Occurrence of S68G and K65R/S68G mutations among various subtypes of HIV-1
The prevalence of S68G mutation and K65R/S68G double mutation among various subtypes of HIV-1 was analyzed in reverse transcriptase inhibitor (RTI)-naive and RTI-treated individuals from the HIV Drug Resistance Database of Stanford University (https://hivdb.stanford.edu/cgi-bin/MutPrevBySubtypeRx.cgi; version of 19 April 2019). For patients having more than one isolates with the same mutation are counted once. Mutations occurring in ≥1% and at least two individuals were included in the analyses. All sequences with a mixture of mutations were excluded from the analyses.
Study participants and sample collection
Four patients infected with CRF01_AE who failed treatment and carried the K65R and S68G double mutations were selected from a large-scale long-term ART cohort at the First Affiliated Hospital, China Medical University (Shenyang, China) (Fig. S1). The patient identification numbers were 301635, 301770, 301844, and 302335. The treatment regimens were tenofovir (TDF), lamivudine (3TC), and efavirenz. Genotypic testing for drug resistance and the viral load of plasma was performed at baseline, and at 6, 7, 10, and 12 months after treatment. Thereafter, all four patients changed treatment regimens. Among the four patients, 301635 and 302335 were followed up four times within 6–7 months post ART, while 301770 and 301844 were followed up five times within 10–12 months post ART. The study protocol was approved by the Ethics Committee of the First Affiliated Hospital of China Medical University. Written informed consent was provided by all patients.
Extraction, amplification, and sequencing of HIV-1 DNA
Viral RNA was extracted from plasma using a QIAamp® RNA Blood Mini kit (Qiagen, Stanford, VA, USA). The partial pol gene (145 bp) was amplified using primers 503b-F (5'-CAAAAATTGGGCCTGAAAATCCATA-3') and 52r-R (5’-TGTGGTATTCCTAATTGAACTTCCCA-3') through nested polymerase chain reaction (PCR). PCR products were purified with an Agencourt AMPure™ kit (Beckman Coulter, Fullerton, CA, USA) and quantified using a Qubit® dsDNA BR Assay kit (Life Technologies, Carlsbad, CA, USA). Subsequently, the PCR products were evaluated with a Bioanalyzer (2100; Agilent Technologies, Santa Clara, CA, USA) to control the size of the amplified fragments, and a library was constructed based on an input of 130–150 ng. A TruSeq™ Nano DNA HT Sample Preparation kit was employed for the construction of a DNA library (Illumina, San Diego, CA, USA). The quality of library construction was monitored using a DNA 7500 kit (Agilent Technologies). The standardized libraries were incorporated into 50% quality-controlled PHIX Libraries (Miseq phix control v3; Illumina). Sequencing was performed after the qualification of all operations.
The FASTQC (version 0.11.5) software was used to evaluate the sequencing quality of the paired-end FASTQ sequence files. MiSeq data analysis was performed using HyDRA Web, an open web portal that offers an automated pipeline for the analysis of next-generation sequencing-derived data related to HIV drug resistance. Advanced options for data quality assurance, filtering, variant calling, and reporting thresholds were modified to customize the analysis. Reads were filtered using a minimum quality score of Q30 and 50 bp in length, an error rate of 0.0021 for the MiSeq platform, a minimum variant quality of Q30, a minimum read depth of 100×, and a minimum allele count of five. HIV-DRMs detected with a frequency >1% were reported based on the default HyDRA Web Mutation Database. This database is a combination of the Stanford 2015 list of HIV-1 DRMs with added annotations from the World Health Organization 2009 list of mutations for the surveillance of transmitted HIV drug resistance [13].
Construction of a mutant clone and phenotypic resistance assay
Protease (PR) genes (codons 1–99) and reverse transcriptase (RT) genes (codons 1–240) were amplified from viral RNA obtained from patients infected with CRF01_AE using nested PCR using primers Round2-F (5'-ATAGCCAAAAATTGCAGGGCCCCTAGRAAAAAG-3') and Round2-R (5’-GTCCTTCCTTTCCACATTTCCA-3’). Patient-derived PCR products containing the K65R/S68G double mutations were cloned into a pNL4-3-ΔE-Luc plasmid through a ApaI/AgeI double-enzyme digestion and T4 ligation strategy to produce the pNL4-3-ΔE-Luc K65R/S68G clone. Subsequently, the S68G mutation was reversed to wild-type through in vitro site-directed mutagenesis using primers G68S-F (5'-AAGAGAAAGGACAGTACCAAATGGAGAAAG-3') and G68S-R(5'-TCTCCATTTGGTACTGTCCTTTCTCTTTAT-3') to produce the pNL4-3-ΔE-Luc K65R clone. Pseudoviruses were packaged in 293T cells with plasmid VSVG and co-transfection of the K65R/S68G clone or K65R/S68G clone [14]. The titers of viral stocks were determined according to the Spearman–Karber method [15].
The susceptibility of the virus to AZT, 3TC, and TDF was calculated as the half maximal inhibitory concentration (IC50). All experiments were conducted in triplicate. The IC50 of the mutant virus was compared with that of the fully susceptible virus (wild-type pseudovirus NL4-3-ΔE-Luc). The degree of resistance of the mutant pseudoviruses was determined by calculating the fold change (FC) in IC50 compared with that of the wild-type pseudoviruses.
Growth competition assay
PR genes and RT genes of the pNL4-3-wildtype plasmid were replaced by patient-derived genes containing K65R/S68G double mutations through an ApaI/AgeI double-enzyme digestion and T4 ligation strategy to produce the pNL4-3-wildtype K65R/S68G mutant infectious clone. Subsequently, the S68G mutation was reverted to wild type through in vitro site-directed mutagenesis to produce the pNL4-3-wildtype K65R mutant infectious clone. The K65R mutant and K65R/S68G mutant viral stocks were produced via transfection of 293T cells, and cultured at 37°C in an atmosphere of 5% CO2 for 48 h. The supernatant was collected for viral titration. The K65R/S68G mutants and K65R mutants viral stocks were diluted and mixed at a 4:6, 1:1, and 9:1 ratio, respectively. Peripheral-blood mononuclear cells (3 × 105) obtained from healthy donors were infected with the same virus titers (multiplicity of infection = 0.05) from each of the competing viruses. All experiments were conducted in triplicate. Viral fitness was determined through full pairwise competitions between all combinations of viruses. On days 3, 5, 7, 10, and 13, half of the culture supernatant was harvested, and viral RNA was extracted. The viral pol gene was amplified and sequenced for Sanger sequencing. The ChromatQuan Internet tool was used to calculate the viral ratio at each time point [16].
Statistical analysis
The chi-squared test was used to evaluate the frequency of S68G mutation between RTI-naive and -treated individuals, and the K65R/S68G double mutation among HIV-1 subtypes. The nonparametric t-test was used to analyze differences in FC between K65R alone and K65R/S68G double mutants. A p < 0.05 denoted statistical significance.