1 Introduction
Primary ciliary dyskinesia (PCD, (MIM: 244400)) is a clinically and genetically heterogeneous motile ciliopathy, mostly following autosomal recessive or, less frequently, X-linked recessive or autosomal dominant inheritance [
1,
2]. Approximately 50% of PCD cases present Kartagener syndrome, characterized by chronic sinusitis, bronchiectasis, and situs inversus [
3]. Clinically, patients with PCD can manifest subfertility due to defective sperm flagella in men or oviduct cilia in women [
2]. Occasionally, hydrocephalus arises from cerebrospinal fluid flow dysfunction due to ependymal cilia dysmotility [
2]. Comprehensive examinations are currently being applied to PCD diagnosis owing to the variability of the underlying pathogenesis of PCD [
1,
4]. Although approximately 50 pathogenic genes have been identified, they only account for approximately 70% of PCD cases [
2].
Motile cilia and sperm flagella are evolutionarily conserved organelles sharing similar microtubule-based axoneme architecture in various species, comprising nine outer doublet microtubules, which encircle a central pair (
i.e., 9+2 structure), except nodal cilia (9+0 arrangement) [
5]. The inner dynein arms (IDAs) and outer dynein arms (ODAs), which are attached to the peripheral microtubules, hydrolyze adenosine triphosphate (ATP) to provide ciliary beating power and modulate beat frequency and pattern by interacting with the axonemal complex [
2,
6]. Dynein axonemal heavy chain 10 (DNAH10) is a subunit dynein heavy chain of IDA that is expressed in the motile cilia of various tissues, including airway epithelial ciliary cells, ventricular epithelial cilia, and sperm flagella [
7,
8]. Emerging evidence suggests that
DNAH family genes may cause complex phenotypes in motile ciliopathies. Several
DNAH family members coding for ODA components in motile cilia, such as
DNAH5,
DNAH9, and
DNAH11, were considered related to PCD [
9–
11]. Moreover, mutations in
DNAH1 and
DNAH6, which encode IDA components, have been reported in PCD or isolated infertility [
12–
15]. Previous studies have reported that
DNAH10 mutations can cause isolated infertility characterized by asthenoteratozoospermia in humans and mice [
16,
17] and heterotaxy or scoliosis in zebrafish [
18,
19]. Based on its essential role in IDA and its widespread expression in motile cilia, we hypothesize that
DNAH10 may also be a candidate pathogenic gene for PCD [
20].
In the present study, we first identified a novel homozygous variant (c.589C > T) of DNAH10 in a patient diagnosed with PCD with microtubular disorganization and IDA defects from a Chinese consanguineous family. Moreover, Dnah10-knockout male mice and Dnah10-knockin male mice harboring a homozygous missense variant (Dnah10M/M), equivalent to that observed in the patient, were constructed using clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated protein 9 (CRISPR-Cas9) technology. Both animal models with Dnah10 deficiency exhibited airway cilia and sperm flagellar ultrastructures and mobility deficiency and recapitulated the complex PCD clinical phenotypes, including hydrocephalus, chronic respiratory infection, and male infertility. These findings strongly suggest that DNAH10 deficiency contributes to PCD in humans and mice.
2 Materials and methods
2.1 Human subjects
A proband with PCD and his family members were recruited from the Second Xiangya Hospital of Central South University (Changsha, China). Clinical diagnostic assessment of PCD, including medical imaging and nasal nitric oxide (nNO) measurement, in addition to electron and immunofluorescence microscopy to analyze the ciliary structure, was performed. PCD diagnosis was established based on the European Respiratory Society guidelines [
4]. This study was approved by the Institutional Ethics Committee of the Second Xiangya Hospital of Central South University (Changsha, China). Written informed consent was obtained from all the patients and healthy controls.
