Conflicts of interest
The authors have no conflicts of interest.
Authors’ contributions: Hong-Li Liu is first author and Hong-Bo Qi obtained the funding. Lan Zhang and Hong-Li Liu designed the study. Hong-Li Liu, Shuai Huang and Jun-Nan Li collected the data. Hong-Li Liu and Lan Zhang analyzed the data. Hong-Li Liu drafted the manuscript. Lan Zhang contributed to the interpretation of the results and critical revision of the manuscript for important intellectual content and approved the final version of the manuscript. All authors have read and approved the final manuscript. Lan Zhang is the study guarantor.