MDS is a clonal hematopoietic stem cell disorder characterized by significant heterogeneity (60). This disease involves ineffective hematopoiesis, cytopenias, morphological developmental abnormalities and transformation to AML (61). Li et al (62) used SPAG6-short hairpin RNA lentiviral vectors to knock down SPAG6 in SKM-1 cells, which resulted in the activation of the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) signaling pathway, indicating that SPAG6 attenuates apoptosis by modulating the TRAIL pathway. Yin et al (63) found that SPAG6 silencing in SKM-1 cells increases phosphatase and tensin homolog deleted on chromosome 10 (PTEN) expression, which induces apoptosis through the PI3K/AKT pathway. Jiang et al (12) showed that SPAG6 mRNA levels were significantly higher in bone marrow cells from patients with MDS and MDS-AML than in those from healthy controls using reverse transcription-quantitative PCR (RT-qPCR). In vitro experiments revealed that SPAG6 knockdown inhibits SKM-1 cell proliferation, causes cell cycle arrests at the G1/S phase and disrupts cell differentiation. Zhang et al (64) showed that SPAG6 silencing induces autophagy through the AMP-activated protein kinase (AMPK)/mTOR/unc-51 like autophagy activating kinase 1 (ULK1) signaling pathway, thereby enhancing SKM-1 cell apoptosis. Collectively, these studies suggest that SPAG6 contributes to MDS pathogenesis and development, indicating its potential as a novel therapeutic target. Luo et al (65) expanded on this understanding from an epigenetic regulation perspective by showing that SPAG6 knockout, combined with the demethylating agent decitabine (DAC), reduces the expression of DNA methyltransferases and methyl-CpG-binding domain proteins. This combination also enhances apoptosis induced by DAC and the histone deacetylase inhibitor LBH589. A study from the same group, by Luo et al (66), demonstrated that suppressing SPAG6 expression in SKM-1 cells enhances DAC-induced apoptosis and promotes PTEN demethylation. Overall, these results support SPAG6 as a target for demethylation therapy in MDS.

AML is an aggressive malignancy involving white blood cells (67,68). It primarily manifests as symptoms associated with bone marrow failure and organ infiltration (69). Luo et al (70) reported that SPAG6 expression in patients with AML positively correlates with risk stratification. Patients with high SPAG6 expression had shorter overall survival than those with low SPAG6 expression. Furthermore, SPAG6 knockdown in the HL60 AML cell line promotes apoptosis and arrests the cell cycle at the G1 phase, thus confirming it as a protumor factor in AML. Steinbach et al (10) found that SPAG6 and six other genes are highly overexpressed in pediatric patients with AML but were normal in patients with sustained complete remission. This suggests that SPAG6 promotes disease progression. SPAG6 was considered a potential indicator for evaluating treatment efficacy and predicting prognosis in pediatric AML (71). Skou et al (72) found that peripheral blood levels of SPAG6 and two other genes predict relapse in pediatric patients with AML and are useful for minimal residual disease (MRD) monitoring in patients lacking leukemia-specific targets. Mu et al (73) identified SPAG6 as a significantly upregulated gene in AML. Its overexpression was negatively correlated with disease prognosis. SPAG6 interacts with and relocates myosin ID (MYO1D) from the cytoplasm to the cell membrane. This activates the PI3K/AKT signaling pathway and extracellular signal-regulated kinase (ERK) pathway, thereby regulating AML growth and prognosis. Thus, SPAG6 may represent a novel therapeutic target for AML.

B-ALL is a genetically heterogeneous malignancy (74) that may be classified into distinct molecular subtypes based on recurrent gene rearrangements, chromosomal abnormalities or specific gene mutations (75). Zhao et al (76) initially reported SPAG6 overexpression in the bone marrow of adult patients with B-ALL, which markedly decreased after treatment and complete remission. Studies using lentiviral transfection to knock down SPAG6 in the human B-ALL cell lines B-ALL-1 and NALM-6 showed significant inhibition of cell proliferation and apoptosis. These results indicate that SPAG6 downregulation attenuates cell proliferation and apoptosis by modulating the transforming growth factor-β (TGF-β)/Smad signaling pathway.

