Unwinding new therapeutic opportunities in rhabdomyosarcoma: the role of RNA helicase DDX5

Rhabdomyosarcoma (RMS) is the most common soft-tissue sarcoma of childhood characterized by the inability to exit the proliferative myoblast-like stage. RMS can be divided in two main histopathological subtypes: alveolar (ARMS), mainly characterized by chromosomal translocations resulting in the oncogenic fusion transcription factors (PAX3- or PAX7-FOXO1); and embryonal (ERMS) characterized by a more heterogeneous genomic profile associated with activation of various tumor-promoting signaling pathways and/or loss of tumor surveillance. Alveolar RMS is the most aggressive subtype, associated with frequent metastasis at the time of diagnosis and limited response to treatment, resulting in poor survival rates, emphasizing the need to develop selective therapeutic strategies for this subset of patients.

Since targeting the oncogenic fusion proteins in ARMS remains a challenge, other therapeutic vulnerabilities resulting from gene expression changes due to specific epigenetic perturbations are progressively being recognized. In this context, DEAD box RNA helicases appear of special interest as new potential pharmacological targets as they play key roles in shaping the transcriptional programs of eukaryotic cells, modulating almost every aspect of RNA metabolism, from transcription to degradation. Accumulating evidence indicate that cancer cells rely on this class of enzymes to sustain aberrant transcriptional programs, however if they play a role in RMS has been not addressed.

Our preliminary results showing that DEAD box helicase 5 (DDX5) is overexpressed in ARMS and that its downregulation induces ARMS growth arrest and apoptosis, both in vitro and in vivo, prompted our interest in investigating it as a potential therapeutic target in ARMS. To this end we propose two inter-related aims:

The first aim will investigate the mechanisms by which DDX5 contributes to the malignant phenotype in ARMS.
To unequivocally identify the transcriptional networks governed by DDX5, we will couple loss-of function experiments with phenotypical characterization and global transcriptional profiling (by RNA-seq). To disclose possible DDX5 functions both at the chromatin level or as a regulator of RNA processing, we will map DDX5 genome-wide localization (ChIP-seq) and its interacting RNAs (CLIP-seq/RIP).

The second aim will assess the preclinical rationale for DDX5 inhibition in the treatment of ARMS. We hypothesize that pharmacological inhibition of the phosphorylated form of DDX5 by the orally available, small molecule inhibitor RX-5902 will result in decreased ARMS proliferation and survival. We will test this both in vitro and in ARMS xenografts in vivo.

Successful completion of our proposal might help opening a new therapeutic venue for ARMS. The activity of DDX5 can be selectively targeted by small molecule inhibitors, some of which already proved clinical efficacy in different cancers.