Sunday, May 11, 2025
5/11/2025
Reprogramming myogenic regulatory factors in RMS to promote differentiation and halt growth - Summary Rhabdomyosarcoma (RMS) accounts for 3-4% of all pediatric cancers, with less than a 30% overall 5-year survival rate for children diagnosed with metastatic RMS. Sarcoma patients also experience higher rates of morbidity and mortality than other cancer patients, and this is particularly evident in children. As a result of their therapies, 42% of childhood cancer survivors experience severe, disabling, or life threatening conditions, including secondary tumors. Thus, there is clearly a need to develop new, more targeted treatment strategies for pediatric tumors such as RMS; treatments that inhibit tumor progression yet confer limited side effects. In many cancers, embryonic programs, including the acquisition of stem/progenitor states, are instituted to contribute to tumor progression. Such reinstatement or retention of developmental programs may be a key driving factor in Embryonal Rhabdomyosarcoma (ERMS, which are generally fusion negative and also referred to as FN-RMS). In RMS, high expression of myogenic-lineage transcription factors (TF), MYOD1 and MYOG, is observed. However, despite high expression of these TFs, RMS cells fail to differentiate. Instead, these TFs drive the RMS malignant phenotype, since knockout of these MRFs results in lethality to the RMS cell. Thus, a key unanswered question in the field is why and how the myogenic regulatory factors (MRFs) depart from their canonical roles as drivers of muscle differentiation to instead maintain RMS cells in an undifferentiated state. In this proposal, we are examining whether SIX1, a critical TF that regulates muscle development, is responsible for globally altering the function of MRFs. Our data show that elevated SIX1 expression promotes FN-RMS progression and growth by promoting an RMS progenitor-like state. Intriguingly, SIX1 knockdown induces a muscle differentiation signature, concomitant with re-localization of key MRFs genome-wide. These data suggest that in FN-RMS, SIX1 overexpression alters MRF function, promoting an RMS progenitor-like phenotype and enhancing tumor growth and progression. Thus, in this proposal we will test the following hypothesis: SIX1 and its obligate cofactors EYA2/3 cooperatively drive the progression of FN-RMS by altering the genomic landscape, causing MRFs to favor growth over differentiation. Since SIX1 expression in differentiated tissues is low, targeting it may therefore be a means to inhibit FN-RMS with limited side effects to untransformed cells where SIX1 is dispensable. Specific aims are as follows: 1) To determine the molecular mechanism by which SIX1 serves as a master regulator of the muscle progenitor vs differentiated state in development and in RMS. 2) To identify the critical SIX1 cofactors (with a focus on EYA proteins) that, when targeted, can induce differentiation and inhibit RMS growth. This work will take advantage of normal developmental regulatory mechanisms to inhibit the tumor, and thus may have limited toxicity due to the paucity of SIX/EYA expression in adult tissue. Our ability to combine zebrafish and human models will maximize the benefits of each model system to rapidly and inexpensively identify means to inhibit RMS growth and progression. Understanding the mechanisms by which RMS cells are trapped in an early developmental state may therefore lead to novel means to target the disease.
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