Andrology-Open Access
Open Access

Primary Ciliary Dyskinesia and Male Infertility: Current Understandings and Future Directions

Niloofar Khoshnam-Rad^{* *}Correspondence: Niloofar Khoshnam-Rad, Department of Clinical Pharmacy, Tehran University of Medical Sciences, Tehran, Iran, Email:

Author info »

Introduction

Primary Ciliary Dyskinesia (PCD) is a rare autosomal recessive genetic disorder characterized by dysfunction of motile cilia. These microscopic, hair-like structures line the epithelium of various organ systems, including the respiratory tract (trachea, bronchi), sinonasal tract (sinuses, Eustachian tubes), otologic system (middle ear), central nervous system (ventricles) and reproductive system (fallopian tubes) [1]. The primary cause of PCD is a defect of motile cilia lining the airways, which can result in ciliary immotility, abnormal beating patterns (ciliary dyskinesia) or complete absence (ciliary aplasia) [2]. PCD is a genetic disorder that has been described globally, affecting males and females equally. Studies of the human genome suggest that PCD occurs in at least 1 in 7,500 people worldwide [3]. Unfortunately, PCD remains largely undiagnosed, emphasizing the need for improved diagnostic strategies.

Recurrent infections of both the upper and lower respiratory tract are the most common characteristics of PCD, although the overall presentation of PCD shows significant individual variation [4]. While most individuals with PCD develop symptoms in childhood, with a typical diagnosis age around 5-5.5 years, some experience a delayed presentation until adulthood, typically diagnosed around 22 years old [5].

Due to potential defects in embryonic nodal cilia, approximately 50% of individuals with PCD experience situs inversus totalis, a condition characterized by reversed internal organ positioning.

When situs inversus, chronic sinusitis and bronchiectasis occur together, an individual is said to have Kartagener syndrome, which is a subgroup of primary ciliary dyskinesia [6].

PCD also significantly impacts fertility in both men and women. In men, the condition often leads to immotile sperm, despite the sperm being alive. Some men with PCD may have motile sperm, but their cilia are immobile. Azoospermia or complete absence of sperm, is also a possibility. Women with PCD also experience decreased fertility, with less than half achieving successful pregnancies. This is because impaired ciliary function in the fallopian tubes can slow the movement of eggs, reducing fertility and very rarely, leading to ectopic pregnancy [7].

In the absence of cystic fibrosis or prematurity, a constellation of symptoms including neonatal respiratory distress, early-onset and persistent cough, chronic rhinosinusitis and chronic otitis media, along with laterality defects like situs inversus or ambiguous situs, should raise a high index of suspicion for PCD [8].

In adults, PCD should be considered in males presenting with dyskinetic spermatozoa and concomitant respiratory symptoms. Similarly, females with unexplained infertility or subfertility, particularly in the context of respiratory complaints, warrant investigation for PCD. Since no single test definitively diagnoses PCD, a combination of tests is necessary for accurate diagnosis. Current management of PCD focuses on individualized approaches, with researches ongoing to identify and develop standardized treatment options for the future. Given the frequent association between ciliary dysfunction and abnormal sperm motility, male PCD patients should be counseled regarding potential infertility. Semen analysis is recommended for further evaluation. In these cases, In vitro Fertilization (IVF) techniques, particularly Intra-Cytoplasmic Sperm Injection (ICSI), have demonstrated efficacy [9].

The aim of this review is to provide an overview of the current understanding of PCD and its impact on male reproductive health, including the potential genetic and molecular mechanisms underlying PCD-associated male infertility, diagnostic approaches and treatment options.

Literature Review

Pathophysiology

Structure and function of motile and immotile cilia: An overview of axoneme and ciliary proteins: PCD is caused by genetic mutations affecting genes responsible for motile cilia formation and function. The axoneme is a cylindrical microtubular structure in these cilia, consisting of nine outer doublets and two central microtubules. The motor force for movement is generated by the coordinated action of outer and inner dynein arms, while the nexin-dynein regulatory complex, radial spokes and the central pair microtubules ensure proper ciliary motion [10]. These structures are also present in the sperm tail axoneme, playing a vital role in generating the propulsive force essential for sperm motility, which is indispensable for male fertility [11].

Given the shared structural foundation of the axoneme in both motile cilia and sperm flagella, male individuals diagnosed with PCD are generally considered to be at a high risk of infertility [12]. However, the overall fertility prognosis for men with PCD remains unclear due to limited data and variability in individual cases. Nevertheless, a single study reported that 75% of couples where the male partner had PCD were diagnosed as infertile [13]. Individuals diagnosed with PCD are likely to exhibit abnormalities in sperm parameters, including oligospermia (low sperm count), asthenozoospermia (decreased motility) and/or teratozoospermia (morphological abnormalities) [14].

