A molecular signature of the Nottingham prognostic index in breast cancer.

The Nottingham prognostic index (NPI) is a widely used clinicopathological staging system for breast cancer prognostication. Using a step-wise classification approach where breast tumor expression profiles were first divided into general "molecular subtypes" [estrogen receptor (ER)+, ER-, ERBB2+], followed by an independent analysis of each subtype, we identified a 62-gene expression signature (NPI-ES) highly correlated to the NPI in ER+ tumors. The NPI-ES classified the majority of ER+ tumors into two distinct groups with high confidence and was significantly correlated to NPI status in two independent sets of ER+ tumors derived from different centers. The NPI-ES is comparable to the classical NPI in identifying patients likely to exhibit a poor clinical prognosis, as well as to a recently described "prognosis signature" for breast cancer. Our findings demonstrate how expression profiling can complement classical staging systems employing histopathological parameters scored over a continuous range of values.