CAXII Is a Surrogate Marker for Luminal Breast Tumors Regulated by ER and GATA3

SIMPLE SUMMARY: Breast cancer is a heterogeneous disease and treatment needs to be adapted to individual tumors. Two thirds of breast tumors may benefit from treatment with drugs targeting a specific protein, the estrogen receptor alpha, a regulator of gene expression activated by female sex hormones. However, a significant percentage of tumors will recur and progress. To help detect more accurately the expression status and activity of this protein, a gene that it upregulates, the progesterone receptor (PR), is routinely used in the clinic as an additional marker. Here, we show that PR status is an imperfect reflection of the expression and/or overall activity of estrogen receptor alpha and identify another marker that can perform this task more consistently. Overall, use of CAXII as a marker of ER(+) tumors should reinforce the diagnosis of ER status and prediction of its activity and enhance the accuracy of hormonal therapy delivery. ABSTRACT: Estrogen receptor alpha (ERα) expression in ~2/3 breast tumors selects patients for hormonal therapies. Tumors negative for ERα but positive for the progesterone receptor (PR, encoded by PGR) have also been candidates for ER-targeting therapies, as PR expression may reflect undetected ER activity. Conversely, PR(−) status in ER(+) tumors predicts a worse therapeutic response. Our analysis of breast tumor transcriptome datasets, however, revealed that in tumors with lower PGR expression, the clinical PR status does not correlate accurately with the expression of ESR1 or of ER target genes, including PGR itself. We identified carbonic anhydrase 12 (CA12) as an estrogen target gene better correlated with ESR1 than PGR, reflecting CA12 regulation by both ERα and the luminal factor and upstream ESR1 regulator GATA3. Immunostaining supported strong positive correlations at the protein level with ERα and GATA3 in a cohort of 118 tumors. Most ER(+)PR(−) tumors expressed CAXII at levels similar to those of ER(+)PR(+) tumors, consistent with observations in tumor transcriptome datasets and with active estrogenic signaling in some ER(+)PR(−) breast cancer cell lines. The few ER(−)PR(+) tumors did not express CAXII or the other luminal markers FOXA1 and GATA3. Overall, CAXII is a luminal marker that can help interpret ER status in single ER/PR positive tumors.