In a 35-year-old woman with hyperprolactinemia (HPRL; 615555), who experienced complete lack of lactation after each of 2 childbirths, Kobayashi et al. (2018) identified compound heterozygosity for a c.511C-T transition (c.511C-T, NM_000949.6) in exon 6 of the PRLR gene, resulting in an arg171-to-ter (R171X) substitution, and a c.806C-T transition in exon 9, resulting in a pro269-to-leu (P269L; 176761.0004) substitution at the docking site for JAK2 (147796) within the highly conserved box 1 motif of the proline-rich domain. R171X was not listed in public variant databases, whereas P269L had been reported as a SNP with a minor allele frequency of 0.00002. The proband's mother, who was heterozygous for R171X, had no history of menstrual irregularities or infertility and had breast-fed each of her 3 children, but stated that she had been concerned about insufficient production of breast milk and supplemented with synthetic milk; lactation ceased spontaneously within 3 months after each childbirth. The proband's fertile and healthy father was heterozygous for the P269L variant; both parents had normal prolactin levels. Functional analysis in transfected HEK293T cells showed that the R171X mutant was present at significantly lower levels than wildtype PRLR and showed no signal at the cell membrane, whereas the P269L variant was detected in both cytoplasm and the cell membrane and showed an elevated level of expression compared to wildtype PRLR. However, neither mutant phosphorylated STAT5 (601511) in response to prolactin, in contrast to the wildtype prolactin receptor; in addition, neither mutant suppressed STAT5 phosphorylation by the wildtype receptor when equimolar amounts of plasmid were transfected. Quantitative assay revealed that the R171X mutant did not respond to prolactin, whereas the P269L mutant exhibited a blunted response that was suppressed when equimolar amounts of R171X and P269L were present. Only the P269L mutant appeared to have a dominant-negative effect, attenuating the response of the wildtype receptor to prolactin by approximately 20%.
