Interaction between RAS and Hedgehog Signaling in Rhabdomyosarcoma

• Applicant: Professorin Dr. Heidi Hahn
• Subject Area: Human Genetics ; Hematology, Oncology ; Pediatric and Adolescent Medicine ; Pathology
• Term: from 2015 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 272016244

Rhabdomyosarcoma (RMS) are the most common soft tissue sarcoma in children. On molecular level the embryonal subtype (ERMS) is characterized by e.g. overexpression of Hedgehog (HH) target genes such as GLI1 and by oncogenic mutations in H-, K- or NRAS (oncRAS). We demonstrate that sporadic ERMS do not show canonical HH signaling activity and thus do not react to SMO inhibitors. However, all oncRAS isoforms suppress the expression of GLI1/GLI1 mRNA/protein in cell lines derived from sporadic ERMS via the MEK/ERK axis. Nevertheless, oncRAS increases the tumorigenicity of the cell lines. Since - according to the literature - ERMS cell lines are sensitive against the GLI inhibitor GANT61, oncRAS apparently is a very strong protumorigenic stimulus that allows for a simultaneous suppression of other protumorigenic stimuli. Since – vice versa - GANT61 increases the phosphorylation of ERK, it is possible that a combination of a GLI- with a MEK-inhibitor may result in very strong antitumoral effects in ERMS. We also have induced the expression of oncRas mutations in ERMS of heterozygous Ptch+/- mice. In contrast to sporadic ERMS the ERMS in this model are initiated by Ptch germline mutations, which is also the case in patients with basal cell nevus syndrome. Ptch-associated ERMS show canonical Hh signaling activity and are sensitive towards a treatment with SMO inhibitors. OncKRas and oncHRas enhance the occurrence and growth of Ptch-associated ERMS, whereas oncNRas does not. Interestingly, these effects only occur when the oncRas mutations are induced in ERMS precursor lesions after birth, but not when they are induced at the full-blown ERMS stage. Together with the fact that oncRas-germline mutations do not result in ERMS in mice, it is possible that oncRas mutations are only advantageous for ERMS growth when they are induced in already initiated ERMS precursor lesions and/or ERMS stem cells. In contrast, they do affect the tumor bulk, at least not in the mouse. Since oncRAS mutations increase the proliferation rate of cell lines derived from sporadic ERMS, it is possible that the cell lines harbor specific, oncRAS-susceptible cellular subpopulations that are simply absent in Ptch-associated ERMS. In addition it is possible that oncRas mutations only affect ERMS growth when induced in a very specific time window during ERMS development, when the tumor cells are susceptible for the mutations. This window may be different for the respective oncRas isoforms, which could explain the lack of growth changes upon an induction of an oncNRas mutation in ERMS precursor lesions in the Ptch mouse model. These questions will be solved with help of the present grant proposal.

• DFG Programme: Research Grants