Homozygous <i>SMAD6</i> variants in two unrelated patients with craniosynostosis and radioulnar synostosis.

Abstract

<i>SMAD6</i> encodes an intracellular inhibitor of the bone morphogenetic protein (BMP) signalling pathway. Until now, rare heterozygous loss-of-function variants in <i>SMAD6</i> were demonstrated to increase the risk of disparate clinical disorders including cardiovascular disease, craniosynostosis and radioulnar synostosis. Only two unrelated patients harbouring biallelic <i>SMAD6</i> variants presenting a complex cardiovascular phenotype and facial dysmorphism have been described. Here, we present the first two patients with craniosynostosis harbouring homozygous <i>SMAD6</i> variants. The male probands, both born to healthy consanguineous parents, were diagnosed with metopic synostosis and bilateral or unilateral radioulnar synostosis. Additionally, one proband had global developmental delay. Echocardiographic evaluation did not reveal cardiac or outflow tract abnormalities. The novel missense (c.[584T>G];[584T>G], p.[(Val195Gly)];[(Val195Gly)]) and missense/splice-site variant (c.[817G>A];[817G>A], r.[(817g>a,817delins[a;817+2_817+228])];[(817g>a,817delins[a;817+2_817+228])], p.[(Glu273Lys,Glu273Serfs*72)];[(Glu273Lys,Glu273Serfs*72)]) both locate in the functional MH1 domain of the protein and have not been reported in gnomAD database. Functional analyses of the variants showed reduced inhibition of BMP signalling or abnormal splicing, respectively, consistent with a hypomorphic mechanism of action. Our data expand the spectrum of variants and phenotypic spectrum associated with homozygous variants of <i>SMAD6</i> to include craniosynostosis.