Tumour Growth and Differentiation in ARMS
PAX3/7 and FOXO1
Epithelial to mesenchymal (EMT) is a multi-step process. During EMT epithelial cells are reprogrammed into a mesenchymal phenotype decreasing cell-to-cell adhesion, cytoskeleton remodeling, basement membrane invasion, acquisition of motility, and a lack of cell polarity (Sannino et al., 2017). This results in normally cohesive cells to shift from their rigid epithelial organization and metastasize to other loci contributing to cancer progression and drug resistance (12). The commitment of cells to the EMT program is orchestrated by specific transcription factors, therefore making transcription factors a major focus of cancer research.
Significantly, ~80% of the ARMS cases manifest due to a chromosomal translocation between paired box protein
PAX3 gene found on chromosome 2 or
PAX7 gene found on chromosome 1, and forkhead box protein
FOXO1 gene on chromosome 13. It consequently results in fusion genes (PAX 3/7-FOXO1) that are considered major drivers of oncogenic activity (7, 13) (
Figure 2). The N-terminal DNA binding domain of
PAX3 or
PAX7 is fused to the C-terminal transactivation domain of
FOXO1 (5, 14).
Transcription factors, known as myogenic factors, not only regulate the physiological process of muscle formation (i.e. myogenesis), but also play an important role in pathologic myogenic differentiation. Accordingly, aberrations within these pathways can lead to development of RMS (15, 16). Previous studies suggest that the development of RMS correlates with a differentiation defect in either stem cells or early progenitor cells such as mesenchymal stem cells (15, 17). ARMS not only has the presence of the PAX3/PAX7 and FOXO1 fusion gene but also display increased levels of MET; a receptor tyrosine kinase (RTK) known to be linked with the metastatic potential of RMS cells (15, 18). Furthermore, recent studies have shown that aggressive ARMS tumors exhibit high expressions of SNAIL which coincidently has a positive correlation to PAX3/7-FOXO1 (15, 19-21). SNAIL has thus become a prominent research focus as it plays a central role in EMT (15, 19).
SNAIL is a zinc finger transcription factor that usually acts as a gene repressor making it a crucial regulator of ARMS growth (20, 21). It is known to be a master regulator of growth and metastasis as it involves the induction of EMT by binding to E-box sequences in the promoter region on genes, and, affects the EZRIN cytoskeleton protein and AKT serine/threonine kinase levels respectively (16, 20, 21).
In ARMS the PAX3/7-FOXO1
fusion gene not only alters the myogenic program and maintains the proliferative state while blocking terminal differentiation, but also results in an overexpression of TGF-β (22, 23). This overexpression of TGF-β results in an upregulation of its two major downstream pathways further allowing SNAIL to promote EMT (
Figure 3). Additionally, SNAIL also regulates the microRNA transcriptome either directly or indirectly through binding their promoter or regulatory regions in RMS cells (16, 19, 21). To this end, gene ontology analysis revealed that SNAIL-miRNA axis does in fact play a role in differentiation, reorganization of actin skeleton and migration (16, 19-21).
ARMS Chemotherapy Drugs and Their Impact in the Tumor Cells Differentiation
Currently, a multiple approaches consisting of a combination of surgery, chemotherapy and/or radiotherapy are being used to treat at risk ARMS patients (34). Due to the shortage of efficacious treatment options there has been insufficient advancements when it comes to treatment options, thus hindering improvement in the outcome of metastatic or relapsed ARMS (35). Intravenous (IV) administration of Vincristine, actinomycin D, and cyclophosphamide (VAC) are considered the classic chemotherapy drugs in North America with only the duration and dosage of each being modified over the last four decades (34). Multiple studies have been conducted to improve treatment plans using a variety of chemotherapeutic agents that target characteristic features of ARMS, including inhibition of myogenic differentiation. Thus, many of these drug strategies focus on the ability to ‘rehabilitate’ differentiation.
Vincristine, Actinomysin D and Cyclophosphamide (VAC)
The VAC regimen consisting of FDA approved drugs; vincristine, actinomycin D, and cyclophosphamide is the standard chemotherapy combination with a response rate of ~70-80% in ARMS patients (36). Vincristine is an anti-tumour vinca alkaloid considered to be a potent microtubule inhibitor (37, 38). It acts by irreversibly binding to and stabilizing tubulin therefore, interfering with microtubule polymerization, inherently preventing mitosis and inhibiting cell growth (39). Actinomycin D (ActD) is a known transcription inhibitor, it binds guanine residues and inhibits the function of DNA-dependent RNA polymerase therefore preventing RNA synthesis (40). This characteristic permits it to act a cytotoxic inducer of apoptosis against tumor cells (40). A study also showed that when administered in low controlled doses, actinomycin D promotes myogenic differentiation while inhibiting proliferation thereby making cells less resistant to chemotherapy (41). Cyclophosphamide is a nitrogen mustard drug that has the ability to alkylate DNA; it does so by metabolizing into it’s active form phosphoramide (42). Phosphoramide has the capability of forming cross-linkages at the guanine N-7 position, both in between and within DNA strands, thus leading to programmed cell death (42).
