The RSPH4A Gene in Primary Ciliary Dyskinesia

Abstract

The radial spoke head protein 4 homolog A (RSPH4A) gene is one of more than 50 genes that cause Primary ciliary dyskinesia (PCD), a rare genetic ciliopathy. Genetic mutations in the RSPH4A gene alter an important protein structure involved in ciliary pathogenesis. Radial spoke proteins, such as RSPH4A, have been conserved across multiple species. In humans, ciliary function deficiency caused by RSPH4A pathogenic variants results in a clinical phenotype characterized by recurrent oto-sino-pulmonary infections. More than 30 pathogenic RSPH4A genetic variants have been associated with PCD. In Puerto Rican Hispanics, a founder mutation (RSPH4A (c.921+3_921+6delAAGT (intronic)) has been described. The spectrum of the RSPH4A PCD phenotype does not include laterality defects, which results in a challenging diagnosis. PCD diagnostic tools can combine transmission electron microscopy (TEM), nasal nitric oxide (nNO), High-Speed Video microscopy Analysis (HSVA), and immunofluorescence. The purpose of this review article is to provide a comprehensive overview of current knowledge about the RSPH4A gene in PCD, ranging from basic science to human clinical phenotype.

Keywords: RSPH4A; cilia; genotype-phenotype relationships; high-speed video microscopy analysis; immunofluorescence; nasal nitric oxide; primary ciliary dyskinesia; transmission electron microscopy.

Figure 1

The Homo sapiens (human) radial spoke head protein 4 homolog A (RSPH4A) protein (Q5TD94). (a) Complete amino acid sequence. (b) Subcellular localization from the COMPARTMENTS database. (c) Three-dimensional AlphaFold structure prediction. The per-residue confidence score is represented in colors (d) The protein-to-protein association network from the STRING database. Lines colors represent known and predicted interaction between proteins (e) RSPH4A gene exon structure and disease-causing variants; red font identify the Puerto Rican founder mutation RSPH4A (c.921+3_921+6delAAGT (intronic)). Figure 1e was created with BioRender.com (accessed on 24 November 2022).

Figure 2

Animal model and orthologs similarity for the human RSPH4A gene. Percentage of the RSPH4A human gene sequence matching the sequence of the orthologs. Created with BioRender.com. Data obtained from GeneCards—the human gene database [7].

Figure 3

Chest X-rays (CXR) of homozygous patients with PCD for the RSPH4A (c.921+3_921+6delAAGT (intronic)) founder mutation. (a) A one-year-old male showing air trapping, (b) a 13-year-old female with perihilar infiltrates, (c) a 38-year-old female with bibasilar atelectasis, and (d) a 51-year-old male with worsening bibasilar atelectasis and fibrotic tissue. No laterality defects are evident. Panel organized using BioRender.com (accessed on 24 November 2022).

Figure 4

High-resolution CT scans (HRCT) of the chest. Homozygous patients with PCD for the RSPH4A (c.921+3_921+6delAAGT (intronic)) founder mutation. (a,b) A 40-year-old female with mild-to-moderate RML and LLL varicose bronchiectasis with a bilateral and peripheral tree-in-bud pattern, and (c,d) a 59-year-old female with increased bronchovascular markings, severe bibasilar bronchiectasis, mucus plugs, and atelectasis, more prominent at LLL. Panel organized using BioRender.com (accessed on 24 November 2022).

Figure 5

Spirometry and forced oscillation technique (FOT) of a homozygous PCD patient with the RSPH4A (c.921+3_921+6delAAGT (intronic)) founder mutation. (a) Airflow limitation with a restrictive pattern is observed in basic spirometry. The flow-volume and time-volume loops are abnormal, with no significant improvement after the bronchodilator. (*) shows values below the lower limit of normal. (b) FOT revealed the presence of respiratory impairment without significant reversibility. Respiratory impedance (Zins) was consistent with severe obstructive disease. Panel organized using BioRender.com (accessed on 24 November 2022).

Figure 6

Transmission Electron Microscopy (TEM). Ciliary biopsies of patients with the RSPH4A (c.921+3_921+6del (intronic)) founder mutation. (a,b) Normal, healthy subject. (c,d) Abnormal central complex microtubule configuration, (9 + 3). (e,f) Abnormal peripheral and central microtubule organization and configuration, (9 + 4). Scale bar for (a,c,e): 50 nm. Illustrations (b,d,f) were created with BioRender.com (accessed on 24 November 2022).

Figure 7

Chemiluminescence detection of nasal nitric oxide (nNO) in a PCD patient homozygous for the RSPH4A (c.921+3_921+6delAAGT (intronic)) founder mutation. Time-lapse from 0 to 90 s: left nostril; 90–200 s: right nostril. The average nNO using the exhalation against resistance technique was 34 nL/min (cutoff < 77 mL/min). ppb = parts per billion. The mean concentration of the right and left nares together times the flow sampling rate (L/min) equals the final nNO value (nL/min).