College of Life Science, Hunan Normal University, Changsha, Hunan 410087, China and Department of Biostatistics and Epidemiology, Georgia Regents University, Augusta, GA 30912-4900, USA.|College of Life Science, Hunan Normal University, Changsha, Hunan 410087, China and Department of Biostatistics and Epidemiology, Georgia Regents University, Augusta, GA 30912-4900, USA.