1. Introduction
Single nucleotide polymorphisms (SNP) are variations in DNA that cause a single nitrogenous base substitution in the gene sequence. Most SNPs are neutral, but some may contribute to the predisposition to disease, consequently acting as genetic markers [
1]; or influence their evolution and serve as useful outcome markers; or even determine the responses to the treatment of diseases such as cancer [
2]. The main advantages of using SNPs as biomarkers are related to their stability (because they have a low mutation rate), high frequency (they are present in more than 1% of the population), and facilitate the optimization of analysis techniques through automation [
3,
4].
Depending on its location, the SNP can promote nucleotide substitutions that lead to different alterations in the DNA; alterations in the formation of proteins (to their structure, function and stability); alterations in protein formation (regarding its structure, function and stability) [
5] and in the regulation of protein-protein interactions [
6]; changes in the mechanisms of splicing, transcription, localization and degradation of mRNAs [
7]; functional changes in transcription factor binding sites, intron/ exon splicing sites, exonic splicing promoter sites, and miRNA binding sites [
7].
The impact of the effects of amino acid substitution on the structure and function of a given protein is essential for a better understanding of the complex mechanisms involved in diseases caused or related to this protein. Likewise, it is important to understand the effect of SNPs on the regulation of gene expression [
3,
4].
Cell adhesion molecules (CAMs) are transmembrane proteins that play important roles in cell-cell communication and interaction [
8,
9,
10]. Selectins are a family of CAMs that are involved in the initial steps of cell adhesion and are particularly important in the immune response. Selectins are able to bind to specific carbohydrates on the surface of different cells, helping to bring these cells closer together. This is important for the immune response, as it allows leukocytes to move from the blood vessels to sites of infection or injury in tissues [
11,
12]. The interaction between selectins and carbohydrates is weak and reversible allowing cells to adhere and detach from each other quickly and efficiently [
11,
12]. In addition to their role in the immune response, selectins have also been implicated in other processes, such as tumor metastasis [
13,
14]. This effect has enabled the development of target-directed drugs that may become important in the treatment of these conditions.
There are three types of selectins: L-selectin, P-selectin, and E-selectin [
12,
15,
16]. L-selectin is encoded by
SELL gene, located on the long arm of chromosome 1 (1q24.2) [
17]. It is believed that the glycosylation patterns of this protein may dictate its functions in the cell, but the mechanisms involved in these functions are still unclear [
17]. Initially its expression was considered to be exclusive to the leukocyte surface, including myeloid cells, naïve T cells and some activated T cells [
17,
18], but this molecule expression has more recently been observed in several types of cancer and other cell types [
13,
14,
19,
20,
21,
22]. Overall, L-selectin favors interactions that allow both leukocytes [
18] and metastatic tumor cells [
19,
22] leave the bloodstream, come into contact with activated endothelial cells and start the rolling process [
15,
18,
23]. In addition to its main role in the process of initial capture of immune cells and cell adhesion, it is also relevant in acute and chronic inflammatory processes [
10].
P-selectin has also been suggested to play a role in cancer [
24,
25] promoting the spread of cancer cells by facilitating their migration using CAM’s migrations system. The protein is encoded by
SELP gene, located on chromosome 1 (1q24.2). It plays a role in the process of blood clotting and inflammation and is found on the surface of platelets and endothelial cells [
26,
27]. In addition, P-selectin has been shown to be involved in the recruitment of immune cells to tumor site [
14,
28]. One of the most important ligands of P-selectin is PSGL-1, a protein encoded by
SELPLG gene, also located on chromosome 12 (12q24.11). It is expressed on the surface of various types of immune cells including T cells, B cells, and neutrophils. Recent studies [
29] have shown that PSGL-1 may promote cancer progression by promoting the adhesion and migration of immune cells to the tumor microenvironment. This can lead to the recruitment of pro-tumor immune cells and the suppression of anti-tumor immune responses, ultimately promoting tumor growth and metastasis [
30]. PSGL-1 has been found to be upregulated in various types of cancer, including breast, lung, and colorectal cancer. This upregulation is associated with more aggressive tumors and poorer prognosis [
30]. In addition, targeting PSGL-1 has been considered a potential approach for cancer immunotherapy [
31,
32,
33,
34,
35,
36,
37,
38].
