Generation of an all-exon Esr2 deleted mouse line: Effects on fertility

Estrogen receptor beta (ERβ), encoded by the Esr2 gene, is one of two nuclear receptors that mediate the functions of the steroid hormone estradiol. The binding of estradiol to the receptor results in enhanced transcription of many genes that have estrogen response elements in promoter or enhancer regions. Several genetically modified mouse lines with mutations or deletions of exons in the Esr2 gene have been developed and results from analysis of these are not completely consistent, especially regarding ERβ′s role in fertility. To address these controversies, we have used the CRISPR/Cas9 genome editing system to make a deletion of the entire Esr2 gene in the mouse genome and determined the effect of this mutation on fertility. We show that female Esr2 deleted mice, Esr2^ΔE1−10, are subfertile at young age, with fewer litters and smaller litter size, and that they become infertile/have severely reduced fertility at around six months of age, while the male Esr2^ΔE1−10 mice are fertile. Ovaries from Esr2^ΔE1−10 mice are smaller than those from wild-type littermates and the morphology of the ovary displays very few corpora lutea, indicating a defect in ovulation. We also show that the estradiol levels are reduced at diestrus, the phase in the estrous cycle when levels are expected to start to increase before ovulation. Our results verify that ERβ has an important function in female reproduction, likely as a regulator of serum estradiol levels, and that its loss does not affect male reproductive function.