E. Langenhoven

Phenotypic variation among familial hypercholesterolemics heterozygous for either one of two Afrikaner founder LDL receptor mutations.

Two common founder-related gene mutations that affect the low-density lipoprotein receptor (LDLR) are responsible for approximately 80% of familial hypercholesterolemia (FH) in South African Afrikaners. The FH Afrikaner-1 (FH1) mutation (Asp206-->Glu) in exon 4 results in defective receptors with approximately 20% of normal activity, whereas the FH Afrikaner-2 (FH2) mutation (Val408-->Met) in exon 9 completely abolishes LDLR activity (< 2% normal activity). We analyzed the contribution of these mutations and other factors on the variation of hypercholesterolemia and clinical features in Afrikaner FH heterozygotes. The type of FH mutation, plasma triglyceride levels, and age of patients each contributed significantly to the variation in hypercholesterolemia, whereas smoking status, high-density lipoprotein cholesterol levels, and gender had no influence. Although all FH heterozygotes had frank hypercholesterolemia, patients with the FH1 mutation had significantly lower cholesterol levels than those with the FH2 mutation. FH1 heterozygotes also tended to have milder clinical features. The differences between the two FH groups could not be explained by a difference in the common apolipoprotein E variants. This study demonstrates that mutational heterogeneity in the LDLR gene influences the phenotypic expression of heterozygous FH.

Phenotypic expression and frequency of familial defective apolipoprotein B-100 in Belgian hypercholesterolemics

DNA screening for apolipoprotein (apo) B mutations causing familial defective apolipoprotein B-100 (FDB) was performed in 87 hyperlipidemic Belgian individuals using heteroduplex analysis. Eighteen FDB heterozygotes from 5 unrelated families were identified. Three of the index cases reported an early family history of premature coronary heart disease (CHD). The frequency of the apo B3500 mutation was 8% in Belgians with type IIa hyperlipidemia, indicating that the prevalence of FDB may be as high as 1 in 250 in the general Belgian population. Plasma lipid levels of the patients identified in the present study are similar to those previously reported for FDB heterozygotes. We compared these data with results obtained in a genotype/phenotype correlation study of heterozygous familial hyper-cholesterolemia (FH) in the Afrikaner population of South Africa. Plasma cholesterol levels in FDB heterozygotes were similar to those reported for FH heterozygotes with defective receptors (Asp206-->Glu, approximately 20% normal receptor activity), but significantly lower than in FH heterozygotes with a mutant protein which virtually lacks receptor activity (Val408-->Met, < 2% normal receptor activity). FDB appears to be a significant genetic cause of hypercholesterolemia in Belgium.

Detection of a frequent polymorphism in exon 10 of the low-density lipoprotein receptor gene

SummaryDNA sequencing of enzymatically-amplified exons of the low-density lipoprotein receptor gene from several individuals revealed a polymorphism in exon 10 of the gene. The codon for arginine 450 was converted from AGG to AGA in some alleles.

Intrafamilial variability in the clinical expression of familial hypercholesterolemia: importance of risk factor determination for genetic counselling

Kotze MJ, Davis HJ, Bissbort S, Langenhoven E, Brusnicky J, Oosthuizen CJJ. Intrafamilial variability in the clinical expression of familial hypercholesterolemia: importance of risk factor determination for genetic counselling.

CpG hotspot mutations at the LDL receptor locus are a frequent cause of familial hypercholesterolemia among South African Indians

Mutation analysis of genomic DNA samples obtained from seven unrelated South African Indians with familial hypercholesterolaemia (FH) revealed two novel and two recurrent missense mutations in the low density lipoprotein receptor (LDLR) gene. The novel mutations are transversions of C to G and A to T at nucleotide positions 1215 (N384K) and 2356 (S765C), respectively. The known mutations were detected in CpG dinucleotides at bases 661 and 682, respectively, in the mutation‐rich exon 4 of the LDLR gene. Mutation D200Y was found in a single FH family, while mutation E207K was detected in two apparently unrelated Indian families on a new mutual haplotype. Analysis of published mutations including our new data has shown that more than 50% of the different LDLR gene mutations identified to date in South African Indians occur at CpG hotspots.