Cilia can be classified into two primary functional categories: Motile and immotile. While these two types have distinct functional roles, they share a remarkable degree of protein composition and structural organization, as well as a common structural foundation known as the axoneme. The axoneme is comprised of nine outer doublet microtubules, which are present in both motile and immotile cilia [15]. This intricate cytoskeletal scaffold plays a critical role in maintaining the structural integrity of the cilium. Despite sharing a fundamental structure, distinct functional roles and diverse tissue distributions characterize different cilia type.

Motile cilia, present on the epithelial surfaces of the upper and lower respiratory tracts, are essential for effective mucociliary clearance. However, compromised ciliary function can predispose individuals to the chronic respiratory infections and inflammation, affecting both the upper and lower airways [16]. Motile cilia are not exclusive to the respiratory tract; their presence extends to various other anatomical locations, notably, motile ciliated cells line the ependyma of the brain and the fallopian tubes. The spermatozoan flagellum shares a fundamental structural core and exhibits remarkably similar motility characteristics to cilia.

Motile cilia possess multimeric dynein arms, which are essential for their characteristic motility. The outer dynein arms generate the force responsible for the sliding movement of adjacent microtubule doublets within the axoneme. Conversely, the inner dynein arms function in concert with the nexin-dynein regulatory complex to regulate ciliary motion [17]. Motile cilia are further characterized by the presence of a central microtubular pair, contributing to the hallmark "9+2" axonemal structure observed in Transmission Electron Microscopy (TEM). This central apparatus fulfills a complex role, including maintaining structural integrity, facilitating force transmission during ciliary beating and ensuring coordinated ciliary motion in a single direction along the airway. Radial spokes project from the central apparatus towards the inner dynein arms, functioning as a communication relay, transmitting signals to the nexin-dynein regulatory complex and thereby regulating inner dynein arms activity [18].

In contrast to the motile cilia found throughout the body, a distinct motile cilium known as the nodal cilium is transiently expressed only during fetal development. Unlike the typical 9+2 structure, nodal cilia exhibit a unique 9+0 microtubular arrangement, lacking a central pair. This specific structure is associated with their rotatory motion, which generates a leftward flow of extracellular fluid across the embryonic node. This flow, in turn, activates a critical signaling cascade that establishes leftright asymmetry, also known as body laterality. Disruption of motile cilia function, including nodal cilia, can lead to laterality defects, with situs inversus totalis being a possible consequence [16].

Primary cilia: Structure, function and primary ciliopathies: Primary cilia are typically immotile, solitary organelles (monocilia) present on the surface of numerous differentiated, non-dividing cells. Most primary cilia exhibit a distinct "9+0" microtubule configuration, lacking the central apparatus and dynein arms responsible for motility. These structures function as essential sensory organelles, adept at sensing and responding to the extracellular environment. They can act as surface mechanoreceptors or chemoreceptors, detecting various stimuli such as changes in osmolality, light, temperature and gravity. Perturbations in primary cilia function, termed primary ciliopathies, are associated with a diverse spectrum of syndromic diseases [19]. While the majority of primary ciliopathies do not directly impact motile ciliary function, rare reports exist of syndromic presentations encompassing characteristics of both primary and motile ciliary dysfunction [20].

Motile cilia in the male reproductive system and implications for PCD: Motile cilia are present in the male reproductive system, specifically lining the efferent ductules that connect the rete testis to the caput epididymis. Their primary function was traditionally thought to aid the transport of the immotile spermatozoa along the ductule. However, recent research suggests that motile cilia generate fluidic turbulence within the lumen, which prevents spermatozoa agglutination during transit.

Spermatozoa acquire motility during maturation in the epididymis and sperm felagella exhibit a similar axonemal ultrastructure to airway cilia, with both sharing the characteristic 9+2 microtubular arrangement and a significant overlap in structural proteins. Despite sharing a fundamental structural blueprint, spermatozoa flagella exhibit distinct functional and structural characteristics compared to motile cilia. These disparities are attributed to variations in gene expression, axoneme protein composition and assembly processes notably, sperm flagella differ from cilia in terms of length and the presence of accessory structures surrounding the axoneme. These unique features contribute to the distinctive motility patterns and specialized functions of each. While motile cilia utilize a coordinated beating pattern consisting of a forward power stroke (propelling mucus) and a recovery return stroke, spermatozoa flagella exhibit a characteristic whip-like motility crucial for their propulsive function. In contrast to the coordinated beating pattern of motile cilia, spermatozoa flagella exhibit undulatory bending waves along their length, generating a propulsive force for forward movement. While respiratory tract cilia affixed to the apical surfaces of the airway epithelium propel overlying fluids, spermatozoa flagella have undergone evolutionary adaptations for propelling the gamete through fluid environments.

