Cooperative effects between two acyclovir resistance loci in herpes simplex virus.

Area of Science:

• Virology
• Molecular Biology
• Antiviral Drug Resistance

Background:

• Herpes simplex virus (HSV) resistance to acyclovir can arise from mutations in viral enzymes.
• Understanding the genetic basis of acyclovir resistance is crucial for developing effective antiviral therapies.
• The SC16 R9C2 mutant exhibits resistance to acyclovir, indicating underlying genetic alterations.

Purpose of the Study:

• To identify and characterize the genetic loci responsible for acyclovir resistance in the SC16 R9C2 mutant.
• To investigate the biochemical properties of mutant thymidine kinase and DNA polymerase.
• To determine the relative contribution of each resistance locus to viral pathogenicity and drug resistance.

Main Methods:

• Genetic recombination was used to generate viral mutants with single resistance loci (mutant thymidine kinase or DNA polymerase).
• Enzyme assays were performed to analyze the properties of wild-type and mutant thymidine kinase and DNA polymerase.
• In vitro (tissue culture) and in vivo (animal models) studies were conducted to assess viral growth and pathogenicity.

Main Results:

• Two independently segregating resistance loci were identified: one in thymidine kinase and one in DNA polymerase.
• Mutant enzymes showed altered affinities for acyclovir or acyclovir triphosphate, and reduced activity.
• Mutants with single or double resistance loci were attenuated in vivo, with the greatest attenuation observed in the DNA polymerase mutant (RSC-26).

Conclusions:

• Both thymidine kinase and DNA polymerase mutations contribute to acyclovir resistance and reduced virulence.
• The resistance and attenuation phenotypes exhibit synergy when both loci are mutated.
• The DNA polymerase mutation confers cross-resistance to other nucleoside analogs, highlighting its critical role in antiviral drug efficacy.