Identification of a Disease-Defining Gene Fusion in Epithelioid Hemangioendothelioma

A newly identified gene fusion defines the vascular cancer epithelioid hemangioendothelioma and encodes a chimeric transcription factor. FISHing for a Gene Fusion Mother was right: There is a time and place for everything. And at the molecular level, inappropriate behavior can have consequences much more severe than being grounded. Using an unbiased deep-sequencing approach coupled with traditional chromosomal karyotyping, Tanas et al. now describe the genes involved in a fusion event that defines epithelioid hemangioendothelioma (EHE), a rare vascular cancer. This genetic aberration may instigate the bad behavior—an improper transcriptional program in endothelial cells. A rare sarcoma, EHE is difficult to diagnose because it shares many characteristics with normal endothelial cells and resembles other abnormal vascular neoplasms, such as epithelioid hemangioma, a benign condition, and epithelioid angiosarcoma, an aggressive vascular cancer. Treatment for patients with localized EHE includes surgical removal, when possible, or liver transplantation in the case of hepatic involvement, and there is no treatment for metastatic disease. To aid in diagnosis and decipher the pathological processes behind this mysterious cancer, researchers and clinicians need a defining biomarker for EHE. Traditional cytogenetic techniques for identifying the genes involved in a genetic translocation are labor-intensive, especially for a rare cancer for which no cell lines are available. So, Tanas et al. took a shortcut; the authors combined cytogenetic methods with deep transcriptome sequencing, which they used to search in an unbiased way for the product of the t(1;3)(p36;q25) chromosomal translocation characteristic of EHE. The translocation involved two genes, WWTR1, which encodes a transcriptional coactivator that is highly expressed in endothelial cells, and CAMTA1, a DNA binding transcriptional regulatory protein that is normally expressed during brain development. The WWTR1/CAMTA1 gene fusion contains the strong endothelial cell promoter of WWTR1, which may drive the inappropriate expression of a protein-encoding fragment of CAMTA1 in endothelial cells. The authors suggest that this promoter switch initiates an ill-suited and ill-timed transcriptional program that may play a role in cancer biology. If this is the case, then the chimeric WWTR1/CAMTA1 transcription factor may represent a therapeutic target for EHE-specific drugs. To aid in disease diagnosis, Tanas et al. also devised a sensitive and specific fluorescence in situ hybridization assay to detect the EHE translocation. Together, these tools should teach researchers about the biology and prognosis of this rare cancer and eventually help bring the bad behavior under control. Integrating transcriptomic sequencing with conventional cytogenetics, we identified WWTR1 (WW domain–containing transcription regulator 1) (3q25) and CAMTA1 (calmodulin-binding transcription activator 1) (1p36) as the two genes involved in the t(1;3)(p36;q25) chromosomal translocation that is characteristic of epithelioid hemangioendothelioma (EHE), a vascular sarcoma. This WWTR1/CAMTA1 gene fusion is under the transcriptional control of the WWTR1 promoter and encodes a putative chimeric transcription factor that joins the amino terminus of WWTR1, a protein that is highly expressed in endothelial cells, in-frame to the carboxyl terminus of CAMTA1, a protein that is normally expressed only in brain. Thus, CAMTA1 expression is activated inappropriately through a promoter-switch mechanism. The gene fusion is present in virtually all EHEs tested but is absent from all other vascular neoplasms, demonstrating it to be a disease-defining genetic alteration. A sensitive and specific break-apart fluorescence in situ hybridization assay was also developed to detect the translocation and will assist in the evaluation of this diagnostically challenging neoplasm. The chimeric WWTR1/CAMTA1 transcription factor may represent a therapeutic target for EHE and offers the opportunity to shed light on the functions of two poorly characterized proteins.