Previous studies have shown that SND1 acts as a nuclease within the RISC, where it associates with small RNAs (such as siRNAs or miRNAs) and ribonucleoproteins to facilitate RNAi-mediated gene silencing (32,33). MTDH has been shown to interact with SND1 and other proteins to form a stable RISC complex (2). The overexpression of either MTDH or SND1 has been associated with the downregulation of several tumour suppressor genes that are targeted by onco-miRNAs. These include genes such as PTEN (targeted by miR-221 and miR-21), CDKN1C (also known as p57; targeted by miR-221), CDKN1A (also known as p21; targeted by miR-106b), SPRY2 (targeted by miR-21) and TGFBR2 (targeted by miR-9) (34). The adenomatous polyposis coli (APC) is essential for maintaining cell polarity and cell-cell adhesion, primarily through regulation of the placement of E-cadherin at the plasma membrane. In colon cancer, reduced levels of APC have been linked with the overexpression of SND1, which has been shown to modulate the expression of APC and other cancer-associated genes. Loss of APC was shown to disrupt the degradation of β-catenin via the proteasome, resulting in its stabilization (33). This stabilized β-catenin subsequently activates the transcription of Wnt pathway target genes, and this process acts a key driver of tumour progression (35). RNA splicing is a vital process in pre-mRNA maturation, during which non-coding intronic regions are excised, and the exonic regions are joined, thereby producing a mature functional mRNA. The spliceosome complex consists of five dynamic small ribonucleoproteins (U1, U2, U4/U6 and U5), along with several non-coding RNAs. SND1 interacts with U5 spliceosomal RNA, a key component of ribonucleoproteins, which facilitates the formation of the spliceosome. This interaction affects the levels of different splice variants, including the production of a variant form of CD44, which thereby increases the motility and invasive behaviour of prostate cancer cells (36).

Furthermore, SND1 has been shown to augment the stability of certain mRNAs, and this is essential for cellular stress responses (37). Under oxidative stress conditions, SND1 and angiotensin II type 1 receptor (AT1R) mRNA are found to co-localize within the stress granules, where SND1 has a critical role in facilitating efficient protein-RNA complex formation (37). Collectively, these findings have demonstrated that SND1 may increase the stability of specific oncogenic mRNAs.

Maintaining genomic stability is essential for cell survival and the prevention of cancer. Eukaryotes have evolved complex and tightly regulated mechanisms which maintain genomic stability, thereby ensuring accurate cell division and precise DNA replication (38). Recently, the association between the expression of lysine demethylase 6A (KDM6A) and the DNA damage pathway in normal tissues has been established. The expression of KDM6A was found to be associated with several key proteins involved in DNA repair, including RAD50, nibrin 1, meiotic recombination 11, DNA polymerase Eta and X-ray repair cross complementing 6 (Ku70). Furthermore, it was demonstrated that the interaction between KDM6A and SND1 facilitates the recruitment of DNA repair factors such as Ku70 and replication protein A to newly synthesized DNA. This study by Wu et al (38) also established that an increased interaction between KDM6A and SND1 contributes to chemoresistance in oesophageal squamous cell carcinoma. Another study revealed that SND1 interacts strongly with another chromatin regulator, namely DNA (cytosine-5)-methyltransferase 3A (DNMT3A) (39). DNMT3A exerts a critical role in the de novo methylation of CpG islands in chromatin and also targets hemimethylated DNA for further methylation (40). Furthermore, the overexpression of DNMT3A has been shown to induce aberrant DNA methylation, oncogene activation and the silencing of tumour suppressor genes, resulting in genomic instability and oncogenesis (41-42). SND1 was found to promote the transcription of DNMT3A by functioning as a chromatin architectural regulator (39). Elevated levels of DNMT3A expression lead to gene methylation and the transcriptional silencing of CDH1, thereby facilitating the metastasis of TNBC (Fig. 3) (39). SND1 has also been found to promote malignant glioma phenotypes by epigenetically inducing topological chromatin interactions that activate downstream Ras homolog family member A (RhoA) transcription. RhoA, in turn, influences the expression of key cell cycle regulators, including cyclin D1, cyclin E1, cyclin-dependent kinase 4 (CDK4) and cyclin-dependent kinase inhibitor 1B, which accelerate the transition from the G₁ to the S phase of the cell cycle, thereby boosting the proliferation of glioma cells (43). In these studies, strong evidence has been provided to suggest that SND1 functions as a novel chromatin architectural modifier, and that it is potentially a promising prognostic marker for cancer and its treatment.

