Brief Introduction of Recombinant Retroviruses

The commonly used recombinant retroviruses are referring to γ-retroviruses (gamma-retroviruses) which are RNA-based viruses belonging to the family of retroviridae. Recombinant retroviruses (rRvs) serve as an efficient tool for delivering genetic material into target cells or organisms and have been widely used in gene delivery experiments.

There are a number of features that make rRvs attractive as gene delivery vehicles. Like recombinant lentiviruses (rLvs), transgene packaged in the rRvs can be stably integrated into host cell chromosomal DNA that allows for long-term, stable expression of introduced genetic elements. In addition, rRvs can theoretically carry payload up to 9 ~ 10 Kb and can deliver big inserts. The main disadvantage of rRvs is that they are uncapable of infecting non-dividing cells due to their large integrase complex which is too big to be transported into the nuclei of the host cells. Another concern is regarding to insertional mutagenesis that may cause safety problem when used in gene therapies.

The general method for producing rRvs is based on co-transfection of the following three plasmids into a packaging cell line such as HEK293T: 1) the transfer plasmid containing long terminal repeat (LTR) sequences, the packaging site (ψ) and the transgene/sgRNA/shRNA expression cassette, 2) the packaging plasmid including gag gene that encodes protease and structural proteins, and pol gene that encodes the reverse transcriptase, 3) the packaging plasmid expressing the envelope protein.

According to the viral backbone, envelope glycoprotein, rRvs can be divided into different subgroups.

1) Subgroups based on the viral backbone:
The most often used rRvs are derived from (Moloney Murine Leukemia Virus (MMLV) and Murine Stem Cell Virus (MSCV). The MMLV-derived retrovirus was the first widely used gene transfer vector. The MSCV retroviral system is a derivative of the MMLV system. Compared with MMLV retroviruses, MSCV retroviruses possess higher efficiency for delivering transgenes into pluripotent cell lines including hematopoietic, embryonic stem (ES), and embryonal carcinoma (EC) cells.

2) Subgroups based on the envelope protein:
There are four main types of envelope proteins used for retroviruses including VSV-G Envelope, 10A1 Envelope, Amphotropic Envelope and Ecotropic Envelope. Each envelope protein has its own unique feature and determine the host range of the retroviruses. The most widely used are the VSV-G envelope protein because of its broad tropism and stability during ultracentrifugation. In contrast to VSV-G, amphotropic and ecotropic viruses are not very stable, and can't be concentrated via ultracentrifugation.

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