risk of breast or ovarian cancer.
My question is: How can we conquer these genetic diseases? What would be the most effective therapeutic means humans can devise to cure them? There are many ways to deal with this. One relatively simple way would be adding the normal version of the abnormally functioning cells. Let’s revisit the sickle-cell example above. Normal red blood cells could be added to manage sickle-cell diseases through blood transfusions. In the context of type I diabetes, this can be achieved through the administration of insulin. However, this type of cure has a critical disadvantage: Patients are required to take this type of medicine for their entire lives.
Ideally, the most fundamental way to attack this problem would be to fix the genome from the beginning. In this way, the original cause of the disease can be removed, and the disease can be cured without need for further medical attention. This had been thought to be an impossibility, but it now seems much more within our reach thanks to two recently emerging biotechnologies: genome editing with engineered nucleases and induced pluripotent stem cells.
The first breakthrough was the development of the genome-editing technology with engineered nucleases. The most famous examples of the gene-editing nucleases are zinc finger nucleases (ZFNs) and tal effector nucleases (TALENs). They are hybrid enzymes, the fusion of the DNA binding protein and the DNA cutting enzyme. The DNA binding part of the enzymes can be easily designed and engineered to be able to bind to only one specific site in the human genome. The nuclease part of the enzyme works like a pair of scissors to cut the DNA into smaller segments. This special hybrid enzyme can be used to cut only a specific site in the genome. When this cut is introduced on the genome by the engineered nucleases, cells detect the damage on the genome and try to fix the damage by various mechanisms, and these cellular repair processes can be used to put the wanted DNA sequences exactly in a desired site. In this way, any mutations on the genome can be corrected using gene-editing nucleases — at least in principle.
However, just correcting mutated sequences in the human genome does not lead to the cure of diseases. There are roughly 100 trillion cells in the human body and each cell has its own genome with exactly the same DNA sequences. Certainly we do not have to correct all the cells’ genomes, but we still have to correct all genomes of a certain type of cell — for instance, red blood cells to cure sickle-cell disease. How can we cope with this issue? The solution for this would be using induced pluripotent stem (iPS) cells. The iPS cell is an artificial pluripotent stem cell derived from an adult somatic cell. For example, adult skin cells can be reprogrammed to go back to the pluripotent state by introducing a specific gene expression. Shinya Yamanaka, who first generated mouse and human iPS cells from skin cells, was awarded the Nobel Prize in Physiology or Medicine for this discovery in 2012.
One important advantage of iPS cells over the human embryonic stem cells is that patient-specific iPS cells can be derived directly from the patient’s own skin cells. Therefore, when iPS cells are used for cell therapy, potential immune rejection can be avoided. Another important advantage is that ethical issues posed by human embryonic stem cell research can be obviated. Human embryonic stem cells are derived from human embryos, which have the potential to develop into a new life, which has been the source of heated ethical debate. However, with iPS cells no such ethical concerns present themselves.
So how do you combine two of the most recent biotechnologies to cure genetic diseases? Let’s get back to the first example, sickle-cell disease. First, skin cells are easily obtained from a sickle-cell disease patient and are reprogrammed to generate iPS cells. By using gene-editing nucleases, the genetic defect of sickle-cell disease can be repaired in iPS cells. Then, this corrected version of iPS cells will be differentiated into hematopoietic stem cells and will be transplanted back into the patient. The transplanted hematopoietic stem cells will be further differentiated into different types of blood cells and eventually normal red blood cells will replace the sickle-shaped red blood cells. This novel approach of using a combination of gene editing and iPS cell therapy can be applied to any genetic disease in principle as long as the cause of the mutation of the disease is known.
However, not a single clinical trial using this approach has yet been implemented. There are still many obstacles to overcome before this can be applied to the real world in clinical trials. It should be ascertained that gene-editing nucleases fix only the target mutation and do not change any other DNA sequences on the genome. Whether iPS cells are suitable for safe use on patients also should be further researched. The optimal transplant method should also be developed for certain types of cells required to cure each genetic disease. More concerted efforts should be dedicated to combining these two emerging technologies and to overcome current obstacles. Advances in combining those technologies would without question help us cure genetic diseases.
Hyung Joo Lee is in the McDonnell International Scholars Academy at Washington University in St. Louis. He received his Master of Science degree in Chemistry in 2009 from Seoul National University in Seoul, South Korea. He is currently a PhD candidate in the Division of Biology & Biomedical Sciences at Washington University’s Graduate School of Arts & Sciences.