OVERCOMING CELLULAR SENESCENCE IN HUMAN CANCER PATHOGENESIS

Elevation of p16, the CDKN2/pl6 tumor suppressor gene (TSG) product, o ccurs at senescence in normal human uroepithelial cells (HUC). Immorta l HUCs and bladder cancer cell lines show either alteration of p16 or pRb, the product of the retinoblastoma (RB) TSG. In addition, many hum an cancers show p16 or pRb alteration along with other genetic alterat ions that we associated with immortalization, including +20q and -3p. These observations led us to hypothesize that p16 elevation plays a cr itical role in senescence cell cycle arrest and that overcoming this b lock is an important step in tumorigenesis in vivo, as well as immorta lization in vitro. Using a novel approach, we tested these hypotheses in the present study by examining p16 and pRb status in primary cultur e (PO) and after passage in vitro of transitional cell carcinoma (TCC) biopsies that represented both superficial bladder tumors and invasiv e bladder cancers. We demonstrated that all superficial TCCs showed el evated p16 after limited passage in vitro and then senesced, like norm al HUCs. In contrast, all muscle invasive TCCs contained either a p16 or a pRb alteration at PO and all spontaneously bypassed senescence (P = 0.001). Comparative genomic hybridization (CGH) was used to identif y regions of chromosome loss or gain in all TCC samples. The applicati on of a statistical model to the CGH data showed a high probability of elevated alteration rates of +20q11-q12 (0.99) and +8p22-pter (0.94) in the immortal muscle invasive TCCs, and of -9q (0.99) in the superfi cial TCCs. Three myoinvasive TCCs lost 3p13-p14. In this study, four o f six myoinvasive TCCs also showed TP53 mutation that associated well with genome instability (P = 0.001), as previously hypothesized. Notab ly, TP53 mutation, which has been used as a marker of tumor progressio n in many human cancers, was less significant in associating with prog ression in this study (P = 0.04) than was p16 or pRb alteration (P = 0 .001). Thus, these data support a new model in which overcoming senesc ence plays a critical role in human cancer pathogenesis and requires a t least two genetic changes that occur in several combinations that ca n include either p16 or pRb loss and at least one additional alteratio n, such as +20q11-q12, -3p13-p14, or -8p21-pter.