In individuals with PCD, there is a hypothesis that motile cilia lining the efferent ductules of the testes exhibit similar dyskinetic characteristics as those observed in the respiratory tract. This potential dysfunction in ciliary motility within the efferent ductules could lead to sperm agglutination, which may contribute to a cascade of detrimental effects, including reduced sperm survival, impaired sperm motility and compromised ability to navigate the female reproductive tract. Moreover, altered sperm axoneme structure in PCD is likely to directly compromise sperm motility. Despite the hypothesized detrimental effects of PCD on sperm function, some men with PCD have been documented to achieve fertilization naturally. This intriguing observation may be attributable to potential genetic heterogeneity, with variations existing between the genes regulating respiratory cilia and those governing sperm flagellar function. These variations could potentially lead to a spectrum of ciliary dysfunction severity, with some individuals exhibiting milder phenotypes that allow for sufficient sperm motility to achieve fertilization [7].

Primary ciliary dyskinesia and male infertility

Clinical implications and genetic counseling for PCD-related male infertility: While male infertility can arise from a diverse array of genetic and non-genetic factors, axonemal defects represent a distinct and readily identifiable cause. Evolving knowledge regarding the association between PCD genes and male infertility is ongoing.

Given the potential for preserved sperm function in some PCD cases, it is essential to assess the male infertility phenotype through laboratory confirmation. Establishing robust genotypephenotype correlations is critical for optimizing patient counseling based on individual PCD genetic test results [14]. Diagnostic approaches and family counseling for PCD should ideally be informed by the specific underlying genetic defect. While an estimated 70% of PCD patients have identifiable mutations in known causative genes, the impact of these mutations on male fertility remains largely undefined. While PCD is associated with an increased risk of fertility issues in both males and females, it is essential to clarify that PCD does not invariably lead to infertility. The decision to pursue Assisted Reproductive Technologies (ART) depends on individual circumstances. Recent advancements in ART offer valuable options for many patients with PCD who may face challenges related to fertility [13].

Prevalence and categories of male infertility, with a focus on PCD: Infertility is defined as the inability to achieve a clinically confirmed pregnancy after one year or more of regular, unprotected sexual intercourse [7]. Approximately 8% to 12% of couples globally experience infertility. Male factor infertility, either as a sole cause or contributing factor, is estimated to be present in roughly 50% of infertile couples.

Male infertility can be classified into four main categories based on the underlying cause: Spermatogenic quantitative defects, ductal obstruction or dysfunction, hypothalamic-pituitary axis disturbances and spermatogenic qualitative defects. These categories can sometimes co-occur and genetic factors are estimated to contribute to roughly 15% of male infertility cases.

PCD affects at least 1 in 7,500 people worldwide [3]. The degree of respiratory tract ciliary dyskinesia in PCD does not predict fertility [13]. Studies suggest that PCD may be a significant factor in male infertility, with up to 83% of men with PCD experiencing some degree of infertility. This is compared to up to 61% of women with PCD.

A systematic review analyzing 52 studies with nearly 2,000 participants found that 13.5% of studies involving adults or adults and children reported on infertility. However, only data from adult participants was analyzed. Among these adults, the prevalence of infertility ranged widely, from 15% to 79% with an average of 30%. When further analyzed by sex in four studies, 58% of females were classified as infertile, while all males evaluated in the three studies reporting male data (100%) were infertile. This finding suggests a significant prevalence of infertility in adults, with a potential disparity between genders, though the limited data on male infertility necessitates further investigation [12].

A retrospective study from 2014-2016 found a high rate of infertility among female patients with PCD-63% compared to the general population (less than 30% for women aged 44-45). Notably, male PCD patients weren't strictly infertile, 24% still fathered children naturally. This suggests that PCD doesn't guarantee infertility in either sex, but rather increases the risk of fertility challenges. This finding contradicts some existing literature that erroneously implies PCD as an automatic cause of infertility [13].

