Abstract 3467: A DAXX-KIFC3 fusion potentiates alternative lengthening of telomeres in osteosarcoma

Abstract

Proliferating cells must enact a program of telomere lengthening to counteract the chromosome end replication problem. In most types of cancer cells, telomeres are maintained through the action of the ribonucleoprotein telomerase, but some cancer cells, particularly those of mesenchymal origin, utilize an alternative method of telomere repair and lengthening termed the alternative lengthening of telomeres (ALT) pathway. Since telomere maintenance is essential for tumor cell immortality, better understanding of the ALT mechanism could potentially reveal drug targets that could be used to develop novel therapies for tumors that use ALT. It has been previously observed that ALT tumors frequently carry mutations in ATRX, which partners with the protein DAXX in a chromatin remodeling complex, but how these mutations facilitate the ALT pathway is not well understood. Work in our lab identified an ALT-positive osteosarcoma cell line, identified here as OS1, in which DAXX has undergone a fusion event with the non-canonical kinesin KIFC3. We find that knockdown of the DAXX-KIFC3 fusion neither impairs ALT nor cell proliferation, suggesting that the fusion represents a loss of function. Furthermore, inducible restoration of wild-type DAXX, reversibly abrogates ALT function in this cell line. One of the hallmarks of ALT is localization of telomeres and DNA recombination machinery to nuclear PML bodies, resulting in formation of ALT- associated PML Bodies, or APBs. Thus it may be considered that changes in PML body composition represent a key aspect of the ALT mechanism. We observe that in OS1 both DAXX and ATRX fail to localize to PML bodies. This finding is consistent with the fact that the DAXX-KIFC3 fusion results in loss of a C-terminal SUMO interaction motif that normally mediates PML body interaction. Leveraging our inducible system, using biochemical and imaging approaches, we are working to define the role of DAXX in maintaining PML body composition. Citation Format: Sarah F. Clatterbuck Soper, Soyeon A. Showman, Kathryn E. Driest, Joshua J. Waterfall, Robert L. Walker, Marbin A. Pineda, Yuelin J. Zhu, Yonghong Wang, Corbin D. Ester, Sven Bilke, Paul S. Meltzer. A DAXX-KIFC3 fusion potentiates alternative lengthening of telomeres in osteosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3467. doi:10.1158/1538-7445.AM2017-3467