Clinically relevant sequence-based genotyping of HBV, HCV, CMV, and HIV

The term 'genotyping' describes the genetic characterization of a genome. The genotype analysis is performed to identify mutations that differentiate one individual or strain from another. The mutations may confer resistance to specific antiviral drugs or they may simply allow classification of a strain as to 'type' and 'subtype'. There are four human viruses for which genotype information is clinically useful. Hepatitis B virus (HBV) infections are being treated with antiretroviral drugs and resistance after prolonged treatment is common. Since HBV cannot be cultured, the only method of detecting resistance-conferring mutations in the genome is a genotypic analysis. Hepatitis C virus (HCV) infection can be cured by treatment with the combination of interferon and ribavirin but certain strains of virus are more resistant to treatment than others. The current recommendations are that all HCV type 1 infections be treated for 12 months whereas other types may be successfully treated in 6 months. Since interferon treatment may have significant side effects, the determination of HCV genotype is an important aspect of this therapeutic regimen. Treatment of cytomegalovirus (CMV) disease with nucleoside analogues occasionally results in resistant virus with mutations in the phosphotransferase gene (UL97) and/or the DNA polymerase gene (UL54) that can be tested with phenotypic or genotypic assays. Since CMV grows very slowly, it may be more clinically useful to perform a rapid genotypic assay although only the UL97 gene can be efficiently genotyped. Finally, the virus for which genotyping has become the standard of care, human immunodeficiency virus type 1 (HIV-1) can now be genotyped routinely by many clinical virology labs experienced with molecular amplification methods and automated DNA sequencing technology. All currently-available antiretroviral drugs are directed against either the protease or reverse transcriptase genes of HIV-1 and the mutations within these genes that confer resistance have been well described. Sequence-based genotyping methods are not necessarily the best approach for routine genotyping of these four viruses, but sequencing is the gold standard from which other methods are developed and against which they are compared.