The exact reasons for this difference between female and male PCD patients and infertility rates are not fully understood, but may be related to the specific roles that cilia play in the male and female reproductive tracts. For example, cilia in the male reproductive tract are essential for sperm motility, while cilia in the female reproductive tract help to transport eggs from the ovaries to the fallopian tubes. It is important to note that the prevalence of PCD in male infertility can vary depending on the population studied and the methods used to diagnose PCD. Additionally, not all men with PCD will be infertile and some may be able to conceive naturally [13]. However, PCD is a significant risk factor for male infertility and men with PCD should be aware of this risk and talk to their doctor about their fertility options.

PCD mutations and their effects on sperm function: While male infertility is frequently observed in PCD patients, the specific mechanisms by which PCD mutations cause impaired sperm function remain under investigation. Recent advancements have identified mutations in approximately 50 genes as causative factors in PCD.

PCD genes are classified into distinct categories: Multiciliogenesis, dynein arm preassembly, Outer Dynein Arm/ Inner Dynein Arm (ODA/IDA), Radial Spokes and Central Pair (RS/CP) and nexin link and microtubular organization genes. While most PCD genes exhibit expression within the testes, notably low levels are observed for the multiciliogenesis gene MCIDAS, the ODA genes DNAH5 and DNAH11 and the RS gene RSPH4A [14].

The consequences of RS/CP gene mutations on male fertility remain unclear, but mutations in the CP genes HYDIN and SPEF2 have been linked to both male infertility and PCD. SPEF2 mutations manifest as Multiple Morphological Abnormalities of the sperm Flagella (MMAF) and mutations in other CP components, such as AK7, ARMC2 and CFAP69, have also been associated with MMAF. An emerging association between AK7 and PCD has also been reported.

Mutations in the RS genes RSPH1, RSPH3 and RSPH9 have been associated with male infertility. Low expression of RSPH4A in the testis suggests it may not be essential for sperm production, while RSPH6A, predominantly expressed in the testis, may potentially compensate for the function of RSPH4A in this tissue.

Previous research and gene expression data suggest a potential link between mutations in dynein arm preassembly genes and impaired sperm motility, which may be associated with male infertility. TTC12 deficiency leads to a complete absence of ODAs and IDAs in sperm flagella, while motile cilia exhibit only partial IDA loss. Observations of variable fertility in PCD patients carrying mutations in ODA-coding genes support the hypothesis of distinct dynein arm complexes functioning in sperm and motile cilia [13,14].

While motile cilia and sperm tail axonemes share similar ultrastructures, recent evidence suggests distinct protein compositions in their dynein arm components. Notably, DNAH5 and DNAH11, the motile cilia dynein heavy chains, appear absent in sperm, suggesting that specific mutations in these genes may not contribute to male infertility in PCD patients. Mutations in DNAH1, DNAH2, DNAH8 and DNAH17 have been associated with predominantly male infertility, potentially accompanied by mild PCD symptoms.

DNAH9 is classified as a PCD gene, essential for ODA assembly in motile cilia and associated with mild PCD manifestations [18]. Two recent case reports documented asthenospermia, characterized by impaired sperm motility, in individuals harboring DNAH9 mutations, suggesting a possible association with male infertility.

PCD gene mutations can also impact the function of the Efferent Ductules (ED), where sperm navigate through ciliated tubules facilitating sperm concentration. The ED epithelium likely generates fluid turbulence, promoting sperm compaction via absorption by neighboring non-ciliated cells. MCIDAS, CCNO and GEMC1, while crucial for motile cilia function, are not expressed in the testis. Depletion of these genes in animal models resulted in azoospermia, suggesting their potential role in ED function and male fertility.

In summary, the effects of PCD mutations on sperm function, sperm motility and ED function play a significant role in male infertility. Further investigation and understanding of the impact of these specific mutations can help optimize genetic counseling for patients and inform potential therapeutic strategies.

Discussion

Diagnosis

Diagnosis of primary ciliary dyskinesia: Despite the lack of a single definitive diagnostic test for PCD, the PICADAR score is a valuable tool that helps identify individuals with a high likelihood of PCD, prompting further investigation. In adult patients, a suspicion of PCD should arise in males presenting with dyskinetic spermatozoa alongside respiratory complaints and in females with unexplained infertility or subfertility, especially those who also experience respiratory symptoms. A definitive diagnosis of PCD typically requires a multi-modal approach due to the limitations of individual tests [8]. While tests measuring nasal nitric oxide and mucociliary clearance can offer initial evaluation, definitive diagnosis typically necessitates confirmation through tests assessing ciliary function and ultrastructure.

