International Journal of Molecular Sciences Review Emerging Genotype-Phenotype Relationships in Primary Ciliary Dyskinesia Steven K Brennan 1,* , Thomas W Ferkol 1 and Stephanie D Davis 2 1 Department of Pediatrics, Division of Allergy and Pulmonary Medicine, Campus Box 8116, Washington University School of Medicine, 660 South Euclid Avenue, St. Louis, MO 63110, USA; [email protected] 2 Department of Pediatrics, University of North Carolina School of Medicine, 101 Manning Drive, Chapel Hill, NC 27514, USA; [email protected] * Correspondence: [email protected] Abstract: Primary ciliary dyskinesia (PCD) is a rare inherited condition affecting motile cilia and leading to organ laterality defects, recurrent sino-pulmonary infections, bronchiectasis, and severe lung disease. Research over the past twenty years has revealed variability in clinical presentations, ranging from mild to more severe phenotypes. Genotype and phenotype relationships have emerged. The increasing availability of genetic panels for PCD continue to redefine these genotype-phenotype relationships and reveal milder forms of disease that had previously gone unrecognized. Keywords: primary ciliary dyskinesia; molecular genetics; genotype-phenotype association Citation: Brennan, S.K.; Ferkol, T.W.; 1. Introduction Davis, S.D. Emerging Genotype- Phenotype Relationships in Primary Motile ciliopathies are a group of disorders where motile cilia lining embryonic cells, Ciliary Dyskinesia. Int. J. Mol. Sci. airways, sinus and inner ear cavities, and ventricles in the brain are dysfunctional. The 2021, 22, 8272. https://doi.org/ primary function of these hair-like projections is to facilitate the movement of fluid (i.e., 10.3390/ijms22158272 mucus that lines the airways, cerebrospinal fluid) within various body cavities to be either recirculated or expelled [1]. Research into the structure and function of cilia in the last Academic Editors: Michał P. Witt, two decades has provided insights into the complex arrangement of the individual cilia Ewa Zi˛etkiewiczand Zuzanna components, and how genetic variation impacts component assembly and subsequent cilia Bukowy-Bieryłło function. This review will provide an overview of cilia structure and function, the genes involved in cilia assembly, and the phenotypic consequences of genetic variants in primary Received: 1 July 2021 ciliary dyskinesia (PCD). Accepted: 26 July 2021 Published: 31 July 2021 2. Overview of Primary Ciliary Dyskinesia Presenting frequently in an autosomal recessive manner, PCD is a rare inherited Publisher’s Note: MDPI stays neutral disorder of motile cilia with an estimated incidence of 1:10,000 to 1:20,000 [2]. The condition with regard to jurisdictional claims in typically presents early in life, with many infants experiencing respiratory distress at birth published maps and institutional affil- despite being born at term [3]. Symptoms of PCD are directly related to ineffective or absent iations. beating of motile cilia in the embryonic node, respiratory, reproductive, and neurologic systems. This dysfunction results in impaired mucus clearance in the airways, abnormal sperm movement, or altered fluid flow in the fallopian tubes and ventricles within the brain. Affected people can present with a variable phenotype that can be severe or mild, leading Copyright: © 2021 by the authors. to diagnostic challenges. Symptoms include chronic daily wet cough, chronic rhinorrhea, Licensee MDPI, Basel, Switzerland. recurrent sinusitis, recurrent otitis media, bronchiectasis, infertility (or subfertility), situs This article is an open access article abnormalities, and rarely hydrocephalus [4]. Sino-pulmonary symptoms are typically distributed under the terms and the most common manifestations of the condition. Approximately fifty percent of people conditions of the Creative Commons with PCD will present with situs inversus, and heterotaxy is more common in this disease. Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ Practitioners who see people with these complaints should have a high degree of suspicion 4.0/). for PCD so early identification and diagnostic testing can be performed. Int. J. Mol. Sci. 2021, 22, 8272. https://doi.org/10.3390/ijms22158272 https://www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 2 of 15 PCD will present with situs inversus, and heterotaxy is more common in this disease. Prac- titioners who see people with these complaints should have a high degree of suspicion for Int. J. Mol. Sci. 2021, 22, 8272 PCD so early identification and diagnostic testing can be performed. 