NAB2-STAT6 drives an EGR1-dependent neuroendocrine program in Solitary Fibrous Tumors (ATAC-Seq)
The pathogenesis of many rare tumor types is poorly understood, preventing the design of effective treatments. Solitary Fibrous Tumors (SFTs) are neoplasms of mesenchymal origin that affect 1/1,000,000  individuals every year and are clinically assimilated to sarcomas. SFTs are commonly found throughout the body and can be surgically removed upon diagnosis. However, 30-40% of tumors become aggressive and can locally relapse or metastasize. There are no effective treatments for malignant SFTs to date. The molecular hallmark of SFTs is a gene fusion between the NAB2 and STAT6 loci on chromosome 12, resulting in a chimeric protein of poorly characterized function called NAB2-STAT6. We use primary samples and an inducible cell model to discover that NAB2-STAT6 operates as a transcriptional coactivator for a specific set of enhancers and promoters that are normally targeted by the EGR1 transcription factor. In physiological conditions, NAB2 is primarily localized to the cytoplasm and only a small nuclear fraction is available to operate as a co-activator of EGR1 targets. NAB2-STAT6 redirects NAB1, NAB2, and additional EGR1 to the nucleus and bolster the expression of neuronal EGR1 targets. The STAT6 moiety of the fusion protein is a major driver of its nuclear localization and further contributes to NAB2's co-activating abilities. In primary tumors, NAB2-STAT6 activates a neuroendocrine gene signature that sets it apart from most sarcomas. These discoveries provide new insight into the pathogenesis of SFTs and reveal new targets with therapeutic potential. Overall design: Omni-ATAC-seq was done in U2OS cells infected with doxycycline inducible NAB2-STAT6 expression plasmids without doxycycline treatment and after 2 days of 1ug/mL doxycycline.