The Origin of EFNB1 Mutations in Craniofrontonasal Syndrome: Frequent Somatic Mosaicism and Explanation of the Paucity of Carrier Males

Abstract

Craniofrontonasal syndrome (CFNS) is an X-linked disorder that exhibits a paradoxical sex reversal in phenotypic severity: females characteristically have frontonasal dysplasia, craniosynostosis, and additional minor malformations, but males are usually mildly affected with hypertelorism only. Despite this, males appear underrepresented in CFNS pedigrees, with carrier males encountered infrequently compared with affected females. To investigate these unusual genetic features of CFNS, we exploited the recent discovery of causative mutations in the EFNB1 gene, which encodes ephrin-B1, to survey the molecular alterations in 59 families (39 newly investigated and 20 published elsewhere). We identified the first complete deletions of EFNB1, catalogued 27 novel intragenic mutations, and used Pyrosequencing and analysis of nearby polymorphic alleles to quantify mosaic cases and to determine the parental origin of verified germline mutations. Somatic mosaicism was demonstrated in 6 of 53 informative families, and, of 17 germline mutations in individuals for whom the parental origin of mutation could be demonstrated, 15 arose from the father. We conclude that the major factor accounting for the relative scarcity of carrier males is the bias toward mutations in the paternal germline (which present as affected female offspring) combined with reduced reproductive fitness in affected females. Postzygotic mutations also contribute to the female preponderance, whereas true nonpenetrance in males who are hemizygous for an EFNB1 mutation appears unusual. These results highlight the importance of considering possible origins of mutation in the counseling of families with CFNS and provide a generally applicable approach to the combined analysis of mosaic and germline mutations.

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

“…This probably explains the majority of the B20% of apparent CFNS patients without an EFNB1 mutation. 21,23,25 The results from this study make the diagnosis of some patients presented in literature therefore doubtful, based on their different facial proportions and dissimilar phenotype. 7,14,30 In other studies, some patients were classified as having frontonasal dysplasia, whereas they actually match the typical phenotype of CFNS.…”

“…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.…”

“…[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.…”

“…10 An explanation for the few severely affected males reported in literature [6][7][8]32 could be a mosaicism in these patients, in which the wild type to mutant ratio should be similar to that in heterozygous CFNS females. 23,26,33 Additional mechanisms were recently added to the phenotypic manifestation. Not only cellular interference, but also an impaired signaling capacity of ephrin-B1 and improper regulation of gap junctional communication could be responsible for the pathogenic process in CFNS expression.…”

“…26 Taken together, around 20% of the patients screened for CFNS did not display a mutation in the EFNB1 gene. 21,23,25 Multiple explanations have been proposed, one of which is misdiagnosis of some of the included patients. 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.…”