DNA extraction

Genomic DNA was extracted from the blood samples using a method described previously (12). Briefly, cell lysates from the blood samples were generated following treatment with Tris-lysis buffer, lysozyme (R&D Systems, Inc., Minneapolis, MN, USA) and proteinase K (Solarbio Life Sciences, Beijing, China). Subsequent to overnight incubation at 56°C, the genomic DNA was isolated using the phenol-chloroform method and then purified through a silica gel column, lywallzyme solution (20 mg/ml in 0.6 M mannitol; Guangdong Institute of Microbiology, Guangzhou, China) and cetyltrimethylammonium bromide (CTAB; Nanjing Aocheng Chemical Co., Ltd., Nanjing, China). The resultant DNA pellet was washed with 70% ethanol and dissolved in deionized H₂O.