Sarah F. Clatterbuck Soper and Paul S. Meltzer

• Submitter: Sarah Clatterbuck Soper (Postdoc or fellow)
• email: sarah.clatterbucksoper@nih.gov

To counteract the end-replication problem, proliferating cells must enact a program of telomere maintenance. In 10-15% of tumors telomeres are lengthened not by telomerase but through a DNA-templated process termed Alternative Lengthening of Telomeres or ALT. ALT is associated with mutations in the ATRX/DAXX/H3.3 histone chaperone complex, which is responsible for deposition of non-replicative histone variant H3.3 at heterochromatic regions of the genome including telomeres. ATRX and DAXX are multifunctional proteins, and recent findings suggest that other chaperones can compensate for H3.3 deposition at telomeres, leading us to ask which functions of this complex are essential for ALT suppression. To answer this question, we have leveraged the G292 cell line, in which ATRX is wild type but DAXX has undergone a fusion event with the non-canonical kinesin KIFC3 (1). Restoration of wild-type DAXX in G292 localizes ATRX and abrogates ALT. Using this model system, we tested the ability of a panel of cancer-associated DAXX missense variants to suppress ALT. We find that mutations in the DAXX H3.3 binding domain also undermine the ability of DAXX to bind and localize ATRX, and thus fail to suppress ALT. Surprisingly, a DAXX variant deficient in ATRX binding appears to recruit endogenous ATRX-DAXX complexes in trans to substantially suppress ALT. These findings raise the question of whether DAXX-DAXX interactions may be essential for forming the protein assemblies that suppress ALT.

1 Yost, K. E. et al. Rapid and reversible suppression of ALT by DAXX in osteosarcoma cells. Sci Rep 9, 4544, doi:10.1038/s41598-019-41058-8 (2019).