Currently, identifying pathogenic variants within the genes responsible for PCD is the most definitive step in the diagnostic process. Given the complexity of PCD diagnosis, referral to a specialist with expertise in this area is typically recommended for a comprehensive evaluation. A summary of diagnostic test is provided in Table 1.

Table 1: Diagnostic tests for primary ciliary dyskinesia (pcd) and their characteristics.

Semen analysis and sperm viability testing: Semen analysis is recommended as an initial fertility assessment for individuals with PCD. This assessment, evaluates sperm concentration, motility and morphology to categorize sperm quality. In individuals with PCD, semen analysis may reveal low sperm count, reduced motility and/or abnormal morphology [14].

While semen analysis offers a general assessment of sperm quality, additional evaluation of sperm viability is crucial in PCD patients with immotile sperm. “Viable sperm" refer to sperm that are capable of fertilizing an egg and initiating embryonic development. This is because fertilization rates with immotile sperm are significantly lower compared to motile sperm.

Several methods can be employed to assess human sperm viability, including:

Chemical sperm activation: This technique evaluates sperm motility potential through the use of specific chemicals.

Light microscopy with eosin staining: This simple test differentiates live (unstained) from dead (stained) sperm based on plasma membrane integrity.

Hypo-Osmotic Swelling Test (HOST): This method assesses the functional integrity of the sperm membrane by observing the tail swelling response in a low-solute environment.

Sybr-14/propidium iodide assay: This flow cytometry-based technique differentiates viable (Sybr-14 positive) from dead (propidium iodide positive) sperm based on membrane permeability [11].

Sperm Tail Flexibility Test (STFT): This test evaluates the tail bending ability of sperm, which is crucial for successful fertilization.

Laser-Assisted Immotile Sperm Selection (LAISS): This advanced method utilizes a laser to isolate viable immotile sperm for potential use in assisted reproductive technologies.

These methods primarily assess the integrity of the plasma membrane, which is essential for various critical sperm functions during fertilization, including capacitation, acrosome reaction, hypermotility and fusion with the oocyte [11].

Among available methods for viable sperm selection, LAISS appears to offer a combination of simplicity, safety and reliability. Notably, HOS test and sperm activation rely on chemicals potentially affecting embryo development. Additionally, HOST and STFT exhibit limitations in processing frozen-thawed sperm and STFT requires significant technical expertise. Compared to the tail flexibility test, LAISS demonstrates superior reliability.

PCD patients commonly have elevated Sperm DNA Fragmentation (SDF) rates, which often occur with low sperm viability and poor semen quality. SDF levels above 30% may indicate lower pregnancy rates following Intrauterine Insemination (IUI). Consequently, incorporating SDF analysis prior to ICSI in PCD patients may be beneficial to optimize treatment success. The Sperm Chromatin Structure Assay (SCSA), a widely used test to assess DNA Fragmentation Index (DFI), provides a reliable approach for evaluating sperm DNA integrity in this context.

Treatment

Treatment options and considerations for PCD associated male infertility: Semen analysis is a crucial tool in developing individualized treatment plans for PCD and male fertility, as it can provide insights into the likelihood of natural conception or success with Assisted Reproductive Technologies (ART), such as ICSI. Laboratory confirmation of male infertility and understanding the genotype-phenotype correlation through genetic testing are also essential, as male infertility can be caused by various genetic and non-genetic factors, including axonemal defects in PCD cases.

Family counseling for PCD patients should be tailored based on the specific genetic underpinnings of the individual case [14]. Addressing modifiable lifestyle factors, such as smoking, substance use, obesity, alcohol consumption, stress and certain medications, is also important for patients with subfertility or mild semen abnormalities. Diagnostic semen analysis gives insights to the likely chance of a couple conceiving naturally or by ART. Observational studies have suggested that smoking, drug use, obesity, alcohol, stress and medication are associated with poor sperm quality in men. Thus, the risk factors for poor sperm quality should be eliminated prior to any other treatment considerations.

PCD often results in reduced sperm motility, which can necessitate ART for successful fertilization. Both In vitro Fertilization (IVF) and ICSI are options for low sperm count or motility but viable sperm, with ICSI being the preferred method for immotile sperm. While a high proportion of sperm with abnormal morphology might not significantly affect the success rates of ART, achieving successful fertilization becomes less likely if only morphologically abnormal sperm are present [14]. Sperm motility is typically a strong predictor of success in ART; however, PCD patients often exhibit immotile sperm, necessitating a shift in focus to sperm viability assessment. This evaluation is crucial, as poor quality sperm can negatively impact pregnancy rates and potentially lead to the development of aneuploid embryos with abnormal chromosome numbers.

