RB expression confers sensitivity to CDK4/6 inhibitor–mediated radiosensitization across breast cancer subtypes
Andrea M. Pesch, … , James M. Rae, Corey W. Speers
Andrea M. Pesch, … , James M. Rae, Corey W. Speers
Published December 21, 2021
Citation Information: JCI Insight. 2022;7(3):e154402. https://doi.org/10.1172/jci.insight.154402.
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Research Article Oncology

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

Abstract

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 (CDK4/6i) 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 CDK4/6i + RT (R2 = 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 CDK4/6i 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 CDK4/6i + RT to difficult-to-control RB-intact breast cancers (including TNBC) and nominate RB status as a predictive biomarker of therapeutic efficacy.

Authors

Andrea M. Pesch, Nicole H. Hirsh, Anna R. Michmerhuizen, Kassidy M. Jungles, Kari Wilder-Romans, Benjamin C. Chandler, Meilan Liu, Lynn M. Lerner, Charles A. Nino, Connor Ward, Erin F. Cobain, Theodore S. Lawrence, Lori J. Pierce, James M. Rae, Corey W. Speers

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Figure 1

CDK4/6 inhibition with palbociclib radiosensitizes TNBC with WT RB1.

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CDK4/6 inhibition with palbociclib radiosensitizes TNBC with WT RB1. (A)...
(A) Cell viability (n = 3, graphed as average ± SEM) was measured in RB WT (solid lines) and RB-null (dashed lines) TNBC cell lines 72 hours after treatment with palbociclib. (B) Western blots were used to assess RB and estrogen receptor (ER) protein expression in various breast cancer cell line models. (C) RB expression was quantified using ImageJ and the correlation coefficient between RB expression and mean radiation enhancement ratios (rER) (highest concentration of palbociclib for each cell line) were compared between ER+ (solid triangles), WT RB1 TNBC (solid circles), or RB-null TNBC (open circles). (D–I) Clonogenic survival assays were performed in the RB WT TNBC cell lines MDA-MB-231 (D), CAL-120 (E), CAl-51 (F), and SUM-159 (G), as well as the RB-null TNBC cell lines MDA-MB-468 (H), and CAL-851 (I) cells with a 1-hour palbociclib pretreatment. Survival fraction of cells was calculated for each cell line at 2 Gy as the mean of 3 independent experiments and mean rER from 3 independent experiments are shown. A 1-way ANOVA with Dunnett’s post hoc test was used to compare palbociclib-pretreated groups to the vehicle-pretreated group. *P < 0.05; **P < 0.01.