The receptors for gibbon ape leukemia virus and amphotropic murine leukemia virus are not downregulated in productively infected cells

Targeted Suicide Gene Therapy with Retroviral Replicating Vectors for Experimental Canine Cancers

Cancer in dogs has increased in recent years and is a leading cause of death. We have developed a retroviral replicating vector (RRV) that specifically targets cancer cells for infection and replication. RRV carrying a suicide gene induced synchronized killing of cancer cells when administered with a prodrug after infection. In this study, we evaluated two distinct RRVs derived from amphotropic murine leukemia virus (AMLV) and gibbon ape leukemia virus (GALV) in canine tumor models both in vitro and in vivo. Despite low infection rates in normal canine cells, both RRVs efficiently infected and replicated within all the canine tumor cells tested. The efficient intratumoral spread of the RRVs after their intratumoral injection was also demonstrated in nude mouse models of subcutaneous canine tumor xenografts. When both RRVs encoded a yeast cytosine deaminase suicide gene, which converts the prodrug 5-fluorocytosine (5-FC) to the active drug 5-fluorouracil, they caused tumor-cell-specific 5-FC-induced killing of the canine tumor cells in vitro. Furthermore, in the AZACF- and AZACH-cell subcutaneous tumor xenograft models, both RRVs exerted significant antitumor effects. These results suggest that RRV-mediated suicide gene therapy is a novel therapeutic approach to canine cancers.

Retroviral Replicating Vectors Mediated Prodrug Activator Gene Therapy in a Gastric Cancer Model

Retroviral replicating vectors (RRVs) selectively replicate and can specifically introduce prodrug-activating genes into tumor cells, whereby subsequent prodrug administration induces the death of the infected tumor cells. We assessed the ability of two distinct RRVs generated from amphotropic murine leukemia virus (AMLV) and gibbon ape leukemia virus (GALV), which infect cells via type-III sodium-dependent phosphate transporters, PiT-2 and PiT-1, respectively, to infect human gastric cancer (GC) cells. A quantitative RT-PCR showed that all tested GC cell lines had higher expression levels of PiT-2 than PiT-1. Accordingly, AMLV, encoding a green fluorescent protein gene, infected and replicated more efficiently than GALV in most GC cell lines, whereas both RRVs had a low infection rate in human fibroblasts. RRV encoding a cytosine deaminase prodrug activator gene, which converts the prodrug 5-flucytosine (5-FC) to the active drug 5-fluorouracil, showed that AMLV promoted superior 5-FC-induced cytotoxicity compared with GALV, which correlated with the viral receptor expression level and viral spread. In MKN-74 subcutaneous xenograft models, AMLV had significant antitumor effects compared with GALV. Furthermore, in the MKN-74 recurrent tumor model in which 5-FC was discontinued, the resumption of 5-FC administration reduced the tumor volume. Thus, RRV-mediated prodrug activator gene therapy might be beneficial for treating human GC.

Unique Structure and Distinctive Properties of the Ancient and Ubiquitous Gamma-Type Envelope Glycoprotein

After the onset of the AIDS pandemic, HIV-1 (genus Lentivirus) became the predominant model for studying retrovirus Env glycoproteins and their role in entry. However, HIV Env is an inadequate model for understanding entry of viruses in the Alpharetrovirus, Gammaretrovirus and Deltaretrovirus genera. For example, oncogenic model system viruses such as Rous sarcoma virus (RSV, Alpharetrovirus), murine leukemia virus (MLV, Gammaretrovirus) and human T-cell leukemia viruses (HTLV-I and HTLV-II, Deltaretrovirus) encode Envs that are structurally and functionally distinct from HIV Env. We refer to these as Gamma-type Envs. Gamma-type Envs are probably the most widespread retroviral Envs in nature. They are found in exogenous and endogenous retroviruses representing a broad spectrum of vertebrate hosts including amphibians, birds, reptiles, mammals and fish. In endogenous form, gamma-type Envs have been evolutionarily coopted numerous times, most notably as placental syncytins (e.g., human SYNC1 and SYNC2). Remarkably, gamma-type Envs are also found outside of the Retroviridae. Gp2 proteins of filoviruses (e.g., Ebolavirus) and snake arenaviruses in the genus Reptarenavirus are gamma-type Env homologs, products of ancient recombination events involving viruses of different Baltimore classes. Distinctive hallmarks of gamma-type Envs include a labile disulfide bond linking the surface and transmembrane subunits, a multi-stage attachment and fusion mechanism, a highly conserved (but poorly understood) "immunosuppressive domain", and activation by the viral protease during virion maturation. Here, we synthesize work from diverse retrovirus model systems to illustrate these distinctive properties and to highlight avenues for further exploration of gamma-type Env structure and function.

Split genome-based retroviral replicating vectors achieve efficient gene delivery and therapeutic effect in a human glioblastoma xenograft model

The murine leukemia virus-based semi-retroviral replicating vectors (MuLV-based sRRV) had been developed to improve safety and transgene capacity for cancer gene therapy. However, despite the apparent advantages of the sRRV, improvements in the in vivo transduction efficiency are still required to deliver therapeutic genes efficiently for clinical use. In this study, we established a gibbon ape leukemia virus (GaLV) envelopepseudotyped semi-replication-competent retrovirus vector system (spRRV) which is composed of two transcomplementing replication-defective retroviral vectors termed MuLV-Gag-Pol and GaLV-Env. We found that the spRRV shows considerable improvement in efficiencies of gene transfer and spreading in both human glioblastoma cells and pre-established human glioblastoma mouse model compared with an sRRV system. When treated with ganciclovir after intratumoral injection of each vector system into pre-established U-87 MG glioblastomas, the group of mice injected with spRRV expressing the herpes simplex virus type 1-thymidine kinase (HSV1-tk) gene showed a survival rate of 100% for more than 150 days, but all control groups of mice (HSV1-tk/PBS-treated and GFP/GCV-treated groups) died within 45 days after tumor injection. In conclusion, these findings sug-gest that intratumoral delivery of the HSV1-tk gene by the spRRV system is worthy of development in clinical trials for the treatment of malignant solid tumors. [BMB Reports 2022; 55(12): 615-620].

