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The study of viral genetics falls into two general areas: (1) mutations and their effect on replication and pathogenesis and (2) the interaction of two genetically distinct viruses that infect the same cell. In addition, viruses serve as vectors in gene therapy and in recombinant vaccines, two areas that hold great promise for the treatment of genetic diseases and the prevention of infectious diseases.
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Mutations in viral DNA and RNA occur by the same processes of base substitution, deletion, and frameshift as those described for bacteria in Chapter 4. Probably the most important practical use of mutations is in the production of vaccines containing live, attenuated virus. These attenuated mutants have lost their pathogenicity but have retained their antigenicity; therefore, they induce immunity without causing disease.
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There are two other kinds of mutants of interest. The first are antigenic variants such as those that occur frequently with influenza viruses, which have an altered surface protein and are therefore no longer inhibited by a person’s preexisting antibody. The variant can thus cause disease, whereas the original strain cannot. Human immunodeficiency virus and hepatitis C virus also produce many antigenic variants. These viruses have an “error-prone” polymerase that causes the mutations. The second are drug-resistant mutants, which are insensitive to an antiviral drug because the target of the drug, usually a viral enzyme, has been modified.
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Conditional lethal mutations are extremely valuable in determining the function of viral genes. These mutations function normally under permissive conditions but fail to replicate or to express the mutant gene under restrictive conditions. For example, temperature-sensitive conditional lethal mutants express their phenotype normally at a low (permissive) temperature, but at a higher (restrictive) temperature, the mutant gene product is inactive. To give a specific example, temperature-sensitive mutants of Rous sarcoma virus can transform cells to malignancy at the permissive temperature of 37°C. When the transformed cells are grown at the restrictive temperature of 41°C, their phenotype reverts to normal appearance and behavior. The malignant phenotype is regained when the permissive temperature is restored.
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Note that temperature-sensitive mutants have now entered clinical practice. Temperature-sensitive mutants of influenza virus are now being used to make a vaccine, because this virus will grow in the cooler, upper airways where it causes few symptoms and induces antibodies, but it will not grow in the warmer, lower airways where it can cause pneumonia (see Table 30–1).
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Some deletion mutants have the unusual property of being defective interfering particles. They are defective ...