4. Discussion
CELSR, planar cell polarity organizer, forms trans-dimers and is involved in the organization of polarized structures between neighboring cells [
14,
15] and have a coordinating effect in the propagation of a signal from cell to cell [
16,
17]. Animal models revealed, that these process are necessary for the proper formation of the neural tube and its derivatives[
18,
19], stereocilia and neurons in the inner ear[
20,
21], ciliated cells of the fallopian tube[
22] and other tubular organs, hair[
23], and endothelial valve cells[
24]. Nevertheless, the functioning of CELSR family proteins at the molecular level remains largely unclear. Like most adhesion GPCRs, they are orphan receptors with an unknown activation mechanism. Even the assumption of a connection between CELSR and heterotrimeric G-protein is based only on the presence of a 7-TM domain characteristic of GPCRs, and the site that interacts with any G-protein has not yet been identified. Experimental data indicate that adhesion G protein–coupled receptors (ADGRs) can activate signaling events unrelated to G-proteins but based on homophilic trans-interactions necessary for planar cell polarity[
15].
Disruptions in CELSR1 in humans are associated with the development of lymphatic malformation 9 (LM9, OMIM: 619319). The expression of
Celsr1 in mouse vessels begins during the formation of valves at embryonic day 16 (E16-E16.5) [
24], which is approximately equivalent to 23 weeks of gestation in humans [
25]. During this process, endothelial cells elongate, undergo reorientation, and coordinated migration into the lumen of the vessel, where they initiate valve leaflet formation. Knockout of
Celsr1 in mice leads to disruption of intercellular contacts, endothelial cells are unable to rearrange and adopt a perpendicular orientation for valve formation, leading to aplasia and impeding the ability of the lymphatic system to collect and drain fluid from tissues, ultimately resulting in lymphedema[
26]. However, Celsr1 is necessary for development but not for maintaining the structure of lymphatic vessels - knockouts after valve formation do not have a significant impact on vessel structure[
24].
CELSR proteins are non-classical cadherins that do not interact with catenins, unlike classical representatives. The structure of CELSRs includes a large ectodomain, with 8 or 9
N-terminal cadherin repeats, up to 8 epidermal growth factor-like domains, two laminin G-type repeats, an EGF-like laminin, one hormone receptor motif (HRM), and a GPCR Autoproteolysis Induction (GAIN) domain. The membrane part consists of a 7-transmembrane domain (7-TM), and the intracellular cytoplasmic tail has 300-600 amino acid residues [
15]. The GPS region in the GAIN domain of most ADGRs is responsible for autoproteolysis in the extracellular domain, which occurs in the ER during receptor biosynthesis. Mature ADGRs are usually cleaved and exist as a non-covalently bound complex of N- and C-terminal fragments, but some of them, including CELSR1, do not cleave due to the absence of a consensus catalytic sequence (Leu↓Thr/Ser/Cys) in GPS [
15]. Cadherin-like, EGF-like, and laminin-like repeats confer adhesive properties and facilitate intercellular communication, while GAIN and laminin domains can bind unidentified ligands.
Transmembrane domain of CELSR1 is relatively small (~8% of all amino acids), and contains few known mutations, especially those associated with diseases. We discovered rare variant, c.7664A>C, resulted in the substitution of histidine to proline at position 2555 (H2555P) of the intracellular loop in the CELSR1 transmembrane domain in a brother and sister with lower extremity lymphedema caused by aplasia and hypoplasia of lymphatic structures. The oedema in both children appeared in the first year of life, and LD was not diagnosed in of other family members. Despite the low/unknown frequency of the detected polymorphism in the population, which is one of the evidences of pathogenicity [
13], identified variant cannot be confidently classified as pathogenic or neutral. The possible pathogenicity of H2555P, in addition to its frequency of occurrence, is evidenced by its presence in two close relatives with primary lymphedema and by the results of
in silico analysis predicting a damaging/negative effect of the mutation on protein function. However, no missense mutation in the transmembrane domain of CELSR1 has been previously classified as pathogenic in LM, and no functional analysis of such alterations has been performed. The pathogenic/likely pathogenic findings closest to p.H2555 include p.A2510P/V and p.D2601N, revealed in the patients with congenital heart defects and primary angle closure glaucoma [
27,
28,
29].
To date, less than 10 experimental studies have been published describing the genetic features and clinical-phenotypic characteristics of CELSR1-associated lymphedema (
Table 2). It has been shown that the disease belongs to forms with early manifestation (first decade of life), characterized by incomplete penetrance and variable expression, predominantly affecting females.
Both our patients demonstrate hypoplasia and aplasia of lymphatic vessels in lower extremities. Such a structure of the lymphatic system, onset of lymphedema at birth or in infancy without syndromic features and systemic involvement is associated with the number of conditions, including Lymphatic malformation 1 (Milroy’s lymphedema, OMIM 153100, 5q35.3, FLT4), Lymphatic malformation 2 (OMIM 611944, 6q16.2-q22.1), Lymphatic malformation 3 (OMIM 615907, 1q42.13, GJC2), Lymphatic malformation 4 (OMIM 615907, 4q34.3, VEGFC), Lymphatic malformation 5 (OMIM 153200, not mapped), Lymphatic malformation 9 (OMIM 619319, 22q13.31 CELSR1), Lymphatic malformation 10 (OMIM 619369, 8p23.1, ANGPT2).
Aplasia or hypoplasia of initial lymphatic vessels is well-known clinical feature of Milroy’s lymphedema, as well as blood vessel abnormalities, including prominent leg veins and saphenous venous reflux [
35,
36,
37]. In our study, both siblings have been presented by aplasia of lymphatic vessels according to MRL data (
Figure 2). Additionally, male patient had prominent vein on left leg, however nail abnormalities or venous insufficiency were not identified in any of the children. Clinical features of another PLD types are much less studied, so it’s not possible to make a more accurate comparison in phenotypes. However, we did not find any pathogenic changes in coding sequences of genes, associated with primary lymphedema, except of CELSR1, in our patients.
Author Contributions
Conceptualization, R.M., Yu.F. and A.R.; methodology, Yu.F. and R.M..; validation, A.R. and R.M.; formal analysis, A.I., E.B., M. Zh.; investigation, Yu.F., A.F., A.I..; resources, A.R.; writing—original draft preparation, Yu.F., A.I.; writing—review and editing, R.M., M.Zh. and A.R; visualization, A.F.; supervision, R.M. and A.R.; funding acquisition, A.R. All authors have read and agreed to the published version of the manuscript.