TSPYL5 Depletion Induces Specific Death of ALT Cells through USP7-Dependent Proteasomal Degradation of POT1

A significant fraction (similar to 10%) of cancer cells maintain their telomere length via a telomerase-independent mechanismknown as alternative lengthening of telomeres (ALT). There are no known molecular, ALT-specific, therapeutic targets. We have identified TSPYL5 (testis-specific Y-encoded-like protein 5) as a PML body component, co-localizing with ALT telomeres and critical for ALT(+) cell viability. TSPYL5 was described as an inhibitor of the USP7 deubiquitinase. We report that TSPYL5 prevents the poly-ubiquitination of POT1-a shelterin component-and protects POT1 from proteasomal degradation exclusively in ALT(+) cells. USP7 depletion rescued POT1 poly-ubiquitination and loss, suggesting that the deubiquitinase activates POT1 E3 ubiquitin ligase(s). Similarly, PML depletion suppressed POT1 poly-ubiquitination, suggesting an interplay between USP7 and PML to trigger POT1 degradation in TSPYL5-depleted ALT(+) cells. We demonstrate that ALT telomeres need to be protected from POT1 degradation in ALT-associated PML bodies and identify TSPYL5 as an ALT(+) cancer-specific therapeutic target.