4 | DISCUSSION
Mutations of the EDAR gene can result in
HED(A.K. et al., 2017;
Feng et al., 2018). The Human Gene
Mutation Database (HGMD Professional, 2018.3) has 62 registered
pathogenic variants of EDAR , of which 50 have the HED
phenotype(Parveen et al., 2019). In
recent years, several EDAR mutations in NSTA patients have been
identified(Arte et al., 2013;
Jonsson et al., 2018;
Mumtaz et al., 2020;
Zeng et al., 2017). In our study, a newEDAR missense mutation c.338 G>A(Cys113Tyr) was
detected by whole exome sequencing in Patient 4 and his mother. A
cross-species alignment of amino acid sequences of EDAR showed
that p.C113 was conserved across seven species, implying the high
pathogenicity of the variant. Our result expands the mutation spectrum
of EDAR .
EDAR is a type I transmembrane protein and a member of the TNF receptor
superfamily. It has a cysteine-rich domain in the extracellular region
(encoded by exons 3, 4 and 5), as well as a death domain in its
intracellular region (encoded by exon 12). EDAR interacts with
extracellular EDA1 and intracellular EDARADD via the extra- and
intracellular regions to form a complex. This in turn activates
downstream nuclear factor kappa B (NF-κB) to mediate transcription of
the target gene(Kumar, Eby, Sinha, Jasmin,
& Chaudhary, 2001; Masui et al., 2011;
Okita, Asano, Yasuno, & Shimomura, 2019;
Outi et al., 2001;
Parveen et al., 2019). The p.Cys113Tyr
(c.338G>A) mutation in exon 4 occurs in a region that
encodes the cysteine-rich domain that binds to EDA1. We speculated that
the p.Cys113Tyr substitution of EDAR might alter its affinity with EDA1,
so that it instead binds to EDARADD, ultimately affecting the activation
of downstream nuclear factor (NF)-κB.
By studying the information of all NSTA patients with EDARmutations, we found that more than half (5/8) of EDAR mutations
in NSTA patients were concentrated in the death domain encoded by exon
12, and the majority of mutations (6/8) were missense mutations (Table
1). We also observed that the most common missing teeth were the
maxillary lateral incisors, followed by the mandibular lateral incisors
and the mandibular central incisors. The maxillary first premolars and
mandibular canines were the least likely to be affected (Table 2; Figure
4). It seems that anterior teeth, especially the incisors, are sensitive
to mutations in the EDAR gene. Arte et al.
(Arte et al., 2013)and Mumtaz et al.
(Mumtaz et al., 2020) found that both
deciduous and permanent incisors were involved most commonly in those
EDAR-related TA cases, is similar to our finding. In particular, the
number of permanent teeth lost in patients with EDAR -related NSTA
ranged from 2 to 10. In the present study, the proband and his mother
had 18 and 17 permanent teeth missing, respectively. This differs from
previous studies, but our participants presented with no signs of other
ectodermal hypoplasias, such as in the hair or skin. We hypothesized
that the differences in the number of missing teeth might be associated
with single nucleotide polymorphisms or epigenetic factors.
According to M. Yu et al.(M. Yu et al.,
2019), over the past two decades 198 different mutations had been
detected that are responsible for NSTA, of which 27 are derived fromEDA1 . Previous studies revealed a clear link between the genotype
and phenotype for congenital tooth
deficiency(Han et al., 2008;
He, Liu, Han, Liu, & Feng, 2016;
Wong et al., 2018;
Zhang et al., 2011). Han et
al.(Han et al., 2008) studied 24 NSTA
patients with EDA1 mutations and conducted statistical analysis
on the number of missing teeth in each position of dentition. They found
that the most likely missing teeth were the lateral incisors followed by
the mandibular central incisors; and the least likely missing teeth were
the maxillary central incisors and first permanent molars. The results
of He et al.(He et al., 2016) also
confirmed that these characteristics were specific phenotypes of NSTA
caused by EDA1 mutations.
In our research, three mutations were found in the EDA1 gene of
oligodontia Patient 1-3. The probands shared the EDA1 mutations
with their mother (Figure 3), indicating that the mutant alleles were
inherited from the maternal line in Family 1, 2, and 3. Two of the
mutations (c.865C>T and c.866G>A) observed in
this study were located in the TNF domain of EDA1 . The TNF
homology domain forms a homotrimer, which is required for interaction
with the receptor at the monomer–monomer
interface(Hymowitz et al., 2000). Song et
al.(Song et al., 2009) carried out
structural analysis on the EDA1 protein and found that in the wild-type
3D EDA1 structure, the Arg289 residue is located at the outer surface of
the homotrimer and makes structural hydrogen bonds with Asn272. In
addition, Arg289 forms hydrogen bonds and electrostatic interactions
with Glu308 of the adjacent homotrimer(s) to stabilize the multi-trimer
asymmetric unit(Lee et al., 2014;
Ruiz-Heiland et al., 2016;
Song et al., 2009). Variants at the p.289
location would abolish these interactions and reduce protein stability.
Therefore, codon 289 mutations in the TNF domain partly impact on the
binding of EDA protein to EDA receptor.
However, it is worth noting that although all changes occurred at p.289
of EDA1 , the phenotypes of patients were slightly different. We
performed a genotype-phenotype analysis and found that all maxillary
lateral incisors are affected, followed by mandibular incisors and
mandibular second premolars. In contrast, the second molars, mandibular
first molars and maxillary central incisors were less affected. It is
particularly interesting that the maxillary first molars were present in
all patients. Our phenotypic analysis of the mutations at codon 289 ofEDA1 is consistent with the typical phenotype resulting fromEDA1 gene mutations. However, the mechanism that causes the
slight changes to the phenotype of teeth loss due to protein mutations
remains to be studied.