In February of 2013, a study was published by a group of researchers at the Autonomous University of Barcelona that recounted their successful treatment of diabetes in dogs with a single dose of gene therapy. A supplementary video to the paper shows one of the diabetic beagles, pre-treatment, lying down in a corner, looking tired, thin, and sad. When the video shows the same dog seven weeks post-treatment, the cameraman can hardly hold the camera still because the dog is trying so enthusiastically to lick his hand. He pants excitedly, trots around the room with energy, and tells us so when he barks. Diabetics watching those videos will find it hard not to see part of themselves in those dogs, and be excited by this news. When the nurse gave my diagnosis of diabetes at age ten, it was small consolation, after the news that I would need to inject insulin multiple times a day in order to manage the disease, that researchers were probably only five to ten years away from finding a cure. This was ten years ago, and still, it seems that every breakthrough regarding diabetes cures comes with a caveat: human applications are at least five to ten years away. I wish I could ask that nurse now – where is the cure?
Diabetes is a tricky disease. Type-1 diabetes is an autoimmune disorder, meaning that the causative agent lies within. The body’s own immune response systematically destroys the insulin-producing cells of the pancreas, known as the islet cells (Bluestone). In a healthy person, these cells act as glucose thermostats, detecting glucose deposited in the blood from digested food and releasing insulin to keep blood glucoses level within a narrow static range (Bluestone). Glucose is too big of a molecule to diffuse through cell membranes by themselves, so insulin is required to act as a “key” to open cell’s “doors” and let the glucose in so that it may be metabolized and used for energy (Bluestone). Without insulin, glucose accumulates in the blood (hyperglycemia), and the body is never able to use any of the energy it consumes.
Fortunately, things have improved greatly for diabetics in the last century. The first corrections of hyperglycemia were done in dogs treated with pancreatic extract, a treatment which later evolved into the insulin-injection therapy that is prevalent today (Callejas). Injection of insulin harvested from cows became a standard treatment until researchers figured out how to engineer bacteria with the human insulin gene and use the microbes as insulin factories (Bluestone). Advances in technology and decreasing size of simple computers have allowed electronic insulin pumps to become popular (Bluestone). The latest trend in diabetes treatment is the use of continuous glucose monitors, which measure glucose levels in real time through a subcutaneous sensor - a huge increase in information about a person’s metabolism from just a few finger sticks a day (Bluestone).
But even with these ever-improving treatment options, glycemic control is still not perfect. Patient non-compliance, availability, and the myriad factors that affect glucose levels beyond insulin all contribute to the overall fact that diabetic treatment therapies will never be perfect (Callejas). This is a big deal, because persistently unstable blood sugars are risky. The morbidity rates associated with diabetes is not due to islet cell destruction itself but rather to the multiple microvascular, macrovascular, and neurological complications associated with hyperglycemia (Callejas). On the other hand, hyper-vigilant insulin therapy can push patients into dangerously low blood sugar levels, which can result in coma and death (Callejas). Understanding the pathology of diabetes suggests a few strategies for cures. Transplantation of islet cells into the body has seen some success, but is challenged by two huge problems – availability of suitable donors, and the body’s immune response (Edmond). Transplanted organs fall victim to the same immune response that destroyed the patient’s own pancreas in the first place – recognition of antigens on the organ cells as foreign, and their subsequent destruction (Samson). In order for these transplants to be viable, heavy doses of immunosuppressants are required for the remainder of the patient’s life (Roberston).
Another strategy is the clever manipulation of other cells (those that aren’t destroyed by a malfunctioning immune system) to do the islet cell’s job. Several stem-cell or gene-therapy based strategies have been developed to create non-pancreatic sources of insulin production (Chan). These only work so well – the challenge here is to create a cell that produces insulin when the body needs it, as opposed to just secreting the hormone all the time. (Gavriliuc). Typically, these strategies use an adenovirus vector to insert new genes into existing cells. Adenoviruses are infectious particles that operate by injecting their DNA into a cell and excising into the host’s genome (St George).
What researchers at the Autonomous University of Barcelona have done is design an elegant detection-and response system. They use the adenovirus gene therapy approach to inject two genes into the skeletal muscle tissue in the legs of their dog (Callejas). Insulin, the gene directly responsible for allowing glucose intake into the cells, and glucokinase, the protein that responds to glucose concentration to stimulate insulin activity responsively, work in synergy to create a dynamic insulin release system.
