Genetic Pathways of Human Cytomegalovirus Drug Resistance

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Project Summary Toxicity and drug resistance limit the efficacy of antiviral therapy for cytomegalovirus (CMV) disease in immunosuppressed hosts such as cancer and transplant recipients. Current therapy includes DNA polymerase inhibitors ganciclovir, foscarnet and cidofovir. The terminase inhibitor letermovir was recently approved for prophylaxis, and the UL97 kinase inhibitor maribavir is in late stage trial as alternative therapy. In clinical practice, CMV drug resistance cannot be diagnosed by direct phenotypic testing of viral isolates, and instead is dependent on an accurate correlation of detected viral mutations and associated resistance phenotypes. The enduring objective of this research is to determine the genetic mechanisms of CMV drug resistance in order to improve clinical diagnosis and the development of alternate therapies. Many resistance mutations and polymorphisms in the CMV UL97 kinase, UL54 DNA polymerase and UL56/UL89/UL51 terminase genes have been and continue to be phenotyped, with diagnostically important new resistance loci being identified during each project period. Technical advances in recombinant phenotyping and whole genome deep sequencing are facilitating this research. In the upcoming project period, new focus areas include the characterization of mutations selected after drug exposure in genes outside of known antiviral targets, and analysis of the effects of baseline viral strain variation on drug susceptibility. Specific aims are (1) continued evaluation of the evolution and phenotypes of CMV mutations that develop after in vitro or in vivo exposure to antiviral compounds and combinations; (2) evaluate the phenotypic significance of genetic changes identified by viral whole genome deep sequencing that are outside of known antiviral target genes, and (3) determine the impact of baseline viral strain on drug susceptibility phenotypes by characterization of alternative viral clones and host cells.