MM is a hematologic malignancy characterized by the malignant clonal proliferation of plasma cells (77). It accounts for >10% of all hematopoietic malignancies (78). Li et al (79) conducted a bioinformatics analysis for plasma cell tumor tissues and bone marrow samples from patients with MM. Significant SPAG6 expression was observed in MM cell lines, plasma cell tumor tissues and patient bone marrow. Increased SPAG6 mRNA levels were positively correlated with elevated hypercalcemia, increased plasma cell proportion and the severity of skeletal infiltration. Functional experiments further revealed that SPAG6 overexpression enhances MM cell proliferation, migration and antiapoptotic capacity in vitro, whereas its downregulation showed inhibitory effects. A direct interaction was confirmed between SPAG6 and dual-specificity phosphatase 1 (DUSP1), which attenuates the expression of downstream molecules in the mitogen-activated protein kinase (MAPK)/ERK signaling pathway. These results suggest that SPAG6 plays an important role in MM development by attenuating the DUSP1-MAPK/ERK axis and may be an effective therapeutic target.

MPN is a malignant clonal disorder caused by somatic mutations in hematopoietic stem/progenitor cells (80). It is characterized by the abnormal increase in peripheral blood cell counts and bone marrow fibrosis (81). Xia et al (82) showed that SPAG6 is expressed by MPN cells at the mRNA and protein level, with the highest expression observed in nucleated erythroid precursor cells and megakaryocytes. They hypothesized that abnormal SPAG6 expression contributes to the development of MPN and suggested that it may serve as a novel tumor marker for BCR::ABL1-negative MPN. Conversely, Ding et al (83) observed the significant upregulation of SPAG6 mRNA in primary MPN cells and MPN-derived leukemia cell lines. In vitro studies revealed that forced SPAG6 expression enhances clonogenic potential and accelerates the G1-to-S phase transition. Conversely, downregulating SPAG6 enhances interferon-α (IFN-α)-mediated apoptosis promotion and cycle arrest through the signal transducer and transcription activator 1 (STAT1) pathway. Furthermore, the expression of SPAG6 protein decreased concomitantly following the inhibition of STAT1 signaling.

BL is a highly aggressive malignancy originating from mature B cells (84). It is characterized by distinct clinical and morphological features, a germinal center B-cell immunophenotype, high proliferative activity and MYC rearrangements involving the immunoglobulin gene loci (85). Zhang et al (11) demonstrated that suppressing SPAG6 expression reduced the viability of Daudi and Raji cells. Conversely, PTEN inhibition using small inhibitory RNA or the specific PTEN inhibitor SF1670 restored proliferation and promoted apoptosis induced by SPAG6 deficiency in vitro and in vivo. These results suggest that SPAG6 promotes proliferation and suppresses apoptosis in BL cells through the PTEN/PI3K/AKT signaling pathway and further indicate that SPAG6 contributes to BL progression and may serve as a prognostic biomarker for these patients. The mechanisms of action and clinical significance of SPAG6 in various hematological tumor types are summarized in Table II.

Breast cancer is a highly heterogeneous malignant tumor (87). Its occurrence and development are driven by genetic and environmental factors, making it one of the leading causes of cancer-related death in women (88). Circular RNAs (circRNAs) play an important regulatory role in tumor progression (89). Fan et al (90) showed that circMYH9 enhances SPAG6 mRNA stability by recruiting the EIF4A3 protein, thus promoting its expression. SPAG6 overexpression reverses the inhibitory effect of circMYH9 knockdown on the malignant phenotype of breast cancer cells. Furthermore, circMYH9 knockout inhibits PI3K/AKT signaling by upregulating PTEN expression, which is similarly antagonized by SPAG6 overexpression. In addition, circMYH9 modulates the PTEN/PI3K/AKT signaling pathway through the EIF4A3-SPAG6 axis, thereby promoting the malignant progression of breast cancer cells.

