FOXL2 drives the differentiation of supporting gonadal cells in early ovarian development.

Background: Forkhead box L2 (FOXL2) is a transcription factor from the forkhead box family primarily expressed in the pituitary, ovaries, and eyelids. Human mutations in FOXL2 cause blepharophimosis, ptosis, epicanthus and inversus syndrome (BPES), which can be associated with primary ovarian insufficiency, and is indirectly linked with differences of sex development (DSD). Animal studies have shown the crucial role that FOXL2 plays in the development, function, and maintenance of the ovary as well as in sex determination. However, the specific role of FOXL2 in early human somatic cell ovarian development is largely unknown.

Methods: In this study, we utilised CRISPR/Cas9 genome activation and a previously published in-house 14-day gonadal differentiation protocol to study the role of FOXL2.

Results: Our results demonstrate that FOXL2 downregulates coelomic epithelial markers GATA4 and LHX9, female gonadal markers RSPO1 and WNT4, and male gonadal markers SOX9, NR0B1 and DHH. The differentially expressed genes were mostly associated with Kyoto encyclopaedia of genes and genomes (KEGG) pathways relating to cell adhesion molecules and gene ontology (GO) pathways relating to extracellular matrix and junction formation. Furthermore, a comparative analysis with existing single cell RNA sequencing data from human in vivo-derived samples elucidated that FOXL2 initiates the downregulation of coelomic epithelial genes GATA4, LHX9 and UPK3B at day 6. By day 8, the genes ARX and GATA2 are transiently upregulated by FOXL2 induction and then downregulated as the genes LGR5, TSPAN8, OSR1 and TAC1 become upregulated.

Conclusions: These findings suggest that FOXL2 facilitates the exit of differentiating cells from the coelomic epithelium and initially drives them towards a transitional identity before progressing into early supporting gonadal-like cells. The findings of this study significantly advance our understanding of normal gonadal development which can be used as a basis to elucidate pathological gonadal development underlying BPES.