Primary Ciliary Dyskinesia (PCD) via the RSPH4A Gene

Primary ciliary dyskinesia (PCD) is a genetically heterogeneous disorder affecting the function of motile cilia (reviewed by Leigh et al. 2009). Motile cilia line the upper and lower respiratory airways, the ventricular system of the brain and spinal cord, and the female fallopian tubes. They are also components of the male sperm flagellum and required for sperm motility. Ciliary movement sweeps mucus, dirt, and bacteria out of the lungs, nasal passageways, and ear canals, thus protecting them from recurrent infections. In the developing embryo, nodal cilia generate a rotational motion that determines the position of the internal organs. Without functional nodal cilia, thoracoabdominal orientation is random. The hallmark features of PCD are neonatal respiratory distress, chronic coughing, and recurrent sinus or ear infections; 80-100% of all PCD patients have one or more of these symptoms. In about 50% of individuals with PCD, the major visceral organs are reversed from their normal positions (also called situs inversus or Kartagener’s syndrome). Fetal cerebral ventriculomegaly and hydrocephalus can also occur due to impaired circulation of the cerebrospinal fluid. In adults with PCD, male infertility and female sub-fertility are also common features. Prompt diagnosis of PCD is critical for the prevention of secondary respiratory complications, such as bronchiectasis, pneumonia, or progressive loss of lung function.

Motile cilia are complex structures composed of roughly 250 proteins (reviewed in Ferkol & Leigh 2006). Planar motion cilia (i.e. from the respiratory tract, brain, and reproductive tract) consist of nine microtubule doublets that surround a central core of two microtubules (9+2 configuration). Rotary motion cilia (i.e. those in the embryonal node) lack the central core microtubules (9+0 configuration). All motile cilia have inner and outer dynein arms attached at regular intervals to the nine peripheral microtubule doublets, which serve as molecular motors that drive microtubule sliding. For 9+2 cilia, radial spokes form a signal-transduction scaffold between the peripheral dynein arms and the central-core microtubule pair, giving these cilia their characteristic planar (i.e. forward and backward) motion. Recessive variants in two genes encoding radial spoke proteins, RSPH4A and RSPH9, have recently been found to cause PCD (Castleman et al. 2009). The respiratory cilia in these patients either lack the central-core microtubules, having a 9+0 configuration, or have a single peripheral microtubule pair transposed into the center, resulting in an abnormal 8+1 configuration. Remarkably, the cilia in these patients have normal beat frequencies but display a rotational beat pattern, rather than the normal forward and backward planar motion. RSPH4A is comprised of 6 exons and, to date, just three nonsense variants (two in exon 1 and one in exon 3) have been described in 5 unrelated families (Castleman et al. 2009). All patients with RSPH4A variants had cilia with transposed microtubules (i.e. 8+1 configuration).

This test is predicted to detect at least one causative variant in ~1-2% of all patients diagnosed with PCD and ~75% of PCD patients with central-core microtubule defects (Castleman et al. Am J Hum Genet 84:197-209, 2009).

This test provides full coverage of all coding exons of the RSPH4A gene plus 10 bases of flanking noncoding DNA in all available transcripts along with other non-coding regions in which pathogenic variants have been identified at PreventionGenetics or reported elsewhere. We define full coverage as >20X NGS reads or Sanger sequencing. PGnome panels typically provide slightly increased coverage over the PGxome equivalent. PGnome sequencing panels have the added benefit of additional analysis and reporting of deep intronic regions (where applicable).

Dependent on the sequencing backbone selected for this testing, discounted reflex testing to any other similar backbone-based test is available (i.e., PGxome panel to whole PGxome; PGnome panel to whole PGnome).