BUBR1 and MAD2: Novel Markers for Predicting Benefit from Adjuvant Anthracyclines?

Abstract

Abstract Background: We previously demonstrated the importance of duplication of CEP17 (C17) in predicting sensitivity to anthracyclines in two retrospective adjuvant trials (BR9601 and MA.5). Spindle checkpoint failure, and resultant chromosome instability (CIN), could be a mechanism for this observation. BubR1 and MAD2 are key components of the spindle assembly checkpoint which prevent progression to anaphase. Overexpression of BUBR1 or a reduction in MAD2 is associated with CIN and polysomy. Unlike MAD2, BubR1 competitively binds, phosphorylates and stabilises p53 during mitosis, at a site overlapping that of MDM2. Therefore, BubR1 is implicated in both spindle assembly and DNA damage checkpoints. We investigated BubR1 and MAD2 as predictive markers of anthracycline sensitivity in breast cancer.Methods: We evaluated cytoplasmic BubR1 and MAD2 protein expression in 321 tumours from patients in the BR9601 trial, which compared E-CMF with CMF as adjuvant therapy for breast cancer. Overall survival (OS) and relapse-free survival (RFS) were estimated using Kaplan-Meier curves.Results: BubR1 and MAD2 overexpression were detected in 49.2% and 25.1% of tumors, respectively. Both MAD2 and BubR1 overexpression were associated with ER negativity (p<0.0001, p=0.013) and high grade (p<0.0001, p=0.029). BubR1 overexpression was associated with C17 (p=0.0003) and high proliferation (Ki67, p<0.0001); similar trends were noted for MAD2. BubR1 overexpression was associated with reduced RFS (HR: 1.58, 95%C.I. 1.06-2.37, p=0.024) and OS (HR: 1.831 95%C.I. 1.161-2.890), whereas there was no clear association with outcome for MAD2.Interaction tests revealed no significant evidence that the benefit of the addition of epirubicin to CMF was confined to those tumours with either high levels of BubR1 or low levels of MAD2 (BubR1 HR: 1.70 95%C.I. 0.74-3.87 and MAD2 HR: 0.573 95%C.I. 0.143-2.296). However, combining overexpression of BubR1 with low expression of MAD2 selects for 33.0% (98/297) of patients and is associated overall with a poorer RFS (HR: 0.602 95%C.I. 0.402-0.902, p=0.014) but an increased RFS benefit from the addition of anthracycline to CMF (HR: 0.409 95%C.I. 0.208-0.803) than those with any other combination of BubR1 and MAD2 expression (HR: 0.718 95%C.I. 0.423-1.216). Similar results were noted with OS. However, neither treatment by marker interaction reached significance (RFS HR: 1.73 95%C.I. 0.74-1.08, p=0.208; OS HR 1.820 95%C.I. 0.707-4.686, p=0.215).Conclusion: BubR1 overexpression is associated with an aggressive phenotype and decreased RFS and OS in BR9601 patients. Tumours with BubR1 overexpression and/or low MAD2 expression showed a potential increased sensitivity to anthracycline therapy compared to those with low BubR1 levels and/or high MAD2 expression. Small trials (BR9601) are useful for hypothesis generating; further work in larger patient cohorts such as MA.5 or NEAT is warranted. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 2124.