Sorting out sortilin.

G association studies have the potential to lead to identification of novel pathways and mechanisms for the pathogenesis of common complex diseases. Previous genome-wide association studies have identified a locus on 1p13 as a risk locus for dyslipidemia and myocardial infarction. However, the responsible mechanisms have remained elusive. Two recent reports implicate SORT1 as the causative gene underlying a human chromosome 1 LDL/coronary artery disease locus but come to opposite conclusions concerning SORTILIN-1’s role in the regulation of VLDL secretion. Human genome wide association studies have had spectacular success in identifying single nucleotide polymorphisms (SNPs) in or near genes associated with variation in plasma lipid and lipoprotein levels. More than 100 loci have been found that account for a significant part of the genetic variation in triglyceride, low-density lipoprotein (LDL), and high-density lipoprotein cholesterol levels. Many of these loci involve novel genes or regions. Genome-wide association studies of coronary artery disease or myocardial infarction have identified a smaller number of genetic loci, some of which are also associated with changes in traditional lipoprotein risk factors. A notable example has been a widely replicated chromosome 1p13 locus that has been associated with both myocardial infarction and LDL cholesterol levels and that does not contain a gene known to give rise to a Mendelian disorder affecting LDL cholesterol levels. The major alleles at this locus are present in about 65% to 80% of whites, and homozygosity for the major alleles, as opposed to homozygosity for the minor alleles, are associated with a 20% to 40% increase in the risk of myocardial infarction and up to 16 mg/dL higher LDL cholesterol levels. The relevant SNPs are localized in a gene cluster containing 4 genes, CELSR2, PSRC1, MYBPHL, and SORT1. A recent study has shown that the SNPs most strongly associated with LDL levels are localized to an intergenic region between CELSR2 and PSRC1 and that the minor allele SNP with strongest association creates a functional C/EBPa binding site. The authors provide evidence that this leads to an increase in the expression of SORT1 in human hepatocytes. Studies using forced overor underexpression in mice demonstrate that higher SORTILIN-1 levels are associated with reduced hepatic very low-density lipoprotein (VLDL) secretion and lower LDL cholesterol levels, matching findings that lower levels of hepatic SORT1 messenger RNA (mRNA) are associated with higher levels of LDL cholesterol in humans. SORTILIN-1 is one of five members of a Vps10p domain receptor family, often found in the trans-Golgi network and early endosomes. The VPS10 domain is a 10-bladed betapropeller. SORTILIN-1 is made as a proprotein that is cleaved in the Golgi by proprotein convertases, allowing it to bind ligand. SORTILIN-1 appears to be a multiligand receptor and has been reported to bind LPL and ApoAV. SORTILIN-1 is involved in the formation and insulinresponsiveness of GLUT-4 storage vesicles during adipocyte differention. In the brain, SORTILIN-1 may form part of a signaling complex that regulates cell survival. The in vivo relevance of these multifaceted properties remains uncertain. In a tour de force of genetic, biochemical, and animal experiments, Musunuru et al implicate SORT1 as the causative gene at the 1p13 locus. While genetic fine mapping led to the identification of 6 SNPs with similar lowest P values for association with LDL cholesterol in white populations, evaluation of the same SNPs in African-Americans revealed a single SNP associated with the lowest P values. When a 6-Kbp DNA element representing a haplotype bloc containing the minor alleles was placed in front of the luciferase gene, there was higher activity compared with the element containing the major alleles. Strikingly, when each variant SNP was changed from the minor to the major haplotype version, only the SNP with strongest association in AfricanAmericans led to the expected decrease in luciferase activity. This also led to the loss of C/EBPalpha binding and activity. These findings strongly suggest that Musunuru et al have identified the causative SNP that changes a G to a T in an enhancer-like element located between the CELSR2 and PSRC1 genes. They also show that the minor allele is associated with more than 12-fold higher levels of SORT1 mRNA, 12-fold higher levels of PSRC1 mRNA, and no significant change for CELSR2 mRNA levels in the liver. Notably, however, there was no differential expression of any of these genes in adipose tissue. The authors provide limited evidence that the minor allele also results in higher induction of SORT1 in response to transfection of CEBPA using a human stem cell model partially differentiated toward hepatocytes. In a separate report, Kjolby et al also focus on Sort1 as a candidate gene and implicate Sort1 in the regulation of VLDL secretion and LDL cholesterol levels, using Sort1 knockout mice and Sort1 overexpression in mouse models and cultured hepatocytes. In contrast to the conclusions of Musunuru, The opinions expressed in this Commentary are not necessarily those of the editors or of the American Heart Association. Commentary is edited by Aruni Bhatnagar and Ali J. Marian. Correspondence to Alan R. Tall and Ding Ai, Division of Molecular Medicine, Department of Medicine, Columbia University Medical Center. E-mail [email protected] (Circ Res. 2011;108:158-160.) © 2011 American Heart Association, Inc.