Haplotype analysis of the low density lipoprotein receptor (LDLR) gene was performed in Norwegian subjects heterozygous for familial hypercholesterolemia (FH). Southern blot analysis of genomic DNA, using an exon 18 specific probe and the restriction enzyme NcoI, showed that two out of 57 unrelated FH subjects had an abnormal 3.6 kb band. Further analyses revealed that this abnormal band was due to a 9.6 kb deletion that included exons 16 and 17. The 5' deletion breakpoint was after 245 bp of intron 15, and the 3' deletion breakpoint was in exon 18 after nucleotide 3390 of cDNA. Thus, both the membrane-spanning and cytoplasmatic domains of the receptor had been deleted. A polymerase chain reaction (PCR) method was developed to identify this deletion among other Norwegian FH subjects. As a result of this screening one additional subject was found out of 124 subjects screened. Thus, three out of 181 (1.7%) unrelated Norwegian FH subject possessed this deletion. The deletion was found on the same haplotype in the three unrelated subjects, suggesting a common mutagenic event. The deletion is identical to a deletion (FH-Helsinki) that is very common among Finnish FH subjects. However, it is not yet known whether the mutations evolved separately in the two countries.
Familial hypercholesterolaemia is an autosomal dominant disorder characterized by hypercholesterolaemia, xanthomas and premature coronary heart disease. Treatment of hypercholesterolemia is effective and consists of dietary changes and lipid lowering drugs. Only a minor proportion of familial hypercholesterolaemia patients are adequately treated, however. One explanation for this is assumed to be the relatively vague clinical diagnostic criteria applied. Because familial hypercholesterolaemia is caused by a mutation in the gene encoding the low density lipoprotein (LDL) receptor, mutation analysis of this gene could form the basis for specific diagnosis. 29 different mutations in the LDL receptor gene have been found to cause familial hypercholesterolaemia among Norwegian patients, and a total of 681 patients from 322 unrelated families have been provided with a molecular genetic diagnosis. We conclude that the use of molecular genetic analysis is feasible, and should be used clinically.
Familial defective apolipoprotein B-100 (FDB) is caused by a mutation in codon 3500 of the apo B gene. It is inherited in a co-dominant fashion and is characterized by hypercholesterolaemia. Thus, FDB has similar features to familial hypercholesterolaemia (FH). In order to investigate whether some of the Norwegian subjects diagnosed as having FH actually have FDB, we have screened 208 Norwegian FH heterozygotes for the apo B-3500 mutation. One of the subjects possessed the mutation which was on a haplotype compatible with the mutation-bearing haplotype found in other populations. Although, hypercholesterolaemia segregated with haplotypes both at the apolipoprotein B and low density lipoprotein (LDL) receptor loci in the proband's family, LDL receptor analysis revealed that the proband was not doubly heterozygous for FDB and FH.
Hereditary hemorrhagic telangiectasia (HHT, Osler-Weber-Rendu disease) is an autosomal dominant inherited disease defined by the presence of epistaxis and mucocutaneous telangiectasias and arteriovenous malformations (AVMs) in internal organs. In most families (~85%), HHT is caused by mutations in the ENG (HHT1) or the ACVRL1 (HHT2) genes. Here, we report the results of genetic testing of 113 Norwegian families with suspected or definite HHT. Variants in ENG and ACVRL1 were found in 105 families (42 ENG, 63 ACVRL1), including six novel variants of uncertain pathogenic significance. Mutation types were similar to previous reports with more missense variants in ACVRL1 and more nonsense, frameshift and splice-site mutations in ENG. Thirty-two variants were novel in this study. The preponderance of ACVRL1 mutations was due to founder mutations, specifically, c.830C>A (p.Thr277Lys), which was found in 24 families from the same geographical area of Norway. We discuss the importance of founder mutations and present a thorough evaluation of missense and splice-site variants.
DNA from 20 unrelated familial hypercholesterolemia (FH) subjects were studied by analysis of single-strand conformation polymorphisms (SSCP) for mutations in exon 3 of the low density lipoprotein (LDL) receptor gene. Four different SSCP patterns were observed. The underlying mutations were characterized by DNA sequencing. One pattern represented the wild-type sequence. Another pattern represented a C-->G mutation (FH-Svartor) that changes codon 78 into the amber stop codon. The two other patterns represented heterozygosity and homozygosity, respectively, for a G-->A splice donor mutation (FH-Elverum) in intron 3. Based upon two PCR-based assays, the frequencies of FH-Svartor and FH-Elverum among 267 unrelated FH subjects, were 8% and 25%, respectively. FH Svartor was located on a chromosome with haplotype 3 in all five families where haplotype analysis were performed. FH Elverum was located on haplotype 2 in 16 out of 20 families. The two mutations must be considered founder mutations in the Norwegian population, and their existence will be clinically useful in diagnosing FH. The presence of two founder mutations together with previously published data on the prevalence of FH in Norway, indicate that FH may be a more common disease in Norway than previously thought.