Characterization of two constitutively active prolactin receptor variants in a cohort of 95 women with multiple breast fibroadenomas

The role of prolactin (PRL) and its receptor (hPRLR) in promoting breast tumors is debated. We recently identified a gain-of-function hPRLR variant (I146L) in four women with multiple breast fibroadenomas (MFA) and no control subject.The specific aims were to describe this cohort of women presenting with MFA to identify and functionally characterize germline variants of hPRL/hPRLR genes and compare phenotypes of all patients.Ninety-five patients prospectively underwent clinical examination, breast ultrasonography, magnetic resonance imaging, and hormonal evaluation of gonadal and lactotrope functions. We analyzed hPRL/hPRLR coding sequences and made comparisons with a control population of 194 women. Functional characterization of hPRLR variants was performed. Pathology and immunochemistry were systematically carried out after surgical removal of tumors.One third of patients had a family history of breast disease. No hormonal imbalance was observed, except 30.7% of explosive stimulated PRL. Prolactin receptor variants were identified in exon 5 (I76V: 10 patients, eight controls) and exon 10 (one patient, no control). Both I146L and I76V variants exhibited constitutive activity. Pathology showed common fibroadenomas and identified six benign phyllodes tumors. Estrogen and progesterone receptors were detected in 85 and 98% of samples, respectively. Ki-67 median staining was less than 5%. No phenotypic difference was observed between carriers and noncarriers of either hPRLR variant.We present the largest population with MFA ever described, 15% of which had a hPRLR exhibiting basal activity in vitro. This questions the involvement of the hPRLR in MFA etiology and the potential relevance of therapeutic inhibition of PRLR signaling in patients.