Biallelic Variants in Plexin B2 (PLXNB2) Cause Amelogenesis Imperfecta, Hearing Loss and Intellectual Disability

BackgroundPlexins are large transmembrane receptors for the semaphorin family of signalling proteins. Semaphorin-plexin signalling controls cellular interactions that are critical during development as well as in adult life stages. Nine plexin genes have been identified in humans, but despite the apparent importance of plexins in development, only biallelicPLXND1andPLXNA1variants have so far been associated with Mendelian genetic disease.MethodsEight individuals from six families presented with a recessively inherited variable clinical condition, with core features of amelogenesis imperfecta (AI) and sensorineural hearing loss (SNHL), with variable intellectual disability. Probands were investigated by exome or genome sequencing. Common variants and those unlikely to affect function were excluded. Variants consistent with autosomal recessive inheritance were prioritised. Variant segregation analysis was performed by Sanger sequencing. RNA expression analysis was conducted in C57Bl6 mice.ResultsRare biallelic pathogenic variants in plexin B2 (PLXNB2), a large transmembrane semaphorin receptor protein, were found to segregate with disease in all six families. The variants identified include missense, nonsense, splicing changes and a multiexon deletion.Plxnb2expression was detected in differentiating ameloblasts.ConclusionWe identify rare biallelic pathogenic variants inPLXNB2as a cause of a new autosomal recessive, phenotypically diverse syndrome with AI and SNHL as core features. Intellectual disability, ocular disease, ear developmental abnormalities and lymphoedema were also present in multiple cases. The variable syndromic human phenotype overlaps with that seen inPlxnb2knockout mice, and, together with the rarity of humanPLXNB2variants, may explain why pathogenic variants inPLXNB2have not been reported previously.