Background: Pheochromocytomas or paragangliomas (PPGLs) caused by mutations in the β-subunit of the succinate dehydrogenase (SDHB) have the highest metastatic rate, for which effective systemic therapy is lacking. We have created the first vertebrate animal model that mimics the metabolic effects of SDHB-associated PPGLs. Homozygous sdhb^-/- larvae are viable, but exhibit a shorter lifespan. Biochemical analysis revealed decreased mitochondrial complex ΙΙ activity and significant succinate accumulation in sdhb^-/- larvae. This allows us for the first time to study the in vivo metabolic effects of the lack of sdhb in further detail.

Aim: Further characterization of sdhb zebrafish mutants to unravel underlying pathogenic mechanisms behind SDHB-associated PPGLs.

Results: Although morphological analysis revealed no differences in amount and structure of the mitochondria, clear defects in energy metabolism and swimming behaviour were observed in sdhb^-/- mutants using behavioural analysis. Besides, non-inflated swim bladders were observed in 60% of the sdhb^-/- mutants. Reactive oxygen species (ROS) levels were increased in homozygous sdhb^-/- larvae compared to heterozygous and wild-type siblings indicative for oxidative stress. Transcriptomics analysis revealed significant differences in pathways involved in TCA-cycle and the electron transport chain. The previously identified hypermethylated regions of tumour suppressor genes in human SDHB tumours (e.g. RASSF1A, CDKN2A, RARB, TNFRSF10D, CDH1 and APC) are for the majority also differentially expressed in our sdhb mutant larvae.

Conclusion: Further characterization of homozygous sdhb zebrafish validates the relevance of this model for SDHB-associated PPGLs and reveals several therapeutic targets that can be evaluated in  our newly developed semi high-throughput drug screening platform using zebrafish larvae.