Department of Statistics, Virginia Tech, 250 Drillfield Drive, Blacksburg, 24061, VA, USA. xwwu@vt.edu.|Virginia Bioinformatics Institute, Virginia Tech, 1015 Life Science Circle, Blacksburg, 24061, VA, USA. masun@vt.edu.|Department of Statistics, Virginia Tech, 250 Drillfield Drive, Blacksburg, 24061, VA, USA. hongxiao@vt.edu.|Virginia Bioinformatics Institute, Virginia Tech, 1015 Life Science Circle, Blacksburg, 24061, VA, USA. davidxie@vt.edu.