Mechanistic Interrogation of a Gene-by-Environment Interaction Informs Segment-specific Predisposition to Thoracic Aortic Aneurysm in Heritable Aortopathies

Description of Background: The mechanism for regional vulnerability to thoracic aortic aneurysm is poorly understood. Bicuspid aortic valve with distal ascending aortic aneurysm (BAV/DAscAA) is the most common heritable aneurysm condition with a strong male bias. Unlike Marfan syndrome (MFS), which is characterized by aortic root aneurysm, little is known about the genetic and mechanistic causes of BAV/DAscAA although rare cases have been attributed to haploinsufficiency in NOTCH1. In a mouse model of MFS, calcium channel blocker (CCB) exposure shifts vulnerability to the DAscAA in an ERK1/2-, AT1R-, and PKC- dependent manner.

Methods: We applied ascending aortic RNA-sequencing to identify gene expression changes that occur in tight temporal sequence with conditional provocations including CCB exposure with or without concomitant rescue upon ERK1/2 antagonism. Candidate pathways and downstream mechanistic hypotheses were interrogated with targeted pharmacologic and/or genetic manipulations.

Results: Pathway analysis revealed enrichment of differentially expressed genes in the NOTCH or androgen receptor signaling pathways. Treatment of MFS mice with CCBs and the NOTCH antagonist DBZ resulted in hyper-acute acceleration of DAscAo growth and rupture. This could be prevented by concomitant ERK1/2, PKC, or AT1R antagonism, and the more severe phenotype observed in males could be normalized by androgen receptor antagonism. Further mechanistic interrogation led us to develop and rigorously test a hypothesis accounting for regional vulnerability that collapses upon composite expression of Rgs genes, encoding potent negative regulators of AT1R. CCBs or DBZ selectively suppress Rgs4 or Rgs5 expression, respectively, while selective deletion of the Rgs4 gene fully phenocopied the deleterious influence of CCBs. Both genes show natural low expression in the DAscAo and are suppressed in Marfan syndrome.

Conclusion: These data elucidate a mechanism for regional predisposition to thoracic aortic aneurysm and show that NOTCH signaling is protective in the face of underlying disease predisposition. Identical results were found in mouse models of other inherited aortopathies including Loeys-Dietz syndrome.

Learning Objective: Describe the phenotypic variability in heritable thoracic aortic aneurysm diseases

Define the molecular pathways that are protective and pathogenic in aortic aneurysm and how they contribute to regional disease vulnerability

Identify opportunities for therapeutic intervention in the pathogenesis of thoracic aortic aneurysm