ABSTRACT
Primary ciliary dyskinesia (PCD) is a rare autosomal-recessive condition resulting from structural and/or functional defects of the axoneme in motile cilia and sperm flagella. The great majority of mutations identified so far involve genes whose defects result in dynein-arm anomalies. By contrast, PCD due to CC/RS defects (those in the central complex [CC] and radial spokes [RSs]), which might be difficult to diagnose, remains mostly unexplained. We identified non-ambiguous RSPH3 mutations in 5 of 48 independent families affected by CC/RS defects. RSPH3, whose ortholog in the flagellated alga Chlamydomonas reinhardtii encodes a RS-stalk protein, is mainly expressed in respiratory and testicular cells. Its protein product, which localizes within the cilia of respiratory epithelial cells, was undetectable in airway cells from an individual with RSPH3 mutations and in whom RSPH23 (a RS-neck protein) and RSPH1 and RSPH4A (RS-head proteins) were found to be still present within cilia. In the case of RSPH3 mutations, high-speed-videomicroscopy analyses revealed the coexistence of immotile cilia and motile cilia with movements of reduced amplitude. A striking feature of the ultrastructural phenotype associated with RSPH3 mutations is the near absence of detectable RSs in all cilia in combination with a variable proportion of cilia with CC defects. Overall, this study shows that RSPH3 mutations contribute to disease in more than 10% of PCD-affected individuals with CC/RS defects, thereby allowing an accurate diagnosis to be made in such cases. It also unveils the key role of RSPH3 in the proper building of RSs and the CC in humans.
