Gudnason, Vilmundur Gardar; (1995) Sequence variation in the LDL receptor gene, and its effect on plasma lipid levels. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Two hundred patients with familial hypercholesterolaemia (FH) were examined for known mutations in the 3 part of exon 4 of the LDL receptor gene by electrophoresis of a Ddel digested PCR amplified fragment. Fifteen individuals (7.5%) were identified with 4 different mutations (of which 2 were novel) in this region of the gene. DNA from 311 patients with FH were then subjected to analysis of the 3'part of exon 4 by single strand conformation polymorphism which had been specifically adapted for sensitive detection of sequence variation in this region of the LDL receptor gene. A total of 29 patients or 9.3% (including the previous 15 patients) were identified, with a total of 6 mutations (additional two novel ones) in a region of 50 bases in the 3'part of exon 4. To examine the impact of the mutations on plasma cholesterol concentration, patients were grouped according to the localisation and nature of the mutations they carried. Four groups were compared, those with a defective protein and a mutation in repeat 5 of the ligand binding domain, those with a defective protein due to mutations elsewhere in the gene, those with a null mutation producing no protein at all and those with no detected mutation. Those individuals with null mutations had the highest plasma cholesterol concentration (11.31 mmol/l) which was similar to that in patients with any mutation in repeat 5 producing defective protein (11.23 mmol/l). These levels were both statistically significantly higher than in patients with mutations elsewhere in the gene producing a defective protein (9.64 mmol/l), or in the whole group (9.37 mmol/l). This suggests that the nature of the mutation in the LDL receptor gene is to some extent determining the phenotype of patients with FH. In addition to mutations in the LDL receptor gene that cause FH there may be mutations affecting plasma cholesterol concentration without causing FH; one such mutation is the common amino acid change A370T. The effect of this amino acid change on the uptake and degradation of LDL was investigated by analysis in CHO cells transfected with the human LDL receptor cDNA with either the T370 or the A3 70 allele. There was no detectable difference in receptor activity between the two alleles within the limits of the sensitivity of these assays. The effect of this polymorphism on plasma cholesterol concentration was also examined in three samples. In an Icelandic sample of 318 men and women the T370 allele was associated with 8.9% higher mean plasma cholesterol concentration in the men which was statistically significant. In contrast, women carrying the T370 allele, had 6% lower mean plasma cholesterol concentration. In a sample of offspring of men that had had a myocardial infarction before 55 years of age, the same trend was seen. Female offspring carrying the T370 allele had 8.8% lower, and male offspring carrying the T370 allele had 5.6% higher mean plasma cholesterol concentration, though this difference only reached statistical significance in the female. In addition, the frequency of the rare T370 allele was statistically significantly higher in the offspring of cases than in the controls, 0.06 vs 0.042 respectively (p<0.05). In the third sample examined, which consisted of 706 British healthy men that were followed for 4 years, the T370 allele was associated with 7.7% higher mean plasma cholesterol concentration at baseline. When only those for whom data were available at all visits were examined, those with the T370 allele had 2.8% lower mean plasma cholesterol concentration. Those individuals carrying the T370 allele who were lost on follow up had 0.5 mmol/1 higher mean plasma cholesterol concentration than those with an A3 70 allele only. These results suggest that the A370T polymorphism may have a small impact on the development of hypercholesterolaemia and the risk of coronary artery disease, but the mechanisms of these effect is yet to be determined.

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