Cancer is the second leading cause of death globally after cardiovascular diseases, and is responsible for an estimated 9.6 million deaths in 2018. Globally, about 1 in 6 deaths is due to cancer. (WHO website). Cancer is a genetic disease which is a result of changes in an organism genetic material such as DNA leading to uncontrollable growth of cells; the mass formed by the transformed cells (tumor) is able to avoid the immune system. Scientists and researchers are making continuous efforts to find a treatment for this deadly disease. They now combine several treatment strategies such as kinase inhibitor, mitotic disruptor, HDAC inhibitors, radiotherapy with new genetic therapy to treat cancer. There are a lot of studies to show that cancer can be treated by using these methods.
Practically, genes are responsible for every part of cell function. If something happens to genes and causes disorders like, missing genes or overactivity, this will appear as diseases. Genes therapy is a process that has attracted a lot of research in recent years to treat diseases by putting them right and solving these mistakes.
There are many gene therapy treatment approaches, some of them can change the irregular or missing genes to normal genes that can do their function in the right way for overactivity disorders researchers use gene therapy to control their operations. Also, it can be used to change all the gene expression with new and strange genes. ( Williams,2018)
Approaches of gene therapy treatment:
Genetically Engineered Viruses
This approach uses specially modified viruses (called oncolytic viruses OV). These viruses are adjusted to attack cancer cells and once they infect these cells they produce proteins, consequently, they destroy cancer cells without harming the regular cells. Scientists use the natural properties of cancer cells such as the defiance to apoptosis, growth suppression, and defects in signaling pathways like interferon pathways (IFN) which makes them more liable to viral infection. OVs can specifically attack cancer cells and start their lysis. This process stimulates cancer cells to release substances that activate the immune mechanism against cancer, these materials are damage-associated molecular patterns (DAMP) and tumor-associated antigens (TAAs), these materials response through direct recognition and killing of tumor cells by T cells, and release antibodies and cytokines that enhance the ability of cell lysis. A schematic mechanism of the killing of cancer cells by the OVs is shown in Fig. 1 (Lathwal et al.2020)
OVs have properties such as infecting cancer cells without harming healthy cells, enhancing the immune response against cancer, and they are also adjustable to improve their effectiveness and targeting tumors, all of this makes them perfect anti-cancer molecule.
This approach uses several kinds of virus to deliver specific drugs to the target issue. The table below demonstrates information on the preferred mode of delivery and different chemotherapeutic drugs used in combination with OV. It shows that adenovirus is the most virus that deliver more drugs than other viruses and HSV is the second one. Also, it shows the preferred mode of administration of the OV which present that the intratumoral mode of delivery was the most prominent one adopted for many OVs except for the vaccinia virus. (Lathwal et al.2020)
In this approach scientists put a new and foreign gene into cancer cells or into the tissues that surround them. This method causes cells to die or block cancer cells and tissues from receiving blood and nutrients they need to live and increase. Researchers use many factors to transfer genes to the target cells that include viral factor such as Retrovirus, Adenoviruses, Herpes simplex virus (HSV), Adeno-associated virus (AAV) and Poxvirus (vaccina virus). However, using viruses led to many dilemma “including immunogenicity, insertional mutagenesis, as well as reports of deaths following the administration of viral vectors for gene delivery. Also, the limited capacity of viruses for gene delivery and expensive production methods of engineered viruses for large-scale production has hampered their application as a promising vector” (Mohammadinejad et al, 2020). Because of these issues, scientists study another non-viral factor for example peptides, lipids, and polymers. The nucleic acid materials have negative charge, while non-viral substances like cationic polymers, have positive charge due to the amino group in their structure as a result they make polyelectrolyte complexes (i.e., polyplex). These compounds have cationic nature due to nano-sized particles which have toxic effect. Researchers look for another structure more suitable for human usage. There is a novel strategy by making synthetic viruses contain of virus part that enable to carry nucleic acid materials and the other part is targeting ligands that guide them to the target.