Tumours are able to evade immune system attacks through various mechanisms, including limiting antigen recognition, suppressing immune responses and promoting T-cell exhaustion (44). One of the key factors contributing to tumour immune escape is the reduced ability of CD8⁺ T cells to recognize tumour cells due to defects in the surface expression of MHC-I molecules. This defect has been observed in 20-60% of common solid tumours, including melanoma and cancers of the lung, breast, kidney, prostate and bladder (5,45,46).

A previous study revealed that SND1 promotes tumour cell immune escape via inhibiting the MHC-I antigen presentation signalling pathway, which leads to an impaired CD8⁺ T cell-mediated antitumour response in the tumour microenvironment (4). This effect is mediated through the downregulation of the MHC-I heavy chain molecule, facilitated by the ERAD pathway. ERAD is responsible for targeting misfolded proteins in the ER for ubiquitination and subsequent degradation by the proteasome. SND1 can capture the nascent MHC-I heavy chain and direct it to the ERAD-mediated proteasomal degradation pathway (4). This process disrupts the proper assembly of the heavy chain with β2-microglobulin (β2m) in the ER lumen, thereby impairing antigen presentation. Consequently, tumour cells become less susceptible to immune surveillance, and exhibit a diminished capacity to present antigens to cytotoxic CD8⁺ T cells. Additionally, a decreased expression of the transporter associated with antigen processing (TAP) protein is considered to be one of the key mechanisms of tumour immune evasion (47,48). Two subtypes of the TAP protein (TAP1 and TAP2) have been shown to be mainly associated with other proteins for the purpose of loading the peptides to the MHC class I-β2m complex, and the antigens are subsequently presented on the cell surface (49-52). A recent study identified that SND1 interacts with TAP1/2 proteins to destabilize the complex with the assistance of the MTDH protein, and this process resulted in a decrease in the expression of TAP1/2 in tumour cells, which were thereby able to evade immune surveillance (12) (Fig. 3). Furthermore, SND1 knockdown in tumour cells led to an increase in TAP1/2 expression levels, which subsequently resulted in an enhanced tumour antigen presentation to CD8⁺ T cells (12). A potential association between immune cell infiltration levels and SND1 gene expression across various cancer types has also been identified through extensive analysis. A negative correlation was observed between CD8⁺ T-cell infiltration and SND1 expression, whereas a positive correlation was identified between cancer-associated fibroblasts and SND1 expression in the majority of different cancer types (3). Recently, epigallocatechin has been reported to block the interaction between SND1-MHC-I, resulting in enhanced MHC-I presentation and an increased CD8⁺ T-cell response within the tumour microenvironment in a xenograft model (53).

SND1 performs a crucial role in promoting metastasis and angiogenesis by activating EMT, cell invasion and cell migration, which are essential for its oncogenic function. In the case of colorectal cancer, SND1 interacts with the histone chaperone and transcription elongation factor SPT6 to co-control the expression of human telomerase reverse transcriptase (hTERT) and cell proliferation (54). Briefly, SPT6 and SND1 work in concert to promote cancer progression by targeting hTERT (54). Similarly, in HCC, SND1 has been shown to promote angiogenesis by activating a linear signalling pathway that comprises NF-κB, miRNA-221, angiogenin and C-X-C motif chemokine ligand 16, and it also promotes EMT via the AT1R and TGFβ signalling pathway (55-57).

