SAT-042 Pituitary Expression Of Isl1 Prevents Ectopic, Foxa1 And Foxj1 Mediated Differentiation Of Progenitors Into Rathke’S Cleft Cysts

Islet 1 (ISL1) is a LIM homeodomain transcription factor expressed in the developing pituitary gland. Its expression overlaps with the progenitor markers SOX2 and PROP1 and with the differentiation markers NR5A1 and CGA, or alpha-subunit. We conditionally knocked out Isl1 using several Cre recombinase drivers and found increased pituitary progenitor cell apoptosis and reduced thyrotroph and gonadotroph differentiation, which results in pituitary-thyroid axis dysfunction and reduced body size in adolescent mice. Elimination of Isl1 with Prop1-Cre results in the development of multiple Rathke’s cleft cysts in pituitaries of mutant mice postnatally, with 100% penetrance. Rathke’s cleft cysts are common intracranial lesions that are thought to originate from embryonic oral epithelium. They can be asymptomatic or cause pituitary dysfunction, headache, visual problems, and diabetes insipidus. The Prop1-cre induced mouse Rathke's cleft cysts mimic the histology of human cysts in that they are comprised of single-layered, ciliated epithelial cells that express the characteristic pathologic markers, cytokeratin 8 and acetylated tubulin. Moreover, they lack expression of B-raf, a marker of craniopharyngiomas, another important pituitary pathology of embryological origin. RNA-Seq analysis of newborn, mutant mouse pituitaries identified ectopic expression of Foxj1 and Foxa1. Normally, the pioneer transcription factor FOXA1 antagonizes epithelial to mesenchymal-like transitions by positively regulating E-cadherin expression and maintaining epithelial-like fate, and FOXJ1 drives a ciliogenic gene expression program. This suggests that ectopic FOXA1 and FOXJ1 may drive Rathke's cleft cyst formation. To assess whether our model of Rathke's cleft cyst formation accurately predicts the mechanisms underlying this pathology in humans, we obtained tissues from patients that underwent surgery for problematic cysts, and we analyzed expression of FOXA1, FOXJ1 and progenitor markers SOX2 and SOX9. We found significant, consistent, ectopic expression of FOXA1 and FOXJ1 in human cysts and variable expression of progenitor markers, mirroring the findings in mice. Together, these studies support a model whereby expression of ISL1 in pituitary progenitors is necessary to drive differentiation into thyrotropes and gonadotropes, and without this essential driver, progenitors can activate FOXA1 and FOXJ1, permitting them to differentiate along an oral epithelial cell fate, producing mucinous cysts. Thus, Isl1 expression in pituitary progenitor cells is necessary to suppress ectopic cell fates and promote appropriate thyrotroph and gonadotroph differentiation. This pituitary-specific Isl1 mouse knockout sheds light on the etiology of Rathke's cleft cysts and the role. of ISL1 in normal pituitary development.