Identification of five prolactin receptor variants with diverse effects on receptor signalling

The prolactin receptor (PRLR) signals predominantly through the JAK2-STAT5 pathway regulating multiple physiological functions relating to fertility, lactation, and metabolism. Understanding of PRLR signalling is incompletely defined, with progress hampered by a lack of reported disease-associated variants in the genes for the prolactin hormone (PRL) and/or PRLR. To date, two common germline PRLR variants are reported to demonstrate constitutive activity, with one, Ile146Leu, overrepresented in benign breast disease, whilst a rare activating variant, Asn492Ile, is reported to be associated with an increased incidence of prolactinoma. In contrast, an inactivating germline heterozygous PRLR variant (His188Arg) was reported in a kindred with hyperprolactinaemia whilst an inactivating compound heterozygous PRLR variant (Pro269Leu/Arg171Stop) was reported in an individual with hyperprolactinaemia and agalactia. We hypothesised that additional rare germline PRLR variants, identified from large-scale sequencing projects (ExAC and GnomAD) may be associated with altered in vitro PRLR signaling activity. We therefore evaluated >300 previously uncharacterised non-synonymous, germline PRLR variants and selected ten variants for in vitro analysis based on protein prediction algorithms, proximity to known functional domains and structural modelling. Five variants, including extracellular, transmembrane and intracellular domain variants were associated with altered responses when compared to the wild-type receptor. These altered responses included both loss- and gain-of-function activities related to STAT5 signalling, Akt and FOXO1 activity as well as proliferation and apoptosis. These studies provide further insight into PRLR structure-function and indicate that rare germline PRLR variants may have diverse modulating effects on PRLR signalling, although the physiological relevance of such alterations remain to be defined.