Aptamer Selection Against Angiostatin for the Control of Micrometastases

Tianlu Ma
August 29, 2011
Fall 2011
R50, RNA, Angiostatin

According to the American Cancer Society, there is a 1 in 4 chance of a male dying from cancer, while the odds for the female population is 1 in 5 (“Lifetime Risk”). For most cancer patients, especially those that have already beat the odds once, the possibility that, even after intensive treatment and surgery, tumors might develop again is horrifying. Currently, research into angiogenesis and angiostatin could lead to the inhibition of these tumors and help relieve the pain of those with cancer.

Angiogenesis is the growth of new blood vessels, such as those needed to ferry nutrients to a wound site, or, alternatively, to cancer tumors (Wahl, 2004). In 1994, angiostatin was discovered to play a pivotal role in the inhibition of angiogenesis, and thus the suppression of micrometastases, dormant secondary tumors (O’Reilly, 1994). Before the discovery of angiostatin, the reoccurrence of tumors was believed to be because the surgeon left tumor cells behind when the primary tumor was taken out. However, the primary tumor actually secreted angiostatin, thus suppressing the micrometastases (Wahl, 2004). When the primary tumor was removed, there was a drop in the level of angiostatin circulation, which lead to angiogenesis and thus the formation of secondary tumors (Wahl, 2004).

Aptamers are singled-stranded nucleic acids that have high binding affinity for various targets (Girardot, 2010). Aptamers are very useful for target selection due to their ability to be stably stored for long periods of time, transported at room temperatures, and undergo reversible denaturation (Huang, 2009). In addition, aptamers are synthesized and selected in vitro, can tolerate many pH levels and salt concentrations, and can be easily tagged and labeled (Girardot, 2010). These aptamers are selected from random-sequence pools through systematic evolution of ligands by exponential enrichment (SELEX) (Girardot, 2010). SELEX for a RNA pool includes binding and selection, reverse transcription and amplification, and transcription to provide RNA for the next round of selection.

Specific Aim 1: Aptamer selection as a diagnosis to detect the position of micrometastases.

Angiostatin can specifically select for micrometastases because of the low extracellular pH in the tumor microenvironment (Wahl, 2004). An RNA aptamer designed to select for angiostatin under conditions of low body pH could then detect where the micrometastases are located. This diagnostic tool would then allow the surgeon to remove those micro-tumors along with the primary tumor, eliminating the need for future surgery.

Specific Aim 2: Modifying the aptamer for drug delivery.

A specific enough aptamer, i.e., one that will only bind to angiostatin under the conditions of the tumor microenvironment, could also serve as a drug delivery system. It might be possible to attach other molecules, which would be released once the aptamer binds to the angiostatin, to the aptamer. These molecules could be those that promote cell death, or inhibit tumor growth in some other way. This would allow for the elimination of the micrometastases without invasive surgery.

Figure 1. Pathway for angiogenesis for tumor cells. Angiostatin most likely is involved in the step between the cancer cell and the endothelial cell. This would lead the potential aptamer to the cancer cell, thus identifying where the micrometastases are and possibly delivering drugs that promote the death of those tumor cells. Adapted from the National Cancer Institute’s Understanding Cancer Series: Angiogenesis, Slide 20.

The discovery of an aptamer for angiostatin will help greatly in reducing the number of patients who have to head back to their oncologist for a second round of chemotherapy and surgery. The aptamer would bind to angiostatin, thus leading to the position of the micrometastases. Additionally, if a drug is developed that specifically targets tumor cells, the aptamer could also serve as a drug delivery system, possibly eliminating the need for invasive surgery and chemotherapy.

Angiostatin (CAT # 4919-100) can be obtained at BioVision (800-891-9699) at a price of $195 per 100 ug. Currently, there are some available in the aptamer -80 storage.

References

Girardot, M., Gareil, P., and Varenne, A. (2010). “Interaction study of a lysozyme-binding aptamer with mono- and divalent cations by ACE.” Electrophoresis. 31: 546-555.

Huang, H., Jie, G., Cui, R., and Zhu, J. (2009). “DNA aptamer-based detection of lysozyme by an electrochemiluminescence assay coupled to quantum dots.” Electrochemistry Communications. 11: 816-818.

“Lifetime Risk of Developing or Dying from Cancer.” Learn About Cancer. Retrieved August 28, 2011, from http://www.cancer.org/Cancer/CancerBasics/lifetime-probability-of-developing-or-dying-from-cancer.

O’Reilly, M.S., et al. (1994). “Angiostatin: A Novel Angiogenesis Inhibitor that Mediates the Suppression of Metastases by a Lewis Lung Carcinoma.” Cell. 79: 315-328.

Wahl, M.L., Moser, T.L., and Pizzo, S.V. (2004). “Angiostatin and Anti-angiogenic Therapy in Human Disease.” Recent Progress in Hormone Research. 59: 73-104.

Click HERE for proposal.

Click HERE for Progress Report 1.

Click HERE for Final Manuscript.