Glioblastoma Multiforme (GBM) is a highly aggressive brain tumor for which there is no treatment. Oncolytic Viruses (OVs) are attenuated tumor-selective viruses designed to replicate in and kill tumor cells without affecting normal tissue. Herpes Simplex Virus type-1 derived oncolytic vectors (oHSV-1) are a promising and safe alternative to current GBM therapies, but the deletions that render these oHSV-1 incapable of replicating in normal tissue also compromise their lytic activity in tumor cells. Consequently, these viruses fail to effectively eradicate the tumor.
In this study, I developed a new oHSV-1, named ICP4-miR124t, whose selective replication in GBM cells does not rely on defective genes, but rather is mediated by micro (mi)RNAs differentially expressed between normal tissue and GBM. MiRNAs recognize complementary target sequences in mRNAs resulting in repression of the mRNA. MiR124 is one of the most abundant miRNAs in developing and mature neurons, but is absent in GBM since its presence is incompatible with the tumor phenotype and viability. After introducing four tandem copies of the miR124 target sequence in the 3’UTR of the HSV-1 essential ICP4 gene, I observed robust replication of the resulting ICP4-miR124t oHSV-1 in primary glioblastoma cells, while viral growth was highly impaired in the same cells upon induction of mir124. Toxicity tests involving intracranial injection of immunocompetent and immunodeficient mice showed a dramatic reduction in vector toxicity compared to unregulated control virus. Moreover, ICP4-miR124t-injected animals showed a loss of viral DNA in the brain over time, whereas an increase was observed in control virus injected animals.
In conclusion the ICP4-miR124t oHSV-1 developed in this study is promising for GBM treatment because it is capable of killing miR124-negative tumor cells as efficiently as wild-type HSV-1, while lacking toxicity for normal brain. These results are of major Public Health importance not only as a potentially more effective alternative to current inefficient GBM treatments, but also as a new paradigm to treat other kinds of tumors by taking advantage of the differential miRNA expression between tumor and normal tissue
