A virus that can heal you?!: A superhero not a villain

Can we cure some of the worst diseases known to humanity? It certainly can be made possible using gene therapy, which is often credited as the future of healthcare. I am Amina and I will mainly be discussing my research project for my master’s year, which is to develop a new approach to improve gene therapy using a virus known as adenovirus.

Improving gene delivery by adenovirus serotype 5

Gene therapy is a novel treatment option that operates by replacing or altering genes that are absent or defective and whose absence or defect may be partially or wholly responsible for a disease. Gene therapy typically takes advantage of a virus’ natural ability to infect cells to deliver therapeutic genes to a tissue of interest to treat or even potentially cure diseases such as cancer, haemophilia, and cystic fibrosis. Nearly all diseases have a genetic component even common diseases such as high blood pressure and diabetes!

So, you have probably come across many individuals affected by a disease that can be treated with gene therapy. However, unfortunately gene therapy is not as safe or efficient as initially believed and this is because the viruses that are supposed to carry the therapeutic gene to the target tissue alter their delivery route.

Consider this: you order food from uber eats for lunch; however, the driver drops your food off at someone else’s place so you’re right where you started but even worse: hungry and concerned about your food. Similarly, the virus may accumulate in the wrong organ without alleviating the problem and may even cause toxicity.

Adenovirus serotype 5 (Ad5), is currently the most common gene delivery vehicle used in cancer clinical trials because it is a well-studied respiratory virus that is safe, easy to produce, and most importantly is able to infect a wide variety of cells through common cell surface proteins. Ad5 has three main surface proteins that are responsible for its interactions: the hexon of which there are 240, the penton, and the fibre (Figure 1).

Figure 1 Creative Commons licence (Lee et al, 2017). A depiction of adenovirus serotype 5 and its surface proteins; the hexon, penton and fibre.

Even though Ad5 seems to have so much potential, it has been discovered that even if targeted, Ad5 mainly accumulates in the liver which can lead to severe consequences such as liver toxicity. It turns out that this is because once Ad5 is in the blood it binds to a blood clotting protein known as factor X, pronounced factor ten (Roman numerals am I right?), which can be split into four domains; the glutamic acid (Gla) domain, epidermal growth factor domains 1 and 2 (EGF1 & EGF2) domain and the serine protease (SP) domain (Figure 2).

Figure 2 Creative Commons licence (Waddington et al, 2008). Factor X and its four domains; the serine protease/SP domain, the epidermal growth factor/EGF domains and the glutamic acid/Gla domain.

Factor X simultaneously binds to the Ad5 hexon through its Gla domain and to proteins on the liver cell surface known as heparan sulphate proteoglycans through its SP domain. In this way, factor X subsequently bridges Ad5 to the liver (Figure 3).

Scientists have therefore attempted to detarget Ad5 from the liver by producing an Ad5 variant without the hexon protein that would therefore not be able to bind factor X. However, by preventing factor X from binding to Ad5 surface, it makes the virus susceptible to immune attack by our own immune system (Figure 3). Recent attempts to retargeting Ad5 still involve preventing the binding of factor X but without success.

Figure 3 Factor X binds to adenovirus serotype 5 and heparan sulphate proteoglycans on the liver cell surface, bridging the virus to the liver. Natural factor X covers the adenoviral surface and protects it from immune attack.

So, we propose a new way to improve gene delivery by Ad5 and that is to take advantage of the ability of Ad5 to bind factor X to retarget it. We will do this by producing a modified factor X with increased affinity (binding) to Ad5 because we don’t want our modified factor X to dissociate from Ad5 after injection, as it will then be replaced by our own natural factor X in the blood and retarget Ad5 to the liver once again. To produce factor X with higher affinity, we will generate several factor X mutants with modifications within the Gla domain and test each for binding to Ad5 hexon using previously established methods of gene mutation, protein production and protein binding.

Figure 4 Depiction of potential mutant factor X with a modified glutamic acid/Gla domain, the epidermal growth factor 1 and 2/EGF1 and 2 domains, removed serine protease/SP domain and added retargeting domain/RD i.e., a protein with affinity for a specific cell surface protein.

By using a modified factor X, we will not only be able to detarget Ad5 from the liver by removing the SP domain and retarget Ad5 to the tissue of interest by adding a retargeting domain which binds to specific molecule on the target cell surface (Figure 4), but we can prevent the destruction of the virus by the immune system and overcome limitations associated with gene delivery by Ad5. In this way, we can potentially increase the use of gene therapy to treat or cure disease. Will we be able to revolutionise treatment using gene therapy? Will we be able to cure cancer?

Even though we may not be able to increase affinity or prevent the immune destruction of Ad5, we must still try this new approach because even a failed attempt may contribute to gene therapy research- the sky is the limit!

Uber eats finally arrives at your location; you are no longer hungry…

Further reading

  • Adenovirus Serotype 5 Hexon Mediates Liver Gene Transfer’, Cell, 132(3), pp. 397–409. Waddington, S., McVey, J., Bhella, D., Parker, A., Barker, K., Atoda, H., Pink, R., Buckley, S., Greig, J., Denby, L., Custers, J., Morita, T., Francischetti, I., Monteiro, R., Barouch, D., van Rooijen, N., Napoli, C., Havenga, M., Nicklin, S. and Baker, A., (2008).
    • Discovered that factor X binds to and targets adenovirus to the liver
  • Biodistribution and retargeting of FX-binding ablated adenovirus serotype 5 vectors’, Blood. American Society of Hematology, 116(15), pp. 2656–2664. Alba, R., Bradshaw, A., Coughlan, L., Denby, L., McDonald, R., Waddington, S., Buckley, S., Greig, J., Parker, A., Miller, A., Wang, H., Lieber, A., van Rooijen, N., McVey, J., Nicklin, S. and Baker, A., (2010).
    • Attempted to retarget adenovirus by modifying hexon
  • Coagulation factor X shields adenovirus type 5 from attack by natural antibodies and complement’, Nature Medicine. Nature Publishing Group, 19(4), pp. 452–457. Xu, Z., Qiu, Q., Tian, J., Smith, J., Conenello, G., Morita, T. and Byrnes, A., (2013).
    • Without factor X binding, adenovirus is susceptible to immune attack
  • Adenovirus-mediated gene delivery: Potential applications for gene and cell-based therapies in the new era of personalized medicine’, Genes and Diseases. pp. 43–63. – uses of adenovirus in medicine. Lee, C., Bishop, E., Zhang, R., Yu, X., Farina, E., Yan, S., Zhao, C., Zeng, Z., Shu, Y., Wu, X., Lei, J., Li, Y., Zhang, W., Yang, C., Wu, K., Wu, Y., Ho, S., Athiviraham, A., Lee, M., Wolf, J., Reid, R. and He, T., (2017).
    • Use of adenovirus in medicine
  • Gene Therapy: Medlineplus Genetics NIH‘, [online] Medlineplus.gov. Available at: https://medlineplus.gov/genetics/understanding/therapy/  [Accessed 27 December 2020].
    • Nice introduction to gene therapy