Characterisation of an Adult Zebrafish Model for <i>SDHB</i>-Associated Phaeochromocytomas and Paragangliomas

Phaeochromocytomas and paragangliomas (PPGLs) are rare neuroendocrine tumours arising from chromaffin cells. Pathogenic variants in the gene succinate dehydrogenase subunit B (SDHB) are associated with malignancy and poor prognosis. When metastases arise, limited treatment options are available. The pathomechanism of SDHB-associated PPGL remains largely unknown, and the lack of suitable models hinders therapy development. Germline heterozygous SDHB pathogenic variants predispose to developing PPGLs with a life-long penetrance of around 50%. To mimic the human disease phenotype, we characterised adult heterozygous sdhb mutant zebrafish as a potential model to study SDHB-related PPGLs. Adult sdhb mutant zebrafish did not develop an obvious tumour phenotype and were anatomically and histologically like their wild-type siblings. However, sdhb mutants showed significantly increased succinate levels, a major hallmark of SDHB-related PPGLs. While basal activity was increased during day periods in mutants, mitochondrial complex activity and catecholamine metabolite levels were not significantly different. In conclusion, we characterised an adult in vivo zebrafish model, genetically resembling human carriers. Adult heterozygous sdhb mutants mimicked their human counterparts, showing systemic elevation of succinate levels despite the absence of a tumour phenotype. This model forms a promising basis for developing a full tumour phenotype and gaining knowledge of the pathomechanism behind SDHB-related PPGLs

Mechanism of Action of a Nanomolar Potent, Allosteric Antagonist of the Thyroid-Stimulating Hormone Receptor

Background and purpose  Graves' disease (GD) is an autoimmune disease in which the thyroid is overactive, producing excessive amounts of thyroid hormones, caused by TSHR-stimulating immunoglobulins (TSIs). A large proportion of GD patients also suffer from thyroid eye disease (Graves' ophthalmopathy or GO), with the TSIs considered to activate TSHRs in orbital tissue also. We recently developed LMW TSHR antagonists as a novel therapeutic strategy for the treatment of GD and GO. In the present study, we determined the molecular pharmacology of a prototypic, nanomolar potent LMW TSHR antagonist, Org 274179-0. Experimental approach  First, we determined the potency and efficacy of Org 274179-0 in antagonizing TSH- and TSI-induced TSHR signaling and its cross-reactivity at the related FSHR and LHR. Second, we explored in depth the allosteric mode of interaction of Org 274179-0. Third, we determined whether Org 274179-0 is an inverse agonist at five naturally occurring, constitutively active TSHR mutants. Key results  Org 274179-0 fully inhibited TSH (and TSI)-mediated TSHR activation with nanomolar potency without hardly affecting the potency of TSH, in accordance with an allosteric mechanism of action. On the reverse, increasing levels of TSHR stimulation only marginally reduced the antagonistic potency of Org 274179-0. Finally, Org 274179-0 fully blocked the increased basal activity of all tested constitutively active TSHR mutants with nanomolar potencies. Conclusions and implications  We conclude that nanomolar potent TSHR antagonists like Org 274179-0 have the potential of being developed to treat GD and GO