Motile cilia structure and function in patients with mutations in the outer dynein arm heavy chain DNAH9

Primary ciliary dyskinesia (PCD) is a rare inherited respiratory condition. The outer dynein arm (ODA) is a dynein motor which drives ciliary beating. The ODA is composed of 3 dynein heavy chains (DNAH5, DNAH11, DNAH9). DNAH5 and DNAH11 are the most commonly mutated PCD genes. Variants in DNAH9 have been suggested as a cause of PCD previously but disregarded due to lack of phenotypic evidence. Variants in DNAH9 were identified using next generation sequencing in 3 patients (2 families), all with situs inversus. Candidate variants in DNAH9 were confirmed by Sanger sequencing and familial segregation analysis. Nasal nitric oxide, high speed video, immunofluorescence and electron microscopy diagnostic investigations for PCD were performed. All 3 individuals had nasal nitric oxide levels within the normal range (>77nl/min), normal ciliary beat frequency and evidence of mucociliary clearance. Ciliary dyskinesia was detected by high speed video microscopy with a subtle but distinctive beat pattern affecting the distal portion of the cilium. Immunofluorescence revealed absence of DNAH9 protein and a reduction of DNAH5 protein at the distal portion of the cilia. DNAH5 was present proximally. A significant deficiency in the ODA volume was detected at the distal part of the axoneme in patients with DNAH9 defects by high resolution 3D electron microscopy (electron tomography) reflecting the known protein position of DNAH9. In conclusion: DNAH9 mutations result in subtle motile cilia defects which are detectable by immunofluorescence and 3D electron tomography but result in normal or subtle diagnostic investigations.