Connecting centrosomes, cilia and the DNA damage response

The suppression of genome instability by the DNA damage response (DDR) is critical for preventing human diseases characterized by developmental defects, infertility and in some cases, elevated cancer predisposition. After DNA damage, the activation of the related ATM and ATR kinases is crucial to amplify a signal transduction cascade required for the proper cellular responses to ensure genomic integrity. Mutations in ATR and other genes involved in the DDR have been identified in patients with Seckel syndrome, that is characterized by microcephaly and dwarfism. As defects in many centrosomal proteins also underlie Seckel Syndrome, it has been suggested that a crosstalk between the DDR and centrosome may be important. Supporting this idea, several MCPH/Seckel proteins, such as MCPH1, CEP152, and CEP63 have been implicated in both centrosomal and DDR functions and many DNA repair factors have been identified at the centrosome. In this thesis, we describe the in vivo functional characterization of two proteins implicated in the DDR, CEP63 and GEMC1. We found that they both have distinct critical roles in preventing multisystem pathology in mice. CEP63 is a centrosome protein that facilitates centriole duplication through the recruitment of CEP152 in the centrosome and has been implicated in the DDR as an ATR/ATM target gene in Xenopus. Human CEP63 mutations cause Seckel syndrome, characterized by growth retardation, microcephaly and mental retardation, although the pathological outcomes are milder than those associated with mutations in ATR. In mice, deficiency in Cep63 leads to microcephaly due to the attrition of neural progenitor cells. Using genetic analysis, we showed that this was through a p53-mediated cell death pathway triggered by centrosome-based mitotic errors and independent of the ATM and CHK2 kinases, that activate p53 after DNA damage. GEMC1, a protein that belongs to the Geminin superfamily together with MCIDAS, was originally identified in Xenopus as a pro-replication factor. The characterization of mice lacking GEMC1 revealed its crucial role in regulating two differentiation programs in mammals, multiciliogenesis and spermatogenesis. Gemc1-deficient mice are growth impaired, develop hydrocephaly and are infertile due to defects in the formation of multiciliated epithelial cells in the brain, respiratory tract, oviducts and efferent ducts of the epididymis. We demonstrated that GEMC1 acts at the top of the transcriptional cascade that drives multiciliogenesis and, like MCIDAS, it controls transcription through the association with E2F4/5 and the cofactor DP1. In addition, we found that Gemc1-deficient mice are infertile due to defects in the terminal differentiation of spermatozoa, a process known as spermiogenesis. Although no patients harboring GEMC1 mutations have been identified to date, we believe that Gemc1 is a good candidate for the rare mucociliary clearance disorder referred as Reduced Generation of Multiple Motile Cilia (RGMC) cauterized by defects in multiciliated cell development. To date, mutations in only two genes have been implicated in RGMC in humans, MCIDAS and CCNO. Thus, the generation of these animal models has provided new insights into the molecular functions of CEP63 and GEMC1 in many tissues and expanded our knowledge of the etiology of pathologies associated with rare human diseases.