In men with PCD, azoospermia is rare, but oligospermia is often reported. In the case of azoospermia, testicular sperm can be retrieved for the female partner to be used during ICSI. Sperm can be retrieved by Testicular Sperm Aspiration (TESA), which involves aspiration of testicular tissue using needles or by surgical procedures, conventional Testicular Sperm Extraction (cTESE) and microdissection Testicular Sperm Extraction (mTESE). In mTESE, light microscopy is used to identify engorged seminiferous tubules, which are then dissected and inspected intraoperatively for any sperm. This method is superior for the identification of viable sperm when compared to TESA and cTESE. The use of testicular sperm can also be an option when the sperm viability is low in ejaculated sperm.

In all cases, patients should be informed about the possibility to pass on the PCD mutations to the next generation and the risks involved in any treatment techniques. This empowers patients to make informed decisions about their reproductive health and family planning.

Treatment success: The treatment pathway should be decided for each patient based on the outcome of the semen analysis. In PCD patients, a 55% fertilization rate was reported for ejaculated sperm and 65% for testicular sperm. Pregnancy rates vary between 35% and 45% for ejaculated and testicular sperm, respectively [7]. In the case that good quality viable sperm can be detected in the ejaculate, the selected viable sperm can be successfully used for ICSI. However, low sperm quality or count in ejaculated sperm decreases the fertilization success and Testicular Sperm Retrieval (TSR) could be recommended. ICSI success rates exhibit significant variability between male partners. A key factor contributing to this variation appears to be sperm viability, potentially influenced by the duration of sperm transit through the epididymis. Prolonged transit times might lead to increased sperm DNA fragmentation and hinder pronucleus formation, particularly in older sperm. Research has revealed that individuals with high sperm viability generally experience satisfactory fertilization rates. However, those with low sperm viability can significantly enhance their treatment outcomes by opting for testicular sperm retrieval. Testicular sperm are typically highly viable, with sperm viability analysis revealing a minimal difference of less than 5% between ejaculated and testicular sperm, and in some cases, a substantial difference of up to 95%. These findings suggest that, in addition to severe oligospermia or azoospermia patients who lack sufficient sperm for ICSI, PCD patients struggling with low sperm viability may also benefit from testicular sperm extraction to improve their treatment success [11].

Research suggests that the specific type of ultrastructural sperm defects may influence ART success rates. For instance, central pair defects have been associated with lower clinical pregnancy rates.

TSR can be considered for men with infertility resulting from low sperm viability in ejaculated sperm. Although testicular sperm generally exhibit higher viability compared to ejaculated sperm, the specific viability rates can vary based on several factors, including the underlying cause of infertility and the specific technique employed for viability assessment. Studies have reported a range of viability differences between ejaculated and testicular sperm.

Some research involving a small number of patients has pointed to enhanced techniques in ICSI, such as the use of ionophores immediately following the procedure, as well as viability and DNA fragmentation tests.

While advancements in ART have greatly improved the outlook for ICSI in male patients with PCD, further investigation is necessary to fully understand the most effective treatments and enhance the fertility treatment process for PCD patients.

Future directions

Potential new treatment approaches: The identification of specific genotypes and underlying mechanisms in PCD has paved the way for personalized medicine, with the ultimate aim of restoring ciliary function. Gene therapy has emerged as a promising approach for personalized medicine in PCD, aiming to restore ciliary function by replacing or repairing mutated genes through the delivery of functional copies of the affected gene. The accessibility of the respiratory system allows for efficient vector delivery. However, current gene therapy faces limitations, including large PCD gene sizes exceeding the capacity of commonly used vectors and potential disruptions in protein production due to differences between natural regulatory elements and promoters used in viral vectors.

To address these challenges, researchers are exploring gene editing tools like Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR), the use of which involves making precise cuts in the DNA and introducing targeted modifications. However, achieving high transfection rates in target cells, establishing long-term gene expression, mitigating immune responses and ensuring safety remain key concerns for successful clinical application.

Transcript therapy or RNA therapy, offers an alternative approach for PCD treatment, as it avoids modifications to the patient's genomic DNA and is generally considered reversible. This approach can be categorized into three groups: mRNA delivery, siRNA-mediated protein degradation and other nucleic acid targeting mechanisms.

