Affiliations: 1) Cleveland Clinic Lerner Research Institute; 2) Cleveland Clinic Lerner College of Medicine of Case Western Reserve University
Keywords: o. stem cells; c. neural stem cells
Microcephaly is a rare disorder in which babies are born with head sizes that are approximately three standard deviations below the mean. Mutations in twenty-five proteins (termed microcephaly proteins), including three condensin subunits, have been shown to cause microcephaly in humans and in mouse models. However, the mechanisms by which microcephaly protein depletion and/or dysregulation leads to the development of microcephaly are not well understood. Condensins are conserved, multi-subunit complexes that are important for regulating chromosome organization. Our lab discovered that condensins also repress the expression and movement of retrotransposons in both Drosophila melanogaster and in human cells.
To determine whether increased retrotransposon expression and activity might contribute to the development of condensin-deficient microcephaly, we first used the UAS-GAL4 system to deplete condensin proteins in specific cell types of the developing Drosophila brain. Depletion of various condensin proteins in stem cells (neuroblasts), but not glial cells or post-mitotic cells resulted in significantly smaller adult brains and adult heads. We observed increased transcript levels of several families of retrotransposons and endogenous retroviruses (ERVs) in larval brains deficient for the condensin II subunit dCAP-D3. Experiments using gypsy-CLEVR reporter stocks demonstrated that replication of the gypsy retrotransposable element/ ERV is prevalent in wild-type larval neuroblasts, and is significantly increased in dCAP-D3-deficient neuroblasts. Further, dCAP-D3-deficient larval neuroblasts exhibited increased lagging chromosomes, a phenotype observed in microcephaly patients harboring condensin mutations. dCAP-D3-deficient pupal brains also exhibit increased cell death. Excitingly, condensin-deficient microcephaly was almost completely rescued by allowing flies to develop on food containing Nucleoside Reverse Transcriptase Inhibitors (NRTIs), which inhibit retrotransposition and ERV replication. Together, these findings suggest that condensins may repress retrotransposon and ERV expression and activity in the developing Drosophila brain to prevent microcephaly. We will also present data which suggests that other microcephaly proteins may be involved in these processes.