Mucociliary clearance (MCC) is the primary innate defense mechanism of the lung. The functional components are the protective mucous layer, the airway surface liquid layer, and the cilia on the surface of ciliated cells. The cilia are specialized organelles that beat in metachronal waves to propel pathogens and inhaled particles trapped in the mucous layer out of the airways. In health this clearance mechanism is effective, but in patients with primary cilia dyskinesia (PCD) the cilia are abnormal, resulting in deficient MCC and chronic lung disease. This demonstrates the critical importance of the cilia for human health. In this review, we summarize the current knowledge of the components of the MCC apparatus, focusing on the role of cilia in MCC.

Primary ciliary dyskinesia (PCD) is characterized by chronic airway disease, reduced fertility, and randomization of the left/right body axis. It is caused by defects of motile cilia and sperm flagella. We screened a cohort of affected individuals that lack an obvious axonemal defect for pathogenic variants using whole exome capture, next generation sequencing, and bioinformatic analysis assuming an autosomal recessive trait. We identified one subject with an apparently homozygous nonsense variant [(c.1762C>T), p.(Arg588*)] in the uncharacterized CFAP57 gene. Interestingly, the variant results in the skipping of exon 11 (58 amino acids), which may be due to disruption of an exonic splicing enhancer. In normal human nasal epithelial cells, CFAP57 localizes throughout the ciliary axoneme. Nasal cells from the PCD patient express a shorter, mutant version of CFAP57 and the protein is not incorporated into the axoneme. The missing 58 amino acids include portions of WD repeats that may be important for loading onto the intraflagellar transport (IFT) complexes for transport or docking onto the axoneme. A reduced beat frequency and an alteration in ciliary waveform was observed. Knockdown of CFAP57 in human tracheobronchial epithelial cells (hTECs) recapitulates these findings. Phylogenetic analysis showed that CFAP57 is highly conserved in organisms that assemble motile cilia. CFAP57 is allelic with the BOP2/IDA8/FAP57 gene identified previously in Chlamydomonas reinhardtii. Two independent, insertional fap57 Chlamydomonas mutant strains show reduced swimming velocity and altered waveforms. Tandem mass tag (TMT) mass spectroscopy shows that FAP57 is missing, and the “g” inner dyneins (DHC7 and DHC3) and the “d” inner dynein (DHC2) are reduced, but the FAP57 paralog FBB7 is increased. Together, our data identify a homozygous variant in CFAP57 that causes PCD that is likely due to a defect in the inner dynein arm assembly process. Primary ciliary dyskinesia (PCD) is a rare genetic disease that affects the function of motile cilia. The estimated incidence is about 1 in 15,000 births, but diagnosis can be difficult since it is genetically heterogenous. At present, about 30% of PCD patients lack a genetic diagnosis. By applying whole exome sequencing and bioinformatic analysis, we identified a variant in an uncharacterized gene, CFAP57, in a subject previously diagnosed with PCD. This is the first reported example of PCD caused by a mutation that likely only affects a subset of the inner dynein arms. These findings demonstrate the effectiveness of whole exome sequencing and bioinformatic analysis for identifying pathogenic variants in rare genetic diseases like PCD, expand our understanding of how dynein arms are positioned during cilia assembly, and identify a variant in CFAP57 that causes PCD.

Teratomas represent a critical interface between stem cells, differentiation and tumorigenesis. These tumors are composed of cell types representing all three germ layers reflecting the pluripotent nature of their cell of origin. The study of these curious tumors became possible when Leroy Stevens identified the 129 mouse strain as a model of spontaneous testicular teratoma and later isolated a substrain carrying the Ter mutation, a potent modifier of tumor incidence. Early studies with 129 mice lead to the discovery of embryonal carcinoma (EC) cells which played a foundational role in the embryonic stem (ES) cell field and the study of pluripotency. The cells of origin of testicular teratomas are germ cells. During early development, primordial germ cells diverge from somatic differentiation and establish their pluripotent nature, maintaining or re-expressing core pluripotency genes; Oct4, Sox2 and Nanog. It is believed that a misregulation of male germ cell pluripotency plays a critical role in teratoma development. Several mouse models of teratoma development have now been identified, including a chromosome substitution strain, 129-Chr19^MOLF, conditional Dmrt1 and Pten alleles and the Ter mutation in the Dnd1 gene. However, it is still unknown what role somatic cells and/or physiology play in the sensitivity to teratoma development. These unusual tumors may hold the key to understanding how pluripotency is regulated in vivo.