Telomere dysfunction is typically studied under conditions in which a component of the six-subunit shelterin complex that protects chromosome ends is disrupted. The nature of spontaneous telomere dysfunction is less well understood. Here we report that immortalized human cell lines lacking wild-type p53 function spontaneously show many telomeres with a DNA damage response (DDR), commonly affecting only one sister chromatid and not associated with increased chromosome end-joining. DDR(+) telomeres represent an intermediate configuration between the fully capped and uncapped (fusogenic) states. In telomerase activity-positive (TA(+)) cells, DDR is associated with low TA and short telomeres. In cells using the alternative lengthening of telomeres mechanism (ALT(+)), DDR is partly independent of telomere length, mostly affects leading strand-replicated telomeres, and can be partly suppressed by TRF2 overexpression. In ALT(+) (but not TA(+)) cells, DDR(+) telomeres preferentially associate with large foci of extrachromosomal telomeric DNA and recombination proteins. DDR(+) telomeres therefore arise through different mechanisms in TA(+) and ALT(+) cells and have different consequences.

Five dysfunctional telomeres predict onset of senescence in human cellsReplicative senescence is triggered by DNA damage response foci associated with telomeres. Reddel and colleagues now establish that a threshold of five damaged telomeres exists to induce senescence in normal cells and that end-to-end chromosome fusion is not required for senescence induction.

Mortalin/mthsp70 (HSPA9) is a stress chaperone enriched in many cancers that has been implicated in carcinogenesis by promoting cell proliferation and survival. In this study, we examined the clinical relevance of mortalin upregulation in carcinogenesis. Consistent with high mortalin expression in various human tumors and cell lines, we found that mortalin overexpression increased the migration and invasiveness of breast cancer cells. Expression analyses revealed that proteins involved in focal adhesion, PI3K-Akt, and JAK-STAT signaling, all known to play key roles in cell migration and epithelial-tomesenchymal transition (EMT), were upregulated in mortalinexpressing cancer cells. We further determined that expression levels of the mesenchymal markers vimentin (VIM), fibronectin (FN1), b-catenin (CTNNB1), CK14 (KRT14), and hnRNP-K were also increased upon mortalin overexpression, whereas the epithelial markers E-cadherin (CDH1), CK8 (KRT8), and CK18 (KRT18) were downregulated. Furthermore, shRNAmediated and pharmacologic inhibition of mortalin suppressed the migration and invasive capacity of cancer cells and was associated with a diminished EMT gene signature. Taken together, these findings support a role for mortalin in the induction of EMT, prompting further investigation of its therapeutic value in metastatic disease models. Cancer Res; 76(9);