Glioblastoma (GBM) is the most frequent high grade brain tumor in adults. Treatment consists of a combination of maximal surgical resection, radiotherapy and chemotherapy, but prognosis is still poor. Therefore, new therapeutic strategies with other biologically-based therapies is essential to improve the survival of patients with GBM, currently restricted to 15-17 months. This urgent need for better therapies has motivated many studies to pursuit new targets in GBM. Our laboratory demonstrated higher expression of CD99 mRNA in GBM samples when compared to lower grade astrocytomas and non-neoplastic brain tissue. CD99 is a membrane molecule and acts as a key regulator in several biological processes, including cell adhesion, migration, apoptosis, differentiation of T cells and thymocytes, diapedesis of lymphocytes to inflamed vascular endothelium, maintenance of cellular morphology and regulation of intracellular membrane protein trafficking. CD99 expression has been described in many types of cancers, with distinct function and signaling pathways, and its role is not completely elucidated yet, including in GBM.We confirmed the higher CD99 expression in TCGA dataset of GBM cases when compared to grade II and III astrocytomas. Furthermore, amongst GBM cases, Proneural molecular subtype presented lower CD99 expression compared to Mensechymal, Classic and Neural subtypes. We performed functional assays knocking down CD99 with two different shRNAs in the human GBM cell line U87MG. The CD99 silencing efficiency was 80 and 97%, for shRNA_1 and shRNA_2, respectively, confirmed at transcriptional level by quantitative real time PCR and at protein level by Western blot. Significant reduction of migration and invasion of CD99 knocked-down U87MG cells was observed. Additionally, cell shrinkage with relevant reduction of cell area was detected after the silencing of this target, with decreased CD99 localization at lamellipodia by immunocytochemistry. These findings pointed out the cytoskeleton rearrangement with reduction in adhesion sites. Further expression profile analysis after CD99 knockdown by a cancer panel RNAseq confirmed its involvement in cell adhesion, regulation of actin cytoskeleton and ECM-receptor interaction in the KEGG pathways analysis. Interestingly, CD99 knockdown impacted in the reduction of cell adhesion in a distinct quantitative pattern, suggesting that the result is dose dependent. Refinement of this aspect is ongoing in further in vivo assays which will better clarify the CD99 role in tumor cell migration/invasion. Altogether, our results suggest CD99 as a potential new therapeutic target to control GBM infiltrative process, allowing a more efficient treatment outcome.

Citation Format: Lais Cavalca Cardoso, Suely Kazue Marie, Roseli Silva Soares, Sueli M. Oba-Shinjo. CD99 expression in astrocytomas and functional analysis in glioblastoma cell line [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1075.