Transforming genomic grade index (GGI) into a user-friendly qRT-PCR tool which will assist clinicians and patients in optimizing treatment of early breast cancer (BC).

Durbecq V, Toussaint J, Haibe-Kains B, Desmedt C, Rouas G, Larsimont D, Buyse M, Bontempi G, Piccart M and Sotiriou C

Background: We have recently shown that proliferation captured by the genomic grade index (GGI) is the most important prognostic factor in BC, far beyond estrogen receptor (ER) status and may encompass a significant portion of the predictive power of many previously published prognostic signatures. Furthermore, we have demonstrated that GGI can identify two clinically distinct ER-positive molecular subtypes in a simple and highly reproducible manner across multiple datasets. The aims of this study were to convert and validate by qRT-PCR assay the prognostic value of the GGI using frozen (FS) and paraffin-embedded tumor samples (FFPE) from early BC patients.

Methods: We first developed a qRT-PCR assay based on 8 selected GGI genes involved in different phases of the cell cycle and 4 reference genes. We then tested its accuracy and concordance with the original microarray derived GGI using a BC population from which FS, FFPE tissues and microarray data were available (N=30). Finally, we assessed its prognostic value on an independent ER-positive tamoxifen only treated BC population (N=82).

Results: A statistically significant correlation was observed between GGI generated by microarray and qRT-PCR assay using FS material (r=0.91; p<0.0001) as well as between GGI using qRT-PCR derived from FS and FFPE tumor samples (r=0.86; p<0.0001). A high GGI levels assessed by qRT-PCR (GGRI) was associated with a higher risk of recurrence in the ER-positive tamoxifen only treated patients [HR= 7.021 (95% CI: 1.245-4.321), p=0.008] in accordance with our previous microarray results. Interestingly, in a multivariate analysis, GGRI remained significant (HR=4.22 (95% CI: 1.031-3.710), p=0.04) together with age (<50y) and tumor size (>2cm).

Conclusions: In this study, we report a qRT-PCR assay based on a limited number of genes, which recapitulates in an accurate and reproducible manner the prognostic power of the microarray derived GGI using both FS and FFPE tumor samples. Further validation of its prognostic value is currently ongoing on two independent populations totaling over 500 BC patients.