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The aim of this study was to characterize Human Cytomegalovirus (HCMV) drug resistance mutations in UL97 and UL54 genes in allogeneic hematopoietic stem cell transplant (allo-HSCT) recipients in Portugal. We have performed a retrospective study with 22 patients from a cohort of patients with different haematological malignancies submitted to allo-HSCT between 2010 and 2014. Patients were selected according to clinical and laboratory data of HCMV infection and management. HCMV resistance mutations were characterized by sequencing of UL97 and UL54 genes. Sequence data were compared with: 1) HCMV genome reference strain AD169; and also 2) UL97 from Merlin strain (GenBank: AY446894.2), and UL54 from TB40/E strain (GenBank: ABV71585.1). Resistance mutations were identified in seven patients (32%): five harboured resistance mutations in UL97: A594V (n = 2), C592G (n = 1), L595W (n = 1), and C603W (n = 1); and two harboured resistance mutations in UL54: P522S and L957F, one in each patient. Several natural polymorphisms and unknown mutations were found in both UL97 and UL54, with the majority of the patients harbouring more than one unknown mutation in UL97 but only one in UL54. No simultaneous mutations were found. This is the first study in Portugal to characterize HCMV UL97 and UL54 sequences and to identify HCMV drug-resistance mutations in allo-HSCT patients. The UL97 resistance mutations found were amongst the most frequent resistant mutations, while UL54 L957F mutation was here reported for the first time in a clinical specimen. This information provides important information regarding HCMV strains and antiviral resistance in our population., (Copyright © 2016 Elsevier B.V. All rights reserved.)

Human cytomegalovirus (HCMV) infection is a major cause of morbidity and mortality in allogeneic hematopoietic stem cell transplant recipients. The significant clinical impact of HCMV infection and progression to HCMV disease among allogeneic hematopoietic stem cell transplant recipients has been reduced by prophylactic, preemptive, and curative treatments using ganciclovir, valganciclovir, foscarnet, and cidofovir. Resistance to (val)ganciclovir results from mutations localized in HCMV UL97 gene (encoding the pUL97 phosphotransferase), UL54 gene (encoding the pUL54 DNA polymerase), or both genes, whereas foscarnet and cidofovir resistance results from mutations localized within UL54 gene only. This review is focused on HCMV antiviral drug resistance, including the functions of target genes of antivirals, the mechanisms of antiviral resistance, the different mutations in pUL97 and pUL54 that have been identified in either clinical isolates or laboratory strains, and their impact on HCMV susceptibility to antiviral drugs. It emphasizes the importance of proving that observed genetic changes confer resistance so they can be distinguished from polymorphisms. Because of the emergence of HCMV resistance to currently available drugs, novel drugs are urgently needed for the therapeutic management of HCMV-resistant infections in hematopoietic stem cell transplant patients., (Copyright © 2016 John Wiley & Sons, Ltd.)