Relationship of Phenotypic Variation with Mechanisms of Craniofacial Development in Two Connexin‐43 Mutant Mouse Models

Background The skull is complex in its development and function. This complexity makes it vulnerable to disruptive forces, and upwards of one third of congenital abnormalities in humans involve the craniofacial complex. Abnormalities in the skull can be viewed as extreme examples of variation, and it is generally assumed that both normal and extreme variation has a developmental basis. If this assumption is correct, then we expect that patterns of variation will be similarly structured if the developmental inputs are similar, and that phenotypic variation can then be used to uncover potential developmental disruptions in cases of aberrant development. The focus of this study is to test the hypothesis that phenotypic variation is structured through developmental processes using mouse models of normal and abnormal skull phenotypes. Methods In humans, reduced connexin‐43 (Cx43) function caused by mutations results in distinct anomalies of the facial skeleton, and mouse models of reduced Cx43 function display a similar phenotype. We use two mouse models with mutations to the Gja1 gene encoding for the gap‐junctional protein connexin‐43 (Cx43 I130T/+ : 50% channel function, Cx43 G60S/+ : 15–20% channel function) to test the relationship between connexin‐43 function, and variation. Geometric morphometric analyses were done on 3D landmark data from μCT scans of mutant skulls (20μm) and their wild type littermates at post‐natal day zero and three months of age (N=30) to assess and compare mean shape, phenotypic variation and covariation among genotypes. Results As expected, both mutant models exhibit significantly altered skull morphology and greater phenotypic variation at both P0 and three months. While morphological changes and variation are more severe in the Cx43 G60S/+ mice than the Cx43 I130T/+ mice, the mean phenotypic changes and patterns of variation are similar in both mutants. Unexpectedly, the greatest phenotypic changes are within the cranial base in both mutants and at both time points, whereas changes to facial morphology are only evident in the three month mice. Significance Our findings indicate that reduced Cx43 function causes distinct skull anomalies in a dosage‐dependent manner. Given that the same skull structures are disrupted in both of our mutants, that variation is increased in both mutants with dosage effect and that this variation is structured the same in each model, our results support the hypothesis that phenotypic variation is structured through developmental processes. Additionally, our results uncovered the previously undocumented involvement of the cranial base, and the relatively late‐timing of alterations to the facial skeleton of adult mice in response to reduced Cx43 function. These findings potentially point to two skeletal disruptions caused by reduced Cx43: 1. An early disruption of skeletal morphogenesis of the cranial base and 2. Altered bone remodelling in the facial skeleton. This study bolsters the idea that study of phenotypic variation is a useful tool to help us uncover and pinpoint developmental disruptions. Support or Funding Information Research was funding through a grant from NSERC This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .

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