Abstract A003: ASAP1 regulates myogenic differentiation in rhabdomyosarcoma by modulating YAP localization

Despite aggressive, multimodal therapies, the prognosis of patients with refractory or recurrent rhabdomyosarcoma (RMS) has not improved in four decades. RMS is thought to arise due to defective differentiation in skeletal muscle precursor cells. Differentiation-inducing therapy may improve outcomes for RMS patients with advanced disease. In RAS-mutant PAX fusion-negative RMS (FN-FMS), targeting ERK1/2 activation through MEK1/2 inhibition (MEKi) induces differentiation, slows tumor growth, and extends survival in preclinical studies. However, the duration of therapeutic response is short lived. Identifying additional targets for differentiation therapy is necessary. We propose that ASAP1, an Arf GTPase-activating protein (Arf GAP) highly expressed in FN-RMS and implicated in breast and colorectal cancer progression as well as osteogenic and retinal endothelium differentiation, contributes to FN-RMS differentiation. We find that knockdown (KD) of ASAP1 inhibits differentiation in myoblasts and FN-RMS cells, and its overexpression enhances differentiation. Moreover, myogenic differentiation-associated genes are not enriched upon loss of ASAP1. We discover that KD of ASAP1 homologs, ASAP2 and ASAP3, also blocks differentiation. However, loss of a paralogous Arf GAP, ARAP1, does not, indicating that ASAP regulates differentiation through a mechanism not explained by GAP activity alone. Interestingly, KD of Arf1 or Arf5, small GTPases inactivated by ASAP, also blocks differentiation of FN-RMS, suggesting a novel relationship between Arf and ASAP. Using RNAseq, qPCR, and immunoblotting techniques, we show that loss of ASAP blocks myogenic transcription factor expression. To determine the mechanism by which ASAP1 regulates myogenic transcription factor expression, we investigate the PI3K/AKT, MAPK, and Hippo pathways, which are known to be regulated by ASAP1. While the PI3K/AKT and MAPK pathways are unaffected, downstream components of the Hippo pathway are modulated by ASAP1 KD in FN-RMS cells treated with MEKi. YAP and TAZ are transcriptional coactivators that promote proliferation. Upon Hippo pathway activation, YAP/TAZ activity is blocked by phosphorylation at a nuclear exclusion site. Using cell fractionation and immunoblotting techniques, we find that induction of differentiation by MEKi increases TAZ phosphorylation, excluding it from the nucleus and blocking pro-proliferative transcription. Further, upon ASAP1 KD, TAZ phosphorylation is blocked, restoring nuclear localization, and inhibiting MEKi-induced differentiation. In conclusion, we discover that ASAP1 regulates MEKi-induced differentiation of FN-RMS cells by modulating TAZ localization and supports targeting the YAP pathway as a strategy for FN-RMS differentiation therapy. Our work also identifies ASAP1 as a potential effector of Arf1 activity, a novel interaction of these two proteins. The results described herein provide a deeper understanding of differentiation in FN-RMS and establish the groundwork for advancing differentiation therapy in FN-RMS.

Citation Format: Katie E. Hebron, Olivia Feehan-Nelson, Angela Kim, Xiaoying Jian, Sofia A Girald, Paul Randazzo, Marielle E. Yohe. ASAP1 regulates myogenic differentiation in rhabdomyosarcoma by modulating YAP localization [abstract]. In: Proceedings of the AACR Special Conference: Sarcomas; 2022 May 9-12; Montreal, QC, Canada. Philadelphia (PA): AACR; Clin Cancer Res 2022;28(18_Suppl):Abstract nr A003.

Abstract IA023: Therapeutic efficacy of trametinib and ganitumab in RAS-mutated rhabdomyosarcoma

Background: PAX-fusion negative rhabdomyosarcoma (FN RMS) is driven by alterations in the RAS/MAP kinase pathway and is partially responsive to MEK inhibition. Overexpression of IGF1R and its ligands is also observed in FN RMS. Preclinical and clinical studies have suggested that IGF1R is itself an important target in FN RMS. Our previous studies revealed preclinical efficacy of the MEK1/2 inhibitor, trametinib, and an IGF1R inhibitor, BMS75807, but this combination was not pursued clinically due to excessive toxicity in preclinical murine models. Here, we sought to identify a combination of an MEK1/2 inhibitor and IGF1R inhibitor that would be better tolerated in murine models and effective in both cell line and patient derived xenograft models of RAS-mutant FN RMS. Methods: Using proliferation and apoptosis assays, we studied the factorial effects of trametinib and ganitumab (AMG 479), a monoclonal antibody with specificity for human and murine IGF1R, in a panel of RAS-mutant FN RMS cell lines. The molecular mechanism of the observed synergy was determined using conventional and capillary immunoassays. The efficacy and tolerability of the combination was assessed using a panel of RAS-mutated cell-line and patient-derived RMS xenograft models. Results: Treatment with trametinib and ganitumab resulted in synergistic cellular growth inhibition in all cell lines tested and inhibition of tumor growth in five out of six models of RAS-mutant RMS. Evidence suggests that the combination had little effect on body weight loss, thrombocytopenia, neutropenia, or hyperinsulinemia in tumor-bearing SCID beige mice. Mechanistically, ganitumab treatment prevented the AKT phosphorylation that is induced by MEK inhibition alone. Therapeutic response to the combination was observed in models with an intact PI3K/PTEN axis. Conclusions: We demonstrate that combined trametinib and ganitumab is effective in a genomically diverse panel of RAS-mutated FN RMS preclinical models. The trametinib/ganitumab combination also likely has an improved tolerability profile compared to other IGF1R/MEK inhibitor combinations. These data support testing this combination in a phase I/II clinical trial for pediatric patients with relapsed or refractory RAS-mutated FN RMS.