E-selectin is also expressed on endothelial cells and is involved in the recruitment of immune cells to sites of inflammation [
12]. It is encoded by
SELE gene, located on the long arm of chromosome 1 (1q24.2). E-selectin plays an important role in the process of inflammation by facilitating the adhesion and migration of immune cells to sites of inflammation [
39]. Recent studies have shown that E-selectin may also be involved in cancer, since it was found to be upregulated in various types of cancer including breast [
24,
40], lung [
41], and pancreatic cancer [
42], and its expression levels are associated with more aggressive tumors and poorer prognosis in cancer patients [
43].
Because of their relevant role in the migration of immune system and metastatic tumor cells, the study of SNPs in SELL, SELP, SELE and SELPLG is fundamental for the identification of potential biomarkers of susceptibility, diagnosis, prognosis and even possible therapeutic targets for cancer. The objective of this study was to understand the alterations caused by the presence of polymorphisms in the DNA structure of these genes and the consequent alterations in the morphology and function of the corresponding proteins through bioinformatics tools.
4. Discussion
For a long time, selectins were thought to be unique to the immune system or correlated cells. However, more recent solid evidence has demonstrated their overexpression in tumor cells, suggesting that these CAMs play an important role in metastatic pathways [
61]. Using a wide repertoire of bioinformatics tools, we identified a series of SNPs that, due to their MAF>0.1, may be significantly present in the population and correlate with several diseases, including cancer. Our bioinformatic analysis demonstrated that rs2229569, rs1131498, rs4987360, rs4987301 and rs2205849 of
SELL gene; rs3917777, rs2205894 and rs2205893 of
SELP gene; rs7138370, rs7300972 and rs2228315 of
SELPLG gene and; rs1534904 and rs5368 of
SELE gene may be promising biomarkers of diseases, especially in cancer patients, considering the role of selectins (L-selectin, P-selectin and E-selectin) and PSGL-1 in these conditions.
L-selectin (
SELL) expression has been related to several types of cancers such as endometrial [
62,
63], breast [
64] and thyroid [
20]. An in-silico study [
64] using the TCGA and On-comine databases found higher expression of
SELL in tumor tissues, suggesting that L-selectin could be a biomarker of inflammatory microenvironment. In addition, the authors observed higher
SELL expression in breast cancer patients with better outcome. Kobawala et al. [
20] analyzed 150 patients with thyroid nodules (83 papillary thyroid carcinomas - PTC and 67 benign nodules) by ELISA and immunohistochemistry techniques and observed a higher protein expression of L-selectin in cells and higher serum levels in PTC patients compared to benign thyroid diseases, suggesting a possible role of this adhesion molecule in the development of thyroid cancer.
Despite several studies reporting gene and protein expression of L-selectin, the literature is still scarce with regard to the study of its polymorphisms in cancer. We demonstrated that rs2229569 (exonic; G/A - P213S and G/T - P213T) may alter the structure and folding of DNA and promote physical-chemical alterations in protein structure with decreased stability and possible protein functional alterations. Similarly, rs1131498 was shown to be able to modify binding patterns with adjacent amino acids, thus altering protein structure at the molecular and atomic level. We also did not find reports in the literature of other possible cancer biomarkers, such as the intronic polymorphisms rs4987360 (A/G), rs4987301 (G/A and G/T) and rs2205849 (T/C), which were considered deleterious by all the tools that we used.