2.2 Exome sequencing and bioinformatics analysis
Peripheral venous blood samples were collected from the patient and his family. Genomic DNA was extracted from the patient using the DNeasy Blood & Tissue Kit (51104, QIAGEN) as directed and used for subsequent exon sequencing. Library capture, sequencing, and data analysis were performed by Novogene Bioinformatics Institute, Beijing, China as described previously [
21]. Briefly, the genomic DNA of the patient was captured using the Agilent SureSelect Human All Exon V6 Kit (G9706K, Agilent Technologies) and sequenced on an Illumina HiSeq 4000. After quality control, the sequencing reads were aligned to the National Center for Biotechnology Information human reference genome (GRCh37/hg19) using Burrows–Wheeler Aligner. ANNOVAR was used to annotate variant call format files.
Single-nucleotide polymorphisms (SNPs) and short insertion–deletions (indels) were filtered as follows: (1) variants with a minor allele frequency below 1% in the 1000 Genomes Project, Genome Aggregation Database (gnomAD), and in-house database of Novogene were assessed; (2) noncoding and intronic variants were excluded; (3) synonymous missense variants were filtered; (4) homozygous variants from the homozygous regions identified using the Automap algorithm were retained [
22]; and (5) a PCD or PCD-candidate gene panel derived from the literature was used to identify disease-related variants [
2,
20].
2.3 Sanger sequencing
Sanger sequencing was performed in the patient to validate the variants. Primer sequences were designed using an online primer design tool. The primer sequences for the Sanger sequencing of humans and mice are displayed in Table S1.
2.4 Immunostaining
Bronchial epithelial tissues from the proband and a healthy control were obtained by fiber bronchoscopy and fixed in 4% paraformaldehyde overnight at 4 °C. Immunostaining on the slides was performed as described previously [
23]. Briefly, the slides were incubated overnight at 4 °C with primary antibodies: DNAH10 (HPA039066, Sigma-Aldrich, 1:100), DNAH5 (HPA037470, Sigma-Aldrich, 1:100), DNALI1 (ab155490, 1:100, Abcam, UK), and anti-α-tubulin (T9026, Sigma-Aldrich, 1:500). Antibody binding was detected using Alexa Fluor 488 anti-mouse IgG (ab150113, Abcam, 1:500, UK) and Alexa Fluor 555 anti-rabbit IgG (A31572, Invitrogen, 1:500). The slides were incubated for 2 h at 37 °C and stained with 2-(4-amidinophenyl)-1H-indole-6-carboxamidine for 5 min at room temperature. Fluorescence signals were photographed using an Olympus BX53 fluorescence microscope and analyzed using the cellSens Dimension software (Olympus BX53).
2.5 Semen and sperm morphological analysis
Semen samples were obtained from the proband and a healthy control after 5–7 days of sexual abstinence and from
Dnah10-knockin mice. Three independent semen analyses were performed according to the World Health Organization (WHO) guidelines [
24]. Morphological defects of the flagella were classified as absent, short, bent, coiled, or irregular in width as determined by Papanicolaou staining [
25]. The percentages of morphologically normal and abnormal spermatozoa were calculated according to the WHO guidelines [
26].
2.6 Mouse models
Dnah10M/M and
Dnah10-knockout mice were constructed using the CRISPR-Cas9 system using previously published methods [
27]. We designed a single-guide RNA (sgRNA-1: GATTCATTAAGAACTCATCGCGG) to target exon 6 of
Dnah10 with a mutation (c.778CGC > TGG) equivalent to that observed in our patient (c.589C > T) to generate
Dnah10M/M mice.
Dnah10-knockout mice against exons 2–5 were reported in our previous study [
16].
We transcribed Cas9 mRNA and sgRNAs using T7 RNA polymerase in vitro and then mixed and co-microinjected them into fertilized oocytes of C57BL/6J mice. The founder animals and their offspring were genotyped via polymerase chain reaction (PCR) and Sanger sequencing of the tail genomic DNA using the specific primers listed in Table S1. All animal procedures were conducted according to the protocols established by the Guide for the Care and Use of Laboratory Animals of the National Institutes of Health and the Institutional Animal Care and Use Committee of Central South University.
