Probing for a deeper understanding of rhabdomyosarcoma: insights from complementary model systems

Abstract

Rhabdomyosarcoma (RMS) is a mesenchymal malignancy composed of neoplastic primitive precursor cells that exhibit histological features of myogenic differentiation. Despite intensive conventional multimodal therapy, patients with high-risk RMS typically suffer from aggressive disease. The lack of directed therapies against RMS emphasizes the need to further uncover the molecular underpinnings of the disease. In this Review, we discuss the notable advances in the model systems now available to probe for new RMS-targetable pathogenetic mechanisms, and the possibilities for enhanced RMS therapeutics and improved clinical outcomes.

Figure 1. PAX3 and PAX7 fusion proteins

Shown are the wild-type (part a) and the fusion (part b) proteins generated owing to the chromosomal translocations in alveolar rhabdomyosarcoma (ARMS). Detection of the PAX3–FOXO1 (forkhead box O1) and PAX7–FOXO1 translocations by either reverse transcription polymerase chain reaction (RT-PCR) or fluorescence in situ hybridization (FISH) distinguishes fusion-positive ARMS from fusion-negative ARMS. Fusion-positive ARMS has a worse prognosis, whereas fusion-negative ARMS as a class is more similar clinically and molecularly to embryonal rhabdomyosarcoma (ERMS). It remains unclear whether the rare variant PAX3 fusions behave in a clinically aggressive manner and recapitulate the molecular features similar to PAX3–FOXO1- or PAX7–FOXO1-containing ARMS. Matching colours represent homologous domains; for INO80D, the regions with no known domain structure are brown in colour. bHLH, basic helix–loop–helix; FH, forkhead DNA-binding domain; HD, homeodomain; NCOA, nuclear receptor co-activator; OP, octapeptide motif; PAX, paired box; PD, paired domain; TAD, transcriptional activation domain.

Figure 2. RMS model systems along the myogenic continuum

The developmental stages of muscle cells from which rhabdomyosarcoma (RMS)-like tumours can arise and the models that have been used to demonstrate the potential cellular origins are shown. Prenatal cellular differentiation is shown in a box with dotted lines. Using targeted approaches, oncogenes are activated and/or tumour suppressors deleted in specific cell populations during muscle development. Mouse models utilize the Cre–lox system, and Cre drivers are controlled by promoters from transcription factors that regulate myogenesis. Drosophila melanogaster models use the GAL4–UAS system to direct the expression of PAX7–FOXO1 in skeletal muscle. Zebrafish models express KRAS^G12D directly from tissue-specific promoters. The diversity of these models and the heterogeneity of the resulting histologies suggest that there are many potential permissive states during muscle development for RMS genesis. The genetic makeup of each model is shown along with its corresponding myogenic cellular source. Also illustrated is the mouse model arising from the non-muscle adipose protein 2 (aP2) lineage. Coloured circles adjacent to the genotypes indicate the different forms of tumours that arise in their corresponding models. ARMS, alveolar RMS; cdh15, cadherin 15; Cdkn2a, cyclin-dependent kinase inhibitor 2A; ERMS, embryonal RMS; FOXO1, forkhead box O1; LSL, lox–STOP–lox; Mhc, myosin heavy chain; Myf, myogenic factor; mylz2, myosin light chain 2; MyoD, myogenic differentiation 1; NOS, not otherwise specified; PAX, paired box; Ptch1, Patched 1; rag2, recombination activating gene 2; Smo, Smoothened.

Figure 3. Complementary models of RMS and their advantages

Genetically modified cells, Drosophila melanogaster, zebrafish and mice have been utilized to model rhabdomyosarcoma (RMS). Mice are used for both xenograft and orthotopic model systems. The advantages of each model have been listed. Skeletal muscle is incredibly evolutionarily conserved in structure and function, as well as in the developmental genetic programme. Thus, RMS was exquisitely well primed to be interrogated by such a diverse array of model systems. All RMS subtypes are represented in cell lines, genetically engineered mice and xenograft mouse models. To date, only embryonal RMS (ERMS) is modelled in zebrafish and only alveolar RMS (ARMS) in Drosophila.

Figure 4. RMS pathways for targeted therapy

Diverse pathways have been implicated in rhabdomyosarcoma (RMS) biology and offer potential for targeted therapeutic intervention. The diversity of targets and multitude of agents illustrates the need to leverage model systems to focus clinical trial efforts in the small RMS patient population. The drugs that target different signalling pathways are listed in boxes adjacent to the corresponding proteins, whereas other targets are listed in an independent box. ALK, anaplastic lymphoma kinase; ANG1, angiopoietin 1; BAD, BCL-2-associated agonist of cell death; CCND, cyclin D; CDK, cyclin-dependent kinase; CTLA4, cytotoxic T lymphocyte-associated antigen 4; EGFR, epidermal growth factor receptor; FGFR4, fibroblast growth factor receptor 4; FOXO1; forkhead box O1; GSK3, glycogen synthase kinase 3; HDAC, histone deacetylase; IGF1R, insulin-like growth factor 1 receptor; NICD, Notch intracellular domain; NF1, neurofibromin 1; PAX, paired box; PD1, programmed cell death protein 1; PDGFR, platelet-derived growth factor receptor; PLK1, Polo-like kinase 1; PTCH, Patched; RB1, retinoblastoma 1; RBPJ, recombination signal binding protein for immunoglobulin kJ region (also known as CSL); RHEB, RAS homologue enriched in brain; SHH, sonic hedgehog; SMO, Smoothened; TSC2, tuberin; VEGFR2, vascular endothelial growth factor receptor 2.