When insulin alone is expressed without glucokinase, it does allow glucose to be brought into the cells (Callejas). However, when presented with a huge influx of sugar in the blood, as there is whenever a meal is eaten, insulin action struggles to compensate (Callejas). A basal level of insulin is required by cells as prerequisite for glucose uptake from the blood to occur in the first place (Callejas). In addition to other roles, insulin in the bloodstream binds to receptors on cells that stimulate the movement of glucose transporter proteins from the interior of the cell to the outer membrane where they can be accessed by glucose (Callejas). When hyperglycemia occurs (as it is bound to after ingesting food), glucokinase helps to cope with the load (Callejas). Glucokinase transforms glucose into glucose-6-phosphate, a similar metabolite. This small but important change in the structure of the glucose molecule prevents it from diffusing from the cell back into the bloodstream and driving the diffusion of glucose through the membrane transports (Callejas). The experimenters included several controls, including some dogs managed with traditional insulin-injection therapy, who did not achieve glucose control. A dog injected with insulin gene therapy alone achieved only moderate improvement in fasting glucose control (Callejas). Those injected with only the glucokinase gene did not experience any improvement in glycemic control (Callejas).
Those injected with both genes, however, experienced quick return to normal fasting glucose levels, and maintained control after eating (Callejas). Additional modifications to this treatment, such as doubling the gene therapy dose, administering dosage after 5 months of insulin-injection therapy, and the usage of optimized-protein vectors, showed better results still (Callejas). Using only a single dosage of gene therapy, researchers effectively cured these dogs of their induced diabetes. Their blood sugar levels were normalized, insulin action restored, and these dogs experienced no hypoglycemic episodes for a full four years after therapy (Callejas).
In some sense, this form of gene therapy is less of a diabetes cure than a diabetes correction. However, as the end consequence and end goal for a diabetic is the same – insulin independence – the distinction is unimportant. It is not fixing the problem at its root – it is not restoring the function of the beta cells or fixing a faulty immune response. But it is providing a mechanism for gluco-normity in a far more accurate and convenient method than any other method currently in use (Callejas). No immunosuppressants are required, and risk of hypoglycemia is fairly low, in contrast to insulin therapy.
The study by Bosch et al. is the first use of gene therapy to cure diabetes in a large animal model. In the translation of basic research to clinical practice, preliminary trials in dogs have historically been a major intermediate step (Callejas). Gene transfer approaches have been used to cure hemophilia B and Leber’s Congenital Amourosis before – in both cases, successful application in dogs preceded successful application in humans (Callejas). The authors conclude their paper with assertion that this study lays the foundation for applications of this technique in veterinary medicine and “possibly” to humans.
The road to clinical implementation is a long one, and perhaps this is why five to ten years away is always the working estimate on a cure. As researchers struggle to understand the complex interactions of different systems involved with the genesis of this disease, new approaches are always being modified, re-tested, and re-drawn. What may seem promising at one point, later turns out to be implausible because of something new just discovered. Despite this, it is necessary to believe that a cure is possible in order for research towards it to be worthwhile.
The next phase of the study, the researchers say, is to do tests to determine the optimal dosage and ratios of glucokinase and insulin action (Callejas). Clinical veterinary studies on diabetic pet guide dogs will also soon begin, allowing the researchers to test this approach in dogs with naturally-occurring diabetes and an irregular lifestyle, unlike the tightly-monitored beagles in this study (Callejas). It’s a small but necessary step, and will probably take several more years to complete. It’s the next step, though, and hopefully the end is somewhere in sight.
Bluestone, J. A.; Herold, K.; Eisenbarth, G. (2010). “Genetics, pathogenesis and clinical interventions in type 1 diabetes”. Nature 464 (7293): 1293.
Callejas, D; et al (2013). “Treatment of Diabetes and Long-term Survival Following Insulin and Glucokinase Gene Therapy”. Diabetes. Epub. '
Chan, L.; Yechoor, V. (2005). “Gene therapy progress and prospects: gene therapy for diabetes mellitus”. Gene Therapy 12, 101-107.
Edmond, R. A., et al. (2002). “Successful islet transplantation: continued insulin reserve provides long-term glycemic control. Diabetes 51, 2148 – 2157.
Gavriliuc; et al (2010). “The race to cure diabetes: how far are we from a breakthrough?” Fiziologia – Physiology 2010.20.1(65).
Robertson, Paul E (2010). “Islet transplantation a decade later and strategies for filling a half-full glass”. Diabetes 59: 1285–1291.
Samson SL, Chan L: Gene therapy for diabetes: reinventing the islet. Trends Endocrinol Metab 2006, 17(3):92–100
St George, JA (2003). “Gene therapy progress and prospects: Adenovirus vectors”. Gene Therapy 10, 1135 – 1141.