CD99 contributes to the EWS::FLI1 transcriptome by specifically affecting FOXM1-targets involved in the G2/M cell cycle phase, thus influencing the Ewing sarcoma genetic landscape.

Ewing sarcoma (EwS), a highly aggressive malignancy affecting children and young adults, is primarily driven by a distinctive oncogenic fusion, the EWSR1-ETS, whose activity is a key source of epigenetic and clinical heterogeneity. CD99 is constantly present in EwS cells, known to modulate the EwS genetic profile and tumor malignancy. However, the relevance of CD99 alone, or in association with EWSR1-ETS chimeras, is poorly understood. We explored the dynamic relationship between CD99 and EWS::FLI1, the main fusion observed in EwS, by means of model systems with inducible expression of either molecule. The transcriptomic dynamics of cells with or without expression of EWS::FLI1 or CD99 were analyzed and correlated with tumor cell growth. The CD99-associated EwS gene profile was found to have commonalities with the profile induced by EWS::FLI1, but also peculiar differences. Both EWS::FLI1 and CD99 are regulated targets of the DREAM complex, but the CD99 expression specifically impacted genes that are the targets of FOXM1 and are involved in the setting of the G2/M phase of the cell cycle. Most CD99-regulated FOXM1-targeted genes were found to correlate with bad prognosis in two public clinical datasets (R2 platform), further supporting the clinical relevance of CD99-mediated regulation of EwS gene expression.