Congenital bicuspid aortic valve (BAV) is a significant risk factor for valvular calcification, occurring about 20 years earlier than people with normal tricuspid aortic valve as they age. In fact, despite its prevalence of only 1-2% in the population, BAV may account for 50% of aortic valve stenosis (1).

 
Two potential mechanisms could account for the propensity of patients with BAV to develop valve calcification. First, genetic mutations that  account for some of the cases of BAV disease, may also be associated with valvular calcification (1). NOTCH1 mutation is one such candidate causing early developmental defect in the aortic valve, while later causing de-repression of calcium deposition (2). A mutation of the gene for endothelial nitric oxide synthase (eNOS) involved in preventing calcification in animal and tissue experiments may be another factor (3,4).

 
Besides genetic explanations, alteration in the mechanical force environments of the BAV itself likely plays an important part in the premature degeneration and calcification of the valve (1). Stenotic and skewed forward flow along with increased jet velocity may increase shear forces on the valve. The resultant inflammatory response and apoptosis could lead to a diseased valve, not unlike what may be seen with tricuspid aortic valve under similar circumstances (5).

Perhaps more fascinating is the observation that fluid shear itself may influence bone morphogenetic protein expression, further contributing to valvular calcification (6).

Bonus Pearl: Did you know that the risk of infective endocarditis may be much higher (>20-fold) among patients with BAV compared to those with triscuspid aortic valve (7)?

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References
1. Yap CH, Saikrishanan N, Tamilselvan G, et al. The congenital bicuspid aortic valve can experience high-frequency unsteady shear stresses on its leaflet surface. Am J Physiol Heart Circ Physiol 2012; 303:H721-H731. doi:10.1152/ajpheart.00829.2011. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3468455/
2. Nigam V, Srivastava D. Notch 1 represses osteogenic pathways in aortic valve cells. J Mol Cell Cardiol 2009;47:828-34. https://www.ncbi.nlm.nih.gov/pubmed/19695258
3. Rajamannan NM, Subramanian M, Stock SR, et al. Atorvastatin inhibits calcification and enhances nitric oxide synthase production in the hypercholesterolaemic aortic valve disease. Heart 2005;91:806-10. https://www.ncbi.nlm.nih.gov/pubmed/15894785
4. Kennedy JA, Hua X, Mishra K, et al. Inhibition of calcifying nodule formation in cultured porcine aortic valve cells by nitric oxide donors. Eur J Pharmacol 2009;602:28-35. https://www.ncbi.nlm.nih.gov/pubmed/19056377
5. Wallby L, Janerot-Sjöberg B, Steffensen T, Broqvist M. T lymphocyte infiltration in non-rheumatic aortic stenosis: a comparative descriptive study between tricuspid and bicuspid aortic valves. Heart 88: 348–351, 2002. https://www.ncbi.nlm.nih.gov/pubmed/12231589
6. Sorescu GP, Song H, Tressel SL, et al. Bone morphogenic protein 4 produced in endothelial cells by oscillatory shear stress induces monocyte adhesion by stimulating reactive oxygen species production from a nox1-based NADPH oxidase. Circ Res 2004;84:773-79. https://www.ncbi.nlm.nih.gov/pubmed/15388638
7. Kiyota Y, Corte AD, Vieira VM, et al. Risk and outcomes of aortic valve endocarditis among patients with bicuspid and tricuspid aortic valves. Open Heart J 2017;4:e000545. Doi:10.1136/opnhrt-2016-000545. https://openheart.bmj.com/content/openhrt/4/1/openhrt-2016-000545.full.pdf