Submission Details

Primary Ciliary Dyskinesia (PCD) is a genetically heterogeneous group of disorders, usually beginning in early childhood, characterized by chronic cough, recurrent infections of the upper and lower respiratory tract, randomization of left/right body symmetry, and subfertility (PMID: 32943623). A subset of PCD cases have situs abnormalities and/or complex congenital heart defects (PMID: 17515466). The PCD phenotype results from structural and/or functional abnormalities of motile cilia and flagella. Ciliary motility is powered by a set of axoneme-specific dynein motor complexes. The cytoplasmic assembly of these large multiprotein complexes is assisted by dynein axonemal assembly factors and their subsequent transport into cilia is facilitated by proteins involved in the intraflagellar transport (IFT) system. Variants in genes encoding proteins involved in the assembly or transport of these complexes can result in the improper assembly of dynein chains or impaired transport of complexes into the axoneme, causing defects in outer dynein arms (ODAs) and/or inner dynein arms (IDAs) that can result in ciliary immotility or dysfunction (PMID: 31624012).

The DAW1 (Dynein Assembly Factor with WD Repeats 1) gene, also referred to as ODA16 and WDR69, encodes a transport adaptor for the multi-subunit IFT-B1 complex, facilitating the efficient transport of ODAs along the axoneme (PMIDs: 18852297, 30133350). In 2022, Leslie et al. described a motile ciliopathy characterized by laterality defects and subtle ciliary beat abnormalities caused by biallelic variants in DAW1 in two unrelated families with an autosomal recessive mode of inheritance (PMID: 36074124). The first family had phenotypes suggestive of a mild form of PCD, including a laterality defect (situs inversus) and oto-sinopulmonary phenotypes, whlie the second family had laterality and complex congenital heart defects but no respiratory symptoms. These findings matched earlier work in 2017 by Jin et al. reporting two probands with biallelic DAW1 variants and laterality and cardiac phenotypes, but no mention of respiratory symptoms (PMID: 28991257).

Upon review of the genetic evidence available for DAW1, the ClinGen Motile Ciliopathy GCEP concluded that the molecular mechanism (biallelic loss of function) and the mode of inheritance (autosomal recessive) are consistent among unrelated patients carrying DAW1 variants. However, the variability in respiratory phenotypes reported among patients was notable and suggestive of reduced evidence of a DAW1 relationship to PCD as opposed to an isolated laterality defect. PCD is a motile ciliopathy with a respiratory phenotype. Therefore, per criteria outlined by the ClinGen Lumping & Splitting Working Group, DAW1 was recommended for split curation for two distinct disease assertions; primary ciliary dyskinesia 52 (MONDO: 0957922, OMIM # 620570) and visceral heterotaxy (MONDO: 0018677). This curation has evaluated all of the available genetic evidence for DAW1, however, scoring has only been reported for cases relevant to a gene-disease association between DAW1 and primary ciliary dyskinesia 52.

Genetic evidence from one proband from the Leslie et al. publication (PMID: 36074124) has been scored in this curation. High speed video microscopy of respiratory cells from this patient showed a subtle reduction of ciliary beating amplitude, an impaired recovery stroke, and a resulting stiff beating pattern (PMID: 36074124). This ciliary phenotype is consistent with the laterality and mild respiratory symptoms described for this patient and her sisters. The proband carries a homozygous missense variant, NM_178821.3(DAW1):c.427A>G (p.Asn143Asp), shown to be a loss-of-function variant through rescue experiments in a zebrafish DAW1 knockout model system (PMID: 36074124).

The gene-disease association between DAW1 and PCD 52 is supported by gene expression data that shows high DAW1 levels in ciliated tissues such as lung, testis, and fallopian tube (PMID: 23715323), consistent with the proposed role of DAW1 in the transport of ODAs in motile cilia and flagella (PMID: 32943623). RNA in situ hybridization studies of embryonic mice show that Daw1 is expressed in the embryonic node (PMID: 36074124), consistent with Daw1's role in left-right organization. The efficient transport of ODAs into cilia is mediated by the interaction of DAW1 with both ODAs and the IFT46 protein of the multi-subunit IFT-B1 complex (PMID: 18852297, 30133350). DAW1 plays a role as a linker protein between these complexes. IFT46 is a core component of the IFT machinery and is required for the formation of all cilia. In Xenopus, knockdown of IFT46 results in a shorter body axis as well as the formation of fewer and shorter cilia (PMIDs: 27320864, 33628615). Yeast two-hybrid experiments confirm an in vivo interaction between ODA16 (human) and IFT46 (mouse) (PMID: 18852297). Co-immunoprecipitation studies in Chlamydomonas confirm that ODA16 physically interacts with ODAs (PMID: 28298440). RNAi knockdown of daw1 in the planarian Schmidtea mediterranea resulted in locomotion defects and protonephridial dysfunction (edema) (PMID: 32359074). These phenotypes are indicative of ciliary dysfunction in cells functionally dependent on motile cilia: the planarian epidermis and the protonephridial excretory system. Zebrafish wdr69 antisense morpholino knockdown produced embryos with phenotypes consistent with defective cilia in multiple tissues: curled tail, pronephric cysts, pericardial edema, and extra otoliths (PMID: 20568242). Embryos were shown to have defective ciliary motility in the Kupffer’s vesicle and developed left-right asymmetries in the heart and gut. Electron microscopy of Kupffer’s vesicle cilia showed an absence of ODAs, confirming that Wdr69 (DAW1) is needed for ODA assembly in the embryonic axoneme. Mice homozygous for a Wdr69 loss-of-function missense variant were studied as a mammalian PCD model (PMID: 28289722). The wdr69–/– mouse model recapitulates phenotypes one might associate with a PCD patient with ODA defects. Wdr69–/– mice respiratory cells show an absence or reduction in ODAs as well as dyskinetic ciliary motion, despite normal ciliated area and ciliary beat frequency. The absence of ODAs is consistent with the role DAW1 is thought to play in the transport of ODAs along the axoneme. These mice have impaired mucociliary clearance and laterality defects such as situs inversus and heterotaxy with complex congenital heart defects. Most of these mice die before term from complex congenital heart defects or soon after birth from hydrocephalus.

This curation scored sufficient points (6.1) for a MODERATE gene-disease clinical validity classification, based largely on evidence supporting the role of DAW1 in the transport of ODAs through the IFT system and robust evidence suggesting that loss-of-function variants in DAW1 have an impact on the cilia of humans and animal models. However, the genetic evidence supporting an association between DAW1 and primary ciliary dyskinesia 52 in humans is limited. Primary Ciliary Dyskinesia typically has a respiratory phenotype due to defects in mucociliary clearance. The phenotypic evidence supporting a DAW1-PCD 52 association is based on three older sisters who self-reported mostly childhood respiratory symptoms as part of a larger study of laterality disorders. Pending more clinical information on the respiratory phenotypes of these patients carrying the p.Asn143Asp variant in DAW1 or the reporting of additional patients carrying biallelic DAW1 variants with laterality and respiratory symptoms, the classification of DAW1 in relation to primary ciliary dyskinesia 52 has been changed to LIMITED. This classification was approved by the ClinGen Motile Ciliopathy GCEP on June 13, 2024 (SOP Version 10).