RB expression confers sensitivity to CDK4/6 inhibitor-mediated radiosensitization across breast cancer subtypes

Standard radiation therapy (RT) does not reliably provide locoregional control for women with multinode-positive breast cancer and triple-negative breast cancer (TNBC). We hypothesized that CDK4/6 inhibition (COK4/6i) would increase the radiosensitivity not only of estrogen receptor-positive (ER+) cells, but also of TNBC that expresses retinoblastoma (RB) protein. We found that CDK4/6i radiosensitized RB WT TNBC (n = 4, radiation enhancement ratio (rER): 1.49-2.22) but failed to radiosensitize RB-null TNBC (n = 3, rER: 0.84-1.00). RB expression predicted response to COK4/6i + RT (R-2 = 0.84), and radiosensitization was lost in ER+/TNBC cells (rER: 0.88-1.13) after RB1 knockdown in isogenic and nonisogenic models. CDK4/6i suppressed homologous recombination (HR) in RB WT cells but not in RB-null cells or isogenic models of RB1 loss; HR competency was rescued with RB reexpression. Radiosensitization was independent of nonhomologous end joining and the known effects of COK4/6i on cell cycle arrest. Mechanistically, RB and RAD51 interact in vitro to promote HR repair. CDK4/6i produced RB-dependent radiosensitization in TNBC xenografts but not in isogenic RB1-null xenografts. Our data provide the preclinical rationale for a clinical trial expanding the use of CEIK4/6i + RT to difficult-to-control RB-intact breast cancers (including TNBC) and nominate RB status as a predictive biomarker of therapeutic efficacy.

Automated summary

Standard radiation therapy (RT) is unreliable in providing locoregional control for women with multinode-positive breast cancer and triple-negative breast cancer (TNBC). CDK4/6 inhibition (COK4/6i) can increase the radiosensitivity of RB wild-type (WT) TNBC, but not RB-null TNBC. RB expression predicts response to COK4/6i + RT, and radiosensitization is lost when RB1 is knocked down in isogenic and nonisogenic models. Radiosensitization is independent of nonhomologous end joining and cell cycle arrest. RB interacts with RAD51 in vitro to promote homologous recombination repair. RB-dependent radiosensitization is observed in TNBC xenografts but not in isogenic RB1-null xenografts.