MON-163 Expression of Odd Skipped Related 1 (Osr1) in the Mouse Uterus Increases During the Implantation Window and is Responsive to Progesterone

Abstract Disclosure: S.A. Pereira: None. A. Kim: None. G.H. Cherfane: None. A. Lofrano-Porto: None. R.S. Carroll: None. W. Kuohung: None. U.B. Kaiser: None. Background: Infertility affects 10% of those seeking pregnancy, and treatment of infertility may reveal recurrent implantation failure (RIF) or result in ectopic pregnancy (EP). Gaps persist in our understanding of the etiology of these disorders, hampering clinical management. We previously characterized a missense mutation in OSR1 identified in a proband and two sisters with impaired Mullerian duct (MD) development, uterine hypoplasia with estrogen-unresponsive endometrium, and EP. We also showed that OSR1/Osr1 is expressed in adult human and mouse endometrium and that Osr1 knockout mouse embryos have MD developmental abnormalities. We hypothesized that Osr1 expression in uteri and oviducts is sex steroid mediated, varying across the mouse estrous cycle and responding to sex steroid treatment.Methods: Adult wild-type C57BL/6 mice were euthanized during three distinct phases of the estrous cycle—proestrus, estrus, and diestrus—and uteri and oviducts were harvested for total RNA extraction and analysis of Osr1 mRNA levels by RT-qPCR. To assess Osr1 expression during endometrial proliferation and differentiation, uteri were collected from four groups: 1) intact females in diestrus, 2) 14 days post-ovariectomy (OVX), 3) 14 days post-OVX plus 3 days of estradiol (E2, 100 ng/day, proliferation phase), and 4) 14 days post-OVX plus 3 days of E2 followed by a 3-day rest and then 8 days of E2 (10 ng/day) and progesterone (P4, 1 mg/day, differentiation phase). Expression of uterine P4-induced genes (Ihh and Gata2) and of oviductal E2-induced genes (Dcpp3 and Igfbp3) were assessed as controls for estrous cycle phases and hormonal replacement. Results: Across the estrous cycle, Osr1 mRNA levels in mouse uteri were higher in diestrus compared to proestrus and estrus, but did not vary in oviducts. Expression of P4-induced genes in uteri was higher in diestrus than in proestrus or estrus. In the oviducts, expression of E2-induced genes was higher in phases with higher E2 levels (i.e., proestrus and estrus) compared to diestrus. In the model of endometrial proliferation and differentiation, uterine Osr1 expression was lower 14 days post-OVX compared to diestrus and recovered after 8 days of E2+P4 treatment, but not after 3 days of E2 alone. Expression of P4-induced genes was higher in diestrus and after E2+P4 treatment compared to both the 14-day post-OVX and E2 only groups.Conclusions: While Osr1 expression in oviducts was stable across the estrous cycle, it varied in the uterus with highest expression in diestrus, the implantation window in mice. Uterine Osr1 expression was reduced in the absence of sex steroids and restored following E2+P4 treatment. Higher Osr1 expression correlated with increased expression of P4-induced genes. Our data suggest that Osr1 may play a role in implantation in mice. Future studies will assess whether OSR1 expression is altered in the endometrium of patients with RIF or EP. Presentation: Monday, July 14, 2025