Neurodevelopment: Johnson et al.

Determining Pathogenesis of a Rare Pediatric Intellectual Disability and Progressive Microcephaly Syndrome
Katherine Johnson1, Leen Schafer2, Gene Yeo2, Ashleigh Schaffer1

1Department of Genetics and Genome Sciences, Case Western Reserve University School of Medicine, Cleveland, OH 44106, USA.
2Department of Cellular and Molecular Medicine, Stem Cell Program and Institute for Genomic Medicine, University of California San Diego, La Jolla, California, USA.

    Human mutations in nuclear proteins that regulate mRNA export have been shown to be causative for multiple rare pediatric intellectual disability and progressive microcephaly syndromes. This complex is known to facilitate nuclear to cytoplasmic mRNA export and the cellular processes of transcription elongation and genome stability; however, the function of each protein in the complex remains unknown. We aim to resolve the temporal and spatial expression of these proteins and determine their requirement during neurogenesis. We will also investigate the mechanism of disease for the known pathogenic human variants using in-vitro and in-vivo approaches. 
To determine protein expression during neurogenesis, we have developed a mouse model harboring a proximal V5 tag, which we will characterize in conjunction with brain specific cell-type markers to identify vulnerable cell types to loss of this complex during brain development. To test the requirement for these proteins during neurogenesis, we will characterize a knockout mouse model by assessing brain morphology and changes in proliferation, R-loop formation, and apoptosis. To create a cellular model to study the molecular mechanism of this syndrome,  we have generated primary mouse neuronal progenitor cells lines from multiple V5/- and V5/+ embryos. Once we have validated the phenotypes we see in-vivo are also present in the in-vitro system, we will assess mRNA export by collecting whole cell lysate RNA, as well as nuclear and cytoplasmic fractions of RNA for sequencing. If mRNA export is defective in mutant cells, we predict we will see retention of cytoplasmic RNAs within the nucleus. Since this protein complex is also known to play a role in transcriptional elongation and splicing, we may observe changes in mRNA expression levels and splicing. Overall, we aim to characterize the role of this complex in brain development, as well as characterize the pathogenic mechanisms leading to pediatric brain disease.