Inherited Variants in SCARB1 Cause Severe Early-Onset Coronary Artery Disease

Abstract

Rationale: Coronary artery disease (CAD) is a pervasive and critical healthcare problem. Elevated high density lipoprotein-associated cholesterol (HDL-C) is associated with improved atherosclerotic cardiovascular disease (ASCVD) outcomes on a population level, but clinical trials aimed at HDL-C elevation have not succeeded in improving ASCVD event risk. Nevertheless, human variants in the HDL receptor, encoded by SCARB1, are associated with dyslipidemia, suggesting that HDL metabolism, not HDL-C, is a suitable target for therapy. However, variants in SCARB1 have never been directly attributed to CAD by Mendelian inheritance. Objective: To determine if compound heterozygous variants in SCARB1 cause disease in two brothers with severe, early-onset CAD. Methods and Results: Using whole exome sequencing, we have identified rare, compound heterozygous variants in SCARB1 that segregate with severe, premature CAD, following patterns of Mendelian inheritance. Using induced pluripotent stem cell-derived hepatocyte-like cells (iPSC-HLCs) from the proband, we discovered the maternal variant (c.754_755delinsC) to be the first identified SCARB1 null allele, characterized by the absence of RNA and protein expression. Further, we demonstrate that the variant on the paternal allele (c.956G>T (p.G319V)) results in decreased cholesterol uptake, decreased SR-BI:HDL binding, and increased affinity for SR-BI dimerization. Finally, we generated a p.G319V knock-in mouse model that displays nearly 100% homozygous lethality and elevated plasma cholesterol in heterozygous animals, confirming pathogenicity of this variant. Conclusions: In summary, our data provide the first molecular mechanism to show the Mendelian inheritance of CAD as a result of human SCARB1 variants. The rarity of these variants supports pathogenicity in this family. Furthermore, SR-BI p.G319V, which has previously been reported benign in the context of heterozygosity, was uniquely presented alongside a null allele, demonstrating the disease-contributing capability of loss-of-function SCARB1 variants within the population.

Good Cholesterol Gone Bad? HDL and COVID-19

“…Studies in mice showed that the absence of functional SR-B1 leads to increased plasma cholesterol concentrations, which is associated with enlarged HDL particles with increased unesterified cholesterol content and altered apolipoprotein composition [52][53][54][55][56]. Although inactivating mutations in the SCARB1 gene in humans are rare, similar increases in HDL cholesterol as well as early-onset coronary artery atherosclerotic disease were reported in human compound heterozygotes carrying two inactivating mutations in SCARB1 [57]. This is similar to findings in mice in which SR-B1 is knocked out in the context of other atherogenic mutations [54,58,59].…”

SR-B1’s Next Top Model: Structural Perspectives on the Functions of the HDL Receptor

“…Several polymorphisms of human SR-B1 have been identified in the human SCARB1 gene, including nine coding mutants: V32M, V111M, S112F, V135I, R174C, T175A, P297S, G319V, and P376L [ 21 ••, 22 ••, 76 , 77 , 92 •, 93 •, 94 •]. The locations of these mutants are mapped to our homology model (Fig.…”

“…V111 is predicted to be solvent exposed within an extracellular β-strand near the N-TMD and may impact SR-B1 membrane dynamics. G319V is within a solvent-exposed loop in our model and in vitro studies showed that G319V-SR-B1 exhibited a decreased capacity to mediate HDL-CE uptake compared to wild-type SR-B1, despite comparable levels of cell surface expression [ 22 ••]. Information from G319V-SR-B1 cellular assays, combined with a recent clinical observation of recurrent myocardial infarctions in a young adult with the G319V-SR-B1 mutation [ 22 ••], highlights the delicate balance of cholesterol homeostasis maintained by SR-B1.…”

“…The mechanisms by which the G319V variant is so damaging remain unclear. Koenig et al utilized homology modeling to propose that G319 resides in an unspecified extracellular dimerization interface near the tri-helix apical bundle [ 22 ••]. They further suggest that the G319V mutation stabilizes dimers via hydrophobic interactions; however, assays to validate these claims were not presented.…”

“…Alternatively, SR-B1 overexpression in wild-type mice results in decreased HDL-C levels [ 16 – 19 ] and less atherosclerotic plaque formation [ 20 ]. In addition, all nine identified human variants of SR-B1 result in elevated plasma HDL-C levels, with two variants (P376L- [ 21 ••] and G319V-SR-B1 [ 22 ••]) directly increasing the risk of ASCVD. Combining these clinical observations with mouse studies, SR-B1’s importance in maintaining appropriate HDL-C levels and mitigating ASCVD risk becomes increasingly clear.…”