Abstract
<div>Abstract<p><b>Purpose:</b> Histologic tumor grade is a well-established prognostic factor for breast cancer, and tumor grade–associated genes are the common denominator of many prognostic gene signatures. The objectives of this study are as follows: (<i>a</i>) to develop a simple gene expression index for tumor grade (molecular grade index or MGI), and (<i>b</i>) to determine whether MGI and our previously described <i>HOXB13:IL17BR</i> index together provide improved prognostic information.</p><p><b>Experimental Design:</b> From our previously published list of genes whose expression correlates with both tumor grade and tumor stage progression, we selected five cell cycle–related genes to build MGI and evaluated MGI in two publicly available microarray data sets totaling 410 patients. Using two additional cohorts (<i>n</i> = 323), we developed a real-time reverse transcription PCR assay for MGI, validated its prognostic utility, and examined its interaction with <i>HOXB13:IL17BR</i>.</p><p><b>Results:</b> MGI performed consistently as a strong prognostic factor and was comparable with a more complex 97-gene genomic grade index in multiple data sets. In patients treated with endocrine therapy, MGI and <i>HOXB13:IL17BR</i> modified each other's prognostic performance. High MGI was associated with significantly worse outcome only in combination with high <i>HOXB13:IL17BR,</i> and likewise, high <i>HOXB13:IL17BR</i> was significantly associated with poor outcome only in combination with high MGI.</p><p><b>Conclusions:</b> We developed and validated a five-gene reverse transcription PCR assay for MGI suitable for analyzing routine formalin-fixed paraffin-embedded clinical samples. The combination of MGI and <i>HOXB13:IL17BR</i> outperforms either alone and identifies a subgroup (∼30%) of early stage estrogen receptor–positive breast cancer patients with very poor outcome despite endocrine therapy.</p></div>
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