To characterize circulating oestrogen receptor ( ER) mutants and splice variants in men with advanced prostate cancer.Sequential blood samples were obtained from men with advanced prostate cancer, and from healthy controls. Blood-derived RNA samples were analysed using droplet digital PCR for the presence of six ERα mutations (E380Q, L536Q, Y537C, Y537S, Y537N and D538G), and six ERα and ERβ splice variants (ERα-66, ERα-36, ERβ1, ERβ2, ERβ4ERβ5).A total of 94 samples were collected from 42 men with advanced prostate cancer. Four mutations (E380Q, L536Q, Y537S and D538G) and all six splice variants were detected in patient samples. Splice variants were detectable in non-cancer control samples. The presence of ER mutations was associated with bone metastases and castration resistance. ERβ splice variant concentrations decreased after successive lines of treatment.The ER mutations were detectable in plasma from patients with advanced prostate cancer. ER splice variants were frequently detected in both men with and without prostate cancer.