All three of these drugs act differentially to collective halt tumour growth. This is achieved by either preventing cell division, stopping metastasis or facilitating cell death, therefore proving to be an effective chemotherapy standard for ARMS patients. (
Figure 6)
Cabozantinib (XL184)
As discussed above, in ARMS MET signalling promotes growth and proliferation, inhibits myogenic differentiation, and increases its metastatic potential (35). The receptor tyrosine kinase inhibitor, Cabozantinib (XL184), upstream of the MET pathway, is used to counteract and impair tumor cell proliferation and angiogenesis, promoting myogenic differentiation (35). Cabozantinib is administered orally and is currently in phase II clinical trials with a recommended dosage of 40 mg/m2/day (43) (
Figure 7).
Bortezomib
Another characteristic feature of ARMS is that it exhibits decreased levels of apoptosis and is successful in evading cell cycle arrest (34). In vitro studies have shown that when treated with Bortezomib, a protease inhibitor that functions by inhibiting the 26S proteosome, ARMS cell lines express increased levels of apoptosis and cell cycle arrest (35).
Vinorelbine
A recent clinical trial was conducted to test the effectiveness of Vinorelbine, a second generation semisynthetic vinca alkaloid with antimitotic and anticancer properties (9). Vinorelbine acts by inhibiting microtubule dynamics by binding microtubular proteins in the mitotic spindle preventing chromosomal segregation and triggering the cancerous cells to undergo apoptosis (9). The results of the trial indicated that Vinorelbine had significant affects both when used as a single agent and in combination, therefore making it an effective chemotherapeutic drug (9). The recommended combination therapy is a low dose of cyclophosphamide ~25mg/m2 per day for 28 days with the administration of 25mg/m2 of vinorelbine on days 1, 8 and 15 (44).
AZD1775
Wee1 kinase is a cell cycle regulator and it acts by the inhibition of cyclin dependent kinase 1 and phosphorylation (35). AZD1775 is a selective tyrosine kinase inhibitor which has been reported to inhibit the growth of several sarcoma cell lines, however it’s role is ARMS is still misunderstood (35).
A recent study illustrated that AZD1775 could serve as a viable therapeutic agent in combination with conventional chemotherapy (35). The Wee 1 kinase is a cell cycle regulator that specifically maintains the cell in the G2/M phase therefore providing sufficient time to repair DNA prior to undergoing mitosis (Kahen et al., 2016). When inhibited by AZD1775 however, the CDK1/2 activity takes over unchecked, allowing the cells to prematurely progress through the G2/M phase and undergo mitosis. The outcome of this leads to DNA strand breaks, mitotic dysfunction and cell death (35).
Previous literature has also shown that when used in combination with gemcitabine, AZD1775 lead to not only a delay in tumor growth but also smaller tumors in osteosarcoma cells (35). It is also hypothesized that if combined with conventional chemotherapy, they could work synergistically to make the DNA damage more potent (35). These findings indicate potential of this agent for improved treatment strategies for ARMS, once evaluated more in future studies(35).
Entinostat, Panobinopstat and Vorinostat
Histone deacetylase (HDAC) is an epigenetic marker that when inhibited has antitumour effects, as has been demonstrated using an RMS model (34). Agents such as entinostat, panobinopstat and vorinostat are known HDAC inhibitors and have been reported to delay tumor growth in RMS xenografts (34). In addition to disrupting transcriptional complexes leading to suppression of key oncogenic genes, HDAC inhibitors also have the capability to induce transcriptional stress resulting in their terminal differentiation or apoptotic cell death (34) (
Figure 8).
Critotinib, Bevacizumab (mAb) and Regorafenib
The constitutive activation of receptor tyrosine kinases (RTK) such as ALK, MET, VEGFR is known to promote tumor progression in ARMS, by reprogramming many intracellular pathways, such as those involved in differentiation (34). Currently, there are two main strategies for targeting RTK’s: small molecule kinase inhibitors and immunotherapy-like monoclonal antibodies. Critotinib, Bevacizumab (mAb) and Regorafenib namely are a few examples of RTK inhibitors that have successfully induced tumor regression in preclinical models (34).
