The Effects of HER2 on CDK4/6 Activity in Breast Cancer

CDK4/6 inhibitors have been used to treat hormone receptor-positive HER2-negative advanced breast cancer. Their benefit in HER2-positive breast cancer has not been determined yet. In this study, we investigated the effects of HER2 on CDK4/6 activity by assessing the level of downstream phosphorylated retinoblastoma protein (pRb) in HER2-positive breast cancer (HER2 positivity is defined by immunohistochemical study or FISH, regardless of ER status) to determine if these cases may be responsive to CDK4/6 inhibitors.One hundred and thirty cases of breast biopsies with invasive carcinoma were collected, including 77 cases of HER2+ (39 cases of ER +PR±HER2+ and 38 cases of ER-PR-HER2+) and 53 cases of HER2- (ER-PR-HER2-) breast cancer. Immunohistochemical study of pRb was performed and the pRb level was assessed by H-score (intensity x percentage of positive cells).The pRb H-score ranges from 3 to 270. The average H-scores for the ER-PR-HER2+, ER+PR±HER2+ and ER-PR-HER2- groups are 115.8 ± 75.8, 93.1 ± 68.6 and 63.1 ± 65.6, respectively. By comparison, HER2+ cases have significantly higher pRb levels than HER2- cases (P = .001). Among HER2+ cases, there was a trend of positive correlation between the HER2 gene copy number, and the pRb level although not statistically significant (r = 0.192, 95% CI, [-0.033, 0.399], P = .09).In breast cancer, HER2 positivity leads to significantly higher levels of CDK4/6 activity as reflexed by pRb. Breast cancer that is positive for HER2 may respond to CDK4/6 inhibitors and pRb may potentially be used as a biomarker to predict the responsiveness.