Mizuki Azuma

Ewing sarcoma is the second most common bone tumor in children. The common molecular abnormality in Ewing sarcoma is the t(11; 22) chromosomal translocation, which results in the expression of a chimeric fusion protein containing EWSR1-derived sequences at the amino terminus fused to the carboxyl-terminus of the FLI1. Our previous study demonstrated that EWSR1-FLI1 and wildtype EWSR1 biochemically interact with each other. As a result, this interaction leads to the inhibition of EWSR1 activity by EWSR1-FLI1 in a dominant negative manner. Given the importance of EWSR1, my research goal is to elucidate how EWSR1 prevents the development of Ewing sarcoma, while also understanding how EWSR1-FLI1 promotes tumorigenesis.

The function of EWSR1 is crucial in maintaining mitosis, chromosomal integrity and preventing tumorigenesis

The EWSR1 is a multifunctional protein that regulates splicing, transcription and DNA damage. Our previous study showcased a unique function of EWSR1, essential for driving faithful mitosis by interacting with Aurora B, the key kinase that drives mitosis. Compromising the EWSR1-Aurora B pathway leads to the induction of mitotic dysfunction and aneuploidy. We have also demonstrated thatewsr1a (homologue of human EWSR1) homozygous and heterozygous zebrafish mutants with tp53 mutant background display a higher incidence of tumorigenesis compared to wildtype fish. This study highlighted the role of EWSR1 in preventing tumorigenesis. Based on these studies, we are investigating whether and how the induction of aneuploidy contributes to the relapse and metastasis of Ewing sarcoma.

During the studies described above, we found that the Arginine (R) located at the 565^th amino acid of EWSR1 (EWSR1:R565) is essential for its interaction with Aurora B kinase. The EWSR1:R565 is located in the RGG3 domain, which is comprised of repeats of Arg (R) and Gly (G). The RGG3 domain is an internally disorganized domain, yet it is known to interact with RNA or protein. We are investigating whether post translational modification (PTM) of this domain facilitates the formation of the structure, ultimately promoting interaction with Aurora B.

The function of EWSR1 in inhibiting differentiation of chondrocytes

Endochondral ossification is a developmental process in long bones. This process involves the migration and condensation of mesenchymal cells, differentiation into prehypertrophic (progenitor) chondrocytes, further differentiation from prehypertrophic chondrocytes to hypertrophic (mature) chondrocytes, and ultimately the calcification of bones. Using the zebrafishewsr1a mutant, we discovered that the Ewsr1a protein interacts with the  transcription factor, Sex-determining Region Y (SRY) box 9 (SOX9). In the study, we demonstrated that this interaction promotes differentiation of the prehypertrophic chondrocytes to hypertrophic chondrocytes. We believe that this observation provides crucial piece of the puzzle in understanding the pathogenesis of Ewing sarcoma because the differentiation of tumor cells is inhibited. Currently, we are investigating the molecular mechanism of how EWSR1-SOX9 activates the differentiation of chondrocytes.