2 of 15 3. Cilia Structure and Function 3. Cilia Structure3.1. and Overview Function 3.1. Overview Cilia are organized structures of microtubules which protrude from the cell surface, Cilia are organizedserving many structures physiologic of microtubules functions. Cilia which are protrude grouped from into categories the cell surface, as motile or non-mo- serving many physiologictile, and can functions.have either Cilia a 9+2 are or 9+0 grouped arrangement into categories of microtubule as motile doublets or non- (Figure 1) [1]. motile, and can have either a 9+2 or 9+0 arrangement of microtubule doublets (Figure1) [1]. Figure 1. Diagram of basic cilia structure in 9+2 and 9+0 microtubule arrangements. MotileFigure cilia 1. Diagram possessing of basic the cilia 9+2 structure arrangement in 9+2 and are 9+0 found microtubule primarily arrangements. in the upper and lower respiratory tract, female reproductive tract, and spermatozoan. The primary function of motile cilia with theMotile 9+2 cilia arrangement possessing is tothe move 9+2 arrangement liquid and/or are mucous found parallel primarily to thein the upper and cells’ surface orlower in the respiratory case of spermatozoan tract, female flagellum, reproductive these tract, cilia and lead spermatozoan. to the propulsion The primary func- of the sperm. Motiletion of ciliamotile possessing cilia with thethe 9+09+2 arrangement is are tofound move inliquid the embryonicand/or mucous parallel to node and play athe primary cells’ surface role in determiningor in the case left-right of spermatozoan asymmetry flagellum, of organs these during cilia human lead to the propul- development bysion directing of the sperm. fluid flow Motile across cilia the possessing cell surface the [ 59+0]. This arrangement is achieved are through found in a the embryonic rotational motionnode of and the cilium,play a primary which contrasts role in determining to the “wave-like” left-right motion asymmetry of the motileof organs during hu- 9+2 cilia. Finally,man primary development cilia are usuallyby directing solitary, fluid nonmotile flow across organelles the cell that surface typically [5]. have This is achieved a “9+0” microtubulethrough configuration. a rotational Present motion on of the the surface cilium, of which most non-dividingcontrasts to the cells, “wave-like” these motion of structures sensethe the motile extracellular 9+2 cilia. environment. Finally, primary Additionally, cilia are theyusually regulate solitary, developmental nonmotile organelles that pathways, andtypically defects lead have to a a “9+0” growing microtubule number of configuration. complex clinically Present varied on the conditions. surface of most non-di- Affected individualsviding can cells, have these diverse structures features, sense such the as extr variableacellular mental environment. disabilities, Additionally, skeletal they reg- anomalies, obesity,ulate retinal developmental degeneration, pathways, and polycystic and defects kidneys lead to [6 a]. growing While overlapping number of complex clini- conditions havecally been varied described, conditions. the role Affected of these individuals structures is ca notn have in fluid diverse movement features, and such as variable thus are not directlymental implicated disabilities, in theskeletal pathogenesis anomalies, of motileobesity, ciliopathies retinal degeneration, [7]. and polycystic kid- neys [6]. While overlapping conditions have been described, the role of these structures is 3.2. Respiratory Cilia not in fluid movement and thus are not directly implicated in the pathogenesis of motile Cilia liningciliopathies the upper and[7]. lower respiratory tracts have a 9+2 configuration of micro- tubules that extend from the apical cell surface. Hundreds of these cilia are present on each ciliated cell3.2. and Respiratory they move Cilia in a coordinated fashion, leading to effective mucociliary clearance or movement of mucus from the lower to the upper respiratory tract, out of the Cilia lining the upper and lower respiratory tracts have a 9+2 configuration of micro- sinus ostia, or out of the middle ear space. The cilia structure is stabilized by hundreds of tubules that extend from the apical cell surface. Hundreds of these cilia are present on proteins whose alteration can produce the phenotype seen in PCD. A normally structured each ciliated cell and they move in a coordinated fashion, leading to effective mucociliary cilium shows an arrangement of nine microtubule doublets that are arranged along the clearance or movement of mucus from the lower to the upper respiratory tract, out of the circumference of the cilia along with another pair in the center referred to as the central sinus ostia, or out of the middle ear space. The cilia structure is stabilized by hundreds of apparatus or central complex. The outer doublets are anchored to the central complex by proteins whose alteration can produce the phenotype