2.7 Transmission electron microscopy (TEM)
Respiratory cilia and spermatozoa from human and mice were treated as described previously [
16,
28]. Briefly, respiratory cilia or spermatozoa were fixed with glutaraldehyde (G1102, Servicebio) and osmium tetroxide, post-fixed with sucrose and osmium oxide, and dehydrated with gradient ethanol. Subsequently, 80 nm-thick sections were contrasted with lead citrate and uranyl acetate. Finally, digital images were captured using an HT7700 Hitachi electron microscope (HT7700, Hitachi) and a MegaView III digital camera (Olympus Optical).
2.8 High-speed video microscopy (HSVM) analysis
Tracheal ciliated epithelia from mice for HSVM analysis were photographed using an upright Olympus BX53 microscope (BX53, Olympus) with a 40× objective lens. Videos were recorded using a semiconductor camera (Prime BSI™, Photometrics) at a rate of 500 fps at 37 °C. Only intact ciliated edges ( > 50 μm) were used for functional analysis. Five separate ciliated epithelial strips from the mucus-free regions were measured. Ciliary beat frequency (CBF) was calculated using a validated automated open-source software (CiliarMove version 1, Microsoft).
2.9 Real-time quantitative polymerase chain reaction (RT-qPCR)
Total RNAs from the trachea and testes of adult Dnah10-knockin mice and Dnah10-knockout mice were extracted using an RNA extraction and purification kit (K0731, Thermo Fisher Scientific) for RT-qPCR. Approximately 1 µg RNA was reverse-transcribed into cDNA using EZscript. A reverse-transcription mix (EZB-RT2GQ, EZBioscience) was used according to the manufacturer’s instructions. The target mRNA expression levels were normalized to those of the mouse Gapdh primers using the primers listed in Table S2.
2.10 Statistical analyses
Statistical analyses were performed using the SPSS version 17.0 and GraphPad Prism version 8.4.0. Descriptive statistics of the variables are presented as mean ± standard deviation or counts (n) and proportions (%). Chi-square test was used for categorical data. Student’s t-test was used for continuous variables. The significance level was set at P < 0.05.
3 Results
3.1 Clinical assessment
A 28-year-old man from a consanguineous family presented with complaints of recurrent cough, purulent sputum, and nasal congestion since childhood (Fig.1). His early deceased sister was diagnosed with Kartagener syndrome (including chronic sinusitis, bronchiectasis, and situs inversus). The proband had no history of trauma, poliomyelitis, tuberculosis, or toxic exposure. High-resolution computed tomography of the paranasal sinuses, chest, and abdomen of the proband revealed chronic rhinosinusitis (Fig.1), bronchiectasis, dextrocardia (Fig.1), and abdominal situs inversus (Fig.1). The nNO production rate of the proband was 24 nL/min, which is far below the reference nNO cutoff value of PCD (77 nL/min) [
29]. TEM analysis of respiratory cilia revealed disorganized 9+2 structure and IDA defects (Fig.1–1G). Routine semen analysis of the spermatozoa of the proband revealed severe asthenoteratozoospermia with completely immotile spermatozoa and a comparatively low sperm count. Papanicolaou staining of the proband revealed that most spermatozoa manifested absent, short, bent, and coiled flagella (Fig.1, Table S3). Similarly, TEM results showed a highly disorganized 9+2 structure and IDA defects in the mutant sperm flagella of the proband (Fig.1–1K). In summary, the proband presented complex PCD phenotypes, including sinusitis, bronchiectasis, situs inversus, and asthenoteratozoospermia.
3.2 Identification of a novel homozygous variant of DNAH10 in a patient with PCD
Whole exome sequencing was performed to identify the potential genetic source of the PCD. In accordance with our established protocol for investigating potential genes associated with PCD, we detected a homozygous missense variant of DNAH10 (NC_000012.12:g.123781230C > T, NM_207437.3:c.589C > T, NP_997320.2:p. R197W) in the proband (IV-4) (Fig.1). Sanger sequencing verified that the proband carried a homozygous variant, whereas the unaffected parents had a heterozygous variant (Fig.2). Homozygosity mapping confirmed that the homozygous region in chromosome 12 contained a variant of DNAH10 in the proband (Fig. S1). Homozygosity mapping showed that the largest homozygous fragment contained the PCD pathogenic genes DNAH5 and CCNO, but none of these two gene variants was found in the whole exome sequencing data. Homozygous fragments of other chromosomes were found to have known PCD causative genes, but no homozygous SNP or indel variants were found in any of them (Tables S4 and S5). The DNAH10 variant was rare in public human databases, including the 1000 Genomes Project and gnomAD (Table S6). The DNAH10 missense variant was also predicted to be disease causing using the Sorting Intolerant From Tolerant, PolyPhen-2, Protein Variation Effect Analyzer, and Combined Annotation Dependent Depletion tools (Table S6).
