Bicuspid Aortic Valve Thoracic Aortic Aneurysm is Associated With Apolipoprotein E and Osteopontin but Not Notch 1 Gene Dysregulation

Thoracic aortic aneurysm (TAA) develops in about 35% of those with bicuspid aortic valve (BAV) but the cause is unkown. Some studies have identified Notch1 gene mutations in BAV while expression of osteopontin ( OPN ) gene has been reported to be significantly elevated in BAV associated TAA. Furthermore OPN gene expression has been shown to regulate apoptosis and through a CD44 antigen dependent mechanism that regulates autophagy. Apolipoprotein E ( ApoE ) gene mutations have been correlated with atherosclerosis and with BAV stenosis however an association with development of TAA in BAV is not clear. We have previously shown using electron microscopy a significant increase of VSMC apoptosis and autophagy in BAV TAA. To determine the role of Notch1, OPN and ApoE gene function in BAV associated TAA we measured mRNA expression using RT PCR methods. Immunohistochemistry was used to analyse aortic wall degeneration and VSMC loss (OPN, CD44, apoptosis and autophagy). TAA tissue was obtained from 9 subjects (7M, 2F; 57±19yr) with BAV at the time of surgery. Samples of control aorta were obtained from 5 organ donor subjects (TAC; 2M, 3F; 54±13yr). There was significant increase in mRNA expression of OPN (1.42±0.69 vs 0.24±0.09; mean±SD, P≤0.05) and ApoE (1.38±0.55 vs 0.25±0.57; P≤0.05) but not Notch1 (1.24±0.16 vs 0.91±0.23) in BAV VSMC compared to controls. BAV VSMC apoptosis (≤ 30% of total VSMCs; caspase-3+) and autophagy (≥50% of total VSMCs; LC3+) was observed but not in controls. Notch1 protein staining was negligible but there was strong OPN expression in the subendothelial layer co-localised with inner elastic lamina degeneration. Focal OPN expression in medial layer was co-localised with VSMC apoptosis and some autophagy. CD44 expression in BAV TAA was minimal and no co-localisation was found with VSMC autophagy. In summary up-regulation of OPN and ApoE but not Notch1 mRNA was found in BAV TAA. This was associated with OPN protein up-regulation that co-localised with areas of VSMC apoptosis and autophagy in the absence of CD44. The findings suggest a potentially important role for OPN and ApoE gene function in VSMC death independent of CD44 in BAV associated TAA