Rb Immunohistochemistry Detects CDK4/6 Inhibitor Resistance Missed by NGS in Metastatic Breast Cancer
In a cohort of 50 ER-positive metastatic breast cancers, Rb loss was detected by IHC in 20% of cases, but NGS identified pathogenic RB1 alterations in only 33% of those Rb-deficient tumours. All Rb-loss cases showed p16 positivity, and 40% had neuroendocrine features. The study demonstrates that Rb IHC paired with p16 offers a rapid, cost-effective screen for CDK4/6 inhibitor resistance that conventional sequencing may miss.
The original study
Rb expression in metastatic ER-positive breast cancer: implications for precision oncology.
- Authors
- Morrar D, Ross D, Razavi P, Brogi E, Pareja F, Wen HY, et al.
- Journal
- Breast cancer research and treatment
- PMID
- 41872428
Original abstract
PURPOSE: The retinoblastoma protein (Rb) is a critical cell-cycle regulator, and its loss of function can lead to resistance to CDK4/6 inhibitors (CDK4/6i), which are the standard first-line treatment for estrogen receptor (ER)-positive metastatic breast carcinoma (mBC). Thus, identifying Rb-deficient mBC is crucial for optimal personalized breast cancer management. This study aimed to determine the prevalence of Rb loss by immunohistochemistry (IHC) in a cohort of ER + mBC and to assess its correlation with RB1 genetic inactivation. METHODS: We analyzed Rb IHC in 50 consecutive ER-positive mBC. Histopathologic and clinical features were analyzed. p16 IHC was performed in a subset of Rb-deficient cases. Targeted next-generation tumor-normal sequencing (NGS) data using MSK-IMPACT were retrospectively analyzed in 38 mBCs. RESULTS: Rb loss was identified in 20% (10/50) of mBC, and was either partial (8%, 4/50) or complete (12%, 6/50). In all evaluable cases (100%, 9/9), Rb loss was associated with p16 positivity. Neuroendocrine (NE) features were observed in 40% (4/10) of mBCs with Rb loss. MSK-IMPACT data were available for six Rb-deficient cases and revealed pathogenic RB1 alterations in two (33%). None of the tumors with preserved Rb expression (80%, 40/50) showed RB1 genetic alterations or NE features. Notably, one mBC case demonstrated disease progression on CDK4/6 inhibitor therapy, accompanied by acquired Rb loss and acquisition of an NE phenotype. CONCLUSION: Rb loss in mBC can be reliably detected by Rb IHC, especially when interpreted alongside p16, offering a rapid and cost-effective means of assessing Rb status. This approach may identify Rb-deficient tumors that are missed by conventional methods, such as next-generation sequencing, and help guide personalized therapeutic strategies in patients with mBC.