Mutations of ephrin-B1 (EFNB1), a marker of tissue boundary formation, cause craniofrontonasal syndrome

Abstract

Craniofrontonasal syndrome (CFNS) is an X-linked developmental disorder that shows paradoxically greater severity in heterozygous females than in hemizygous males. Females have frontonasal dysplasia and coronal craniosynostosis (fusion of the coronal sutures); in males, hypertelorism is the only typical manifestation. Here, we show that the classical female CFNS phenotype is caused by heterozygous loss-of-function mutations in EFNB1, which encodes a member of the ephrin family of transmembrane ligands for Eph receptor tyrosine kinases. In mice, the orthologous Efnb1 gene is expressed in the frontonasal neural crest and demarcates the position of the future coronal suture. Although EFNB1 is X-inactivated, we did not observe markedly skewed X-inactivation in either blood or cranial periosteum from females with CFNS, indicating that lack of ephrin-B1 does not compromise cell viability in these tissues. We propose that in heterozygous females, patchwork loss of ephrin-B1 disturbs tissue boundary formation at the developing coronal suture, whereas in males deficient in ephrin-B1, an alternative mechanism maintains the normal boundary. This is the only known mutation in the ephrin͞Eph receptor signaling system in humans and provides clues to the biogenesis of craniosynostosis.

Phenotypes of craniofrontonasal syndrome in patients with a pathogenic mutation in EFNB1

“…A few other features that were not evaluated in this study are presented in the literature: myoclonus, poor hearing, pelvic kidney, bilateral vesico-ureteral reflux, hip girdle anomalies, 11 median cleft lip/palate, 8 asymmetric mandible. 9 Additional features that have been reported in other studies of patients with EFNB1 mutations include: diaphragmatic hernia, 18,19,21,27 dysplastic clavicles and clavicle pseudoarthrosis, [8][9][10][11][12]18,20,23 accessory nipples, 10 high arched palate, 4,9-12,32 uterus arcuatus, 10 duplication of uterus, kidneys and ureters, 10 and low posterior hairline. 8,12,22 Furthermore, some studies state that CFNS patients have a normal intelligence, 8,10,12,15 whereas others claim that some may have learning difficulties to a variable degree.…”

“…[2][3][4][6][7][8][9] The mystery was unraveled by a combination of results of multiple studies. [16][17][18] The disease locus was finally claimed to be within Xq13.1 and loss of function mutations in EFNB1 were proven to cause CFNS. 10,[18][19][20][21][22][23][24][25][26][27][28][29][30] EFNB1 encodes ephrin-B1, which is a transmembrane ligand for Eph receptor tyrosine kinases.…”

“…[16][17][18] The disease locus was finally claimed to be within Xq13.1 and loss of function mutations in EFNB1 were proven to cause CFNS. 10,[18][19][20][21][22][23][24][25][26][27][28][29][30] EFNB1 encodes ephrin-B1, which is a transmembrane ligand for Eph receptor tyrosine kinases. Because of random X-inactivation heterozygous females are uniquely mosaic and by consequence a cell either does or does not produce a functional protein.…”

“…25 Studies following the discovery of the causal gene predominantly describe the location of new mutations, combined with a brief outline of phenotypic features of small families or cohorts. [18][19][20]22,27,29,30 Detailed overviews of phenotypic features of large cohorts of possible CFNS patients do exist, 3,4,[6][7][8][9][10]12,30 but it is not clear what proportion of these patients carry an EFNB1 mutation. The reports in the older literature could therefore cause confusion by reporting patients who might be improperly classified as CFNS patients.…”

Controlled levels of canonical Wnt signaling are required for neural crest migration

“…are linked with failure of cranial NC migration (Twigg et al, 2004;Merrill et al, 2006), while overexpression of cytoplasmic domain of ephrin-B2 rescues cranial NC migration in ephrin-B2 knockdown mice (Adams et al, 2001). Furthermore, in zebrafish ephrin-B2 has been shown to interact with gap junction protein; connexin-43 and regulate its distribution in migratory cranial NC cells (Mellitzer et al, 1999).…”