Compartmentalization of DHCR7 links 7-DHC metabolism to B-ring unsaturated steroid synthesis in the equine fetal gonad

The endocrinology of equine pregnancy is unique, but many of its unusual phenomena closely resemble pregnancy in women. Placental estrone synthesis uses fetal adrenal androgen precursors in women, but from the fetal gonads in mares, peaking midway through the 11-month gestation. Unique to equine pregnancies, other estrogens like equilin, typified by an unsaturated B-ring sterol structure, are synthesized from 7-dehydrocholesterol (7-DHC), the immediate precursor to cholesterol, without cholesterol formation, peaking later in gestation. Currently, the spatial and temporal regulation underlying the redirection of 7-DHC from cholesterol to B-ring unsaturated steroid synthesis in equine fetal gonads remains unknown. Here, we investigate the developmental dynamics of the equine fetal gonads from the fourth to eleventh gestational month using RNA sequencing, immunofluorescence imaging, RNAScope, and gas chromatography-mass spectrometry. Our results suggest that placental estrone synthesis correlates with the expression of the last enzyme in the cholesterol synthesis pathway, 7-dehydrocholesterol reductase (DHCR7) in the fetal gonad, and its downregulation may drive the accumulation of 7-DHC and the synthesis of B-ring unsaturated steroids. B-ring unsaturated steroid secretion is the hallmark of Smith-Lemli-Opitz Syndrome in humans, resulting from mutations in the gene encoding DHCR7 in affected patients. This naturally occurring phenomenon in equine pregnancies may offer a unique comparative model for understanding the metabolic consequences of 7-DHC accumulation in these patients and a platform for drug development.