Haplotype associations of three DNA polymorphisms at the human low density lipoprotein receptor gene locus in familial hypercholesterolaemia.

The frequency and inheritance of three restriction fragment length polymorphisms (RFLPs) of the low density lipoprotein (LDL) receptor gene were investigated in 27 South African families with familial hypercholesterolaemia. Four haplotypes, defined by the enzymes PvuII, StuI, and NcoI, were found to segregate in this population. The frequency of the rare allele detected by NcoI was found to be 0.53 in 45 unrelated familial hypercholesterolaemic (FH) patients compared to 0.33 in 60 normal controls (p less than 0.005). In 71% of the families studied, a haplotype with common alleles for PvuII and StuI and the rare allele for NcoI cosegregated with the defective gene. In 20% of the families, a second haplotype with rare alleles for PvuII and StuI and common allele for NcoI segregated with FH. In these families the haplotypes unambiguously cosegregate with the disease and can therefore be used for early diagnosis of FH.

Two novel point mutations causing receptor-negative familial hypercholesterolemia in a South African Indian homozygote

Two novel point mutations have been identified in the low density lipoprotein receptor (LDLR) gene of a South African Indian patient with a clinical diagnosis of homozygous familial hypercholesterolemia (FH). The patient is a compound heterozygote, whose paternally-inherited allele has a single base substitution of A to T at position + 1. This conversion of the initiation codon ATG (methionine) to TTG (leucine) would abolish initiation of translation at the normal site, and consequently the synthesis of any normal LDLR molecules. The second mutation identified is a C to A base change at nucleotide position 1176 in exon 8, which creates a stop codon at cysteine-371. Except for previously-described polymorphisms in specific regions of the LDLR gene, the mutations identified in exons 1 and 8 were the only variants observed by screening enzymatically amplified genomic DNA comprising the entire coding and promoter region of the LDLR gene by combined heteroduplex-single-strand conformation polymorphism analysis and by direct sequencing. Cultured cells from the proband expressed no functional LDLR activity and contained no receptor protein that could be detected by antibody binding. These findings are consistent with the nature of the two base changes identified and provide evidence that the mutations cause FH in the proband and his affected family members. The mutations, designated M-21L and C371X, were absent in 17 apparently unrelated Indian hypercholesterolemics and 200 normal chromosomes screened.

Screening for mutations in exon 4 of the LDL receptor gene: identification of a new deletion mutation.

DNA from 14 unrelated New Zealand familial hypercholesterolaemia (FH) heterozygotes, originating from the United Kingdom, was screened for mutations in exon 4 of the low density lipoprotein receptor (LDLR) gene. One patient was heterozygous for mutation D206E, which was initially identified in South Africa. The chromosomal background of this mutant allele was compatible with that described previously in Afrikaner and English patients, suggesting that this mutation originated in the United Kingdom. The 2 bp deletion in codon 206 and mutations D154N and D200G, previously reported in English FH patients, were not detected in this sample. In one of the patients, however, a new deletion of 7 bp was identified after nucleotide 581 (or 582) in exon 4 of the LDLR gene.

Two novel frameshift mutations in the low density lipoprotein receptor gene generated by endogenous sequence-directed mechanisms

DNA samples from 60 unrelated Belgian hypercholesterolemic patients were subjected to heteroduplex analysis of exon 4 of the low density lipoprotein receptor (LDLR) gene. Aberrant mobility bands were detected in 2 patients and the underlying mutations were characterized by DNA sequence analysis. Both mutations, a 19-bp insertion at codon 141 and a 23-bp deletion at codon 168, produce premature stop codons in the highly conserved ligand binding domain of the mature LDLR. Sequence data indicated that mispairing between short direct repeats during DNA replication is the most probable mechanism by which these mutations could have arisen. Our observations are consistent with an endogenous sequence-directed mechanism of mutagenesis.