How well do you know your mutation? Complex effects of genetic background on expressivity, complementation, and ordering of allelic effects.

For a given gene, different mutations influence organismal phenotypes to varying degrees. However, the expressivity of these variants not only depends on the DNA lesion associated with the mutation, but also on factors including the genetic background and rearing environment. The degree to which these factors influence related alleles, genes, or pathways similarly, and whether similar developmental mechanisms underlie variation in the expressivity of a single allele across conditions and among alleles is poorly understood. Besides their fundamental biological significance, these questions have important implications for the interpretation of functional genetic analyses, for example, if these factors alter the ordering of allelic series or patterns of complementation. We examined the impact of genetic background and rearing environment for a series of mutations spanning the range of phenotypic effects for both the scalloped and vestigial genes, which influence wing development in Drosophila melanogaster. Genetic background and rearing environment influenced the phenotypic outcome of mutations, including intra-genic interactions, particularly for mutations of moderate expressivity. We examined whether cellular correlates (such as cell proliferation during development) of these phenotypic effects matched the observed phenotypic outcome. While cell proliferation decreased with mutations of increasingly severe effects, surprisingly it did not co-vary strongly with the degree of background dependence. We discuss these findings and propose a phenomenological model to aid in understanding the biology of genes, and how this influences our interpretation of allelic effects in genetic analysis.

ß-Catenin/Smad3 Interaction Regulates Transforming Growth Factor-ß-Induced Epithelial to Mesenchymal Transition in the Lens.

Cataracts are the leading cause of blindness worldwide. Although surgery is a successful method to restore vision loss due to cataracts, post-surgical complications can occur, such as secondary cataracts, also known as posterior capsular opacification (PCO). PCO arises when lens epithelial cells (LEC) are left behind in the capsular bag following surgery and are induced to undergo epithelial to mesenchymal transition (EMT). Following EMT, LEC morphology and phenotype are altered leading to a loss of transparency and vision. Transforming growth factor (TGF)-ß-induced signaling through both canonical, TGF-ß/Smad, and non-canonical, ß-catenin/Wnt and Rho/ROCK/MRTF-A, pathways have been shown to be involved in lens EMT, and thus PCO. However, the interactions between these signaling pathways in the lens have not been thoroughly explored. In the current study we use rat LEC explants as an ex vivo model, to examine the interplay between three TGF-ß-mediated pathways using α-smooth muscle actin (α-SMA) as a molecular marker for EMT. We show that Smad3 inhibition via SIS3 prevents nuclear translocation of ß-catenin and MRTF-A, and α-SMA expression, suggesting a key role of Smad3 in regulation of MRTF-A and ß-catenin nuclear transport in LECs. Further, we demonstrate that inhibition of ß-catenin/CBP interaction by ICG-001 decreased the amount of phosphorylated Smad3 upon TGF-ß stimulation in addition to significantly decreasing the expression levels of TGF-ß receptors, TBRII and TBRI. Overall, our findings demonstrate interdependence between the canonical and non-canonical TGF-ß-mediated signaling pathways controlling EMT in the lens.