The p.R197W variant is located in the N-terminal of DNAH10 (Fig.2), which is responsible for dimerizing with other dynein proteins and connecting to the intermediated chain. Notably, the affected DNAH10 residues (R197) were conserved among different species (Fig.2). Modeling using SWISS-MODEL software suggested that R197W mutations in DNAH10 affected intramolecular hydrogen bond formation, which is predicted to affect the hydrophobicity and stability of the alpha helix (Fig.2). Collectively, these data suggest that DNAH10 is an excellent candidate to explain the PCD phenotype of the patient.
3.3 DNAH10 loss in airway cilia in the patient with DNAH10 homozygous variant
The presence and localization of several proteins belonging to different substructures of the axoneme, including DNAH10 and accepted markers of ODA (DNAH5) and IDA (DNALI1) were determined to further explore the ultrastructural defects of respiratory cilia revealed by TEM. Interestingly, compared with normal epithelial cells, in which DNAH10 staining was specifically localized along the ciliary axoneme, DNAH10 staining was absent in the respiratory cilia of the proband (Fig.3 and S2). DNAH5 was intact, whereas DNALI1 was absent in the respiratory epithelial cells of the proband (Fig.3 and 3C).
3.4 PCD phenotype in Dnah10M/M mice
A Dnah10M/M-knockin mouse model harboring a homozygous Dnah10 variant (c.778CGC > TGG, p.R260W) that corresponded to the PCD-affected proband (c.589C > T, p.R197W) was constructed using CRISPR-Cas9 technology to demonstrate whether the DNAH10 variant in the patient with PCD was pathogenic (Fig. S3A). The designed mutation site was confirmed in mouse genomic DNA using PCR amplification and Sanger sequencing (Fig. S3B). Dnah10 mRNA was also nearly absent in the trachea and testes of Dnah10M/M mice as detected by RT-qPCR (Fig. S4A and S4B).
Approximately 7% (3/40) of Dnah10M/M mice presented growth retardation and head enlargement (Fig.4). Evident hydrocephalus signs were suggested by the hematoxylin and eosin (H&E) staining of the brains of 5-week-old Dnah10M/M mice (Fig.4).
We observed chronic lung infection in Dnah10M/M mice by H&E staining, which manifested as lymphocyte and plasma cell aggregation in the lung stroma and pulmonary interstitial hyperplasia when compared with Dnah10+/M mice (Fig.4). Ultrastructural examination of the airway epithelium cilia by TEM revealed microtubular disorganization and IDA defect of the ciliary axoneme, which was similar to that in the affected man (Fig.4–4F). Half of the visual fields of the cilia in the Dnah10M/M mice were totally static or swing in residue pattern (Fig.4), whereas half of the other fields (10/20) remained in subtle reduced beat (CBF=11.0±0.1 Hz) when compared with those in the Dnah10+/M mice (CBF=12.7±0.3 Hz, Fig.4).
Simultaneously, we observed infertility in Dnah10M/M male mice (Fig.5), whereas Dnah10M/M female mice gave birth normally. Similar testicular weights were observed in the Dnah10M/M and Dnah10+/M mice (Fig.5). Using computer-aided sperm analysis, we found that the sperm count in Dnah10M/M mice was remarkably lower compared with that in Dnah10+/M mice (Fig.5), and the sperm was completely immotile in Dnah10M/M male mice (Fig.5). The sperm flagella of Dnah10M/M male mice exhibited multiple morphological abnormalities, including absent, short, coiled, and irregular flagella (Fig.5, Table S7). TEM results showed a disorganized 9+2 axoneme structure and mitochondrial sheath and dense outer fibers in Dnah10M/M male mice (Fig.5–5H). Additionally, laterality defects were not observed in the Dnah10M/M mice. Together, the similar PCD profiles observed in Dnah10M/M mice further indicated that the DNAH10 variant (c.589C > T) was pathogenic for PCD.
