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

AbstractMyocardin-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 withde novovariants inMRTFB(R104G and A91P) and mild dysmorphic features, intellectual disability, global developmental delays, speech apraxia, and impulse control issues. As theMRTFBprotein is highly conserved between vertebrate and invertebrate model organisms, we generated a humanizedDrosophilamodel expressing the humanMRTFBprotein in the same spatial and temporal pattern as the fly gene. Expression of the humanMRTFBR104Gvariant using amrtf-T2A-GAL4line proved to be embryonic lethal. Additional phenotypes were also identified by expressing theMRTFBR104GandMRTFBA91Pvariant in a subset ofDrosophilatissues. Notably, expression within wing tissues resulted in an expansion of intervein tissue, wing vein thickening, shortening or loss of wing veins, and blistering. TheMRTFBR104GandMRTFBA91Pvariants 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,MRTFBN95SandMRTFBR109Q, highlighting that the location of the mutation within the 2nd RPEL domain is critical to the pathogenicity of the variant. These changes suggest that theMRTFBR104GandMRTFBA91Palleles we have identified affect the regulation of the protein and that these variants inMRTFBunderly a novel neurodevelopmental disorder.