Clinical, Biochemical, and Molecular Characterization of Novel Mutations in ABCA1 in Families with Tangier Disease

Abstract

Tangier disease is a rare, autosomal recessive disorder caused by mutations in the ABCA1 gene and is characterized by near absence of plasma high-density lipoprotein cholesterol, accumulation of cholesterol in multiple tissues, peripheral neuropathy, and accelerated atherosclerosis. Here we report three new kindreds with Tangier disease harboring both known and novel mutations in ABCA1. One patient was identified to be homozygous for a nonsense mutation, p.Gln1038*. In a remarkably large Tangier disease pedigree with four affected siblings, we identified compound heterozygosity for previously reported missense variants, p.Arg937Val and p.Thr940Met, and show that both of these mutations result in significantly impaired cholesterol efflux in transfected cells. In a third pedigree, the proband was identified to be compound heterozygous for two novel mutations, a frameshift (p.Ile1200Hisfs*4) and an intronic variant (c.4176-11T>G), that lead to the creation of a cryptic splice site acceptor and premature truncation, p.Ser1392Argfs*6. We demonstrate that this mutation arose de novo, the first demonstration of a pathogenic de novo mutation in ABCA1 associated with Tangier disease. We also report results of glucose tolerance testing in a Tangier disease kindred for the first time, showing a gene-dose relationship between ABCA1 activity and glucose tolerance and suggesting that Tangier disease patients may have substantially impaired islet function. Our findings provide insight into the diverse phenotypic manifestations of this rare disorder, expand the list of pathogenic mutations in ABCA1, and increase our understanding of how specific mutations in this gene lead to abnormal cellular and physiological phenotypes.

Learning from each other: ABC transporter regulation by protein phosphorylation in plant and mammalian systems

“…ABCA1 has a notable role in cholesterol efflux and Tangier disease which is due to harbouring a mutation in the ABCA1 transporter [4][5][6]. Further, member 4 of the A subfamily (ABCA4) acts as a transporter of N-retinylidene-phosphatidylethanolamine (NrPE).…”

Targeted next-generation sequencing to diagnose disorders of HDL cholesterol

“…Immunoblotting and immunofl uorescence in transiently transfected HEK293 cells were carried out as previously described ( 15 ). Cholesterol effl ux assays were performed, as previously described ( 15 ), in transiently transfected HEK293 cells. Data represent the mean ± SD of at least three independent experiments, each performed in triplicate.…”

“…Patients were excluded if they could not speak English or were unable or unwilling to provide written informed consent. As a positive control, we included patients in whom a molecular diagnosis of a Mendelian disorder of HDL had previously been established by Sanger sequencing ( 15 ). Clinical data were abstracted from the patients' medical records.…”

“…We considered a molecular diagnosis to have been made if a patient had a pathogenic or probably pathogenic variant detected in a gene known to cause the phenotype present in that patient (i.e., in a low HDL-C gene in a patient known disease-causing rare mutations in the ABCA1 gene previously identifi ed by Sanger sequencing ( 15 ). This included fi ve individuals heterozygous for missense mutations in ABCA1 and one individual compound heterozygous for mutations in ABCA1 .…”

“…This suggests that the sequencing-based approach described here may be more sensitive to identify patients with rare Mendelian disorders of HDL-C compared with an approach based on classical stigmata of these conditions. Previously, we have employed Sanger sequencing to identify pathogenic mutations in patients with Mendelian disorders of HDL-C ( 15 ). For large genes such as ABCA1 (50 exons), this approach is costly, laborintensive, time-consuming, and generally requires sequential testing of candidate genes if no pathogenic mutation is identifi ed in the initial screening.…”