Identifying MIB1 as a novel gene causing Bicuspid Aortic Valve

Introduction: Bicuspid aortic valve (BAV) is the commonest congenital heart valvular malformation. Although BAV has a major heritable component, most of the involved genes are unknown. Low penetrance and high genetic heterogeneity make identification of causative genes challenging. We used familial and population genetic bioinformatic analysis complemented by mice model and identified MIB1 as a causative gene for BAV.

Material and Methods: In silico analyses of exome sequencing in a familial cohort (28 pedigrees, n=70) were initially used. Next, burden tests were performed to compare rare variants in a replication cohort of 446 BAV cases to unaffected cohort from gnomAD database. Genotyping data of additional 452 BAV cases and 1849 matched controls were used as a second independent replication cohort. The resulted were verified in a signaling assay as well as via in vivo CRISPR-Cas mice models.

Results and Discussion: We identified predicted damaging variants. MIB1 is an E3 ligase essential for NOTCH signaling activation, a key pathway in heart development. BAV cases presented significant enrichment for rare and predicted damaging mutations compared to ethnically matched controls (p=0.02). Genotyping data analysis identified risk genetic haplotypes in MIB1, which were significantly associated with BAV (p=0.017). The identified MIB1 nonsense variants showed loss of interaction with their substrate, failing to activate NOTCH downstream pathway in cell-based signaling assay. Mouse models of Mib1 inactivation and a knock-in of a missense variant found in our BAV family developed BAV.

Conclusions: We identified MIB1 as a novel gene causing BAV using human genetic, cellular, and animal models. Given the importance of the gene in NOTCH signaling, our results underscore the role of this pathway in the pathophysiology of BAV, recognizing the first driver pathway for BAV, which is a potential target for diagnostic and therapeutic interventions.