BRD9 inhibition as potential treatment option for testicular germ cell tumors.

Background: Testicular germ cell tumors (TGCT) are the predominant tumor in younger males. Usually, 5-year survival rates are quite high, but 15-20% of patients with metastatic non-seminomas are resistant to standard cisplatin-based therapy. Interfering with the epigenetic landscape has already been shown to be effective in prostate cancer. BRD9 is an epigenetic reader that is part of a chromatin-remodeling complex involved in regulation of gene expression.

Objectives: Alternative treatment options for therapy-resistant TGCT patients need to be investigated.

Materials and methods: BRD9 expression was analyzed by meta-analysis of microarray data as well as by Western blot and immunohistochemistry of tissue microarrays in TGCT cell lines and TGCT tissues. Viability was assessed by performing XTT-assay to determine the effect of BRD9 inhibition in TGCT cell lines. FACS analysis was used to display changes in cell cycle distribution as well as apoptosis. The impact on transcriptome level of BRD9 inhibition was analyzed by 3'mRNA-sequencing.

Results: BRD9 was heterogeneously expressed in TGCT cell lines and tissues. Nevertheless, inhibition of BRD9 led to a strong decrease in viability. I-BRD9 induced apoptosis as well as cell cycle arrest in G1-phase. On transcriptome level, prominent downregulation of pluripotency markers (NANOG, PRMD14, and KLF4) and upregulation of genes involved in epithelium development were detected.

Discussion: I-BRD9 treatment of TGCT cell lines reduced viability, induced apoptosis and cell cycle arrest while control cells remain only slightly affected. Transcriptomic data indicate exit of pluripotency and differentiation toward the epithelial fate. In fact, loss of pluripotency and differentiation seems to be a common aspect of germ cell tumors (GCT) reacting to drug application.

Conclusion: The data implicate I-BRD9 as a possible treatment alternative for TGCTs.