Aberrant cell segregation in the craniofacial primordium and the emergence of facial dysmorphology in craniofrontonasal syndrome

Abstract

Craniofrontonasal syndrome (CFNS) is a rare X-linked disorder characterized by craniofacial, skeletal, and neurological anomalies and is caused by mutations in EFNB1. Heterozygous females are more severely affected by CFNS than hemizygous males, a phenomenon called cellular interference that results from EPHRIN-B1 mosaicism. In Efnb1 heterozygous mice, mosaicism for EPHRIN-B1 results in cell sorting and more severe phenotypes than Efnb1 hemizygous males, but how craniofacial dysmorphology arises from cell segregation is unknown and CFNS etiology therefore remains poorly understood. Here, we couple geometric morphometric techniques with temporal and spatial interrogation of embryonic cell segregation in mouse mutant models to elucidate mechanisms underlying CFNS pathogenesis. By generating EPHRIN-B1 mosaicism at different developmental timepoints and in specific cell populations, we find that EPHRIN-B1 regulates cell segregation independently in early neural development and later in craniofacial development, correlating with the emergence of quantitative differences in face shape. Whereas specific craniofacial shape changes are qualitatively similar in Efnb1 heterozygous and hemizygous mutant embryos, heterozygous embryos are quantitatively more severely affected, indicating that Efnb1 mosaicism exacerbates loss of function phenotypes rather than having a neomorphic effect. Notably, neural tissue-specific disruption of Efnb1 does not appear to contribute to CFNS craniofacial dysmorphology, but its disruption within neural crest cell-derived mesenchyme results in phenotypes very similar to widespread loss. EPHRIN-B1 can bind and signal with EPHB1, EPHB2, and EPHB3 receptor tyrosine kinases, but the signaling partner(s) relevant to CFNS are unknown. Geometric morphometric analysis of an allelic series of Ephb1; Ephb2; Ephb3 mutant embryos indicates that EPHB2 and EPHB3 are key receptors mediating Efnb1 hemizygous-like phenotypes, but the complete loss of EPHB1-3 does not fully

The First Description of Monozygotic Twin Females Discordant for the Craniofrontonasal Syndrome Phenotype and the Report of Four Novel Pathogenic Variants in the EFNB1 Gene

“…Furthermore, rarely reported mosaic male individuals are more severely affected than their hemizygous counterparts. This is because other ephrin family members can presumably substitute the complete lack of ephrin-B1 in purely hemizygous males (3,7,17). In the medical literature, we have found merely one description of mildly affected CFNS female patient.…”

“…Further reports describing more severely affected males, who all were mosaic for deleterious variants in the EFNB1, strengthen the hypothesis about the described pathomechanism's biological relevance (8). However, the precise molecular explanation for this phenomenon remains not yet fully understood (7).…”

Clinical and molecular characterization of craniofrontonasal syndrome: new symptoms and novel pathogenic variants in the EFNB1 gene

“…Wieacker and Wieland in 2005 explained the above paradox as a cellular interference, which assumes that due to a random X-inactivation, heterozygous females are uniquely mosaic and therefore have both functional and nonfunctional ephrin-B1, a protein which is encoded by the EFNB1 gene [ 5 ]. These two ephrin-B1 forms’ coexistence affects the adhesion and sorting of cells, disrupting normal embryological development [ 6 , 7 ]. Further reports describing more severely affected males, who all were mosaic for deleterious variants in the EFNB1 , strengthen the hypothesis about the described pathomechanism’s biological relevance [ 8 ].…”

“…Further reports describing more severely affected males, who all were mosaic for deleterious variants in the EFNB1 , strengthen the hypothesis about the described pathomechanism’s biological relevance [ 8 ]. However, the precise molecular explanation for this phenomenon remains not yet fully understood [ 7 ].…”

A Novel Nonsense Mutation in ARR4 Leads to X-linked High Myopia: A Genetic Paradox

“…Further proof is needed in the future. In Craniofrontonasal syndrome, the female heterozygote of EFNB1 gene located on X chromosome is usually patient, but the non-mosaic hemizygous male is usually not affected or has only mild symptoms (Niethamer et al, 2020). Some studies believe that this is a kind of "cellular interaction" caused by the mosaic state of EPHRIN-B1 protein, and the mechanism is not clear (Twigg et al, 2004;Wieacker & Wieland, 2005;Wieland et al, 2004).…”