The MUS81 endonuclease is essential for telomerase negative cell proliferation.

A substantial number of human tumors (approximately 10%) are telomerase negative, and cells in such tumors have been proposed to maintain telomere length by the alternative lengthening of telomeres (ALT) pathway. Although details of the molecular mechanism of ALT are largely unknown, previous studies have shown that telomere homologous recombination (HR) is implicated in the ALT pathway. MUS81 is a DNA structure-specific recombination endonuclease and functions on aberrant DNA replication and recombination. Recently, we demonstrate that MUS81 plays a key role in the maintenance of telomeres in ALT cells (Zeng, et al. Nature Cell Biology, 2009). The MUS81 endonuclease specifically localizes to ALT-associated promyelocytic leukemia nuclear bodies (APBs) and interacts with telomeres in ALT cells. Depletion of MUS81 leads to reduced telomere recombination resulting in the growth arrest of ALT cells. The endonuclease activity of MUS81, regulated by its binding partner TRF2, is found to be essential for telomere post-replicative recombination. This study provides the first direct evidence that MUS81 specifically functions on ALT recombination-based cell survival. The specific function of MUS81 on the ALT pathway provides a potential powerful diagnostic marker and a therapeutic target for ALT tumors.