Background: Tumour cell proliferation has emerged as a major prognostic factor in breast cancer. The immunohistochemical (IHC) proliferation marker Ki67 has been extensively investigated, but has not gained widespread clinical acceptance. Phosphohistone H3 (PHH3) is a new immunohistochemical marker for quantifying mitoses, however there is limited information on its prognostic value in breast cancer. This study performed a head-to-head comparison of Ki67 and phosphohistone H3. The aims were to establish the marker of greatest prognostic value, and to compare Ki67 protein expression and Ki67 mRNA levels in breast tumours.
Methods: Tumour samples from 108 women with breast cancer were constructed as tissue microarrays (TMAs) and evaluated using immunohistochemistry. Cox proportional hazards regression was used to evaluate the prognostic significance of Ki67 and phosphohistone H3. C-statistics, integrated discrimination improvement (IDI), and net reclassification improvement (NRI) were used to evaluate the discriminatory ability of various prediction models. Ki67 mRNA from 30 matched fresh/frozen tumour specimens was analysed using quantitative polymerase chain reaction (qPCR), and was compared with Ki67 protein expression.
Results: Phosphohistone H3 had greater prognostic value than Ki67 in a multivariable model that adjusted for traditional prognostic variables in breast cancer (hazard ratio: 3.32; 95\% confidence interval: 1.51--7.30). A risk prediction model that incorporated phosphohistone H3 efficiently separated patients at low risk and high risk of death at 5-years after diagnosis. The correlation between Ki67 protein expression and Ki67 mRNA levels was weak.
Conclusions: Phosphohistone H3 outperforms Ki67 as an independent predictor of survival in breast cancer. Phosphohistone H3 may provide important information for counselling patients about the likely outcome of their disease, and accurately classifying groups of patients for clinical trials.
