Targeting hedgehog signaling reduces self-renewal in embryonal rhabdomyosarcoma

Abstract

Current treatment regimens for rhabdomyosarcoma (RMS), the most common pediatric soft tissue cancer, rely on conventional chemotherapy, and although they show clinical benefit, there is a significant risk of adverse side effects and secondary tumors later in life. Therefore, identifying and targeting sub-populations with higher tumorigenic potential and self-renewing capacity would offer improved patient management strategies. Hedgehog signaling has been linked to the development of embryonal RMS (ERMS) through mouse genetics and rare human syndromes. However, activating mutations in this pathway in sporadic RMS are rare and therefore the contribution of hedgehog signaling to oncogenesis remains unclear. Here, we show by genetic loss- and gain-of-function experiments and the use of clinically relevant small molecule modulators that hedgehog signaling is important for controlling self-renewal of a subpopulation of RMS cells in vitro and tumor initiation in vivo. In addition, hedgehog activity altered chemoresistance, motility and differentiation status. The core stem cell gene NANOG was determined to be important for ERMS self-renewal, possibly acting downstream of hedgehog signaling. Crucially, evaluating the presence of a subpopulation of tumor-propagating cells in patient biopsies identified by GLI1 and NANOG expression had prognostic significance. Hence, this work identifies novel functional aspects of hedgehog signaling in ERMS, redefines the rationale for its targeting as means to control ERMS self-renewal and underscores the importance of studying functional tumor heterogeneity in pediatric cancers.

A Novel Notch–YAP Circuit Drives Stemness and Tumorigenesis in Embryonal Rhabdomyosarcoma

“…This variability suggests that other developmental pathways might cooperate with Notch to support stemness in eRMS. For example, the Hedgehog pathway also supports stem cell properties (74) and has been found critical for Rh36, CCA, and CT-TC cell growth (75). Why there might be different developmental pathways underlying stemness in different eRMS tumors is unknown, however we speculate it may be due to factors including but not limited to patient age, genetic background, primary tumor mutational status, anatomic location, cell of origin, and even culturing in vitro , and emphasizes the need for a large repository of eRMS cell lines and tumors for systematic preclinical studies.…”

Interfering with Hedgehog Pathway: New Avenues for Targeted Therapy in Rhabdomyosarcoma

“…SHH immunoreactivity is not common in RMS and the higher Hh activity in NRMS compared to fusion positive RMS (PRMS) does not correlate with SHH mRNA levels, which are unchanged between the two subtypes [39][40][41]. However, we previously found a positive correlation of IHH and DHH mRNA levels with the expression of Hh target genes in RMS supporting the hypothesis of a ligand-mediated activation of Hh pathway [45]. Therefore, underestimation of the role of the other two Hh ligands might have led to misleading conclusions and further studies need to more carefully elucidate their role in RMS.…”

“…Importantly, one of the biological limitations of RMS-directed therapies is the possible existence of functionally defined sub-populations of cells within the tumor having increased tumorigenicity, self-renewal and chemoresistance [70]. This is prominent for NRMS where the hierarchical model may be applied and has to be taken into account when developing novel treatment strategies [45,[71][72][73][74]. Indeed, we found that Hh inhibition might provide a promising anti-cancer stem cell (CSC) therapy in NRMS, and therefore multistrategy approaches with bulk-reducing drugs may be more effective in tumor eradication, particularly for high-risk group patients [45].…”

“…This is prominent for NRMS where the hierarchical model may be applied and has to be taken into account when developing novel treatment strategies [45,[71][72][73][74]. Indeed, we found that Hh inhibition might provide a promising anti-cancer stem cell (CSC) therapy in NRMS, and therefore multistrategy approaches with bulk-reducing drugs may be more effective in tumor eradication, particularly for high-risk group patients [45]. Accordingly, LDE-225 alone and Vismodegib (Curis/Roche) in combination with a Notch inhibitor (R04929097) have entered a clinical trial for recurrent RMS and adult advanced RMS, respectively [75,76].…”

“…Accordingly, up-regulation of Hh target genes such as GLI1 and PTCH1 has been demonstrated by retrospective analysis of RMS patient samples or in human RMS cell lines [39][40][41][42][43]. Additionally, activation of Hh pathway seems to be specific for fusion negative RMS (NRMS) and significantly identifies patients with poor prognosis [40,41,44,45]. Despite this recognized role of Hh in RMS, the contribution of the ligand-based signaling versus non-canonical pathway activation in sporadic RMS is still unclear.…”