Reporter Transgenes for Monitoring the Antitumor Efficacy of Recombinant Oncolytic Viruses

Abstract

Accurate measurement of tumor size and margins is crucial for successful oncotherapy. In the last decade, non-invasive imaging modalities, including optical imaging using non-radioactive substrates, deep-tissue imaging with radioactive substrates, and magnetic resonance imaging have been developed. Reporter genes play the most important role among visualization tools; their expression in tumors and metastases makes it possible to track changes in the tumor growth and gauge therapy effectiveness. Oncolytic viruses are often chosen as a vector for delivering reporter genes into tumor cells, since oncolytic viruses are tumor-specific, meaning that they infect and lyse tumor cells without damaging normal cells. The choice of reporter transgenes for genetic modification of oncolytic viruses depends on the study objectives and imaging methods used. Optical imaging techniques are suitable for in vitro studies and small animal models, while deep-tissue imaging techniques are used to evaluate virotherapy in large animals and humans. For optical imaging, transgenes of fluorescent proteins, luciferases, and tyrosinases are used; for deep-tissue imaging, the most promising transgene is the sodium/iodide symporter (NIS), which ensures an accumulation of radioactive isotopes in virus-infected tumor cells. Currently, NIS is the only reporter transgene that has been shown to be effective in monitoring tumor virotherapy not only in preclinical but also in clinical studies.

Keywords: NIS; deep-tissue imaging; oncolytic viruses; optical imaging; reporter transgenes; tumor cell.

Fig. 1

Tumor cell imaging methods using oncolytic viruses expressing protein reporter transgenes. Optical imaging methods: (A) – fluorescence imaging: fluorescent proteins emit fluorescence when irradiated with light of a certain wavelength; (B) – bioluminescence imaging: light is produced when exogenous substrates are oxidized by a bioluminescent reporter enzyme; (C) – photoacoustic imaging: a photoacoustic effect is achieved by irradiating a target tissue with a short-pulsed laser. Deep-tissue imaging techniques (D–F) – SPECT, PET; (G) – MRI; (D) – the HSV1-tk enzyme interacts with radioactively labeled substrates and converts them into a metabolite incapable of leaving the cell; (E) – the SSTR2 receptor binds radioactively labeled synthetic peptide substrates; (F) – NIS/NET transporters ensure the absorption and accumulation of radioactive substrates inside the cell; (G) – melanin enhances the magnetic resonance signal