Bicuspid aortic valve (BAV) is the most commonly occurring congenital heart defect in humans. BAVs can be part of a syndrome or as a non-syndromic trait. Importantly, there is a significant sex bias in the incidence of BAV, with approximately 75% of all BAV occurring in males. The sex bias towards males is not understood, but it is presumed that a gene or genes on the X chromosome that are not present on the Y are involved in the higher incidence of BAV in males compared to females, but there have been no definitive studies to show that having two X chromosomes is protective. We have developed a new hypothesis addressing sex bias in BAV based on findings from our recent study of Turner syndrome-related BAV. Turner syndrome is a rare cytogenetic disorder due to the complete or partial loss of the second sex chromosome. They are at substantially increased risk of having a BAV, which occurs in ~25-35% of individuals with Turner syndrome, compared to the risk in the euploid population of about 2% of males and 0.5% of females. Our recent study of BAV in Turner syndrome showed that having a single copy of the X chromosome gene, TIMP1, is associated with a >4-fold increase in the likelihood of having a BAV when compared to Turner syndrome females who carry two copies of TIMP1. Although TIMP1 is not in a pseudoautosomal region, it escapes X-inactivation so both copies are expressed in females. Likewise, euploid males have only one copy of TIMP1 and are 3- times more likely to have a BAV than euploid females. We propose that the increased risk for BAV in euploid males is due, at least in part, to hemizygosity of TIMP1. Accordingly, we hypothesize that euploid females with BAV lack expression from a second copy of TIMP1 due to copy number variation (CNV) or inactivating mutations. The goal of this pilot project is to test this hypothesis by determining if there are CNVs that encompass or affect expression of TIMP1 and/or single nucleotide variants (SNVs) that inactivate a copy of TIMP that are associated with BAV in women.
The aims of this proposal are to 1) use an X chromosome microarray to identify CNVs associated with BAV, and 2/3) targeted DNA sequencing of TIMP1 and TIMP3 to identify SNVs that have the potential to decrease expression. SNVs will be evaluated for the potential to be deleterious using the Combined Annotation Dependent Depletion (CADD) scoring algorithm to evaluate all variants and rank according to the potential to be deleterious. The effect of SNVs will be functionally evaluated using in vitro gene expression analyses. Successful completion of this pilot project will determine if a decrease in TIMP1 expression is associated with risk of BAV in euploid females and are therefore a likely risk factor for increasing risk in males.
Bicuspid aortic valve (BAV) is the most common congenital heart defect in humans. There is a significant sex bias, with 75% of BAV occurring in males. The project aims to determine why males are at a substantially greater risk for BAV than females.
