When viruses come to mind, most people tend to think of their threats against human health. However, scientists are rapidly discovering a new branch of medicine: oncolytics. Oncolytics is the use of viruses to kill cancerous cells, while leaving the surrounding healthy tissue unaffected. Certain forms of cancer are ideal for oncolytic research, due to their location and lack of viable treatment. A sarcoma is a certain type of cancer that comes from mesenchymal tissue (1). Sarcomas can be divided into two categories: soft-tissue or bone (1). While certain types of surgeries can cure sarcoma, other forms are inoperable, and difficult to treat with surgery, chemotherapy, or radiation (1). Thus, sarcomas are optimal for experimental oncolytic research.
In a recent study published by the Journal of Virology, German scientists from the University Hospital Tubingen investigated the effectiveness of the measles vaccine virus (MeV) in treatment of eight types of sarcoma cell lines: HT1080 (human fibrosarcoma), A673 (extraosseous Ewing sarcoma), SRH (sclerosing spindle cell rhabdomyosarcoma), BR and ZF (alveolar rhabdomyosarcoma), SCOS (osteosarcoma), CCS (clear cell sarcoma), and ST (dedifferentiated leiomyosarcoma). Researchers chose to focus on MeV due to its previous oncolytic potential in several types of cancers, such as ovarian carcinoma (2), lymphoma (3), and glioblastoma (4). Typically, oncolytic viruses are equipped with “suicide genes,” which can produce localized chemotherapy in an infected cell (5). In this experiment, the researchers armed MeV with super cytosine deaminase (SCD), which converts 5-fluorocytosine (5-FC) into 5-FU, a type of clinical chemotherapy, thus producing MeV-SCD (6).
The researchers found that MeV-SCD works effectively against five of the eight tested sarcomas: BR, ST, A673, ZF, and HT1080. To start, they infected a sample of each cell line with MeV-SCD, and observed each resulting tumor mass at 96 hours past infection. For this experiment, they infected the cell lines at an MOI (or multiplicity of infection) of 1, which means that the ratio of virus to cell was 1. After 96 hours, cell lines with more than 50% of the original mass were classified as resistant to MeV-SCD virotherapy, whereas cell lines with less than 50% of the original mass were categorized as susceptible to oncolysis. Five of the cell lines were susceptible, with resulting tumor masses of 3-40%, while three were resistant, with tumor masses of 72-95% remaining. Researchers then investigated the addition of 5-FC, the original pro-drug substituted for 5-FU. The susceptible cell lines were infected with MeV-SCD at MOIs of 1, 0.1, and 0.01, while resistant cell lines were infected with MOIs of 1 and 0.1. Varying concentrations of 5-FC were added to the infected cells at three hours past infection. The investigators found that high 5-FC concentrations (1mM) strongly enhanced the oncolytic effects of the susceptible cell lines at MOIs of 0.01 or 0.1. Additionally, the 1mM of 5-FC strongly enhanced oncolytic activity in two of the three originally resistant cell lines (SRH and CCS), while SCOS cells still remained resistant.
After the initial findings, the investigators then researched the mechanisms behind why certain cell lines were more susceptible than others. To start, the scientists took a look at the expression levels of the MeV receptors in the eight different cell lines. They investigated one receptor in particular, CD46, using flow cytometry. They found consistently higher CD46 expressions in the five originally susceptible lines, compared to the three resistant cell lines. The susceptible cell lines also showed higher infection rates than resistant sarcomas. In order to study the infection rate, the investigators introduced a GFP marker gene into the cells, encoding MeV. The GFP gene encodes the green fluorescent protein, thus infected cells turned visibly green, and were analyzed through flow cytometry. Additionally, the susceptible cell lines showed higher rates of viral replication than the resistant sarcomas. The investigators performed a viral growth curve assay for all eight cell lines, and found that viral replication was inhibited in the resistant cell lines. Lastly, the researchers examined a possible correlation between IFN-β mRNA expression and the resistant cell lines. Interferon, or IFN, is the immune system’s response to viral infection. However, data indicated no clear connection between IFN-β expression and the resistant cell lines.
Oncolytics is a novel yet controversial branch of cancer treatment. Although it has a great potential for treating many types of cancers, some scientists warn of possible threats to health. Specifically, there is always a slight chance that the oncolytic virus could possibly mutate, and kill the non-cancerous cells. Clearly, more research can be investigated involving MeV and sarcomas. Perhaps one way to expand upon this research is to take more of an in vivo approach. This way, investigators can observe any damaging effects the oncolytic virus may have on healthy, surrounding tissue. This can help doctors understand potential side effects of oncolytic therapy. Also, researchers may be able to look at inserting different genes into the oncolytic virus. In this experiment, they were able to convert 5-FC into 5-FU, a chemotherapy, but perhaps they could possibly look at the addition of a targeted therapy to the virus. In the future, oncolytics will develop into a unique and novel branch of cancer therapy.
Berchtold S, Lampe J, Weiland T, Smirnow I, Scheicher S, Handgretinger R, Kopp HG, Reiser J, Stubenrauch F, Mayer N, Malek NP, Bitzer M, Lauer UM. (2013) Innate Immune Defense Defines Susceptibility of Sarcoma Cells to Measles Vaccine Virus-Based Oncolysis. Journal of Virology 87(6): 3484-3501.
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