TP53 and osteosarcoma: U-2 OS ALT cells are established from an osteosarcoma but maintain an intact p53 pathway although most other ALT cell lines, including VA-13 and Saos-2, are mutated or null for p53. U-2 OS cells died in culture in response to WRN knockdown after 17 PD accompanied by an increase in cleaved poly-ADP ribose polymerase 1 (PARP1) (∼30% induction compared to camptothecin treatment) following transfection with WRN siRNAs (Fig 2A).