Nonsense mutations, which affect up to 28% of PCD patients, introduce Premature Termination Codons (PTCs) within a gene, causing truncated and often non-functional proteins. These abnormal proteins can harm the cell. As nonsense mutations significantly impact protein function in PCD, research efforts are underway to develop PTC therapeutics.

Several challenges need to be addressed for successful clinical application of above treatments. These challenges include achieving high enough transfection rates in target cells to ensure functional ciliary restoration, establishing methods for long-term gene expression and mitigating the potential for immune responses against the introduced genes or vector components. Additionally, safety concerns regarding off-target effects, as observed in some early gene therapy trials, necessitate rigorous evaluation.

Transcript therapy, also known as RNA therapy, offers an alternative treatment approach for PCD. Unlike gene therapy, it avoids modifications to the patient's genomic DNA. Additionally, RNA therapy is generally considered reversible, potentially reducing the risk of long-term side effects. This approach can be broadly categorized into three groups: Those delivering messenger RNA (mRNA) to encode proteins, those utilizing short interfering RNA (siRNA) to target specific proteins for degradation and those targeting nucleic acids through other mechanisms.

Nonsense mutations, identified in up to 28% of PCD patients, introduce Premature Termination Codons (PTCs) within a gene. These PTCs cause premature termination of protein translation, resulting in truncated and often non-functional proteins. In some cases, these abnormal proteins may even harm the cell. Due to the prevalence of nonsense mutations in various genetic diseases and the significant impact on protein function in PCD, significant research efforts are underway to develop PTC therapeutics.

In conclusion, while significant progress has been made in understanding the underlying mechanisms of PCD and developing potential new treatment approaches, there are still many challenges to be addressed in order to ensure successful clinical application and improve the fertility treatment process for PCD patients. Further investigation and refinement of these emerging therapies is crucial for advancing the field and improving patient outcomes.

The importance of continued research in this area: PCD is a complex genetic disorder with a significant impact on male fertility. While current treatments such as ICSI have shown some success in overcoming fertility issues associated with PCD, there is still much to be learned about the underlying mechanisms of the disorder and the most effective treatment approaches. Furthermore, as our understanding of PCD genetics and pathophysiology continues to evolve, there is a critical need for ongoing research to identify new therapeutic targets and develop personalized treatment strategies for PCD patients. By continuing to investigate the genetic and molecular underpinnings of PCD and its impact on male fertility, we can work towards improving diagnostic accuracy, enhancing treatment outcomes and ultimately improving the quality of life for individuals with PCD.

Conclusion

Primary Ciliary Dyskinesia (PCD) is a rare genetic disorder that affects the motile cilia in various organ systems, including the reproductive system. It significantly impacts male fertility due to defects in sperm production and maturation. Despite its prevalence, PCD remains largely undiagnosed, emphasizing the need for improved diagnostic strategies. Current management of PCD focuses on individualized approaches, with ongoing research to identify and develop standardized treatment options. In vitro fertilization techniques, particularly intracytoplasmic sperm injection, have demonstrated efficacy in overcoming male infertility associated with PCD. Continued research in this field holds promise for developing effective and personalized treatment options for PCD and improving overall reproductive health outcomes.

Author Contributions

Conceptualization, N.K and B.R; Methodology, GR, H.K; Investigation: N.K, G.R and B.R; Writing-original draft preparation, N.K, G.R and M.F; writing-review and editing, N.K and H.A; Project administration, S.M and G.R; Supervision, H.A and N.K. All authors have read and agreed to the published version of the manuscript.

Acknowledgments

None.

Conflict Of Interest Statement

We wish to confirm that there are no known conflicts of interest associated with this work.

Funding Information

We did not receive any fund or any financial support for this study.

References

1. Keicho N, Hijikata M, Miyabayashi A, Wakabayashi K, Yamada H, Ito M, et al. Impact of primary ciliary dyskinesia: Beyond sinobronchial syndrome in Japan. Respir Investig. 2024;62(1):179-186.

   [Crossref] [Google Scholar] [PubMed]

2. Horani A, Ferkol TW. Understanding primary ciliary dyskinesia and other ciliopathies. J Pediatr. 2021;230:15-22.e1.

   [Crossref] [Google Scholar] [PubMed]

3. Hannah WB, Seifert BA, Truty R. The global prevalence and ethnic heterogeneity of primary ciliary dyskinesia gene variants: A genetic database analysis. Lancet Respir Med. 2022;10(5):459-468.

   [Crossref] [Google Scholar] [PubMed]

4. Kim M, Lee MH, Hong SJ, Yu J, Cho J, Suh DI, et al. Clinical manifestations and genotype of primary ciliary dyskinesia diagnosed in korea: Multicenter study. Allergy Asthma Immunol Res. 2023;15(6):757-766.