First, genetic materials are mixed with deliver vectors and formed polyplexes or lipoplexes. Then they cross the extracellular barrier (serum endonucleases). Next, they enter cells through two paths: endocytosis (i.e. clathrin-mediated endocytosis, caveolin-mediated endocytosis, macropinocytosis, and phagocytosis) or nonendocytic pathways (i.e. permeabilization and membrane fusion). Finally, gene expression happened either in the cytosol (siRNA/miRNA) or in the nucleus (pDNA). (Xing et al 2019).
The main factors that made non-viral delivery system effective are the ability to carry the therapeutic nucleic acid to the target issues and release it inside the cells with low toxic effect. Another factor is the size and zeta potential of the complexes, they found that the best size is between 50-100 nm and zeta potential of around ±10 mV. When they choose the best nucleic acid materials, they depend on the aim of the treatment. For example, CYP1A is a significant element of cytochrome P450 enzymes which is highly expressed in lung cancer.” The investigators developed CYP1A1siRNA encapsulated cationic liposomes to inhibit the CYP1A1 gene in vivo. The cationic liposomes carrying CYP1A1siRNA efficiently silenced the CYP1A1 gene and inhibited tumor growth in BALB/c nude xenografts” (Mohammadinejad et al, 2020).
Researchers are developing a lot of new strategies and materials that used in this approach. The efficiently and safety of delivery system are the main standards that moved these accomplishments from trial phase to clinical applications.
This way depends on increasing the ability of the patient’s immunity to fight cancer. It is one type of immunotherapy called CAR T-cell therapy. Researchers modify viruses carrying certain genes with the patient’s T-cell. These viruses go to the target issues and deliver genes to T-cells and they enhance T-cells to produce specific protein called a chimeric antigen receptor, or CAR, on their surface. This CAR helps T-cells to attack and destroy cancer cells.
CARs are composed of an extracellular part that is made of antibody that recognizes and links a particular antigen and intracellular part that is made of T cell signaling proteins that transfer cellular signals. Fig 3 (Lattime et al. 2013)
The extracellular part consists of antigen binding fragment (Fab) of mouse monoclonal antibodies (mAbs) that have high affinity for the specific antigens and this Fab contains of heavy- and light-chain variable domains (VH, VL) which called single-chain variable fragment (scFv). The intracellular part is developed through three generations, first-generation CARs containing the zeta chain of the (T cell receptor) TCR/CD3 complex (CD3z), whereas second-generation CARs consist of CD28 or 4-1BB linked to CD3z. Third-generation CARs consist of two costimulatory domains linked to CD3z. (Lattime et al. 2013)
The premier clinical trials for the first generation were disappointing, in 2011 the second generation which is CAR targeting CD19 was developed and has been as the main model for engineered T cell therapies in cancer. The CD19 is a perfect target because it shows in frequent and high-level expression in B cell without being outside B cell and it is required for normal B cell development in humans. The results of treating patients with relapsed leukemia with targeting CD19 obtain complete rest for most of them. Two forms of resistance appear in this generation, the first one is the loss of the antigenic epitope on CD19 in patients with acute leukemia, the second resistance, shown in patients with chronic lymphocytic leukemia (CLL), is the failure of the CAR T cells to increase after infusion. When they used the third generation of CAR with high affinity scFV, toxicity was appeared especially in lung tissue leading to lung failure and releasing cytokines. Also, when they used lower doses of CAR T cells with low affinity scFV, they were less toxicity but also less efficiency. (June et al.2017)
The future of gene therapy for cancer treatment
