The Role of Castration-Resistant Bmi1+Sox2+ Cells in Driving Recurrence in Prostate Cancer.

<bold>Background: </bold>Recurrence following androgen-deprivation therapy is associated with adverse clinical outcomes in prostate cancer, but the cellular origins and molecular mechanisms underlying this process are poorly defined. We previously identified a population of castration-resistant luminal progenitor cells expressing Bmi1 in the normal mouse prostate that can serve as a cancer cell-of-origin. Here, we investigate the potential of Bmi1-expressing tumor cells that survive castration to initiate recurrence in vivo.<bold>Methods: </bold>We employed lineage retracing in Bmi1-CreER; R26R-confetti; Ptenf/f transgenic mice to mark and follow the fate of emerging recurrent tumor clones after castration. A tissue recombination strategy was used to rescue transgenic mouse prostates by regeneration as grafts in immunodeficient hosts. We also used a small molecule Bmi1 inhibitor, PTC-209, to directly test the role of Bmi1 in recurrence.<bold>Results: </bold>Transgenic prostate tumors (n = 17) regressed upon castration but uniformly recurred within 3 months. Residual regressed tumor lesions exhibited a transient luminal-to-basal phenotypic switch and marked cellular heterogeneity. Additionally, in these lesions, a subpopulation of Bmi1-expressing castration-resistant tumor cells overexpressed the stem cell reprogramming factor Sox2 (mean [SD] = 41.1 [3.8]%, n = 10, P < .001). Bmi1+Sox2+ cells were quiescent (BrdU+Bmi1+Sox2+ at 3.4 [1.5]% vs BrdU+Bmi1+Sox2- at 18.8 [3.4]%, n = 10, P = .009), consistent with a cancer stem cell phenotype. By lineage retracing, we established that recurrence emerges from the Bmi1+ tumor cells in regressed tumors. Furthermore, treatment with the small molecule Bmi1 inhibitor PTC-209 reduced Bmi1+Sox2+ cells (6.1 [1.4]% PTC-209 vs 38.8 [2.3]% vehicle, n = 10, P < .001) and potently suppressed recurrence (retraced clone size = 2.6 [0.5] PTC-209 vs 15.7 [5.9] vehicle, n = 12, P = .04).<bold>Conclusions: </bold>These results illustrate the utility of lineage retracing to define the cellular origins of recurrent prostate cancer and identify Bmi1+Sox2+ cells as a source of recurrence that could be targeted therapeutically. [ABSTRACT FROM AUTHOR]