Gene therapy has entered the forefront of medicine and there may be potential benefits in all fields of healthcare. The potential for gene therapy to target disease has vastly expanded since the first successful human treatment for severe combined immune deficiency (SCID) emerged in 1990.
In podiatric medicine, one proposed target that has devastating consequences is the diabetic foot ulcer. As diabetes mellitus continues to become more common within the podiatric population, the necessity to care for wounds and focus on limb preservation is becoming more essential.
Currently, there are nearly 24 million people who have diabetes mellitus in the United States. Accordingly, nearly 8 percent of the country’s population has diabetes.1 Diabetes not only plagues the geriatric community but the youth of our country. There are a multitude of complications that accompany this disease and podiatric physicians are on the frontline since diabetic foot exams and foot care programs can decrease amputation by 45 to 85 percent.2
There is a whirlwind of research around genetically modified proteins and viruses, and their potential benefits in helping to facilitate healing for diabetic foot ulcers. As the research continues on diabetic wounds, it is also becoming more clear that these wounds have altered blood flow, impairments with fighting microbial pathogens and abnormal chemokine expression, resulting in abnormal inflammatory responses.3
The healing process is very complex and there are a series of steps and certain components that need to be in place to ensure proper wound healing. To this end, Martin, et al., discussed the basic steps of inflammation, the formation of granulation tissue, angiogenesis and tissue remodeling.4 When looking at the gene expression in wound healing and the cascade of components that are necessary at certain moments in time to heal wounds, if one of those components is not present, it could delay the whole process.5
A comparison study of diabetic mice versus non-diabetic mice showed that pertinent components necessary for healing, such as IGF-I and IGF-II mRNA, were delayed in expression for the diabetic mice.6-9 There has been a wide range of research centering on the idea that diabetic wounds express menial levels or potentially fail to express some of the necessary components for healing. In the effort to aid in wound healing for those with diabetes, multiple research studies have looked at engineering these components that could be potentially lacking, delivering them to the vicinity of the wound whether it is via topical application or injection.6-9
A Pertinent Overview Of Gene Therapy
Gene therapy is defined as “treatment of a disease by introducing a new gene into a cell.” 10 One can apply gene therapy to virtually any field of medicine. A gene is a sequence of DNA that “codes” for a certain cell function. The simplest building block of the gene is the nucleotide, which is comprised of DNA. Several genes make up a chromosome. All the chromosomes together constitute the human genome. There are approximately 25,000 genes in each human cell. The Human Genome Project identified nearly all the genes, although their functions have not been clearly defined. 11
One of the more challenging aspects to gene therapy is how to deliver the gene into the host cell. Different vectors can accomplish gene delivery. Transfer of naked DNA on plasmids (double-stranded DNA) has proven inefficient. Initial transfers used a retrovirus (single-stranded RNA) to replace the faulty gene. Adenovirus (double-stranded DNA) vectors have been in use but these vectors are not incorporated into the human genome, resulting in seemingly transient effects. 11 Accordingly, the properties of adenoviruses seem more suited to use in diabetic foot ulcers, in which a transient effect is necessary to heal the wound.
All of the above are considered in vivo gene therapy. In the diabetic foot wound, we also have experience using genetically engineered cells with Dermagraft (Advanced Biohealing) and Apligraf (Organogenesis), which are cultured ex vivo. If these cells were genetically modified prior to implantation, that would be ex vivo gene therapy.
A Closer Look At The Research On Gene Therapy For Podiatric Applications
Research has explored many different proteins in the healing cascade in regard to their potential benefits for diabetic ulcerations. Investigators have found that the topical application of insulin-like growth factors (IGF) I and II, and platelet derived growth factor (PDGF) in diabetic mice increased diabetic wound healing.7 Researchers have also shown that the use of transforming growth factor-alpha and fibroblast growth factor as monotherapies has shown promise in improving wound healing in diabetic mice.
In a study of intradermal injection of vascular endothelial growth factor A (VEGF-A), researchers showed promising results in mice with full thickness wounds by improving the wound healing rate while also inducing angiogenesis.8 While this technique was not on diabetic mice, researchers performed a similar study using the topical application of VEGF on mice and it showed parallel results.
