Discussion
In this study, two compound heterozygous mutations of DNAAF2 were
identified in two siblings with similar characteristic manifestations of
PCD including recurrent pneumonia with atelectasis mainly in right
middle and left lingular lobe, early bronchiectasis, sinusitis, otitis
media, significant reduction in nNO level and complete loss of ODAs and
IDAs of bronchial cilia found by the TEM. One of the mutations is a
nonsense mutation of NM_018139: c.156C>A (p.Y52*) which
has already been reported in a Chinese family and proved to be
pathogenic[5]. Another mutation is a frameshift mutation
(NM_018139: c.177_178insA (p.E60Rfs*3) , which is a novel mutation.
DNAAF2 is located on chromosome 14q21.3 and consists of three exons
encoding cDNAs [6]. The mutation p.E60Rfs*3 locates in exon1 and
near another mutation p.Y52*. Exon1 is consists of 621 amino acids, and
the mutation of E60Rfs*3 leads to early transcription termination after
the 63 amino acids (Fig.6). The p.E60Rfs*3 mutation has been predicted
to be pathogenic with the evidences of PVS1+PM2+PM3+PP4 according to the
standards and guidelines of American College of Medical Genetics and
Genomics (ACMG). Multiple software, such as SIFT, Polyphen‑2 and
Mutation Taster, also predict the p.E60Rfs*3 mutation to be pathogenic.
In addition, none of cystic fibrosis (CF) or primary immune deficiency
(PID) related genes which may also lead to recurrent pneumonia,
atelectasis and bronchiectasis were not detected. Based on the clinical
manifestations and hereditary mode of compound heterozygous mutations
from both parents, it’s reasonable to highly suspected that the novel
p.E60Rfs*3 mutation of DNAAF2 is a disease causing mutation and is
responsible for the pathogenesis of PCD.
The movements of cilia can help to clear airway mucous, cerebrospinal
fluid flow along the brain ventricular system, support the transport of
the oocyte to the uterus, and male germ cell move along the female
reproductive tract [6]. The normal structure of the cilia consists
of the conserved basic 9+2
microtubule-based axonemes and other several functional modules[7].
Members of multi-subunit motor protein complexes, ODAs and IDAs are
responsible for the generation and
regulation of ciliary beating. Dynein arms are ATPase-based
protein-complexes, via ATP hydrolyzed by dynein heavy chain, the paired
microtubules slide relative to
each other, leading to cilia curve over the deformed
microtubules[8-9]. During the process, IDA causes the curvature of
cilia, ODA accelerates active sliding of the outer microtubules, leading
to effective propulsion produced by the cilia effectively paddling
forward and reverting paddling backward. Appropriate rheological
properties and the functional structure of cilia are the basis that
maintain mucociliary clearance which is essential to the respiratory
tract clearance defense[5].
Approximately 40 genes have been reported to be associated with PCD,
including DNAH5, DNAH11, DNAI1, DNAI2, DNAL1, TXNDC3, DNAAF1, and DNAAF2
et al. [1, 6,
10-14].
In 2008, two heterozygous mutations c.C23A [pS8X] and
c.1214-1215insACGATACCTGCGTGGC [p.G406Rfs89X mutations of DNAAF2 were
first identified in two consanguineous families with PCD presented with
recurrent respiratory tract infections, laterality defects and impaired
fertility[6]. DNAAF2 works as a facilitator of dynein pre-assembly.
The disfunction of DNAAF2 is associated with specific defects in the
interaction of intermediate and heavy chains in the cytoplasm which
leads to complete or partial loss of ODAs and IDAs and loss of
motility[6]. After that, only 15 PCD cases have been reported caused
by DNAFF2 with 19 mutations. The
cases of DNAAF2 mutation are
summarized in Tab.1[5-6, 15-27]. As for Chinese children, DNAFF2
gene is rare. As Guan et al reported reported in 81 Chinese children,
genes with the highest incidence of mutations that caused PCD were
DNAH11, followed by HYDIN, DNAH5, CCDC39, DNAH1 and CCNO, no DNAFF2 gene
mutation was detected[28]. In Chinese adult patients, Sun et.at have
identified two heterozygous mutations c.C156A [p.Y52X] and c.C26A
[p.S9X] in the DNAAF2 gene which lead to the defect of outer dynein
arms and inner dynein arms resulting in PCD with the manifestation of
male infertility[5]. Lu C et.at detected another two heterozygous
mutations c.491T>C [p.L164P] and c.822del
[p.A275Profs*10] in two females presents with the bronchiectasis,
sinusitis, situs inversus, and infertility, besides, the one with
c.491T>C mutation had scoliosis[27].
Consequently, we made an early diagnosis of PCD in two siblings with a
novel mutation of the rare DNAFF2 gene. The early stagy of PCD may be
easily misdiagnosed as common pneumonia or sinusitis. When a child
presented with recurrent pneumonia or atelectasis accompanied with
recurrent sinusitis or otitis media, PCD should be considered. The
characteristic HRCT manifestations including atelectasis in the right
middle and left lingular lobe, thickened bronchial wall, and mild
bronchiectasis can provide a diagnostic clue for PCD. Besides, nNO and
TEM of bronchial cilia are useful for the further diagnosis , and
genetic tests can help to confirm the diagnosis.
In conclusion, our study suggests that the p.E60Rfs*3 mutation of DNAAF2
gene is a novel disease causing mutation of PCD. This study enriches our
knowledge of clinical manifestations and genetic information of PCD
caused by DNAAF2.
Acknowledgements We are grateful to the patients and their
family for
their collaboration.
Funding Medical Science and Technology Project of Henan
Province (2018020621); Demonstration
application of remote collaborative service network for complex genetic
diseases (SJPT-03-01).
Conflicts of interest The authors declare that they have no
competeing conflicts.