Citation Format: Marielle E. Yohe, Katie E. Hebron, Xiaolin Wan, Jacob S. Roth, David J. Liewehr, Nancy E. Sealover, Stacey Stauffer, Olivia Feehan-Nelson, Wenyue Sun, Kristine A. Isanogle, Christina M. Robinson, Amy James, Parirokh Awasthi, Priya Shankarappa, Xiaoling Liu, Haiyan Lei, Donna Butcher, Roberta Smith, Elijah F. Edmonson, Jin-Qui Chen, Noemi Kedei, Cody S. Peer, Jack F. Shern, W. Douglas Figg, Lu Chen, Matthew D. Hall, Simone Difillipantonio, Frederic G. Barr, Robert L. Kortum, Angelina V. Vaseva, Javed Khan. Therapeutic efficacy of trametinib and ganitumab in RAS-mutated rhabdomyosarcoma [abstract]. In: Proceedings of the AACR Special Conference: Sarcomas; 2022 May 9-12; Montreal, QC, Canada. Philadelphia (PA): AACR; Clin Cancer Res 2022;28(18_Suppl):Abstract nr IA023.

Abstract B32: ASAP1 regulates differentiation in myoblasts and PAX-FOXO1 fusion-negative rhabdomyosarcoma

Rhabdomyosarcoma (RMS), the most frequently diagnosed soft-tissue sarcoma in children, is caused by a differentiation defect in skeletal muscle precursor cells. Despite aggressive, multimodal therapies, the prognosis for recurrent PAX-FOXO1 fusion-negative RMS (FN-RMS) remains poor. Inducing differentiation in diseases defined by defective differentiation programs has led to curative therapies in cancers such as acute promyelocytic leukemia. A recent study showed that inducing differentiation slowed tumor growth and extended survival in a xenograft model of FN-RMS, identifying differentiation as a promising therapeutic target in FN-RMS. However, genes associated with differentiation are frequently found to be hijacked by tumor cells and repurposed for proinvasive programs. Therefore, better understanding of differentiation signaling and its relation to invasion could reveal novel therapeutic opportunities for patients with advanced FN-RMS. We hypothesize that ASAP1, an Arf GTPase-activating protein implicated in differentiation in normal cells and invasion in carcinoma, promotes progression by controlling proinvasive elements of differentiation signaling through focal adhesion assembly. ASAP1, an Arf GTPase-activating protein (Arf GAP), regulates integrin adhesion complexes, critical regulators of biologic processes such as proliferation, migration, and differentiation that are commonly dysregulated in cancer. ASAP1 is overexpressed in several cancers and correlates with increased metastasis and poor patient prognosis but has also been shown to promote differentiation. The mechanisms by which ASAP1 affects cancer progression and differentiation and the relationship between these effects are not yet understood. We found that ASAP1 is overexpressed in FN-RMS. ASAP1 overexpression inhibits proliferation in myoblasts, but not in FN-RMS. Knockdown of ASAP1 inhibits differentiation in both myoblast and FN-RMS cell lines, while overexpression enhances differentiation. Moreover, gene set enrichment analysis shows that myoblast differentiation-associated genes fail to become enriched upon knockdown of ASAP1. Finally, knockdown of Arf1 and Arf5, established binding partners of ASAP1, also blocks differentiation of FN-RMS cell lines, indicating that ASAP1 may regulate differentiation through its interaction with Arf GTPases. These data support our hypothesis that ASAP1 regulates the continuum of differentiation and invasion in FN-RMS. As a continuing test of the hypothesis, future studies will investigate focal adhesion assembly, dynamics, and signaling, which are processes known to be affected by ASAP1, as a mechanism for ASAP1-mediated regulation of myoblast differentiation.

Citation Format: Katie E. Hebron, Olivia Feehan-Nelson, Xiaoying Jian, Sofia A. Girald, Paul A. Randazzo, Marielle E. Yohe. ASAP1 regulates differentiation in myoblasts and PAX-FOXO1 fusion-negative rhabdomyosarcoma [abstract]. In: Proceedings of the AACR Special Conference on the Advances in Pediatric Cancer Research; 2019 Sep 17-20; Montreal, QC, Canada. Philadelphia (PA): AACR; Cancer Res 2020;80(14 Suppl):Abstract nr B32.