3.5 Dnah10-knockout mice recapitulates PCD phenotype in human
The methods used to construct the
Dnah10-knockout mice were described in our previous report, and the
Dnah10-knockout mice showed evident asthenoteratozoospermia similar to that in humans [
16]. We expanded mouse breeding to perform follow-up observations of the phenotypes of
Dnah10-knockout mice from our previous study. We found that approximately 10% (5/50)
of Dnah10−/− mice exhibited growth retardation and head enlargement (Fig.6). Evident hydrocephalus signs were suggested by the H&E staining of the brains of
Dnah10−/− mice aged 8 weeks (Fig.6). Laterality defects were not observed in
Dnah10−/− mice.
RT-qPCR of Dnah10 showed that Dnah10 expression was considerably abolished in the trachea and testes of Dnah10−/− mice (Fig. S5A and S5B). H&E staining of lung tissues revealed that Dnah10−/− mice showed signs of chronic lung inflammation characterized by lymphocyte and plasma cell-dominated chronic infiltration in the lung stroma and mucus accumulation in small airways (Fig.6). TEM results showed that the cross sections of the axoneme in Dnah10−/− male mice revealed a disorganization of the 9+2 structure and IDA defects (Fig.6–6F). Compared with the normal CBF (12.3±0.3 Hz) of Dnah10+/− mice via HSVM analysis (Fig.6), the cilia of Dnah10−/− mice were totally static or with residual motility in 80% (16/20) of the visual fields and beat slowly (CBF=6±0.5 Hz, Fig.6) in a stiff manner in the other 20% (4/20) of the observed fields. Together, these data indicate that Dnah10 deficiency can cause a PCD phenotype in mice, which is similar to that of the affected man.
4 Discussion
To the best of our knowledge, this study is the first to report DNAH10 deficiency in PCD, characterized by recurrent respiratory infection, laterality defect, and male infertility. Dnah10-knockout mice and Dnah10-knockin mice resembled the features of PCD in the man in this study.
Motile cilia are widely distributed in the respiratory epithelium, ependyma, sperm flagella, oviduct, and embryonic nodes, which provide the driving force for epithelial fluid and nodal flow and sperm flagellar movement [
2,
5].
DNAH family members encode 13 macromolecular motor proteins of IDA (DNAH1, DNAH2, DNAH3, DNAH6, DNAH7, DNAH10, DNAH12, and DNAH14) and ODA (DNAH5, DNAH8, DNAH9, DNAH11, and DNAH17) that hydrolyze ATP and release energy to maintain ciliary beating in a regular pattern and specific frequency. The absence of only one of the many dynein isoforms in human cilia has been associated with pleiotropic symptoms due to defects in ciliary motility [
5]. Previous studies have shown that
DNAH5,
DNAH9, and
DNAH11 are PCD-associated genes [
10,
11,
30].
DNAH1 and
DNAH6 variants have been reported to result in asthenoteratozoospermia, with or without the manifestation of PCD symptoms [
12–
15]. These observations indicate that dynein isoforms play important roles in ciliary and flagellar functions.
Recently,
DNAH10 has been recognized as an isolated asthenoteratozoospermia gene [
16,
17]. However, in our study, the patient carrying the
DNAH10 homozygous variant and
Dnah10-mutant animal models presented with typical PCD symptoms. These phenotypic differences may be due to population heterogeneity or differences in environmental exposure. Interestingly, similar differences between PCD and isolated infertility were observed for other PCD causative genes or isolated asthenoteratozoospermia genes, such as
DNAH1 [
12,
13],
DNAH6 [
14,
15],
SPEF2 [
31,
32], and
CFAP43 [
33,
34]. These different phenotypes may contribute to testis-specific isoforms or homologs between tissues, which may prevent the effect of specific mutations on spermatogenesis or ciliogenesis [
35,
36]. Therefore, the exact mechanisms involved in flagellar and ciliary functions require extensive investigation, which is a necessity in the counseling of patients about their PCD symptoms and men carrying the
DNAH10 variant.
