A single-gene biomarker identifies breast cancers associated with immature cell type and short duration of prior breastfeeding.

The pathogenesis of breast cancers that do not express estrogen receptors or Her-2/neu receptors (ERx/HER2x phenotype) is incompletely understood. We had observed markedly elevated gene expression of gamma-aminobutyric acid type A (GABAA) receptor subunit p (GABAp, GABRP) in some breast cancers with ERx/HER2x phenotype. In this study, transcriptional profiles (TxPs) were obtained from 82 primary invasive breast cancers by oligonucleotide microarrays. Real-time reverse transcription‐polymerase chain reaction (RT‐PCR) was used to measure GABAp gene expression in a separate cohort of 121 invasive breast cancers. GABAp gene expression values from TxP and RT‐PCR were standardized and compared with clinicopathologic characteristics in the 203 patients. GABAp gene expression was increased in 16% of breast cancers (13/82 TxP, 20/ 121 RT‐PCR), particularly in breast cancers with ERx/HER2x phenotype (60%), and breast cancers with basal-like genomic profile (60%). The profile of genes coexpressed with GABAp in these tumors was consistent with an immature cell type. In multivariate linear regression analysis, the level of GABAp gene expression was associated with ERx/HER2x phenotype (P<0.0001), younger age at diagnosis (P=0.0003), and shorter lifetime duration of breastfeeding (£ 6 months) in all women (P=0.017) and specifically in parous women (P=0.013). GABAp gene expression was also associated with combinations of high grade with ERx/HER2x phenotype (P=0.002), and with Hispanic ethnicity (P=0.036). GABAp gene expression is increased in breast cancers of immature (undifferentiated) cell type and is significantly associated with shorter lifetime history of breastfeeding and with high-grade breast cancer in Hispanic women.