Abstract P4-09-06: High levels of Bruton's tyrosine kinase (BTK) protein is associated with favorable outcome in breast cancer
Authors: Chaudhary LN , Peck AR , Yunguang S , Hu H , Kovatich AJ , Hooke JA , Shriver CD , Mitchell EP , Chervoneva I , Rui H 
Published: February 2018

Background: Bruton's tyrosine kinase (BTK) belongs to the TEC family of cytoplasmic tyrosine kinases. BTK promotes survival and proliferation of malignant B-cells. Although BTK is predominantly cytoplasmic in hematopoietic malignancies, BTK may also be localized to cell nuclei. In breast cancer cell lines, BTK was identified as a candidate survival factor based on a large-scale loss-of-function analysis of human tyrosine kinases using RNA interference. We therefore sought to determine whether BTK levels in breast cancer cells were associated with clinical outcome.

Methods: For these studies we applied quantitative immunofluorescence-based immunohistochemistry of breast cancer tissue specimens in tissue microarray (TMA) format. All patients underwent upfront breast surgery; hence there are no confounding effects of neoadjuvant treatment on biomarker levels. Within the cancer cell compartment, BTK protein was expressed primarily in nuclei of scattered breast cancer cells. Because of the scattered BTK expression distribution, we developed an analytical approach that considered all percentiles of cell-by-cell BTK protein expression in nuclei of all cancer cells sampled within a tumor tissue. Tumors from patient Cohort A (training set, N=758) was used to define the optimal dichotomized (high vs low) BTK marker by selecting the percentile that dichotomized to have the highest predictive power for progression free survival (PFS). The prognostic value of the BTK marker was subsequently evaluated in tumors of the independent validation Cohort B (N=316) using Kaplan-Meier curves and the Cox proportional hazards model for PFS.

Results: The analyses of the training set identified the 94th percentile of BTK cell-by-cell distribution as the optimal predictor of recurrence when dichotomized as High if Qn(0.94)&gt;0.369 and as Low if Qn(0.94)≤0.369. For tissue cores in the training set, 467 of 758 (61.6%) were classified as High BTK and 291 of 758 (38.4%) were classified as Low BTK. Application of the optimal cut point from Cohort A to tumors in the validation Cohort B classified 145/316 cores (45.9%) as High BTK and 171/316 cores (54.1%) as Low BTK. High BTK was associated with longer PFS in both training (p=0.001) and validation sets (p=0.048). The result remained similar and significant in the final multivariate Cox model (HR=0.50; 95% CI 0.26-0.97; p=0.039) after adjusting for pathology and treatment variables.

Conclusions: High BTK nuclear protein expression was associated with a significantly more favorable clinical outcome in breast cancer patients, and held up as an independent marker after adjusting for other variables, including hormone receptor status and treatment. Although the association of High BTK is associated with favorable outcome, further studies are warranted to determine whether BTK represents a therapeutic target in breast cancer.

Citation Format: Chaudhary LN, Peck AR, Yunguang S, Hu H, Kovatich AJ, Hooke JA, Shriver CD, Mitchell EP, Chervoneva I, Rui H. High levels of Bruton's tyrosine kinase (BTK) protein is associated with favorable outcome in breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P4-09-06.