Rhabdomyosarcoma

Abstract

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in children and represents a high-grade neoplasm of skeletal myoblast-like cells. Decades of clinical and basic research have gradually improved our understanding of the pathophysiology of RMS and helped to optimize clinical care. The two major subtypes of RMS, originally characterized on the basis of light microscopic features, are driven by fundamentally different molecular mechanisms and pose distinct clinical challenges. Curative therapy depends on control of the primary tumour, which can arise at many distinct anatomical sites, as well as controlling disseminated disease that is known or assumed to be present in every case. Sophisticated risk stratification for children with RMS incorporates various clinical, pathological and molecular features, and that information is used to guide the application of multifaceted therapy. Such therapy has historically included cytotoxic chemotherapy as well as surgery, ionizing radiation or both. This Primer describes our current understanding of RMS epidemiology, disease susceptibility factors, disease mechanisms and elements of clinical care, including diagnostics, risk-based care of newly diagnosed and relapsed disease and the prevention and management of late effects in survivors. We also outline potential opportunities to further translate new biological insights into improved clinical outcomes.

Figure 1:. RMS incidence varies with age and subtype

(A) Graph showing the relative proportion of each RMS subtypes presenting in various age groups. Spindle cell/sclerosing and botryoid forms not shown owing to their relative rarity. Note that relative frequency of PAX7-FOXO1 and PAX3-FOXO1 ARMS across the age spectrum has not been well studied and is captured by SEER. (B) Graph showing how the incidence of each major RMS subtype has changed or remained stable over the past ~30 years. The apparent increased incidence of ARMS since the 1990s, but that may, in part, relate to evolving diagnostic definitions, as described in the main text. Data are from the Surveillance, Epidemiology, and End Results (SEER) April 2008 release. Figure has been reproduced with permission from. ARMS, alveolar rhabdomyosarcoma; ERMS, embryonal rhabdomyosarcoma.

Figure 2:. PAX–FOXO1 fusion gene drives RMS formation

FP RMS is defined by balanced translocations between PAX3, residing in the Giemsa band 35 on the long (q) arm of chromosome 2 (2q35) or PAX7 on chromosome 1 (1q36) with the FOXO1 gene on chromosome 13 (13q14) generating fusion proteins. These balanced translocations generate two derivative (der) chromosomes (left side panel), only one of which encodes for the PAX3 (or PAX7)-FOXO1 fusion mRNA and protein (right side of panel). These fusion proteins contain the amino terminal portion of either PAX3 or, less commonly, PAX7 and the carboxyl portion of FOXO1. The amino terminus of the fusion protein includes motifs needed for DNA binding from the respective PAX gene, and the carboxyl terminus of the fusion protein is felt to alter the transcriptional activation domain of the oncogenic transcription factor. Note that the alternate derivative chromosomes are not known to contribute to RMS pathogenesis. Chrom, chromosome; FP, fusion positive; RMS rhabdomyosarcoma; der; derivative chromosome; DBD, DNA binding domain; TAD, transcriptional activation domain; PB, paired-box domain; HB, homeobox domain; FH, Forkhead-related domain; FKHR, forkhead homolog in RMS (original designation of FOXO1 gene).

Figure 3:. Key functional pathways are perturbed in RMS

Key processes of apoptosis, cell proliferation, cellular differentiation, and epigenetic homeostasis are deregulated by mutation or gene copy-number and/or gene expression alterations in fusion negative (FN) or fusion positive (FP) rhabdomyosarcoma (RMS). In FP RMS, chromosomal translocations result in PAX3–FOXO1 or PAX7–FOXO1 fusion genes. The aberrant PAX3–FOXO1 fusion protein can synergize with loss of p16 or p53 functionality that is associated with CDKN2A gene loss and/or promoter methylation and TP53 mutation. The stability and subcellular localization of the PAX3–FOXO1 protein is dependent on phosphorylation of specific sites and it works in a complex that can include BRD4. The PAX3-FOXO1 containing complex acts as pioneer factor and drives expression of other transcription factors such as MYCN and MYOD1 via super-enhancers that lead to reprogramming of the transcriptional and epigenetic landscape of tumors. The genes encoding MYCN and MYOD1 transcription factors may themselves be genetically amplified or mutated, likely contributing to RMS formation or progression in a subset of cases, respectively. The fusion protein also drives expression of specific receptor tyrosine kinases (RTKs). Overexpression and activating mutations of genes encoding the same RTKs, and mutation of genes encoding downstream signaling components, are seen in FN RMS. Together this leads to frequent activation of PI3K and RAS pathway signaling in FP and FN RMS, which likely contribute to disease pathogenesis by altering cell proliferation, apoptosis, and other metabolic pathways in ways that are not yet precisely defined. Next-generation DNA sequencing and other molecular genetics tools have demonstrated deleterious mutations in genes encoding certain proteins involved in RMS pathogenesis (*). Exactly how these pathways driven RMS pathogenesis is not clear.

Figure 4:. ERMS and ARMS can be distinguished based on histopathology features

Representative photomicrographs of embryonal RMS (ERMS; left panels) and alveolar RMS (ARMS; right panels) following staining with hematoxylin and eosin (H&E; top panels) or with primary antibodies to detect Myogenin (middle panels) or Desmin (bottom panels) to mark the skeletal muscle lineage. ARMS often, but not always, displays loosely associated tumor cells in clusters resembling pulmonary alveoli and robust immunohistochemical staining for Myogenin; however, confirmation by analysis of PAX3–FOXO1 or PAX7–FOXO1 fusion is required to confirm FP state. Original magnification: 400x (Image provided by D. Rakheja, University of Texas Southwestern Medical Center)(contact information: dinesh.rakheja@utsouthwestern.edu)

Figure 5:. PAX–FOXO1 translocation can be detected by FISH

The clinical importance of determining PAX–FOXO1 fusion status in RMS has led to the development of clinical-grade molecular assays, such as fluorescence in situ hybridization (FISH). Here, FOXO1 (13q14) and PAX3 (2q36.1) break-apart probe sets (left and right, respectively) illustrate the splitting of the normally juxtaposed orange and green signals within the neoplastic interphase nuclei of alveolar rhabdomyosarcoma cells, indicating rearrangements of these loci. (Images provided by J. Bridge, University of Nebraska Medical Center)(contact information: jbridge@unmc.edu)

Figure 6:. Survival in children with RMS

A. A graph showing Kaplan-Meier plots for proportion of children surviving when treated according to successive Intergroup Rhabdomyosarcoma Study Group (IRS) studies from 1972-1997. Overall survival improves in successive RMS clinical trials. Multiple factors contributed to the improved survival, as detailed in the text. B. Clinical outcomes in data from IRS-III and IRS-IV clinical trials, showing failure free survival (FFS) in children with RMS. Note that survival for Intermediate Risk group represents average of survival for children with Stage 2 or 3, Group III ERMS (73%) and those with Stage 1-3, Group I-III ARMS (65%). Figure 6 part a reproduced from Ref . Data in figure 6 part b are from ref .