All Trans Retinoic Acid (ATRA)
All trans retinoic acid (ATRA) is an important metabolite of vitamin A mediating functions of growth and development (45). It plays a crucial role in cell proliferation, cell differentiation, apoptosis and embryonic development (46). ATRA primarily exerts its effects through its interactions with nuclear retinoic acid receptors (RARs), which are transcription factors (47). RARs form heterodimers with retinoid X receptors (RXRs) and regulate the transcription of target genes (48, 49). The RAR signalling pathway plays a crucial role in mediating the differentiation-promoting effects of ATRA in ARMS (50). By activating RARs, ATRA can modulate the expression of genes associated with myogenic differentiation; the process by which ARMS cells transform into more mature, muscle-like cells.
ATRA exerts differentiation-inducing effects in ARMS through various mechanisms. For instance, it can act by upregulating MYOD1, a crucial myogenic regulator, promoting commitment to the muscle lineage and facilitating ARMS cell differentiation (51). Moreover, ATRA also targets the hallmark genetic abnormality in ARMS, inhibiting the PAX3-FOXO1 fusion protein's expression and activity, thus disrupting oncogenic signaling and facilitating differentiation (49). Finally, ATRA induces epigenetic changes, such as DNA methylation and histone modifications, influencing the expression of genes related to myogenic differentiation in ARMS cells (52). These mechanisms collectively contribute to ATRA's potential as a therapeutic agent for ARMS differentiation.
Cisplatin
Cisplatin, is a toxic antineoplastic agent, but one of the most heavily employed agents that first came to use in the 1970’s. It operates by inducing DNA damage and subsequent cell cycle arrest, rather than directly promoting differentiation in cancer cells (53). It forms covalent bonds with DNA, leading to the formation of DNA crosslinks and adducts, triggering DNA repair responses and, in ARMS cells, potentially causing apoptosis rather than differentiation (54). Cisplatin is often administered in combination with other chemotherapeutic agents to enhance treatment efficacy and inhibit tumor growth. Additionally, it can act as a radiosensitizer, increasing the sensitivity of cancer cells to radiation therapy, which is commonly part of ARMS treatment (55). The utilization of various therapeutic approaches, including cisplatin, aims to achieve comprehensive control of ARMS, with treatment plans tailored to individual patients in consultation with medical oncologists.
5-Azacytidine
5. -Azacytidine is a demethylating agent which functions by incorporating into DNA during replication and inhibiting DNA methyltransferase activity, resulting in DNA demethylation (56). In ARMS, hypermethylation of specific genes can lead to their silencing, including those associated with differentiation (57). 5-Azacytidine treatment can reverse this process, allowing previously silenced genes linked to myogenic differentiation to be re-expressed. This demethylation also contributes to the restoration of myogenic regulatory factors (MRFs) such as MyoD and myogenin, which are crucial for myogenic differentiation (58). Furthermore, 5-Azacytidine can induce cell cycle arrest and cellular senescence, promoting a more mature cellular state in ARMS (59). However, it is important to note that 5-Azacytidine's effectiveness often varies among patients and is often used in the context of personalized treatment plans for ARMS, typically under clinical trial conditions.
The following table summarizes the key molecular targets of the drugs discussed.
Table 1.
Chemotherapy drugs used for ARMS and their molecular targets.
Table 1.
Chemotherapy drugs used for ARMS and their molecular targets.
Drug/Compound |
Molecular Target |
Reference |
Vincristine, Actinomycin D, and Cyclophosphamide (VAC) |
Microtubule Polymerization, Guanine nucleotide in DNA, cross-linkages with guanine N-7 respectively |
(40, 42) |
Cabozantinib (XL184) |
Tyrosine Kinase (MET) |
(35) |
Bortezomib |
26s proteosome |
(35) |
Vinorelbin |
Microtubular Proteins |
(9) |
AZD1775 |
Wee 1 |
(35) |
Entinostat, Panobinopstat and Vorinostat |
Histone Deacetylase (HDAC) |
(34) |
Critotinib, Bevacizumab (mAb) and Regorafenib |
Receptor Tyrosine Kinae (RTK) |
(34) |
All Trans Retinoic Acid (ATRA) |
retinoic acid receptors (RARs) |
(20) |
Cisplatin |
DNA |
(53) |
5-Azacytidine |
DNA methyltransferase |
(56) |