Endocrine Abstracts

The pituitary gland, is a small midline organ situated at the base of the brain, that acts as a master regulator of multiple physiological functions: such as growth, puberty, metabolism, stress response, reproduction and lactation. The pituitary gland is composed of three lobes: the anterior and intermediate, which form the anterior pituitary (AP and contains hormone producing cells), and the posterior lobe which constitutes the posterior pituitary (PP contains axonal inputs). Many molecules that govern the development of the AP have been identified, and mutations within a number of these molecules have been shown to cause varying pituitary phenotypes, from congenital hypopituitarism to pituitary tumours. Congenital hypopituitarism encompasses a range of disorders that can be manifested as an isolated hormone deficiency, or loss of multiple hormones (combined hypopituitarism), in which two or more hormones are lacking. Severe endocrine dysfunction can also result from pituitary tumours, such as adamantinomatous craniopharyngiomas (ACPs). ACPs are slow-growing tumours that arise from the RP, affect mainly children and often cause lesions in the nearby structures – the hypothalamus and the optic nerves – with life-threatening consequences and high morbidity for the patients. Previously, we have shown that the Wnt/β-catenin pathway needs to be antagonised during early AP development to maintain the appropriate numbers of progenitor cells. Hence, absence of two Wnt/β-catenin antagonising genes, Six3 and Hesx1, in the AP results in a higher proliferation of undifferentiated precursors, severe AP hyperplasia and dwarfism (Gaston-Massuet, Dev Biol, 2008). With this in mind, we studied the effect of the absence of the Wnt/β-catenin downstream effector Tcf3 by conditionally ablating Tcf3 from the AP (Hesx1^Cre/+; Tcf3^flox/flox). We show for the first time a novel role for Tcf3 in the AP development. Absence of Tcf3 results in AP hyperplasia a phenotype that closely resembles that of Hesx1 mutants. Moreover, morphological analyses of second murine model that carries a Tcf3 allele that lacks β-catenin binding domain, Tcf3^ΔN/ΔN, shows normal pituitary development indicating that the function Tcf3, is to repress downstream targets. In contrast, over-activation of the canonical Wnt pathway, by conditional expression of a degradation-resistant form of β-catenin (Ctnnb1^flox(ex3)) in the undifferentiated precursors of the pituitary gland (Hesx1^Cre/+; Ctnnb1^flox(ex3)/+) results in hypopituitarism, severe hyperplasia and adamantinomatous craniopharyngioma ACP- like tumours. This finding, demonstrated for the first time, a causal effect of mutations in β-catenin in ACP, and provided with and novel animal model to further study the ACP pathogenesis (Gaston-Massuet PNAS, 2011). Importantly, only pituitary undifferentiated precursors/stem cells are responsive to mutated β-catenin, which identifies the cell origin of ACP tumours to be pituitary undifferentiated precursor cells that are Sox2^+ve, Sox9^-ve, p27Kip2^+ve. In order to identify a possible therapeutic effects of Wnt/β-catenin inhibition in vivo, we have generated a mouse that antagonises Wnt by expressing Hesx1 from the Rosa26 locus (Hesx1^Cre/+; Ctnnb1^flox(ex3)/+; R26^Hesx1/+). Interestingly, this triple compound mice show a median survival of 29 weeks compared to 12 weeks for the double compound mutants (Hesx1^Cre/+; Ctnnb1^flox(ex3)/+) indicating that in vivo inhibition of Wnt results in amelioration of adamantinomatous craniopharyngioma tumours. Over-expression of Hesx1 in these tumours leads to restoration of wild-type levels of Lef1 and Axin2 (downstream targets of β-catenin) and proliferation, indicating that this inhibition is sufficient to exert and ameliorating effect on ACPs. We have generated an in vitro murine ACP-cell culture assay to identify Wnt β-catenin inhibitory compounds with potential therapeutic effect on ACPs. We have found, that indomethacin and sulindac sulfone have a strong effect on cell colony assay growth, suggesting a positive therapeutic effect of these compounds. Our research indicates that Wnt/β-catenin pathway is crucial for pituitary development. Initially, this pathway needs to be negatively regulated to maintain the progenitor pool. When Wnt/β-catenin is aberrantly expressed by activating mutations, this leads to pituitary tumours such as ACPs.

Declaration of funding.

NIHR-BRC, Wellcome trust and Endocrinology Society.