A. Overexpression of SENP2 in the presence of SUMO-1 haploinsufficiency led to embryonic lethality. The compound SENP2-Tg/SUMO-1+/− mice obtained from crossbreeding between SENP2-Tg mice (line #2839) and SUMO-1+/− mice were present at lower P1 frequency than the expected Mendelian rate (25%). The P1 frequency was analyzed from 7 litters comprising a total of 49 animals. The number shown above each bar indicates the P1 frequency of each corresponding group. **, P<0.01. Chi-square test was used for statistical analysis. B. The transcripts of transgene flag-tagged SUMO-1 were detected in both atria and ventricles of SUMO-1-Tg mouse hearts at E16.5 but not in the WT littermate hearts. GAPDH was used as a control. C. Mortality rate of double SUMO-1-Tg/SENP2-Tg mice was substantially decreased compared with that of single SENP2-Tg mice. Rescue data from line #2839 were compiled from 15 litters of 97 animals in total. Fisher's Exact Probability Test was used for statistical analysis. D. Improved modification of SUMO-1 in double SUMO-1-Tg/SENP2-Tg mouse heart. Western blots were performed on heart extracts from the WT, SENP2-Tg, SUMO-1-Tg and double SUMO-1-Tg/SENP2-Tg mice. Compare SUMO-1 conjugation (upper panel) in lane 2 and lane 4. GAPDH (lower panel) served as a control. E. Decreased penetrance of cardiac defects in the double SUMO-1-Tg/SENP2-Tg mice. The data were compiled from 76 available animals of 10 litters with ages ranging from E14.5 to P2. Fisher's Exact Probability Test was used for statistical analysis. The number shown above each bar indicates the incidence of cardiac defects of that corresponding group. F–G. Improved cardiomyocyte proliferation in the double SUMO-1-Tg/SENP2-Tg mouse hearts. BrdU staining was performed on sections of E14.5 embryonic hearts of the WT, SUMO-1-Tg, SENP2-Tg and double SUMO-1-Tg/SENP2-Tg mice, respectively (F) and statistical analysis was shown in G. n=4 for each group. Representative data were shown. Magnification, 20×. Bar, 200 μm.