Mitochondrial dysfunction is a major contributor towards numerous metabolic diseases, including neurodegenerative disorders, aging and cancer. A recent study (20) demonstrated that SND1 promotes liver cancer through PGAM5-mediated dephosphorylation of serine-637 of dynamin-related protein 1 (DRP1) and mitophagy, as shown in Fig. 3. This study revealed that SND1 enhances mitophagy by promoting the interaction between PGAM5 and DRP1(20). In another study, Shen et al (11) employed immunoprecipitation of MTDH followed by mass spectrometric analysis to identify SND1 as a primary interacting partner of MTDH. Their findings revealed that the MTDH-SND1 complex has a critical role in the progression and metastasis of late-stage breast cancer. Furthermore, a different study (58) revealed a functional interaction between SND1 and the ETS transcription factor (ERG) protein in prostate cancer. In prostate organoid models, wild-type SND1 cells exhibited ERG-driven increases in colony size, an effect that was found to be completely eliminated by knockdown of SND1. Collectively, these results suggested that SND1 is a key mediator of ERG-driven organoid growth in prostate epithelial cells.

Drug resistance presents a major challenge in treating cancer, given that it results in cancer recurrence, cancer dissemination, and death. Transporters, oncogenes, tumour suppressor genes, mitochondrial alteration, DNA repair, autophagy and EMT are involved in the molecular mechanisms underlying multidrug resistance (59-61). The overexpression of SND1 has been demonstrated to be involved in the chemoresistance of various types of cancer. A recent study (62) showed that SND1 binds to the 3'-UTR of GPX4, thereby providing stability, and that this fulfilled an essential role in the development of chemoresistance against cisplatin in bladder cancer. Silencing of SND1 was also found to reverse the cisplatin resistance and trigger cell death via ferroptosis. SND1 has also been shown to be overexpressed in HCC (63), and may be a contributing factor towards the development of resistance in HCC. Another recent study (64) established that the silencing of SND1 expression caused an increase in the protein expression of organic anion transporter 2, which stimulated 5-fluorouracil to inhibit proliferation of the HCC PLC/PRF5 cell line, and PLC/PRF/5 cell growth in a xenograft model. Furthermore, Fu et al (65) investigated the potential role of SND1 in radio-resistance in cervical cancer. SND1 has been shown to contribute to radio-resistance through preferential activation of the ATM pathway by affecting cell-cycle checkpoints and DNA repair to promote cell survival upon the cells receiving DNA damage (65-68). Similarly, another recently published study (69) showed that non-small cell lung cancer (NSCLC) is highly resistant to chemo- or radiation therapy due to crosstalk between SND1 and the programmed cell death-4 protein. Chemosensitivity of NSCLC cells to different chemotherapeutic drugs was also found to be increased following the silencing of SND1(69). Given its consistent overexpression across various types of cancer, SND1 holds promise as a biomarker in the future for the early detection and progression monitoring of malignancies.

Crystal structure analyses have shown that the SN domains of SND1 are responsible for nuclease activity, whereas the Tudor domain is involved in RNA binding (7,15). A primitive inhibitor, 3',5'-deoxythymidine bisphosphate (pdTp), was identified as a competitive inhibitor that targets the SN domain to inhibit the RNA-protein interactions of SND1 in Plasmodium (Table I). A previous study showed that pdTp inhibits the growth of both chloroquine-sensitive and chloroquine-resistant strains of P. falciparum at concentrations ranging from 100-200 µM (78). Moreover, upon treating cells of an HCC cell line (QGY-7703) with pdTp, a significant reduction in cell viability was noted, as well as a decrease in colony-forming potential (79). Furthermore, the administration of different doses of pdTp in human HCC cells (of the QGY-7703 cell line) in a xenograft NSG™ mouse model resulted in the observation of a significant inhibitory effect on tumour progression compared with vehicle and no toxicity was observed. Recently, Lehmusvaara et al (9) identified the top three most effective inhibitors (suramin, NF 023 and PPNDS) of SND1 and RNA-binding activity using a fluorescence polarization-based competitive assay (Table I). Among the identified compounds, suramin proved to be the most potent, inhibiting RNA binding to SND1 with an IC₅₀ of 0.6 µM (9). Inhibition of SND1 binding to RNA by suramin led to an increased expression of the miRNA miR-1-3p and sensitization of SW480 colon cancer cells to navitoclax (a Bcl-2 inhibitor). However, effective and specific molecules that block the interaction of SND1 with RNA have yet to be identified, and subsequent studies should focus on developing novel and more effective compounds for this purpose.