Dynein assembly involves complex processes, such as cytoplasmic preassembly through the aid of chaperoning complexes, transportation into the ciliary/flagellar compartment through interactions with intraflagellar transport (IFT) machinery, and anchoring onto doublets through the aid of proteins that form high-affinity docking sites [
6,
37]. In our study, we found an immotile or retarded beating frequency with a stiff pattern in the respiratory cilia of
Dnah10-mutant mice. Accordingly, ultrastructure examinations revealed that
DNAH10 deficiency led to IDA loss and axoneme disarrangement, and immunostaining revealed that DNALI1 (IDA subunit a–c markers) was also lost in the respiratory cilia, suggesting that DNAH10 may participate in multiple processes as a component of cytoplasmic preassembly, as a cargo of IFT, or as a cooperator with the docking complex. Similar ultrastructural deformities can be observed in mutations of ruler protein genes
CCDC39 and
CCDC40 [
38–
40], which display attachment sites for IDA and nexin–dynein regulatory complexes, and in other IDA-coding genes, such as
DNAH1 [
13] and
DNAH6 [
14]. Therefore, exploring the pathogenesis of
DNAH10 in ciliogenesis and spermatogenesis is worthy of further studies.
Our study found that a small proportion of mice with
Dnah10 deficiency developed hydrocephalus, a severe consequence of cerebrospinal fluid circulation disorders [
41]. Several PCD pathogenic genes demonstrated hydrocephalus phenotypes in humans or mice, such as genes for ciliogenesis (
FOXJ1,
CCNO,
MCIDAS), ODA components (
DNAH5), and cilia- and flagella-associated protein (
CFAP43) [
30,
34,
42–
44]. Hydrocephalus is more common in animal models of PCD than in humans. For example, hydrocephalus was found in homozygous mouse models with
Dnah5 mutation, but the incidence rate of hydrocephalus in patients with PCD was only 2.5% (2/80), which may be related to the wider and shorter aqueduct of the midbrain in humans than in mice [
45]. This difference in humans and mice was also observed in the present study. Our findings suggest that
DNAH10 defects may affect ependymal motile cilia and consequently cause hydrocephalus, expanding the clinical spectrum of PCD.
Mice are the most common mammalian model because of their high similarity in genome, anatomy, and physiology with humans [
46].
Dnah10-knockin and
Dnah10-knockout mice recapitulated the manifestation of chronic respiratory infection and asthenoteratozoospermia, but they did not develop laterality defects, implying variable phenotypes in different species. Additionally, laterality defects in the mouse progeny was difficult to completely observe, which maybe due to the randomization of organ arrangement and the underlying compensation mechanism [
47,
48].
Our study has some limitations. First, we could not assess the ciliary motility of the proband owing to the patient’s refusal. We hope to analyze the CBF and ciliary beat pattern in a large cohort of patients with
DNAH10 deficiency to describe its ciliary beating profile by HSVM analysis in the future. Second, we did not show the absence of DNAH10 at the protein level in mouse models, because a stable and ideal commercial anti-DNAH10 antibody for mice was unavailable. However, we tested
Danh10 variants at the DNA and RNA levels, which could demonstrate the expression deficiency of DNAH10 to some extent. Moreover, PCD is a complex genetic disorder without a definite clinical cure strategy; thus, only symptomatic treatment is provided to patients with PCD. Gene editing through airway organoid construction may bring new treatment strategies for patients with PCD in the future [
49,
50].
In summary, we identified for the first time a novel DNAH10 homozygous variant in a patient with PCD. Genetic evidence from DNAH10-associated patient and Dnah10-knockin and Dnah10-knockout mice strongly suggests that biallelic DNAH10 variants can induce multisystem motile ciliopathies, such as PCD, which is characterized by chronic respiratory infection and infertility due to ultrastructural and functional abnormalities of the cilia and sperm flagella. Our clinical and genetic findings provide a candidate gene for the molecular diagnosis of PCD and extend our knowledge of the pathogenic effects of DNAH10 mutations.