Dual-vector prodrug activator gene therapy using retroviral replicating vectors

Retroviral replicating vectors (RRVs) have been shown to achieve efficient tumor transduction and enhanced therapeutic benefits in a variety of cancer models. In the present study, we evaluated a possible combinatorial effect of prodrug activator genes delivered by two different RRVs derived from amphotropic murine leukemia virus (AMLV) and gibbon ape leukemia virus (GALV) on human hepatocellular carcinoma Hep3B cells. Both RRVs showed efficient replicative spread in culture and can overcame superinfection resistance each other. Notably, the replication and spread of each RRV in culture remained unaffected by pretransduction with the counterpart RRV. We further transduced cells with RRVs which individually possessed the prodrug activator genes yeast cytosine deaminase (CD) and herpes simplex virus thymidine kinase (TK) alone or in combination, and evaluated the cytotoxic effects of RRV-mediated gene therapy with CD and TK in the presence of the respective prodrugs, 5-fluorocytosine and ganciclovir. All combinations of the two prodrug activator genes produced synergistic cytocidal effects, but the combined effects of the different genes were significantly greater than those of the same genes when delivered by two different vectors. The present findings indicate the potential utility of dual-vector gene therapy using two different RRVs carrying different prodrug activator genes.

Efficient tumor transduction and antitumor efficacy in experimental human osteosarcoma using retroviral replicating vectors

Retroviral replicating vectors (RRVs) have achieved efficient tumor transduction and enhanced therapeutic benefit in a wide variety of cancer models. Here, we evaluated two different RRVs derived from amphotropic murine leukemia virus (AMLV) and gibbon ape leukemia virus (GALV), which utilize different cellular receptors (PiT-2 and PiT-1, respectively) for viral entry, in human osteosarcoma cells. Quantitative RT-PCR showed that low levels of expression of both receptors were observed in normal and non-malignant cells. However, high PiT-2 (for AMLV) and low PiT-1 (for GALV) expression was observed in most osteosarcoma cell lines. Accordingly, AMLV expressing the green fluorescent protein gene infected and replicated more efficiently than GALV in most osteosarcoma cell lines. Furthermore, RRVs expressing the cytosine deaminase prodrug activator gene showed differential cytotoxicity that correlated with the results of viral spread. AMLV-RRV-mediated prodrug activator gene therapy achieved significant inhibition of subcutaneous MG-63 tumor growth over GALV in nude mice. These data indicate that AMLV vectors predominate over GALV in human osteosarcoma cells. Moreover, our findings support the potential utility of the two RRVs in personalized cancer virotherapy on the basis of receptor expression.

Permissive XPR1 gammaretrovirus receptors in four mammalian species are functionally distinct in interference tests

Xenotropic/polytropic mouse leukemia viruses (X/P-MLVs) use the XPR1 gammaretrovirus receptor for entry. X/P-MLV host range is defined by usage of naturally occurring restrictive XPR1 receptors, and is governed by polymorphisms in the virus envelope glycoprotein and in XPR1. Here, we examined receptors of four mammalian species permissive to all X/P-MLVs (Mus dunni, human, rabbit, mink). Interference assays showed the four to be functionally distinct. Preinfection with X-MLVs consistently blocked all nine XPR1-dependent viruses, while preinfection with P-MLVs and wild mouse X/P-MLVs produced distinctive interference patterns in the four cells. These patterns indicate shared usage of independent, but not always fully functional, receptor sites. XPR1 sequence comparisons identified candidate sites in receptor-determining regions that correlate with some interference patterns. The evolutionary record suggests that the X/P-MLV tropism variants evolved to adapt to host receptor polymorphisms, to circumvent blocks by competing viruses or to avoid host-encoded envelope glycoproteins acquired for defense.

A mutant retroviral receptor restricts virus superinfection interference and productive infection

Background

Both cell-free and cell-associated infection routes are important for retroviral dissemination. Regardless of the mechanism, the driving force of retroviral entry is the interaction between the viral envelope and its receptor. To date it remains unclear how decreased affinity of viruses for their receptors affects viral cell-free infection, cell-cell transmission, and spreading kinetics. We have previously characterized a mutant form of the amphotropic murine retrovirus receptor human phosphate transporter 2 (PiT2) wherein the single substitution of a glutamic acid for the lysine residue at position 522 of this receptor is sufficient to render it to function as a gibbon ape leukemia virus (GALV) receptor.

Results

In this study we analyzed the binding affinity of the mutant receptor PiT2K522E and determined that it has a 1000 fold decreased GALV envelope binding affinity compared to the GALV wild type receptor. The decreased affinity does not restrict the initiation of cell-free GALV infection. The diminished binding affinity does, however, correlate with a decrease in the ability of GALV to spread in cells expressing this mutant receptor.

Conclusions

The reduced ability of GALV to subsequently spread among cells expressing PiT2K522E is likely resulted from reduced cell-cell transmission, the decreased ability of PiT2K522E-expressing cells to establish superinfection interference, and attendant cytopathic affects.