Introduction to Retrovirus Vectors

Retrovirus vectors are developed from RNA viruses that belong to a viral family Retroviridae. Within the host cell these viruses can synthesize double-stranded DNA (dsDNA) from their RNA genomes using an enzyme known as reverse transcriptase. This DNA can then be integrated into the chromosome of the host cell using another enzyme carried by the virus, known as integrase. Stable integration of the DNA synthesized from the viral genome modifies the host cell, which now synthesizes viral proteins along with host proteins. When the modified host cell divides, the daughter cells retain copies of the viral genes.

The retrovirus genome usually comprises of three open reading frames:

  • Gap - Group-specific antigen: Encodes structural proteins that form the viral capsid.
  • Pol - Polymerase: Encodes the reverse transcriptase, protease, and integrase enzymes.
  • Env - Envelope: Encodes the viral envelope (surface and transmembrane glycoprotein) proteins.

 

Retrovirus Advantages

Retroviruses are the most common gene delivery vectors and are used in over 60-70% of human gene therapy clinical trials. This is attributed to the following reasons:

  • Retroviruses have high efficiency of gene transfer. These vectors allow for stable integration of the transgene into the host genome, facilitating long term expression.
  • Retroviruses can be pseudotyped with env proteins to infect a specific cell type or a broad range of target cells, as required.
  • Retroviruses have a higher packaging capacity (8 kb) compared to adenoviral and AAV vectors.

 

Retrovirus Limitations

Despite their successful use in most gene therapy experiments, retroviruses present a number of problems. One such issue is that retroviruses only infect dividing cells. Owing to this, retroviruses are generally used in ex vivo models, where target cells are extracted from the patient, stimulated to divide in vitro and then transduced with retroviral vectors carrying the therapeutic gene. The transduced cells are then re-introduced into the patient. However, retroviruses can be enabled to target non-dividing cells by either engineering the viral surface proteins to recognize specific proteins on target cells or co-infecting the host with other viruses, such as adenovirus.

Another issue related to the use of retroviruses is the risk of insertional mutagenesis. The integrase enzyme is known to often insert genetic material into an arbitrary position in the host genome (ectopic integration). In such cases, if the transgene is inserted within a tumor suppressor gene sequence or a gene that regulates cell division, it can lead to uncontrolled cell division or cancer. Recently, scientists have come up with a way to mitigate this problem. It has been shown that the use of genome editing technologies or the inclusion of sequences, such as the beta globin locus control region, can direct the integration of a transgene to specific chromosomes in the host genome.

Lately, lentiviral vectors are being considered over retroviral vectors.