Although mammography remains the standard imaging modality for early breast cancer screening, it has certain limitations. Mijnes et al (91) developed an epigenetic analysis method based on cell-free DNA in blood. This minimally invasive technique detects the methylation status of tumor suppressor genes and serves as a liquid biopsy to complement traditional imaging techniques. The combined detection of SPAG6, period circadian regulator 1 (PER1) and inter-alpha-trypsin inhibitor heavy chain 5 (ITIH5) achieved 64% sensitivity for breast cancer detection. Although liquid biopsy has technical challenges, the 'SNiPER' panel, which includes SPAG6, NK2 homeobox 6, ITIH5 and PER1, holds promise (91). Manoochehri et al (92) demonstrated that a methylation scoring model established from the first three differentially methylated regions in the SPAG6, LINC10606 and TBCD/ZNF750 regions yields high sensitivity and specificity for detecting triple-negative breast cancer (TNBC). LINC10606 and TBCD/ZNF750 showed strong discriminatory power in patients with TNBC compared with healthy controls [area under curve (AUC)=0.78 in the test set, AUC=0.74 in the validation set]. Therefore, noninvasive DNA methylation detection may provide novel biomarkers for the early diagnosis of TNBC.

Nasopharyngeal carcinoma is a malignant tumor originating from the mucosal epithelium of the nasopharynx (93), with a predilection for the pharyngeal recess (Rosenmüller's fossa) (94). Zhang et al (95) used machine learning to identify genes associated with nasopharyngeal carcinoma and determined their correlation with the immune microenvironment. Of note, four genes, including SPAG6, exhibited high predictive efficacy (AUC >0.9) in the training and validation sets. The expression of these genes was significantly correlated with the degree of immune cell infiltration, with SPAG6 showing a particularly strong association with the immune infiltration phenotype.

Thyroid cancer is a malignant tumor originating in the thyroid gland (96), and it is the most commonly diagnosed endocrine malignancy worldwide (97). Located in the anterior neck region, the thyroid gland secretes hormones that regulate metabolism (98). Wang (99) proposed that SPAG6 exerts tumor-suppressing effects in thyroid cancer. SPAG6 overexpression inhibited tumor cell invasion and proliferation. Li et al (86) used immunofluorescence techniques to show that SPAG6 expression positively correlates with immune checkpoint molecules in thyroid carcinoma in vitro. SPAG6 overexpression suppresses malignant cell behavior, including reduced proliferation and migration, and affects the functional phenotypes associated with DNA repair, MYC signaling, peroxidase activity and the G2/M checkpoint.

Squamous cell carcinoma is the second most common nonmelanoma skin tumor, which accounts for ~20% of all skin cancers (100). It has strong metastatic potential and is capable of metastasizing to multiple organs in the body, thereby posing a high risk of mortality. An in-depth study of its molecular mechanisms is necessary to establish prevention and treatment strategies (101). Gim et al (102) selected early-stage squamous cell carcinoma tissue samples representing invasive and precancerous regions. Using the NanoString GeoMx Digital Spatial Profiler for spatial transcriptomics analysis, they identified SPAG6 as having the highest absolute log2-fold change in expression among other cancer-associated genes in fibroblasts. SPAG6 was associated with fibroblast development and function. In addition, significant alterations in its expression were evident during the progression of actinic keratosis to squamous cell carcinoma.

Osteosarcoma is the most common primary malignant bone tumor (103). It primarily affects adolescents and young adults and is highly invasive and prone to metastasis. Although surgery combined with chemotherapy significantly improves patient survival, the prognosis for metastatic or recurrent osteosarcoma is poor (104). Bao et al (105) examined SPAG6 expression in tumor tissues from 42 patients with osteosarcoma and 12 osteochondroma control tissues using immunohistochemistry, RT-qPCR and western blot analysis. The SPAG6 protein positivity rate in the osteosarcoma tissues (71.43%) was significantly higher than that in the control tissues (33.33%; P<0.05). Both mRNA and protein levels were markedly increased compared with the levels in adjacent normal tissue. High SPAG6 expression positively correlated with higher pathological grade, metastasis and advanced Enneking stage (P<0.05). SPAG6-positive patients experienced significantly shorter overall survival. These results indicate that SPAG6 overexpression is associated with malignant progression and poor prognosis in osteosarcoma, thus suggesting its potential as a prognostic biomarker.

