De NovoVariants inMRTFBhave gain of function activity inDrosophilaand are associated with a novel neurodevelopmental phenotype with dysmorphic features

Myocardin-Related Transcription Factor B ( MRTFB ) is an important transcriptional regulator which promotes the activity of an estimated 300 genes during different stages of development. Here we report two pediatric probands with de novo variants in MRTFB (R104G and A91P) and mild dysmorphic features, intellectual disability, global developmental delays, speech apraxia, and impulse control issues. As the MRTFB protein is highly conserved between vertebrate and invertebrate model organisms, we generated a humanized Drosophila model expressing the human MRTFB protein in the same spatial and temporal pattern as the fly gene. Expression of the human MRTFB R104G variant using a mrtf-T2A-GAL4 line proved to be embryonic lethal. Additional phenotypes were also identified by expressing the MRTFB R104G and MRTFB A91P variant in a subset of Drosophila tissues. Notably, expression within wing tissues resulted in an expansion of intervein tissue, wing vein thickening, shortening or loss of wing veins, and blistering. The MRTFB R104G and MRTFB A91P variants also display a decreased level of actin binding within critical RPEL domains, resulting in increased transcriptional activity and changes in the organization of the Actin cytoskeleton. These changes were not observed in flies expressing two additional candidate variants, MRTFB N95S and MRTFB R109Q , highlighting that the location of the mutation within the 2nd RPEL domain is critical to the pathogenicity of the variant. These changes suggest that the MRTFB R104G and MRTFB A91P alleles we have identified affect the regulation of the protein and that these variants in MRTFB underly a novel neurodevelopmental disorder.