Utility of ultra-deep sequencing for detection of varicella-zoster virus antiviral resistance mutations.

We report the first application of ultra-deep sequencing (UDS) to varicella-zoster virus (VZV) genotypic antiviral testing in a case of acyclovir-resistant VZV infection initially detected by Sanger sequencing within a deeply immunocompromised heart transplant recipient. As added-value compared to Sanger analysis, UDS revealed complex dynamics of viral population under antiviral pressure. Varicella-zoster virus (VZV) is a ubiquitous human herpesvirus affecting populations worldwide. VZV is commonly acquired in youth whose primary infection usually manifests as benign varicella (chickenpox). After the initial infection, the virus establishes lifelong latency in sensory ganglia leading to a risk of subsequent reactivation. Reactivation usually results in the development of localized herpes zoster (HZ) lesions, a painful skin rash commonly known as shingles (Cohen, 2013). The incidence and severity of HZ increase with impaired specific cell-mediated immunity mainly as a result of increasing age, malignancy, immunodeficiency, organ transplantation, or immunosuppressive drug therapy (Cohen, 2013; Koo et al., 2014; Pavlopoulou et al., 2015). In particular, HZ remains a significant cause of morbidity among solid organ transplant (SOT) recipients, especially in patients undergoing heart transplantation (HT) compared with liver, kidney, or lung transplant recipients (Carby et al., 2007; Koo et al., 2014; Pavlopoulou et al., 2015). These particular individuals are at increased risk of primary infection, reactivation followed by dissemination with visceral involvement and associated with bacterial superinfection, and chronic recurrences (Cohen, 2013). VZV infections may also engender debilitating neuralgia among highly immunocompromised patients (Sampathkumar et al., 2009). HT is also associated with the risk of reactivation of other latent viruses belonging to the Herpesviridae family as herpes simplex virus (HSV). Currently licensed drugs to prevent or to cure HSV- or VZV-associated diseases target the viral DNA polymerase (Pol). Acyclovir (ACV) and its prodrug valacyclovir (VACV) are considered as the first-line therapy, whereas foscarnet (FOS) or cidofovir (CDV) constitute alternative options. After primophosphorylation by the viral thymidine kinase (TK), ACV targets the viral DNA polymerase and inhibits the viral genome replication by a chain termination mechanism. According to this mechanism of action, viral mutations conferring resistance to ACV have been mapped both in TK and Pol encoding genes. Viral mutations conferring resistance to FOS and CDV are only detected in Pol gene. VZV ACV-resistance is mostly mediated by TK alterations, consisting in either translational frameshifts, sometimes associated with premature stop codon, or amino acid substitutions. In the remaining cases, amino acid substitutions are detected within Pol (De et al., 2015; Piret and Boivin, 2014). Classically, Sanger sequencing has been recognized as the gold standard for the detection of drug resistance mutations (DRMs) within VZV TK and Pol genes (Perrier et al., 2016; Piret and Boivin, 2014). However, this approach cannot detect minor variants present at a frequency below 20%. Ultra-deep sequencing (UDS) has an enhanced sensitivity compared to Sanger method and allows quantitative evaluation of the viral mutants (Chin et al., 2013). We report here a case of VZV resistant infection in an HT recipient. Our retrospective study aimed at showing the utility of UDS for DRM detection as a complement of Sanger method.