Uncovering phenotypic expansion in AXIN2-related disorders through precision animal modeling.
PURPOSE: Heterozygous pathogenic variants in AXIN2 (HGNC: 904) cause oligodontia-colorectal cancer syndrome. We identified 5 individuals with de novo heterozygous variants [NM_004655.4:c.196G>A p.(Glu66Lys), c.197A>G p.(Glu66Gly), and c.199G>A p.(Gly67Arg)] in AXIN2. Common phenotypes among these individuals included ectodermal dysplasia, global developmental delay, microcephaly, and limb, ophthalmologic, and genitourinary abnormalities. METHODS: Structural modeling was performed to predict the impact of these variants on AXIN2. A prime editing N1 screen of mouse embryos was performed to test whether the p.Glu66Lys variant produces a phenotype. Drosophila models were used to test the effect of this variant on Wnt signaling. RESULTS: Structural modeling suggests that these variants disrupt AXIN2 binding to tankyrase, which regulates AXIN2 levels through poly-ADP-ribosylation. Heterozygous (p.Glu66Lys) mouse embryos were perinatally lethal with soft palate clefts and skeletal abnormalities. Modeling of the p.Glu66Lys variant in the Drosophila wing suggests gain-of-function or dominant-negative activity compared to reference AXIN2. CONCLUSION: Specific variants in the tankyrase-binding domain of AXIN2 are pathogenic, leading to phenotypic expansion with potential context-dependent effects on AXIN2 function and Wnt signaling. The N1 modeling strategy used to demonstrate variant pathogenicity may be beneficial for resolving other heterozygous variants associated with congenital anomalies.