WEE1 accumulation and deregulation of S-phase proteins mediate MLN4924 potent inhibitory effect on Ewing sarcoma cells.

Ewing sarcoma (ES) is an aggressive bone and soft tissue tumor of children and young adults in which finding effective new targeted therapies is imperative. Here, we report an in-depth preclinical study of the investigational cullin-RING ubiquitin ligase (CRL) inhibitor MLN4924 in ES, as we have recently demonstrated the implication of a CRL component in the ES pathogenesis. First, our results support a high sensitivity of ES cells to MLN4924 growth inhibition both in vitro (14 ES cell lines tested, median IC50=81 nM) and in tumor xenografts (tumor regression achieved with 60 mg/kg BID, subcutaneously, n=9). Second, we report a dual mechanism of action of MLN4924 in ES cells: while a wide range of MLN4924 concentrations (∼30-300 nM) trigger a G2 arrest that can only be rescued by WEE1 kinase inhibition or depletion, saturating doses of the drug (>300 nM) cause a delay in S-phase progression concomitant with unbalanced CDK2-Cyclin E and CDK2-Cyclin A relative levels (accumulation of the first and depletion of the latter). The aberrant presence of CDC6 in the nucleus at late S-phase cell cycle stage confirmed the loss of CDK2-Cyclin A-specific functions. Remarkably, other mechanisms explored (P27 accumulation and DNA damage signaling pathways) were found unable to explain MLN4924 effects, strengthening the specificity of our findings and suggesting the absence of functionality of some CRL substrates accumulated in response to MLN4924. This study renders a rationale for clinical trials and contributes molecular mechanisms for a better understanding of this promising antitumoral agent.

1q gain and CDT2 overexpression underlie an aggressive and highly proliferative form of Ewing sarcoma.

Despite extensive characterization of the role of the EWS-ETS fusions, little is known about secondary genetic alterations and their clinical contribution to Ewing sarcoma (ES). It has been demonstrated that the molecular structure of EWS-ETS lacks prognostic value. Moreover, CDKN2A deletion and TP53 mutation, despite carrying a poor prognosis, are infrequent. In this scenario identifying secondary genetic alterations with a significant prevalence could contribute to understand the molecular mechanisms underlying the most aggressive forms of ES.We screened a 67 ES tumor set for copy number alterations by array comparative genomic hybridization. 1q gain (1qG), detected in 31% of tumor samples, was found markedly associated with relapse and poor overall and disease-free survival and demonstrated a prognostic value independent of classical clinical parameters. Reanalysis of an expression dataset belonging to an independent tumor set (n=37) not only validated this finding but also led us to identify a transcriptomic profile of severe cell cycle deregulation in 1qG ES tumors. Consistently, a higher proliferation rate was detected in this tumor subset by Ki-67 immunohistochemistry. CDT2, a 1q-located candidate gene encoding a protein involved in ubiquitin ligase activity and significantly overexpressed in 1qG ES tumors, was validated in vitro and in vivo proving its major contribution to this molecular and clinical phenotype. This integrative genomic study of 105 ES tumors in overall renders the potential value of 1qG and CDT2 overexpression as prognostic biomarkers and also affords a rationale for the application of already available new therapeutic compounds selectively targeting the protein-ubiquitin machinery.