In recent years, SND1 has attracted significant attention due to the identification of interacting partner proteins and its role in cancer prognosis. Although multiple protein interactions have been reported, the interaction between SND1 and MTDH has been identified with markedly high oncogenic potential in different types of cancer (80). The identification of the SND1-MTDH interaction paved the way for the development of potent and specific inhibitors to block this interaction and to prevent cancer progression (Tables II and III). This progress was facilitated by elucidating the crystal structure of the MTDH-SND1 complex, which revealed a distinct interface between the SN1 and SN2 domains of SND1 and a MTDH peptide motif. The SND1 protein contains two deep pockets that specifically interact with two tryptophan residues of MTDH. Notably, the large and hydrophobic side chains of Trp-394 and Trp-401 of MTDH were found to fit deeply into the two hydrophobic binding pockets of SND1(11). The following approaches have been reported and proposed to prevent the SND1-MTDH interaction.

The interaction between MTDH and SND1 is crucial for breast cancer progression, and targeting this interaction with small chemical compounds may offer therapeutic potential (6,11). A high-throughput screening analysis performed by Shen et al (11) led to the identification of specific inhibitors that can disrupt the SND1-MTDH interaction. Of the three identified compounds (A26, A32, and A36), A26 was further modified, deriving the compounds C26-A2 and C26-A6. C26-A6 has been identified as the first small-molecule inhibitor that specifically targets the interaction between SND1 and MTDH. In in vitro studies, C26-A6 demonstrated strong inhibitory effects in SCP28 breast cancer cells that expressed a split-luciferase reporter system, and this was used to monitor the SND1-MTDH interaction. Furthermore, in vivo experiments in a breast cancer xenograft model showed that C26-A6 significantly suppresses tumour growth. These experiments were performed in 8-week-old female mice across multiple strains, including immunocompromised (NSG™ and nude) and immunocompetent (FVB and BALB/c) mice (Table II).

The discovery of C26-A6 has encouraged researchers to develop additional inhibitors with higher affinity and efficacy. Subsequent all-atom MD simulations in solution revealed that C26-A6 binds more strongly to SND1 compared with C26-A2, since C26-A2 undergoes a 180˚ directional shift during the simulation process (81). Furthermore, C26-A6 was shown to reduce tumour growth and metastasis, while also enhancing the sensitivity of TNBC preclinical models to the chemotherapeutic agent paclitaxel when combined with C26-A6(12). In addition, the MTDH-SND1 complex was shown to suppress tumour antigen presentation and to prevent the infiltration of CD8⁺ T cells within the tumour microenvironment (11). Disrupting the MTDH-SND1 interaction with C26-A6 boosted the immune response, thereby improving the effectiveness of anti-programmed cell death protein 1 (anti-PD-1) therapy in a preclinical metastatic breast cancer model (11).

Another approach utilizing MD simulations was performed to screen over 1 billion compounds from the ZINC15 database, aiming to identify novel small-molecule inhibitors that could inhibit the SND1-MTDH interaction (13). The top 12 potential candidates were identified through virtual screening, and subsequently tested for their ability to bind to SND1 using a surface plasmon resonance-based assay (Table II). Among the 10 best SND1 binders, L5 and L8 were the top hit compounds, with K_d values of 2.64 and 0.2 µM, respectively. Subsequent analysis of L5 for anticancer activity in MDA-MB-231 breast cancer cells revealed an IC₅₀ value of 57 µM.

