This study was supported by grants from the National Key Project of Scientific Research Program in China ( 2006CB910502 , 2006CB910504 ), the 111 Project ( 111-2-12 ), the National Science Foundation of China ( 30770825 , 30901718 ) and the Program for New Century Excellent Talents in University ( NCET-08-0562 ). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.