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Purpose <p>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 <i>RB1</i> genetic inactivation.</p> Methods <p>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.</p> Results <p>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 <i>RB1</i> alterations in two (33%). None of the tumors with preserved Rb expression (80%, 40/50) showed <i>RB1 genetic</i> 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.</p> Conclusion <p>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.</p>

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Rb expression in metastatic ER-positive breast cancer: implications for precision oncology

  • 期刊论文

  • 发表日期：

    2026年3月24日

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  • Doaa Morrar,
  • Dara Ross,
  • Pedram Razavi,
  • Edi Brogi,
  • Fresia Pareja,
  • Hannah Y. Wen,
  • Christopher J. Schwartz

摘要

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.