4. Discussion
WES analysis of eight patients with BCL in this study identified 17 shared somatic variants with moderate or high impact (
GOLIM4, ITM2B, STN1, UNC79, PLEKHG4, BRF1, ENSCAFG00845007156, SEMA6B, DSC1, TNFAIP1, MYLK3, WAPL, ADORA2B, LOXHD1, GP6, AZIN1, and
NCSTN). According to the literature, considering the functions of these genes and related diseases,
GOLIM4, ITM2B, STN1, DSC1, TNFAIP1, WAPL, and
NCSTN can be assumed to play roles as tumor suppressor genes. GOLIM4 (also known as GPP130) is a component of the Golgi transport complex that plays an important role in the transport of Golgi proteins [
15]. Although the Golgi apparatus may be involved in tumor biological processes, the function of GOLIM4 during tumorigenesis remains unclear. Nevertheless, the Golgi apparatus and endosome dysfunction are involved in the progression of various tumors, and increased expression of GOLIM4 has been shown to inhibit cancer cell proliferation, promote apoptosis, and induce G1 phase arrest in human head and neck cancer cell lines [
16]. Integral membrane protein 2B (ITM2B; also known as BRI2) is a type II transmembrane protein that is a substrate for regulated intramembrane proteolysis [
17]. ITM2B induces apoptosis and inhibits proliferation [
18]. Downregulation of ITM2B has been identified in human lung cancer tissues; therefore, ITM2B appears to play a role as a tumor suppressor gene [
19]. STN1, along with CTC1 and TEN1, is a component of the CST (CTC1-STN1-TEN1) complex and is responsible for maintaining telomere and genome integrity [
20]. CST was first identified as a telomere-binding protein complex and functions in telomere replication and protection. CST can mediate end protection at double-strand breaks, likely using a similar strategy to filling in of the telomeric C-strand. Supporting this observation, CST has been shown to promote ploy polymerase inhibitor sensitivity in BRCA1-deficient cancer cells. Given its essential roles in replication and DNA repair, CST is known to be important for genome stability [
21]. DSC1 was predicted to encode a sodium channel based on a high sequence similarity with vertebrate and invertebrate sodium channel genes. In human medicine, decreased expression of DSC1 was related to the poor differentiation and prognosis of head and neck squamous cell carcinoma, lung cancer, melanoma, and colorectal carcinoma [
22,
23,
24,
25]. TNF-α-induced protein 1 (TNFAIP1; B12) gene is highly conserved gene in several species and is known as a tumor suppressor gene.
TNFAIP1 is induced by TNF-α and interleukin-6, and it is mainly involved in DNA synthesis and repair and apoptosis [
26,
27,
28]. In human medicine, breast cancer, gastric carcinomas, and lung cancer are known to be associated with
TNFAIP1 mutations [
29,
30,
31]. WAPL is important in regulating the level of aggregation of chromosomes by separating cohesive loops from chromatin. By separating cohesion from chromatin, WAPL is a regulator of the loading and unloading cycle. The loss of WAPL is known to result in p53-dependent cell cycle arrest [
32,
33]. The role of nicastrin (NCSTN) remains unknown. However, NCSTN is known to be related to AKT and p-AKT, which affect cell proliferation, growth, and differentiation [
34]. In addition, incomplete expression of NCSTN is known to reduce the expression of miR-100-5p, which acts as a tumor suppressor involved in cell self-renewal and wound healing [
35].
On the other hand,
BRF1, SEMA6B, ADORA2B, GP6, and
AZIN1 can be assumed to function as oncogenes. The
BRF1 gene encodes the BRF1 protein in its zinc ribbon domain and directly participates in the process of protein synthesis. Deregulation of BRF1 is associated with cell proliferation, cell transformation, and tumorigenesis. BRF1 is overexpressed in hepatocellular carcinoma, breast cancer, gastric cancer, prostate cancer, and lung cancer in humans [
36,
37,
38,
39]. Semaphorin 6b (SEMA6B) is a member of the semaphoring axon-guidance family and was initially characterized as an axon guidance factor with axon navigation functions but has also been demonstrated to induce or inhibit tumor progression. The overexpression of this gene is related to colorectal cancer in humans [
40,
41,
42].