P-selectin (SELP) is mainly expressed on the surface of activated endothelial cells and platelets [
11,
26]. Its expression upregulation is correlated with the pathogenesis of various diseases, including atherosclerosis [
65], thrombosis [
66], diabetes [
67] and cancer [
14,
24,
25,
28,
68]. Because P-selectin has been shown to promote the formation of cancer metastases by facilitating the adhesion and migration of tumor cells [
24,
25], this molecule has emerged as a potential therapeutic strategy for the treatment of various conditions. Some studies suggested that SNPs of SELP gene could be diagnostic biomarkers of head and neck [
28] and pancreatic cancer [
68]. Our analysis also identified rs3917777 (T/A, T/C and T/G), rs2205894 (T/A and T/G) and rs2205893 (T/A and T/G) as promising biomarkers that have not yet been investigated in cancer patients.
PSGL-1 (P-selectin glycoprotein ligand-1) plays a key role in mediating leukocyte adhesion to activated endothelial cells and platelets, as well as facilitating leukocyte rolling and migration into sites of inflamation [
33] and has been widely studied because of its role as an immune checkpoint and its promising role in immune checkpoint landscaping [
32,
33]. Monoclonal antibodies against PSGL-1 have been shown to inhibit tumor growth and metastasis in preclinical models of cancer [
29,
30,
37]. However, studies on its polymorphisms and its possible clinical use are still scarce. Our data indicate that both rs7138370 and rs7300972 could promote DNA structural and functional alteration; rs2228315 (C/T; M62I) also may promote modifications in DNA and, additionally, modify structure, function, stability, rigidification and interaction with adjacent amino acids, making these SNPs interesting candidates for biomarkers. PSGL1 polymorphisms can help identify response patterns to immunological therapies to which this molecule is targeted and improve the quality of treatment offered to cancer patients.
Expression of E-selectin at significantly higher levels has been related to several types of cancer such as colorectal [
14], gastric [
69] and breast [
24,
40] was correlated with decrease risk of hospitalization or need for respiratory support/death in COVID-19 cases [
70]. Na Li et al [
71], in an in-silico study employing TCGA and the GEPIA server, found higher gene expression in tumor samples when compared to healthy samples, and in of lymph node metastasis in colorectal cancer [
71]. Furthermore, when antitumor drugs were applied, gene expression levels were reduced, suggesting a possible role of E-selectin as an oncogene. Targeting E-selectin has emerged as a potential therapeutic strategy for the treatment of cancer [
72,
73,
74]. Several approaches have been developed to target E-selectin, including monoclonal antibodies and small molecule inhibitors and have shown promise in preclinical studies by reducing tumor growth and metastasis [
75,
76,
77,
78,
79]. SELE polymorphisms have been reported as possible risk indicators for a number of medical conditions such as hypertension in cases of occupational stress [
80], risk of coronary artery disease [
81], enlargement of renal cyst in patients with polycystic kidney disease [
82], type 2 diabetes [
83] and subclinical atherosclerosis and increased platelet activity in systemic lupus erythematosus [
84]. We demonstrated that rs1534904 (T/A and T/G) could provoke important alterations in DNA. This polymorphism has not yet been reported in studies evaluating disease conditions. We also showed that rs5368 (H468Y) may alter the secondary structure of the E-selectin protein and is capable of increase protein stability and flexibility. Zakariya BF et al [
40] found CT heterozygous genotype frequency significantly higher in breast cancer patients, confirming the importance of SELE polymorphisms in cancer risk prediction.
Author Contributions
Conceptualization, LTR, KCP, EST, NEB and LSW; methodology, LTR, DZV and EST; validation, LTR and DZV; formal analysis, LTR and KCP; investigation, LTR and DZV; data curation, LTR; writing—original draft preparation, LTR, KCP, DZV and LSW; writing—review and editing, LTR, NEB and LSW; visualization, LTR, KCP, NEB and LSW; supervision, NEB and LSW; project administration, LTR and DZV; All authors have read and agreed to the published version of the manuscript.