Scientists have agreed that each tumor is different from other tumors. The last two decades have witnessed a lot of experiments to uncover these differences, however cancer and its clinical presentation still need more studies. These differences make designing drugs to treat cancer very hard also some type of cancer such as pancreatic cancer have intrinsic resistance so they do not response to drugs, while other type develop a resistance (intrinsic resistance). To solve this problem, they develop synthetic analogues drugs to improve the effectiveness of drugs and reduce their resistance by modifying the genetic level. The early detection of cancer and nano technology results are very encouraging to improve the median survival rate of the patients and their quality of life but we still need more studies to treat late-stage solid tumors. (Singh& Amiji2020)
They tried to use anti CD19 CAR Engineered T Cells to treat blood cancer patients and the response results were high. In addition, the FDA approved to use anti CD CAR T cells to treat resistant B-cell malignancies, therefore, many biopharmaceutical companies try to develop new products based on CAR T cells. These drugs are still in clinical assessment for the treatment of both liquid and solid tumors. Recently, scientists try to develop new generation of CAR technology to produce smart and multifunction T cells that able to boost complex immune response for each kind of tumor. (Abreu et al 2020)
In conclusion, cancer is a genetic disease and scientists are still looking for treatment. Gene therapy is a very promising techniques but they are still in trail phase so they need more experiments to be use for human. The development of these techniques depends on several factors, including effectiveness in targeting cells, safety, and low toxicity. In addition, non-viral gene carriers may become a powerful and main appliance in gene therapy because of the promising results that scientists have reached. Finally, the applications used to develop gene therapy can be used in other therapeutic applications, such as the use of CAR techniques to develop stem cells.
- David A. Williams, MD.(2018)How is Gene Therapy Being Used to Treat Cancer?.[online].Available from:https://blog.dana-farber.org/insight/2018/04/gene-therapy-used-treatcancer/#:~:text=In%20gene%20transfer%2C%20researchers%20introduce,nutrients%20they%20need%20for%20survival.
- Edmund C. Lattime and Stanton L. Gerson.ed.(2013). Gene Therapy for Cancer 3rd edition. [online]. available from:https://www.sciencedirect.com/book/9780123942951/gene-therapy-of-cancer
- Anjali Lathwal, Rajesh Kumar, Gajendra P.S. Raghava., (2020) OvirusTdb: A database of oncolytic viruses for the advancement of therapeutics in cancer: Virology 548:109-116. [online]. Available from: https://www.sciencedirect.com/science/article/pii/S0042682220301057
- Carl H. June, Roddy S. O’Connor,Omkar U. Kawalekar,Saba Ghassemi, Michael C. Milone(2017) CAR T cell immunotherapy for human cancer:Science,359:1361-1365.[online]. Available from: https://science.sciencemag.org/content/359/6382/1361#BIBL
- Reza Mohammadinejad, Ali Dehshahri,Vijay Sagar Madamsetty ,Masoumeh Zahmatkeshan, Shima Tavakold ,Pooyan Makvandi, Danial Khorsandi, Abbas Pardakhty, Milad Ashrafizadeh,Elham Ghasemipour Afshar Ali Zarrabi. (2020) In vivo gene delivery mediated by non-viral vectors for cancer therapy: Journal of Controlled Release, 325:249-275.[online]. Available from: https://www.sciencedirect.com/science/article/pii/S0168365920303734
- Haonan Xing, Mei Lu, Tianzhi Yang, Hui Liu, Yanping Sun, Xiaoyun Zhao, Hui Xu, Li Yang, Pingtian Din (2019). Structure-function relationships of non-viral gene vectors: Lessons from antimicrobial polymers: Acta Biomaterialia, 86: 15-40 [online]. Available from: https://www.sciencedirect.com/science/article/pii/S1742706118307669#f0010
- Amit Singh and Mansoor Amiji (2020) The future of drug delivery in cancer treatmen. In: Biomaterials for Cancer Therapeutics (Second Edition). Woodhead Publishing: 569-597.
- Teresa R. Abreu, Nuno A. Fonseca, Nélio Gonçalves, João Nuno Moreira (2020). Current challenges and emerging opportunities of CAR-T cell therapies: Journal of Controlled Release, 319:246-261.