Overview

A retrovirus is a type of virus that uses RNA as its genetic material. Thus, retroviruses comprise the most diverse family of enveloped RNA viruses and can be defined by common taxonomic features, including composition, replicative properties and structure. The virions possess an envelope composed of lipid and also contain viral glycoproteins. When a cell becomes infected by a retrovirus, the viral RNA is converted to DNA and the newly formed DNA integrates into the host cell's genetic material. Subsequently, the infected cell generates additional retroviruses, spreading the infection to other cells. Numerous retroviruses are linked to various diseases, such as AIDS and certain types of cancer.

In the 1970s, retroviruses were linked to animal cancers, but their role in human diseases was unclear. These early findings involved exogenous retroviruses, which infect from outside the body, unlike endogenous retroviruses, which are inherited in the human genome. The discovery of interleukin-2 allowed Robert Gallo's team at IRP to grow human T cells in vitro, where they first observed retroviruses in human cells, advancing understanding of their link to human diseases.

Key features of retroviruses

• RNA genome: Single-stranded RNA serves as the genetic template
• Reverse transcription: Viral RNA is converted into DNA by reverse transcriptase
• Genome integration: Viral DNA integrates into the host genome as a provirus

What is the structure and function of retroviruses?

Retroviruses constitute a sizable category of enveloped viruses with a single-stranded RNA genome. The examination of retrovirus proteins and particles has been conducted thoroughly using methods such as nuclear magnetic resonance (NMR), X-ray crystallography, cryo-electron microscopy (cryo-EM) and cryo-electron tomography (cryo-ET).

Retroviruses include key components such as an envelope, a capsid and a single-stranded RNA genome. These viruses infect host cells by fusing with cellular membranes, a process mediated by the envelope glycoprotein (Env), thereby delivering the viral core into the cytoplasm. How does the retrovirus replication cycle work?

Retrovirus replication involves entry into host cells, reverse transcription of viral RNA into DNA, integration of the provirus into the host genome, transcription and translation of viral genes and the final release of new virions through budding.

Retrovirus replication steps:

• Entry: Retroviruses bind to host cell receptors to fuse membranes and release viral RNA into the cytoplasm
• Reverse Transcription: Upon entry, reverse transcriptase converts viral RNA into double-stranded DNA, forming a provirus that can integrate into the host genome
• Integration: The viral DNA integrates into host DNA via integrase, becoming a provirus, which helps pass viral material to daughter cells during division
• Transcription and translation: The host cell machinery transcribes and translates the integrated proviral DNA, creating viral RNA and proteins needed for assembling new viral particles
• Assembly and Budding: Viral components reach the cell membrane, where they assemble into new virions and bud off, acquiring a host-derived covering to infect new target cells

Common Retroviral Infections in Humans

Retrovirus and Cancer

Retroviruses play a significant role in cancer development by integrating viral oncogenes into the host genome, thereby disrupting normal cellular regulation.

Oncogenes are genes that, when mutated or aberrantly activated, can drive the transformation of normal cells into cancerous cells. In the context of retroviral infection, tumor development is often linked to the integration of viral genetic material into the host genome, which can disrupt normal cellular regulation and promote uncontrolled cell proliferation.

Retroviruses contribute to cancer through multiple mechanisms, including activating host oncogenes and expressing viral regulatory proteins that alter gene expression. For example:

• HTLV-1 encodes the Tax protein, which enhances transcriptional activity, promotes cell proliferation and interferes with tumor suppressor pathways, contributing to adult T-cell leukemia
• HIV-1, although it does not carry a classical oncogene, increases cancer risk indirectly by causing immune dysregulation, which reduces immune surveillance against malignant cells

In HIV-associated cancers, co-infections with oncogenic viruses such as HHV-8 further amplify cancer risk, as seen in Kaposi’s sarcoma. Together, these mechanisms highlight how retroviruses can influence cancer development through both direct genetic effects and indirect immune-mediated pathways.

How are retroviruses used in gene therapy?

Retroviral vectors: how they’re made

Retroviral vectors are engineered from proviral sequences to deliver a gene of interest into target cells, often under the control of viral or cellular promoters such as CMV to enhance gene expression. During production, packaging cells generate viral particles by supplying essential proteins from plasmids, resulting in replication-deficient virions that carry the vector genome and integrate into the DNA of dividing target cells, enabling stable gene expression.

Key design and production features include:

• Incorporation of a gene of interest into a modified proviral backbone
• Use of strong promoters (e.g., CMV) to drive gene expression
• Packaging cells that supply viral proteins for vector assembly
• Generation of replication-deficient viral particles for safety
• Stable integration into host genomes of dividing cells

In research and clinical settings, retroviral vectors are widely used for gene transfer applications. They support:

• Developmental research: gene expression in embryonic tissues

• Transgenic model generation: germline modification in animals

• Mechanistic studies: identification of viral integration sites

• Gene therapy applications:

  • Correction of genetic defects (e.g., adenosine deaminase deficiency)
  • Introduction of therapeutic genes for targeted cell modification
  • Enhancement of immune responses in cancer therapies

Despite their advantages, clinical use may be limited by factors such as cell-type specificity and potential safety risks associated with genomic integration.

Prevention and Treatment of Retrovirus Infections

Preventive strategies for retroviral infections, particularly HIV, focus on minimizing transmission risk and improving public awareness. Key measures include:

• Screening blood products to prevent transfusion-related transmission
• Promoting safe sex practices through education initiatives
• Discouraging the sharing of needles to reduce blood-borne exposure

Treatment of retroviral infections relies on antiretroviral therapy (ART), including drugs such as azidothymidine (AZT) and other antiviral agents. These therapies work by inhibiting viral replication, thereby reducing viral load and slowing disease progression. ART is used both as a preventive strategy to limit progression after exposure and as a long-term treatment for established infections.

In clinical settings, particularly for HIV, ART also plays a critical role in managing complications associated with immune suppression. This includes preventing and treating opportunistic infections and certain cancers that arise from weakened immune function. Treatment approaches are typically individualized, based on disease stage, immune status and the presence of co-infections or related conditions.

Current Research and Future Directions

Studies focus on using molecular methods to identify and characterize retroviral infections, improving diagnosis and understanding viral dynamics. Research aims to develop cell lines for high-titer retrovirus production that can transduce rhesus monkey hematopoietic stem cells with consistent results. Scientists are developing methods to improve retrovirus production for gene therapy, aiming to enhance the efficiency and scalability of viral vector manufacturing.

Efforts to prevent retroviral infections focus on developing vaccines and public health measures. Studies investigate vaccines against retroviruses, such as the feline immunodeficiency virus (FIV), to control their spread. Researchers also explore interventions to reduce the impact of retroviral infections, especially regarding human immunodeficiency virus (HIV) and AIDS.