ADORA2B encodes a protein belonging to the G protein-coupled receptor superfamily that plays a role in tissue distribution along with A1, A2A and A3. Abnormal expression of ADORA2B may play a pathophysiological role in some human cancers [
43]. ADORA2B is highly expressed in oral cancer, lung adenocarcinoma, and prostate cancer and promotes proliferation and metastasis of carcinoma cells [
43,
44,
45]. GP6 is a transmembrane protein that is the major signaling receptor for collagen on platelets and regulates several platelet functions, such as adhesion, aggregation, and procoagulant activity [
46,
47]. In addition, GP6 plays a role in supporting platelet adhesion to tumor cells, which is known to be involved in the metastasis of colorectal cancer and breast cancer [
48]. Adenosine to inosine (A-to-I) RNA editing catalyzed by adenosine deaminases acting on RNA enzymes is a post-transcriptional modification that has emerged as a key player in tumorigenesis and cancer progression. AZIN1 has been identified as one of the most frequently occurring A-to-I RNA alterations in colorectal cancer and hepatocellular carcinoma and acts as an oncogene [
49,
50].
The function of three genes (
UNC79, PLEKHG4B, and
ENSCAFG00845007156) and their associations with cancers have not been clearly identified. UNC79 protein forms an NALCN complex with NALCN, FAM155, and UNC80 proteins, which are involved in voltage-gated sodium and calcium channels [
51,
52]. There is no research about the association between
UNC79 mutations and cancers. Mutations in the pleckstrin homology domain-containing family g member 4b (
PLEKHG4B; puratrophin-1) gene are associated with the hereditary neurological disorder autosomal dominant spinocerebellar ataxia. However, the biochemical function of this gene product has not been described [
53]. Moreover, there is no research about the association between
PLEKHG4B mutations and cancers.
ENSCAFG00845007156 is similar to human Aldo-keto reductase family 1 member D1, but its function has not been identified, and protein analysis has not been performed to date.
For both genes (
MYLK3 and
LOXHD1), the function of each gene was not associated with cancers but was associated with diseases other than cancers. Myosin light chain kinase 3 (
MYLK3) is a protein-coding gene that acts as a regulator of the actin cytoskeleton and immune response signaling. WES revealed that MYLK3 mutations are associated with dilated cardiomyopathy in humans [
54].
LOXHD1 encodes a protein consisting of 15 polycystin lipoxygenase α-toxin repeats, which can bind lipids and proteins in other proteins [55]. Mutations in
LOXHD1 cause progressive hearing loss (Grillet et al., 2009). As a result, mutations in tumor suppressor genes can affect cancers such as CL. Therefore,
GOLIM4, ITM2B, STN1, DSC1, TNFAIP1, WAPL, and
NCSTN mutations may be prognostic markers for patients with CL. However, the functions of
UNC79, PLEKHG4, and
ENSCAFG00845007156 and their associations with cancers have not yet been identified. In addition,
MYLK3 and
LOXHD1 mutations appear to have a very low association with CL.
As a result of constructing a PPI network for approximately330 genes targeting 480 shared somatic variants, one large network was formed. A total of seven genes (STN1, AZIN1, ITM2B, ADORA2B, SEMA6B, NCSTN, and DSC1) in the network were genes identified with moderate to high impact mutations. However, none of the many genes observed in the network appear to be directly related to CL.
KEGG pathway analysis revealed that three pathways are associated with at least two mutated genes. Among them, two pathways suspected to be closely related to BCL are the calcium signaling pathway and the platelet activation pathway. Processes such as cell proliferation and death and gene transcription are essential for regulating cellular functions, and tight regulation of calcium signaling is fundamental in this process [
57]. Therefore, changes in calcium signaling can cause various diseases including tumors, and these changes have been confirmed in cancer cell lines [
58]. Platelet function plays an important role not only in hemostasis but also in tumor metastasis [
59]. Some research has demonstrated reduced tumor metastasis after experimentally inducing thrombocytopenia in mouse models [
60]. Therefore, mutations in genes (
ADORA2B, MYLK3, and
GP6) involved in both pathways may be mutations of interest in BCL.
Overall, considering the function of each gene, GOLIM4, ITM2B, STN1, DSC1, TNFAIP1, WAPL, and NCSTN mutations were found to be highly associated with BCL, and ADORA2B, MYLK3, and GP6 mutations were suspected to be associated with BCL through KEGG pathway analysis. Therefore, GOLIM4, ITM2B, STN1, DSC1, TNFAIP1, WAPL, NCSTN, ADORA2B, MYLK3, and GP6 mutations are proposed as candidate mutations associated with BCL.
There are some limitations in this study. First, the patients with BCL in this study were various canine breeds. Different breeds of dogs exhibit different physical characteristics, and these characteristics may be related at a genetic level. Second, the number of patients used in this study was small. The inclusion of more patients will more clearly identify commonly observed somatic variants. Third, a number of germline mutations were observed, but these could not be analyzed due to the vast amount of data. A review of germline mutations in human DLBCL suggests a possible association between germline mutations and lymphoma [
61]; therefore, further studies on germline mutations and CL will be needed.