When it comes to the field of diabetic retinopathy research, Dr Lalit Pukhrambam at Wayne State University is exploring ways to prevent blinding eye diseases due to diabetes by targeting a protein called thioredoxin-interacting protein (TXNIP). Gene therapy is, therefore, a promising option
By way of an introduction, diabetic retinopathy (DR) is a devastating eye disease causing blindness; yet, there is no cure. Thioredoxin-interacting protein (TXNIP) is strongly induced by high glucose and inhibited by insulin. Therefore, chronic hyperglycaemia-induced TXNIP expression in tissues remains elevated in diabetes. TXNIP inhibits the anti-oxidant and thiol reducing the capacity of thioredoxin, thereby, causing cellular oxidative stress, inflammation and cell death. Gene therapy targeting TXNIP is one way to prevent DR.
TXNIPlogy: The study of the role of TXNIP in health and disease
Diabetes global epidemic
Diabetes mellitus (DM) is a metabolic disease in which high blood sugar level persists over a prolonged period. Two main types of DM are Type 1 (insulin-dependent or Juvenile diabetes, an autoimmune disease, ~10%) and Type 2 diabetes (insulin-resistant or adult-onset diabetes, associated mainly with obesity and lifestyle change, ~90%). DR is a severe complication of diabetes causing damage to retinal microvasculature and neurons. It can eventually lead to blindness. DR affects up to 80% of all diabetic patients who have had diabetes for 10 years or more. DR accounts for ~12% of all new cases of blindness each year in the United States and is the leading cause of blindness among the working adult population.
With a global increase in diabetes, obesity and hypertension (Diabesithy, a term I coined for risk factors for DR), the number of people affected by DR will increase in the coming years. Therefore, there is an urgent need for cure or methods to prevent diabetes and its complications.
TXNIP and diabetic retinopathy
Diabetic retinopathy has been defined by complications of retinal microvascular capillaries due to blood vessel leakage/blockade and the development of new and fragile blood vessels (neovascularization). Early DR involves non-proliferative DR (NPDR), which progress to a more serious disease of proliferative DR (PDR). Laser coagulation, anti-inflammatory steroid and anti-VEGF antibody injection into the vitreous are current treatment methods.
However, these treatments do not produce satisfactory results for most patients. Therefore, there is an unmet need for developing effective therapies via the identification of new gene targets and metabolic pathways.
Currently, it is recognised that microvascular damages are late pathologies in DR and there is an early neuronal injury. This is where new therapeutic methods need to be targeted in the early stages of DR, as diabetic eye diseases develop after a prolonged exposure to hyperglycaemia and diabetes duration. Most common retinal cell dysfunctions (including neurons, glia, pigmented epithelium, pericytes and endothelial cells) in hyperglycaemia and/or hyperlipidaemia involve oxidative stress, mitochondrial dysfunction, low-grade inflammation and premature cell death.
Recently, we identified TXNIP as a gene strongly induced by diabetes and high glucose in retinal cells causing oxidative stress, mitophagy dysregulation and inflammation. Knocking down TXNIP by small inhibitory RNAs (RNAi) in the retina prevents early abnormalities of DR, which include capillary basement membrane thickening, glial activation, and neuronal injury. Therefore, we proposed that TXNIP itself and/or its downstream partners, including the NLRP3 inflammasome, are potential targets for gene and drug therapies.
Furthermore, we showed that the TXNIP promoter exists as an opened and poised configuration that it is activated strongly by high glucose and histone deacetylase inhibitors (HDACi). Therefore, this TXNIP promoter may be operably linked with a therapeutic gene or RNAi to increase/decrease gene expression in diabetes, DR and age-related neurodegenerative diseases.
Nucleic acid constructs containing a TXNIP promoter for gene therapy
As mentioned above, TXNIP is induced by high glucose and diabetes in most tissues tested so far, including pancreatic beta and retinal cells. Therefore, the TXNIP promoter may be linked with a therapeutic gene or an RNAi and induce their expression under hyperglycaemia, such as encountered in diabetes or after a meal. In particular, the TXNIP promoter may be operably linked to a gene encoding as follows:
- Insulin or an insulin promoting protein (PDX1);
- A protein that reduces oxidative stress, inflammation and cell death (Trx1 or Gpx4);
- An RNAi that reduces expression of a protein, which promotes oxidative stress, inflammation and cell death (including TXNIP itself and NLRP3); or
- A neurotrophic factor (BDNF or GDNF).
Advantages are that the TXNIP promoter will remain active when hyperglycaemia prevails; while mostly unresponsive under physiological glucose.
Gene delivery
Gene delivery into the retina may be achieved by packaging the TXNIP promoter-gene construct into an appropriate vector, such as recombinant adeno-associated viral vector, rAAV2. The eye provides an exceptional opportunity for gene therapy because it is a closed organ and relatively an immune privileged site. Therefore, cross-contamination from an intravitreal injection to another organ or systemic immune response will be minimal. Additionally, genetic material needed for the retinal gene therapy is small (cost effective) compared to a systemic gene delivery for other organs.
In fact, most current trials in human gene therapy are conducted for retinal diseases, e.g., retinitis pigmentosa. Gene therapy using the TXNIP promoter is simple, yet innovative, potentially mitigating DR progression. Furthermore, gene and tissue bioengineering methods may be applied in ex vivo systems using autologous adipose-derived stromal stem cells (ASCs) or inducible pluripotent stem cells (iPSC) to produce insulin via a TXNIP-promoter-linked insulin gene expression and subsequent subcutaneous transplant of the manufactured cells. These autologous cells, if producing insulin under hyperglycaemia, will avoid immune responses and survive longer in their own subcutaneous environment, as opposed to a pancreatic beta cell transplant.
Conclusion
Diabetes and its complications affect, not only individuals but also families directly and the society at large. One in ten people in the world is considered to have diabetes, yet many are to be diagnosed. The loss in work productivity and economy due to chronic diabetes diseases are enormous. Tinkering with simple, yet innovative, ideas may lead to diabetes cure and/or prevention of organ damages. There are promises in the horizon for gene and cell therapy for curing diabetes or preventing DR. TXNIP may be an answer.
Supports
American Diabetes Association
Juvenile Diabetes Research Foundation
Mid-West Eye Banks
National Kidney Foundation
National Institutes of Health
Research to Prevent Blindness (RPB)
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Lalit Pukhrambam, PhD
Associate Professor
Department of Ophthalmology
Visual and Anatomical Sciences
Wayne State University School of Medicine, Detroit, U.S.
Tel: +1 313 577 5302
