Molecular qRT-PCR grade index: a new tool for breast cancer (BC) patient grading improvement

Toussaint J, Sieuwerts A, Durbecq V, Haibe-Kains B, Berns E, Harris AL, Larsimont D, Piccart MJ, Foekens J, Sotiriou C

Background: We have recently shown that proliferation captured by the GGI is one of the most important prognostic indicators in BC and may encompass a significant portion of the predictive power of several previously published prognostic signatures in particular for ER+ disease. The aims of this study were 1) to convert this microarray index to an index using qRT-PCR and 2) to assess its prognostic and predictive value for tamoxifen response.

Methods: A qRT-PCR genomic grade index (PCR-GGI) was developed based on the expression of 4 genes selected from the GGI microarray signature and 4 reference genes. The accuracy and concordance with the original microarray-derived GGI was assessed using a BC set from which frozen, FFPE tissues and microarray data were available (N = 19). The evaluation of the prognostic value of the PCR-GGI was assessed using a consecutive series of 212 systemically treated early BC patients according to the standard of care at the time of diagnosis using FFPE material. The predictive performance for tamoxifen response was assessed using an ER+ BC population treated either with adjuvant tamoxifen only (n = 141) or first-line tamoxifen for advanced disease (n = 279).

Results: A statistically significant correlation was observed between GGI derived from microarray and qRT-PCR assay using frozen (ρ = 0.95, p < 10-6) as well as FFPE material (ρ = 0.89, p < 10-6). Similarly to our previous microarray results, PCR-GGI redistributed histological grade 2 (HG2) tumors into two subgroups with statistically distinct clinical outcome similar to those of HG1 and HG3 tumors, respectively (HR = 2.27; 95CI: 0.94–5.48, p = 0.068). Of notice, PCR-GGI was more informative than Ki67 (IHC) in discriminating HG2 patients into good and bad prognosis group. Additionally, PCR-GGI identified two distinct ER+ subgroups with statistically different DMFS and response to tamoxifen treatment (PFS) in both early (HR = 2.26; 95CI: 1.075–4.751, p = 0.03) and advanced setting (HR = 1.95; 95CI: 1.49–2.544, p < 10-6) respectively. Interestingly, among the 66% of ER+ node-negative early BC patients assigned as low risk, only 7% exhibited distant recurrence compared to 47% for the high-risk patients at 10 years follow-up.

Conclusions: GGI using a qRT-PCR assay based on a limited number of genes recapitulated in an accurate and reproducible manner the performances of the GGI developed by microarray using both frozen and FFPE tumor samples. PCR-GGI can be used as a powerful tool for BC management.