Abstract
Abstract only Congenital heart defects (CHD), which represent the most common type of congenital defects, are found in 8 out of 1000 births each year in the US. Aneuploidy, defined as an abnormal number of chromosomes in a cell, is the largest contributor to CHD. Defects in mitotic regulators can promote aneuploidy. For instance, mutations in the mitotic checkpoint protein BubR1 can lead to mosaic variegated aneuploidy (MVA). MVA patients display varied phenotypes including growth deficiency and a shortened lifespan. In addition, some patients develop CHDs such as atrioventricular septal defects. To determine whether BubR1 regulates cardiac development we carried out timed matings to harvest heart tissue at various embryonic stages. We found that BubR1 expression dramatically increases in the mouse heart through embryonic development. We engineered a conditional BubR1 knockout mouse model using the Cre-lox system which allows for the generation of cardiac-specific BubR1 knockout animals when crossing this BubR1 fl/fl mice to Nkx2.5-cre mice, which mediates deletion of BubR1 beginning at E7.5. Interestingly, we did not observe any homozygous cardiac-specific BubR1 knockout mice in newborn litters, suggesting that loss of BubR1 in the developing heart is embryonic lethal (n = 120, P < 0.01). We observed that deletion of BubR1 in the embryonic heart leads to cardiac malformations including the absence of trabeculations and reduction in the size of ventricles. Immunofluorescence studies reveal that cardiac-specific BubR1 knockout embryos have an increase in cell death as measured by the apoptosis marker cleaved-caspase 3. These embryos also show an increase in the apoptosis regulator CaMKII, whose upregulation has been previously tied to an increase in cardiomyocyte apoptosis. Therefore, we conclude that BubR1 suppresses the expression of CaMKII in the developing heart to regulate apoptosis during cardiogenesis. Taken together, our data suggest that BubR1 is a critical regulator for cardiac development in vivo , and its mutation or loss may contribute to the development of CHDs.