   [Crossref] [Google Scholar] [PubMed]

5. Kuehni CE, Frischer T, Strippoli MF, Maurer E, Bush A, Nielsen KG, et al. Factors influencing age at diagnosis of primary ciliary dyskinesia in European children. Eur Respir J. 2010;36(6):1248-1258.

   [Crossref] [Google Scholar] [PubMed]

6. Rahimi B, Roostaei G, Kazemizadeh H, Edalatifard M, Khoshnam-Rad N. Persistent cough and situs inversus in a middle-aged female. Respirol Case Reports. 2024;12(2): e01309.

   [Crossref] [Google Scholar] [PubMed]

7. Newman L, Chopra J, Dossett C, Shepherd E, Bercusson A, Carroll M, et al. The impact of primary ciliary dyskinesia on female and male fertility: A narrative review. Hum Reprod Update. 2023;29(3):347-367.

   [Crossref] [Google Scholar] [PubMed]

8. Lucas JS, Barbato A, Collins SA. European respiratory society guidelines for the diagnosis of primary ciliary dyskinesia. Eur Respir J. 2017;49(1): 1601090.

   [Crossref] [Google Scholar] [PubMed]

9. Sha YW, Ding L, Li P. Management of primary ciliary dyskinesia/Kartagener’s syndrome in infertile male patients and current progress in defining the underlying genetic mechanism. Asian J Androl. 2014;16(1):101-106.

   [Crossref] [Google Scholar] [PubMed]

10. Heuser T, Dymek EE, Lin J, Smith EF, Nicastro D. The CSC connects three major axonemal complexes involved in dynein regulation. Mol Biol Cell. 2012;23(16):3143-3155.

    [Crossref] [Google Scholar] [PubMed]

11. Jayasena CN, Sironen A. Diagnostics and management of male infertility in primary ciliary dyskinesia. Diagnostics. 2021;11(9):1550.

    [Crossref] [Google Scholar] [PubMed]

12. Goutaki M, Meier AB, Halbeisen FS, Lucas JS, Dell SD, Maurer E, et al. Clinical manifestations in primary ciliary dyskinesia: Systematic review and meta-analysis. Eur Respir J. 2016;48(4):1081-1095.

    [Crossref] [Google Scholar] [PubMed]

13. Vanaken GJ, Bassinet L, Boon M, Mani R, Honoré I, Papon JF, et al. Infertility in an adult cohort with primary ciliary dyskinesia: Phenotype-gene association. Eur Respir J. 2017;50(5):1700314.

    [Crossref] [Google Scholar] [PubMed]

14. Sironen A, Shoemark A, Patel M, Loebinger MR, Mitchison HM. Sperm defects in primary ciliary dyskinesia and related causes of male infertility. Cell Mol Life Sci. 2020;77(11):2029-2048.

    [Crossref] [Google Scholar] [PubMed]

15. Mitchell DR. The evolution of eukaryotic cilia and flagella as motile and sensory organelles. Adv Exp Med Biol. 2007;607:130-140.

    [Crossref] [Google Scholar] [PubMed]

16. Praveen K, Davis EE, Katsanis N. Unique among ciliopathies: Primary ciliary dyskinesia, a motile cilia disorder. F1000Prime Rep. 2015;7:36.

    [Crossref] [Google Scholar] [PubMed]

17. Heuser T, Raytchev M, Krell J, Porter ME, Nicastro D. The dynein regulatory complex is the nexin link and a major regulatory node in cilia and flagella. J Cell Biol. 2009;187(6):921-933.

    [Crossref] [Google Scholar] [PubMed]

18. Horani A, Ferkol TW. Advances in the genetics of primary ciliary dyskinesia: Clinical implications. Chest. 2018;154(3):645-652.

    [Crossref] [Google Scholar] [PubMed]

19. Gerdes JM, Davis EE, Katsanis N. The vertebrate primary cilium in development, homeostasis and disease. Cell. 2009;137(1):32-45.

    [Crossref] [Google Scholar] [PubMed]

20. Hannah WB, de Brosse S, Kinghorn B, Strausbaugh S, Aitken ML, Rosenfeld M, et al. The expanding phenotype of OFD1-related disorders: Hemizygous loss-of-function variants in three patients with primary ciliary dyskinesia. Mol Genet Genomic Med. 2019;7(9):e911.

    [Crossref] [Google Scholar] [PubMed]