Primary ciliary dyskinesia: From diagnosis to molecular mechanisms.

Primary ciliary dyskinesia (PCD) is a rare autosomal recessive disorder affecting motile cilia. This can lead to neonatal respiratory distress, early onset upper and lower airway infections, laterality abnormalities and sub- or infertility. Although disease progression shows large individual variability, all adult patients eventually develop extensive bronchiectasis. As in cystic fibrosis, early diagnosis and frequent follow-up with microbiological control is the best therapeutic strategy, as other treatment options are lacking. PCD is underdiagnosed and diagnosed late due to clinical unawareness, limited availability of diagnostic tests and difficult interpretation of test results. Diagnosis is currently based on a combination of assessment of ciliary motion and ultrastructure by high-speed video microscopy and electron microscopy, respectively. As nasal nitric oxide is low in almost all PCD patients, these measurements can be used for screening. Although there are 26 PCD genes known so far, the genetic basis of the disease has not been unraveled in an estimated 30-40% of patients. However, the rapid discovery of novel PCD genes in recent years is expected to enable accurate genetic characterization of most patients in the near future. Large-scale use of next-generation sequencing and the availability of large ciliary proteomic and transcriptomic databases accelerate the identification of novel PCD genes, especially those that play a key role in cytoplasmic assembly of ciliary ultrastructural components. These genetic advances are revolutionizing the process of obtaining a molecular diagnosis for PCD as we speak and may ultimately lead to an increased understanding of ciliogenesis and function, providing novel handles for therapeutic interventions in PCD patients.