Submission Details

Primary ciliary dyskinesia (PCD) is a genetically heterogeneous group of disorders, usually beginning in early childhood, characterized by recurrent infections of the upper and lower respiratory tract, randomization of left/right body asymmetry, and subfertility (PMID: 32943623). The PCD phenotype results from structural and/or functional abnormalities of motile cilia and flagella. The most frequently mutated gene (15-24% of Caucasian patients) in PCD is DNAH5 (Dynein Axonemal Heavy Chain 5) which encodes a subunit of the ciliary outer dynein arms (ODAs) and is essential for force generation during ciliary beating (PMIDs: 11788826, 16627867, 19357118). The DNAH5 gene was first reported in relation to PCD in 2000 (Omran et al., PMID: 11062149). The specific disease entity, primary ciliary dyskinesia 3, with or without situs inversus is an autosomal recessive disorder caused by homozygous or compound heterozygous mutation in the DNAH5 gene. DNAH5 variants were first reported in 2002 by Olbrich et al., (PMID: 11788826). Per criteria outlined by the ClinGen Lumping and Splitting Working Group, we found the molecular mechanism and autosomal recessive mode of inheritance to be consistent among unrelated patients, while the phenotypic variability among them appeared to represent a spectrum of disease rather than separate disease entities. Therefore, cases caused by inherited DNAH5 variants have been lumped into a single disease entity, primary ciliary dyskinesia 3 with or without situs inversus (MONDO: 0012085, OMIM #608644).

Evidence from 17 probands in 2 publications have been included in this curation representing 22 unique variants (3 missense, 5 nonsense, 9 frameshift, and 5 splice site) (PMIDs: 11788826, 16627867). Of note, four of these variants have been identified as founder mutations by haplotype analysis: NM_001369.3(DNAH5):c.10815del (p.Pro3606fs), Clin Var ID: 65636, is found in seven PCD families in Hornef et al., (PMID: 1662786). Similarly, NM_001369.3(DNAH5):c.13338+5G>A, Clin Var ID: 572293, is also likely a founder mutation. This variant was identified in more than one unrelated PCD family (PMID:11788826). Variants NM_001369.3(DNAH5):c.5563dup (p.Ile1855fs), Clin Var ID: 407241, and NM_001369.3(DNAH5):c.13458dup (p.Asn4487Ter), Clin Var ID: 350995, have been identified as founder mutations that also occur independently in some families of different haplotype (PMID: 16627867). The mechanism of pathogenicity in DNAH5 mutations appears to be bi-allelic loss of function. There is an abundance of case-level and case-control level evidence available in the literature, but its inclusion in this curation was not necessary to reach the maximum score for genetic evidence (12 points).

This gene-disease association between PCD 3 and DNAH5 is also supported by gene expression data. In situ hybridization analysis of murine Dnah5 expression in the developing and adult respiratory system of mice shows ciliated epithelial expression in the whole bronchial system (embryonic nasopharynx, larynx, trachea and adult lung) (PMID: 12775878). This murine expression pattern is consistent with the PCD phenotype of humans with mutations in DNAH5. Similarly, murine Dnah5 is expressed exclusively at the mouse Left Right Organizer (LRO) during early embryogenesis, consistent with the hypothesis that laterality defects (situs inversus) linked to DNAH5 mutations in humans may result from immotile LRO monocilia with defective outer dynein arms (PMID: 11788826). Distribution of DNAH5 in human respiratory cilia was analyzed by immunofluorescent staining in both healthy control patients and in patients with DNAH5 mutations. DNAH5 staining is observed throughout the respiratory ciliary axoneme and in the microtubule organizing centers in healthy patients, but is undetectable in the ciliary axonemes of DNAH5 mutant patients where protein instead accumulates at the microtubule organizing centers (PMID: 15750039). An example of disease phenotypic variability is seen in one patient with compound heterozygous mutations affecting splicing at the 3' end of the gene (C-terminal mutations), where mutant DNAH5 is absent from the distal part of the ciliary axonemes but is still detectable within the proximal part, as well as in the microtubule organizing centers. This pattern indicates that at least one of these C-terminal DNAH5 variants is still targeted to the site of proximally located ODA complexes (PMID: 16627867).

In addition to gene expression data, there are multiple animal models available to support this gene-disease relationship, including Dnah5mut/mut mice that show most of the pathological phenotypes associated with PCD 3 (PMIDs: 11912187, 31638833, 33561200); mouse respiratory epithelial spheroid cells cultures with Mdnah5 siRNA knockdown that reproduce defects in cilia motility seen in human DNAH5 variants (PMID: 32823934); and embryonic Xenopus with morpholino dnah5 knockdown that support the role of dnah5 in the ciliary motility underlying leftward flow during embryogenesis (PMID: 19450574). In summary, there is definitive evidence supporting a gene-disease relationship between variants in DNAH5 and primary ciliary dyskinesia 3 with or without situs inversus. This has been repeatedly demonstrated in both the research and clinical diagnostic settings, and has been upheld over time. This classification was approved by the ClinGen Motile Ciliopathy GCEP on December 9, 2021 (SOP Version 8).