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Purpose: Primary ciliary dyskinesia (PCD) is a heterogeneous disorder that includes respiratory symptoms, laterality defects, and infertility caused by dysfunction of motile cilia. Most PCD-causing variants result in abnormal outer dynein arms (ODAs), which provide the generative force for respiratory ciliary beating and proper mucociliary clearance., Methods: In addition to studies in mouse and planaria, clinical exome sequencing and functional analyses in human were performed., Results: In this study, we identified homozygous pathogenic variants in CLXN (EFCAB1/ODAD5) in 3 individuals with laterality defects and respiratory symptoms. Consistently, we found that Clxn is expressed in mice left-right organizer. Transmission electron microscopy depicted ODA defects in distal ciliary axonemes. Immunofluorescence microscopy revealed absence of CLXN from the ciliary axonemes, absence of the ODA components DNAH5, DNAI1, and DNAI2 from the distal axonemes, and mislocalization or absence of DNAH9. In addition, CLXN was undetectable in ciliary axonemes of individuals with defects in the ODA-docking machinery: ODAD1, ODAD2, ODAD3, and ODAD4. Furthermore, SMED-EFCAB1-deficient planaria displayed ciliary dysmotility., Conclusion: Our results revealed that pathogenic variants in CLXN cause PCD with defects in the assembly of distal ODAs in the respiratory cilia. CLXN should be referred to as ODA-docking complex-associated protein ODAD5., Competing Interests: Conflict of Interest The authors declare no conflicts of interest., (Copyright © 2023 American College of Medical Genetics and Genomics. Published by Elsevier Inc. All rights reserved.)

Purpose: The clinical spectrum of motile ciliopathies includes laterality defects, hydrocephalus, and infertility as well as primary ciliary dyskinesia when impaired mucociliary clearance results in otosinopulmonary disease. Importantly, approximately 30% of patients with primary ciliary dyskinesia lack a genetic diagnosis., Methods: Clinical, genomic, biochemical, and functional studies were performed alongside in vivo modeling of DAW1 variants., Results: In this study, we identified biallelic DAW1 variants associated with laterality defects and respiratory symptoms compatible with motile cilia dysfunction. In early mouse embryos, we showed that Daw1 expression is limited to distal, motile ciliated cells of the node, consistent with a role in left-right patterning. daw1 mutant zebrafish exhibited reduced cilia motility and left-right patterning defects, including cardiac looping abnormalities. Importantly, these defects were rescued by wild-type, but not mutant daw1, gene expression. In addition, pathogenic DAW1 missense variants displayed reduced protein stability, whereas DAW1 loss-of-function was associated with distal type 2 outer dynein arm assembly defects involving axonemal respiratory cilia proteins, explaining the reduced cilia-induced fluid flow in particle tracking velocimetry experiments., Conclusion: Our data define biallelic DAW1 variants as a cause of human motile ciliopathy and determine that the disease mechanism involves motile cilia dysfunction, explaining the ciliary beating defects observed in affected individuals., Competing Interests: Conflict of Interest The authors declare no conflicts of interest., (Copyright © 2022 The Authors. Published by Elsevier Inc. All rights reserved.)