Lung cancer is one of the most common and deadliest malignant tumors (106). Based on its histology, it may be broadly classified into non-small cell lung cancer (NSCLC; accounting for ~85% of all cases) and SCLC (accounting for ~15% of cases) (107,108). Early detection is important for improving survival outcomes. DNA methylation is an important epigenetic regulatory mechanism that contributes to the development of various malignancies, including lung cancer, by modulating transcriptional activity. It demonstrates significant potential for predicting the early diagnosis, prognosis and treatment response of lung cancer (109).

NSCLC originates from lung tissues, such as the bronchial mucosa, glandular epithelium or pulmonary alveoli. It is the most prevalent type of lung cancer (110) and has a poor prognosis (111). Altenberger et al (112) reported that SPAG6 and LINE-1 type transposase domain containing 1 (L1TD1) mRNA expression is significantly lower in tumor tissues from patients with NSCLC than in normal lung tissues from the same patients. In NSCLC cell lines exhibiting downregulated mRNA expression, treatment with epigenetic modifiers reactivated the expression of these genes. Tumor-specific hypermethylation of SPAG6 and L1TD1 in NSCLC tissues with this methylation pattern effectively distinguished tumors from normal tissues. These results indicate that SPAG6 and L1TD1 undergo tumor-specific methylation in NSCLC, which regulates SPAG6 expression at the transcriptional level through DNA methylation (112).

LUSC is a type of NSCLC characterized by tumor heterogeneity, genetic mutations, cancer stem cells, immune resistance and chemotherapy resistance. Because it is usually diagnosed at an advanced stage, it has a poor prognosis (113). Epigenetic modifications, primarily DNA methylation (112), are associated with genomic instability in LUSC. Wu et al (114) found an effect of SPAG6 DNA methylation on its expression in LUSC. They identified contributors to SPAG6 DNA hypermethylation. For example, DNA methyltransferase 3 b (DNMT3b)-mediated hypermethylation of the SPAG6 promoter in LUSC resulted in SPAG6 downregulation, whereas SPAG6 reversed the malignant phenotype of LUSC cells. Mechanistically, SPAG6 negatively attenuates the JAK/STAT signaling pathway by suppressing the transcriptional activity of STAT1 and STAT3. In addition, SPAG6 expression was found to be positively correlated with immune cell infiltration in LUSC tissues, whereas it was negatively correlated with the expression of immunosuppressive genes, such as cytotoxic T-lymphocyte associated protein 4 and programmed cell death 1. Furthermore, SPAG6 suppressed tumor stem cell properties by downregulating the stemness Nanog homeobox (Nanog), aldehyde dehydrogenase 1 family, member A1 (ALDH1) and Sox2.

HGSOC is the most common and aggressive epithelial ovarian cancer subtype (115). It displays high invasiveness and a poor patient prognosis. A deeper understanding of HGSOC tumorigenesis may provide insights for the development of new therapeutics (116). Coan et al (117) observed SPAG6 expression in ciliated cells of the fallopian tube. Impaired ciliary motility disrupts laminar fluid flow over the tubal epithelium, which may reduce the management of oxidative stress induced by follicular fluid and contribute to tumorigenesis.

Bladder cancer originating from the bladder mucosa (118) is a common malignant tumor of the urinary system (119). Kitchen et al (120) observed frequent methylation in the SPAG6 promoter in bladder cancer tissues. They also observed significantly increased SPAG6 methylation levels in recurrent and advanced bladder cancers, suggesting that SPAG6 functions as a tumor suppressor gene in these tissues. SPAG6 methylation may represent an independent predictor of bladder cancer recurrence and progression (120).

All of these findings indicate that SPAG6 shows diverse regulatory functions across different tumor microenvironments; however, its pathogenic mechanism is primarily attributed to dysregulated expression caused by epigenetic and post-transcriptional regulation, rather than frequent mutations in the gene itself. This characteristic is similar to that of other cancer-testis antigens. Its biological functions within tumor cells and the underlying molecular mechanisms remain to be fully elucidated. Further in-depth studies are needed to elucidate these mechanisms. The mechanisms of action and clinical significance of SPAG6 in different types of solid tumors are summarized in Table III and Fig. 2. Furthermore, Fig. 3 provides a schematic of SPAG6's role and regulation across various cancers.