In regard to targets of current gene therapy intervention, Tissue Repair Company is conducting a nationwide study using an adenovirus vector of topical gene active matrix (GAM) to transfer the PDGF gene into wound cells. 15 Research sponsored by the National Institutes of Health is looking at the intralesional injection of PDGF for venous leg ulcers. 15
VEGF is the target gene in several gene therapy trials for peripheral arterial disease and critical limb ischemia. Viromed is conducting a Phase I trial to evaluate the effect of injecting the VEGF gene intramuscularly.15 Sanofi-Aventis just completed a Phase II trial of VEGF for severe peripheral arterial disease in several European countries and it has now moved on to a Phase III trial in over 30 countries.15
Diabetic neuropathy is the target of two trials (Sangamo Biosciences and Caritas St. Elizabeth Hospital) evaluating the intramuscular injection of VEGF.15
Through proper delivery and therapeutic levels, gene therapy could provide intriguing benefits for the treatment of diabetic foot ulcers, diabetic neuropathy and peripheral arterial disease. In the event there are drastically impressive results in one or two gene therapy trials for these conditions, there would be a rapid acceptance and wide usage of this modality. It would behoove the podiatric physician to be familiar with the process of gene therapy and the disease targets currently under investigation.
Dr. Rogers is Director of the Amputation Prevention Center at Broadlawns Medical Center in Des Moines, Iowa. He directs research at the center and has been an investigator on over 20 clinical trials.
Ms. Lear is a third-year podiatric medical student at Des Moines University College of Podiatric Medicine and Surgery. She has conducted undergraduate research in gene identification and gene therapy.
Dr. Steinberg is an Assistant Professor in the Department of Plastic Surgery at the Georgetown University School of Medicine in Washington, D.C. He is a Fellow of the American College of Foot and Ankle Surgeons.
For further reading, visit www.podiatrytoday.com .
1. Centers for Disease Control and Prevention. www.cdc.gov/media/pressrel/2008/r080624.htm .
2. Rogers LC, Lavery LA, Armstrong DG. The right to bear legs – an amendment to healthcare: How preventing amputations can save billions for the US health-care system. J Am Podiatr Med Assoc 2008:98;3-5
3. Galiano RD, Tepper OM, Pelo CR, Bhatt KA, Callaghan M, Bastidas N, Bunting S, Steinmetz HG, Gurtner GC. Topical vascular endothelial growth factor accelerates diabetic wound healing through increased angiogenesis and by mobilizing and recruiting bone marrow-derived cells. Am J Pathol 2004:164;1935-47
4. Martin P, Hopkinson-Woolley J, McClusky J. Growth factors and cutaneous wound repair. Prog Growth Factor Res 1992;4:25-44
5. Sharma A, Singh AK, Warren J, Thangapazham RL, Maheshwari RK. Differential regulation of angiogenic genes in diabetic wound healing. J Invest Dermatol 2006;126:2323-31
6. Badillo AT, Chung S, Zhang L, Zoltick P, Liechty KW. Lentiviral gene transfer of SDF-1alpha to wounds improves diabetic wound healing. J Surg Res 2007;143:35-42
7. Brown DL, Kane CD, Chernausek SD, Greenhalgh DG. Differential expression and localization of insulin-like growth factors I and II in cutaneous wounds of diabetic and nondiabetic mice. Am J Pathol 1997;151:715-24
8. Buitrago W, Roop D. Cutaneous Gene Therapy for Skin and Systemic Disorders. In: Templeton N (ed.) Gene and Cell Therapy: Therapeutic Mechanisms and Strategies, second edition. Marcel Dekker, New York, NY; 2003; pp. 657-673
9. Keswani SG, Katz AB, Lim FY, Zoltick P, Radu A, Alaee D, Herlyn M, Crombleholme TM. Adenoviral mediated gene transfer of PDGF-B enhances wound healing in type I and type II diabetic wounds. Wound Repair Regen 2004;12:497-504
10. Howard Hughes Medical Institute. Glossary. Definition of gene therapy retrieved at: http://www.hhmi.org/genetictrail/glossary.html On July 1st, 2008.
11. Colavito MC. Gene Therapy. Benjamin Cummings Publishing, San Francisco, CA: 2007.
12. Herweijer, H, Harding C, Hagstrom J, Wolff J. Biological Aspects of Disease, Contribution From Animal Models. Hardwood Academic Publishers, Newark, NJ: 2007
13. Sussman M. Gene Therapy and Cellular Therapy in Cardiac Repair. In: (Leri A, Anversa P, Frishman W, eds.) Cardiovascular Regeneration and Stem Cell Therapy. Blackwell Publishing, Ltd, Malden, MA: 2007
14. Paschen A, Schadendorf D, Weiss S. Bacteria as Vectors for Gene Therapy of Cancer. In: Templeton N (ed.) Gene and Cell Therapy: Therapeutic Mechanisms and Strategies, second edition. Marcel Dekker, New York, NY, 2004, pp. 199-209
15. Clinical trials listed on www.clinicaltrials.gov on July 28, 2008