Sample selection of patient
This retrospective study was performed on patients selected from Ilam University of Medical Sciences in three peaks of COVID-19 infection (Alpha, Delta, and Omicron BA.5) from October 2020 to January 2022. Real-time reverse transcription polymerase chain reaction (rtReal time-PCR) was used to detect SARS-CoV-2 genome in pharyngeal swab specimens. All rtReal time-PCR tests were conducted by expert personnel under the same conditions regarding sampling time, the RNA extraction kit, and the real-time PCR kit.
Only 2,491 participants out of a total of 12,316 patients were considered eligible to participate in the study according to inclusion criteria: (1) providing consent before taking part in the study; (2) having same ethnic background and Iranian nationality; (3) having positive rtReal time-PCR test results and being selected from only one hospital; (4) The patients had no previous history of SARS-CoV-2 infection; (5) having no underlying comorbidities such as chronic obstructive pulmonary disease, cystic fibrosis, cancer, pregnancy, human immunodeficiency virus (HIV), obesity, heart disease, kidney disease, liver disease, and diabetes.
The studied patients were categorized into two groups. Group 1 included recovered patients with any of the signs and symptoms of COVID-19 such as headache, fever, sore throat, muscle pain, malaise, loss of taste and smell, nausea, and diarrhea, but without dyspnea, respiratory distress, or abnormal chest imaging and those with oxygen saturation (SpO2) levels below 94% and displayed signs of lower respiratory illness during clinical evaluation or imaging. These patients were outpatients and were not admitted to the hospital, and samples were taken from them when visiting the hospital.
Group 2 included deceased patients with the following features: SpO2 below 94%, a PaO2/FiO2 ratio below 300 mm Hg, a respiratory rate above 30 breaths per minute, lung infiltrates above 50%, septic shock, multiple organ dysfunction, and respiratory failure necessitating the use of mechanical ventilation.
All clinical parameters including aspartate aminotransferase (AST), alanine aminotransferase (ALT), alkaline phosphatase (ALP), low-density lipoprotein (LDL), high density lipoprotein (HDL), cholesterol, triglyceride (TG), uric acid, serum creatinine, fasting blood glucose (FBS), white blood cells (WBC), platelets, C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), thyroid-stimulating hormone (TSH), thyroxine (T4), triiodothyronine (T3), real-time PCR cycle threshold (Ct) values, and, 25-hydroxyvitamin D were obtained from the electronic medical record system during pre-hospitalization visits.
Genomic DNA extraction and IFITM3 rs6598045 genotyping
Approximately 10 ml of blood samples were collected from infected patients in EDTA tubes. Ficoll density gradient centrifugation (Ficoll-Paque PLUS, GE Healthcare, USA) was used to isolate peripheral blood mononuclear cells (PBMCs) and then the PBMCs were kept at −20 °C until the experiment.
The genomic DNA was extracted from the blood samples with a commercial kit (High Pure PCR Template Preparation Kit, Roche Diagnostics Deutschland and GmbH, Mannheim, Germany), according to the manufacturer's instructions. The yield of DNAs was determined using NanoDrop spectrophotometers (Thermo Scientific, USA), and the quality of DNAs was validated using electrophoresis gel.
The in-house amplification refractory mutation system–polymerase chain reaction (ARMS–PCR) assay was used for IFITM3 rs6598045 genotyping. The IFITM3 rs6598045 5′ ARMS primers were constructed with one base mismatch at the 3′ end. The “A” and “G” allele reverse primers were 5´-TCTTAGCCCTCAGCCCCACT-3´ and 5´-TCTTAGCCCTCAGCCCCACC-3´, respectively, and the forward primer was 5´-GCACCCTCTGAGCATTCCCTG-3´. The PCR product size was 490 bp. Moreover, as an internal control, we applied an 827 bp product with the primer sets of 5´-GCACCCTCTGAGCATTCCCTG-3´ and 5´-CACAGTGACGGTTATGGGAGACG-3´ (Additional file 1: Figure S1). The PCR test was carried out by initial denaturation at 95 °C for 18 min, followed by 40 cycles of 95 °C for 20 s, 63 °C for 25 s, 72 °C for 25 s, and final extension at 72 °C for 10 min.
The 890 bp PCR products were sequenced using the Sanger method on an ABI 3500 DX Genetic Analyzer (ABI, Thermo Fisher Scientific, and Waltham, MA, USA) to verify the ARMS-PCR results. MEGA Version 11.0 (https://www.megasoftware.net/) was used to analyze the raw sequencing data (Additional file 1: Figure S2).
Statistical analyses
The categorical variables as number (%) and continuous variables (numerical variables) as mean ± standard division (SD) were summarized using SPSS version 22.0 (SPSS, Inc, Chicago, IL, USA). The Shapiro–Wilk test was used to evaluate the normality assumptions for all numerical variables. The Mann–Whitney U test was used to compare continuous variables with normally distributed distributions. The Chi-square test was used to compare categorical variables. Independent factors of the likelihood and mortality of COVID-19 were identified using logistic regression-based multivariate models. Also, odds ratio (OR) and associated 95% confidence interval (CI) were calculated. An area under the receiver operating characteristic curve (AUC-ROC) analysis was used to assess the impact of IFITM3 rs6598045 on COVID-19 mortality. A P-value of < 0.05 was regarded statistically significant, and all tests were two-tailed. The online SNPStats tool was used to calculate the allelic frequency of the chosen variant to determine the minor allele frequency (MAF), four inheritance models (dominant, codominant, overdominant, and recessive), and the Hardy–Weinberg equilibrium (HWE). The optimal model was determined using the Akaike Information Criterion (AIC) and the Bayesian Information Criterion (BIC) (http://bioinfo.iconcologia.net/SNPStats).