Lack of CCDC146, a ubiquitous centriole and microtubule-associated protein, leads to non-syndromic male infertility in human and mouse
Genetic mutations are a recurrent cause of male infertility. Multiple morphological abnormalities of the flagellum (MMAF) syndrome is a heterogeneous genetic disease, with which more than 50 genes have been linked. Nevertheless, for 50% of patients with this condition, no genetic cause is identified. From a study of a cohort of 167 MMAF patients, pathogenic bi-allelic mutations were identified in the CCDC146 gene in two patients. This gene encodes a poorly characterized centrosomal protein which we studied in detail here. First, protein localization was studied in two cell lines. We confirmed the centrosomal localization in somatic cells and showed that the protein also presents multiple microtubule-related localizations during mitotic division, suggesting that it is a microtubule-associated protein (MAP). To better understand the function of the protein at the sperm level, and the molecular pathogenesis of infertility associated with CCDC146 mutations, two genetically modified mouse models were created: a Ccdc146 knock-out (KO) and a knock-in (KI) expressing a HA-tagged CCDC146 protein. KO male mice were completely infertile, and sperm exhibited a phenotype identical to our two MMAF patients phenotype with CCDC146 mutations. No other pathology was observed, and the animals were viable. CCDC146 expression starts during late spermiogenesis, at the time of flagellum biogenesis. In the spermatozoon, the protein is conserved but is not localized to centrioles, unlike in somatic cells, rather it is present in the axoneme at the level of microtubule doublets. Expansion microscopy associated with the use of the detergent sarkosyl to solubilize microtubule doublets, suggest that the protein may be a microtubule inner protein (MIP). At the subcellular level, the absence of CCDC146 affected the formation, localization and morphology of all microtubule-based organelles such as the manchette, the head-tail coupling apparatus (HTCA), and the axoneme. Through this study, we have characterized a new genetic cause of infertility, identified a new factor in the formation and/or structure of the sperm axoneme, and demonstrated that the CCDC146 protein plays several cellular roles, depending on the cell type and the stages in the cell cycle.