Smad3 deficiency attenuates disease severity in a zebrafish model for Smad6-related aortic disease

Thoracic aortic dissection (TAD) associates with a high mortality rate. Treatment options are limited and mainly consist of surgical repair at critical aortic diameters as current pharmacological interventions are unable to stop disease progression. Despite the existence of different mouse models for thoracic aortic aneurysm (TAA) and TAD, the underlying disease mechanisms remain elusive. In humans, loss-of-function of SMAD3 or SMAD6 increases the risk for TAA. We therefore targeted both ohnologs of smad3 and smad6 in zebrafish in order to further investigate their contribution to aortic homeostasis. We found an increased diameter of the ventral aorta in smad3a^-/-;smad3b^-/- double knockout (smad3a/b DKO) zebrafish larvae, while smad6a^-/-;smad6b^-/- (smad6a/b DKO) zebrafish larvae have a reduced aortic diameter. Smad3a/b DKO survive normally to adulthood, but smad6a/b DKO die before the age of 8 months due to dissections and ruptures in the ventral aorta. Smad6a/b DKO zebrafish also show hypoplasia of the aortic arches and the distal part of the ventral aorta. Surprisingly, the smad3a^-/-;smad3b^-/-;smad6a^-/-;smad6b^-/- quadruple knockout (qKO) zebrafish model has normal survival and a milder vascular phenotype compared to the smad6a/b DKO. RNA sequencing of zebrafish larvae indicates upregulation of pathways related to melanogenesis, ribosome, blood vessel development and carboxylic acid transport, and downregulation of negative regulation of endopeptidase activity and immune system. Transcriptomic data of damaged aorta compared with healthy control aorta identifies significant differences in oxidative phosphorylation, mitochondrial function, extracellular matrix and the citrate cycle. In conclusion, data from our novel zebrafish models of thoracic aortic dissection and rupture indicate that SMAD3 function has an important modifying effect on the severe aortic manifestations induced by loss of SMAD6.