A Common Functional Polymorphism in the Promoter Region of the Microsomal Triglyceride Transfer Protein Gene Influences Plasma LDL Levels

Abstract

Abstract-Microsomal triglyceride transfer protein (MTP) is required for the assembly and cellular secretion of apolipoprotein B (apoB) -containing lipoproteins from the liver and intestine. The secretion pattern of apoB-containing lipoproteins is likely to influence the VLDL and LDL levels in plasma. By initial opportunistic screening for polymorphic sites in the regulatory region of the MTP gene by gene sequencing in 20 healthy male subjects, a common functional G/T polymorphism was detected 493 bp upstream from the transcriptional start point. There was differential binding of unique nuclear proteins at this site, as shown by electrophoretic mobility shift assay. The G variant seemed to bind two or three nuclear proteins that do not bind to the T variant. Expression studies with minimal promoter constructs linked to the chloramphenicol acetyltransferase reporter and transfected into HepG2 cells revealed marked enhancement of transcriptional activity with the T variant. The prevalence of the MTP promoter genotypes was investigated in a group of 184 healthy, middle-aged white men; the frequency of homozygosity for the MTP Ϫ493 T variant was .06 and the allele frequency of MTP Ϫ493T was .25 in the population. These homozygous subjects had a 22% lower LDL cholesterol concentration than did heterozygotes or subjects homozygous for the MTP Ϫ493 1 Functional MTP is an absolute requirement for the assembly and cellular secretion of apoB-containing lipoproteins. Cells that normally do not secrete apoB can acquire this competence if the genes encoding apoB and MTP are provided by gene transfection, as shown in HeLa cells and COS-1 cells.2,3 Conversely, if MTP activity is inhibited in cells that normally do secrete apoB-containing lipoproteins, the secretion of apoB is drastically reduced.4,5 A complete lack of MTP activity leads to abetalipoproteinemia, 6 a disease caused by mutations in the coding region of the MTP gene. 7-9The promoter region of the MTP gene is highly conserved between species and shows signs of both cell type-specific expression and response to metabolic regulators. The activity of the human MTP promoter is suppressed by insulin and enhanced by cholesterol. 10 The insulin response has been confirmed in HepG2 cells.11 It has also been shown that hamsters fed either a high-fat or a cholesterol-enriched diet have higher concentrations of MTP mRNA.Against this background we hypothesized that genetic variation in MTP expression might influence the plasma concentrations of apoB-containing lipoproteins in humans. We report herein a common functional polymorphism in the promoter region of the MTP gene, of which the rarer allele is linked to low plasma LDL cholesterol concentrations. Methods Human SubjectsA total of 184 healthy white men, aged 30 to 45 years, were recruited at random from a register containing all permanent residents of the Stockholm metropolitan area (response rate of 70%). Men with documented coronary heart disease or any other chronic disease were excluded. The mean age of the study group ...

Cholesterol absorption status and fasting plasma cholesterol are modulated by the microsomal triacylglycerol transfer protein −493 G/T polymorphism and the usual diet in women

“…Both functional genetic variant in the MTP gene and diet are likely to alter the expression and intracellular concentration of MTP and subsequently, the amount of assembled chylomicrons in the small intestine. The lower level of cholesterol absorption in G carrier versus TT women could likely result from the twofold lower activity of the MTP gene promoter bearing the G allele versus T as found in vitro [14]. It has also been reported that the activity of the MTP gene promoter could also be modulated by the -164T [ C polymorphism [34] which was not studied herein.…”

“…It has largely been described that upon controlled dietary interventions dedicated to lower plasma or LDL cholesterol, part of subjects showed a marked response, while others did not exhibit any change [5,11,15,50], thus highly suggesting an important gene-diet interaction. According to other authors, the minor variant -493T allele in MTP promoter has been associated with lower serum LDL cholesterol [2,14,17,19] but not in all studies [38,49]. Finally, a higher cholesterol absorption level in subjects with elevated risk of CHD [32,20] has recently been reported.…”

“…In fact, some other studies have highlighted that polymorphisms in MTP gene can alter various healthrelated indexes such as circulating cholesterol [2,14,17,19] and insulinemia [7]. Both functional genetic variant in the MTP gene and diet are likely to alter the expression and intracellular concentration of MTP and subsequently, the amount of assembled chylomicrons in the small intestine.…”

“…The MTP (or MTTP) -493G/T polymorphisms (rs 1800591, chromosome 4, contig NT 016354.18 position 25043208) were genotyped by a polymerase chain reaction (PCR)-restriction fragment length polymorphism assay, the restriction cleavage being performed by Hph1 enzyme [14]. Primers used for PCR were as follows: forward 5 0 -AG TTTCACACATAAGGACAATCATCTA-3 0 and reverse 5 0 -GGATTTAAATTTAAACTGTTAATTCATATCAC-3 0 (mutated).…”

“…The MTP gene is polymorphic, with several genetic variants especially in the promoter region [14,17]. A number of studies were undertaken to demonstrate associations between this single nucleotide polymorphisms (SNP) at position -493 (G/T) in the promoter region of the MTTP and the concentration of circulating cholesterol but provided inconsistent data [6,7,14,19,38,41,49].…”