Estrogen receptor–α messenger RNA variants that lack exon 5 or exon 7 are coexpressed with wild-type form in human endometrium during all phases of the menstrual cycle

We have assessed the expression levels of messenger RNA for estrogen receptor–α and splice variants lacking exon 5 or exon 7 that presumably exert dominant positive (splice variants lacking exon 5) and negative (splice variants lacking exon 7) effects, respectively, on estrogen responses in the human endometrium. This was a prospective study that was conducted at an academic community-based hospital. The patients, aged 18 to 40 years, underwent hysterectomy for benign gynecologic causes. Eighty-one endometrial specimens (46 proliferative, 35 secretory) were analyzed with the use of reverse transcription–polymerase chain reaction assay for the messenger RNA levels of estrogen receptor–α, and splice variants lacking exon 5 and exon 7. Wild-type estrogen receptor–α and splice variants splice variants lacking exon 5 and lacking exon 7 messenger RNAs were detected in all endometrial specimens throughout the menstrual cycle. In addition, a double-splice estrogen receptor–α messenger RNA variant (splice variants lacking exon 5 and exon 7) was detected at constant low levels of expression. Semiquantitative analysis showed higher levels of estrogen receptor–α messenger RNA in the early and mid proliferative endometrial phases than in late proliferative and secretory endometrium (P