Nkx2-5 and Sarcospan genetically interact in the development of the muscular ventricular septum of the heart.

The muscular ventricular septum separates the flow of oxygenated and de-oxygenated blood in air-breathing vertebrates. Defects within it, termed muscular ventricular septal defects (VSDs), are common, yet less is known about how they arise than rarer heart defects. Mutations of the cardiac transcription factor NKX2-5 cause cardiac malformations, including muscular VSDs. We describe here a genetic interaction between Nkx2-5 and Sarcospan (Sspn) that affects the risk of muscular VSD in mice. Sspn encodes a protein in the dystrophin-glycoprotein complex. Sspn knockout (SspnKO) mice do not have heart defects, but Nkx2-5+/-/SspnKO mutants have a higher incidence of muscular VSD than Nkx2-5+/- mice. Myofibers in the ventricular septum follow a stereotypical pattern that is disrupted around a muscular VSD. Subendocardial myofibers normally run in parallel along the left ventricular outflow tract, but in the Nkx2-5+/-/SspnKO mutant they commonly deviate into the septum even in the absence of a muscular VSD. Thus, Nkx2-5 and Sspn act in a pathway that affects the alignment of myofibers during the development of the ventricular septum. The malalignment may be a consequence of a defect in the coalescence of trabeculae into the developing ventricular septum, which has been hypothesized to be the mechanistic basis of muscular VSDs.

Association of genes with physiological functions by comparative analysis of pooled expression microarray data.

The physiological functions of a tissue in the body are carried out by its complement of expressed genes. Genes that execute a particular function should be more specifically expressed in tissues that perform the function. Given this premise, we mined public microarray expression data to build a database of genes ranked by their specificity of expression in multiple organs. The database permitted the accurate identification of genes and functions known to be specific to individual organs. Next, we used the database to predict transcriptional regulators of brown adipose tissue (BAT) and validated two candidate genes. Based upon hypotheses regarding pathways shared between combinations of BAT or white adipose tissue (WAT) and other organs, we identified genes that met threshold criteria for specific or counterspecific expression in each tissue. By contrasting WAT to the heart and BAT, the two most mitochondria-rich tissues in the body, we discovered a novel function for the transcription factor ESRRG in the induction of BAT genes in white adipocytes. Because the heart and other estrogen-related receptor gamma (ESRRG)-rich tissues do not express BAT markers, we hypothesized that an adipocyte co-regulator acts with ESRRG. By comparing WAT and BAT to the heart, brain, kidney and skeletal muscle, we discovered that an isoform of the transcription factor sterol regulatory element binding transcription factor 1 (SREBF1) induces BAT markers in C2C12 myocytes in the presence of ESRRG. The results demonstrate a straightforward bioinformatic strategy to associate genes with functions. The database upon which the strategy is based is provided so that investigators can perform their own screens.

Complex trait analysis of ventricular septal defects caused by Nkx2-5 mutation.

BACKGROUND: The occurrence of a congenital heart defect has long been thought to have a multifactorial basis, but the evidence is indirect. Complex trait analysis could provide a more nuanced understanding of congenital heart disease. METHODS AND RESULTS: We assessed the role of genetic and environmental factors on the incidence of ventricular septal defects (VSDs) caused by a heterozygous Nkx2-5 knockout mutation. We phenotyped >3100 hearts from a second-generation intercross of the inbred mouse strains C57BL/6 and FVB/N. Genetic linkage analysis mapped loci with lod scores of 5 to 7 on chromosomes 6, 8, and 10 that influence the susceptibility to membranous VSDs in Nkx2-5(+/-) animals. The chromosome 6 locus overlaps one for muscular VSD susceptibility. Multiple logistic regression analysis for environmental variables revealed that maternal age is correlated with the risk of membranous and muscular VSD in Nkx2-5(+/-) but not wild-type animals. The maternal age effect is unrelated to aneuploidy or a genetic polymorphism in the affected individuals. The risk of a VSD is not only complex but dynamic. Whereas the effect of genetic modifiers on risk remains constant, the effect of maternal aging increases over time. CONCLUSIONS: Enumerable factors contribute to the presentation of a congenital heart defect. The factors that modify rather than cause congenital heart disease substantially affect risk in predisposed individuals. Their characterization in a mouse model offers the potential to narrow the search space in human studies and to develop alternative strategies for prevention.