Animal Viruses: Replication, Latency, Cancer, and Prions

Assembly and Release of Animal Viruses

Overview of Viral Assembly and Release

Animal viruses undergo specific steps to assemble and release new virions from host cells. The process varies depending on the type of virus and the host cell involved.

• Assembly Location: Most animal viruses assemble in the nucleus, while some develop solely in the cytoplasm.

• Virion Production: The number of viruses produced depends on the size and initial health of the host cell.

• Persistent Infections: Some viruses cause persistent infections, where virions are produced over a prolonged period.

• Release Mechanisms: Viruses are released by exocytosis or by budding from the cell membrane, which may or may not kill the host cell.

Example: Budding in Enveloped Viruses

Enveloped viruses acquire their envelope by budding through the host cell membrane, incorporating viral proteins and capsid into the envelope.

Pattern of Virion Abundance in Persistent Infections

The abundance of virions in persistent infections follows a pattern:

• Entry: Virus enters the host cell.

• Synthesis and Assembly: Viral components are synthesized and assembled.

• Release: Virions are gradually released over time, maintaining infection.

Latency of Animal Viruses

Latent Viruses and Proviruses

Some animal viruses can remain dormant within host cells for extended periods, a phenomenon known as latency. These viruses are called latent viruses or proviruses.

• Examples: Varicella-zoster virus (chicken pox), Hepatitis B virus, and Herpes simplex virus.

• Characteristics: Latency can be prolonged for years with no viral activity.

• Integration: Integration of provirus into host DNA is permanent, making it difficult to eradicate the virus.

Example: HIV Latency

HIV can integrate its genome into host DNA, remaining latent until reactivation.

Role of Viruses in Cancer

Genetic Control of Cell Division

Cell division in multicellular animals is tightly regulated by genetic mechanisms. Disruption of these controls can lead to uncontrolled cell growth and tumor formation.

• Normal Regulation: Some cells are prevented from dividing, while others are restricted from unlimited division.

• Tumors: Uncontrolled cell division produces neoplastic cells, forming a tumor.

• Benign vs. Malignant Tumors: Benign tumors do not spread, while malignant tumors (cancers) can invade other tissues (metastasis).

Oncogenes and Viral Induction of Cancer

Viruses can contribute to cancer by affecting genes that regulate cell growth, known as oncogenes.

• Oncogenes: Genes that promote cell growth and division.

• Activation: Viral infection or mutation can activate oncogenes, leading to cancer.

• Mechanisms: Viruses may carry oncogenes, promote host oncogenes, or interfere with tumor suppressor genes.

Oncogene Theory (Described Table)

The process of viral induction of cancer involves:

1. Normal state: Proto-oncogenes are repressed.

2. First "hit": Virus inserts promoter near proto-oncogene, activating it.

3. Second "hit": Virus inserts oncogene, leading to production of cancer-causing proteins.

Examples of Viruses in Human Cancer

Viruses are estimated to cause 20–25% of human cancers. Mechanisms include carrying oncogenes, promoting host oncogenes, and interfering with tumor repression.

• Known Cancer-Causing Viruses:

  • Epstein-Barr virus (Burkitt's lymphoma)

  • Human papillomavirus (cervical cancer)

  • Human T-cell leukemia virus (Hodgkin's disease)

  • Kaposi's sarcoma-associated herpesvirus (Kaposi's sarcoma)

Culturing Viruses in the Laboratory

Methods for Culturing Viruses

Viruses cannot grow in standard microbiological media and require host cells for replication.

• Media Types:

  • Mature organisms (bacteria, plants, animals)

  • Embryonated eggs

  • Cell cultures

Viroids

Characteristics of Viroids

Viroids are infectious agents composed of short, circular pieces of single-stranded RNA (ssRNA) that infect plants.

• Lack Capsid: Viroids do not have a protein capsid.

• No Protein Coding: Viroid RNA does not code for proteins.

• Pathogenicity: Viroids adhere to complementary plant RNA, forming double-stranded RNA (dsRNA) that is degraded, leading to disease.

Prions

Nature and Structure of Prions

Prions are infectious proteins that cause neurodegenerative diseases in mammals.

• Structure: Normal prion protein (PrPC) has alpha-helices; infectious prion (PrPSc) has beta-sheets.

• Mechanism: PrPSc induces refolding of normal PrP into the infectious form.

Diseases Caused by Prions

Prions cause transmissible spongiform encephalopathies (TSEs), which are fatal neurodegenerative diseases.

• Examples:

  • Creutzfeldt-Jakob disease (CJD)

  • Scrapie (sheep)

  • Kuru (humans)

  • Chronic wasting disease (CWD, deer)

  • Variant CJD (vCJD)

• Transmission: By ingestion, transplantation, or contact with infected tissues.

• Treatment: No effective treatment for prion diseases.

Characteristics of Prions

• Resistance: Prions are not deactivated by standard sterilization procedures.

• Destruction: Prions are destroyed by incineration or autoclaving in concentrated sodium hydroxide.

• Enzyme Treatment: The European Union has approved the use of specific enzymes to remove prions from medical instruments.

Summary Table: Comparison of Infectious Agents

This table summarizes key differences between viruses, viroids, and prions.

Agent	Genetic Material	Capsid	Protein Coding	Host Range	Diseases
Virus	DNA or RNA	Present	Yes	Animals, plants, bacteria	Influenza, HIV, herpes, etc.
Viroid	ssRNA	Absent	No	Plants	Potato spindle tuber, etc.
Prion	None (protein only)	Absent	No	Mammals	CJD, scrapie, kuru, etc.

Additional info: Academic context and examples have been expanded for clarity and completeness.