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. To unravel the underlying pathogenic mechanisms and to evaluate therapeutic strategies, suitable in vivo models are needed. The available Sdhb knock-out mice models do not mimic the human PPGL phenotype, as also seen for the Von Hippel Lindau gene. In contrast to mice, zebrafish vhl knockouts showed several features of VHL in humans. Despite absence of obvious PPGL-like tumours in vhl mutants, we detected increased metanephrine levels in homogenates of vhl knock-out larvae and adult fish, as well as their swimming water. Similar to VHL, we hypothesized that, possibly, features of SDHB-related diseases in humans could also be replicated in a zebrafish model. 

Aim: To generate and characterize a zebrafish sdhb mutant as a potential model for SDHB-related disease in humans. 

Results: Using CRISPR-cas9 technology, we successfully introduced a germline lesion in the zebrafish sdhb gene resulting in a frameshift and early stop codon. 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 as compared to their heterozygous sdhb^+/- and wild-type siblings. In line with the anticipated inability of our fish to produce sufficient energy, behavioral analysis showed a lower basic activity as well as a faster decrease in activity after labor in our sdhb^-/- fish. These features are similar to those found in patients born with inactive SDHB due to bi-allelic SDHB mutations and as found in SDHB-associated PPGL.

Conclusion: We have created a first vertebrate animal model that mimics the metabolic effects of SDHB-associated PPLGs and allows us for the first time to study the metabolic effects of the lack of sdhb in vivo.