In addition, structure-based virtual screening was performed to target the known active site of the SND1 enzyme, which resulted in the identification of three promising lead molecules (BAS_00381028, BAS_00327287 and BAS_01293454) from the Asinex library (https://www.asinex.com/) (Table II). The binding energy score was calculated and compared with the reference compound C26-A6. These molecules exhibited excellent binding to the SND1 enzyme, and maintained stable docked conformations during MD simulations. Furthermore, the pharmacokinetic properties were also elucidated, and favourable drug-like properties were predicted that could be used in experimental investigations to study their SND1 inhibition potential (14). A MD approach was performed by Shen et al (11), which revealed a novel series of inhibitors for the SND1-MTDH interaction. Using 293 cells stably expressing SND1-Nluc and Cluc-MTDH, this research group identified a markedly potent inhibitor, C19, which disrupted the MTDH-SND1 interaction according to a split-luciferase assay (IC₅₀=487±99 nM). C19 was also shown to inhibit MCF-7 breast cancer cell proliferation, invasion and migration, to arrest the cell cycle, and to induce apoptosis (Table II). Furthermore, C19 has demonstrated promising levels of tumour growth inhibition in MCF-7 xenograft models (82).

Trp-394 and Trp-401 are two crucial tryptophan residues in MTDH that have been found to occupy the binding pocket of SND1, and these tryptophan residues are critical in the SND1-MTDH interaction to promote breast cancer initiation and progression (Fig. 1) (7). A 12-mer (CPP-4-2-2) high-affinity MTDH-like peptide was discovered through utilizing phage display technology to target the MTDH-SND1 complex (Table III). CPP-4-2-2 was found to exert anticancer effects, both in vitro in MDA-MB-231, MCF-7 and MDA-MB-468 breast cancer cell lines and in vivo in an xenograft BALB/c nude mice model via interacting with the SN1/2 domain of SND1 and disrupting the SND1-MTDH interaction, which resulted in SND1 degradation (83). However, the linear peptide CPP-4-2-2 contains a number of notable limitations, including its having an unstable secondary structure, low serum stability, weak cell permeability and poor druggability.

Subsequently, Chen et al (10) utilized a terminal aspartic acid cross-linking strategy (TD method) to overcome the issues associated with linear peptides. This research group effectively created a range of stabilized peptides derived from the MTDH sequence using structure-based design and optimization. Ultimately, they identified the peptides MS2D-cyc4 (K_d=74 nM) and MS2D-cyc6 (K_d=82 nM), which demonstrated exceptional binding affinities, efficient cellular uptake and enhanced serum stability (Table III). Further study of MS2D-cyc4 and MS2D-cyc6 revealed that these peptides possessed appreciable bioactivity in TNBC cells (10). Additionally, a more potent cyclized peptide, NS-E, exhibiting a K_d value of 23.4 nM, was developed, which possessed a 4-fold greater activity compared with MS2D-cyc4 and MS2D-cyc6(84). In spite of its enhanced activity, however, this NS-E exhibited poor cell penetration efficiency, and also demonstrated antitumour effects at relatively high concentrations, with an IC₅₀ value exceeding 100 µM in MDA-MB-231 and 4T1 cells (Table III). To overcome these limitations, a stabilized peptide delivery system was developed which utilized a reversible sulfonium-based peptide carrier (Wpc) to improve the cell permeability of NS-E. The IC₅₀ of the nanomaterial-conjugated peptide (Wpc/NS-E) was found to be reduced to 20 µM in cell-based assays (Table III). The Wpc/NS-E peptide effectively blocked the MTDH-SND1 interaction by targeting SND1, with enhanced delivery into tumour cells. Consequently, the Wpc/NS-E peptide induced apoptosis and significantly inhibited the proliferation, migration and invasion of human MDA-MB-231 and mouse 4T-1 TNBC cells. Moreover, the Wpc/NS-E peptide demonstrated exceptional antitumour activity in a 4T1-TNBC mouse model (84).