Detection and characterization of a novel splice mutation in the LDL receptor intron 12 resulting in two different mutant mRNA variants

Mutant transcripts of the LDL receptor gene: mRNA structure and quantity

mRNA of the low-density lipoprotein receptor (LDLR) gene from 22 heterozygous familial hypercholesterolemic subjects possessing different mutations in this gene was analyzed by Northern blot analysis and reverse transcription-polymerase chain reaction (RT-PCR) in order to detect abnormally spliced transcripts. These analyses revealed abnormally spliced transcripts for the two splice-site mutations 1359-1G-->A and 1705 + 1G-->T. The abnormally spliced transcript for mutation 1359-1G-->A was caused by activation of a cryptic acceptor splice site in exon 10. As a result, seven nucleotides of exon 10 were deleted. For mutation 1705 + 1G-->T, two mutant transcripts were observed. In the first transcript, exon 10 was spliced to exon 13, and in the second transcript intron 11 was retained. The relative amount of mutant transcripts from 14 of the 22 subjects was determined by use of an RT-PCR-based method. Quantitation of the relative amounts of mutant transcripts for five missense mutations resulted in a mean value (+/-SD) of 52.8% (+/-4.55). In comparison, quantitation of the relative amounts of mutant transcripts for five nonsense mutations resulted in a mean value of 31.8% (+/-6.91). This value was significantly lower than the value of 54.2% (+/-2.38) obtained for nine healthy subjects (P < 0.0001). The relative amount of mutant transcripts for the 1705 + 1G-->T mutation was 36%. Thus, transcripts from alleles containing premature stop codons are present in reduced amounts, whereas transcripts from alleles containing missense mutations are present in normal amounts. These findings underscore the importance of determining how mutations affect mRNA structure and quantity in order to understand how mutations cause disease.

Effects of a 9.6-kb deletion of the LDL receptor gene (FH Helsinki) on structure and levels of mRNA

FH Helsinki is a deletion of the low-density lipoprotein receptor (LDLR) gene that deletes 9.6 kb from intron 15 to exon 18. Screening for mutant transcripts by Northern blot analysis from a patient heterozygous for FH Helsinki revealed two mutant transcripts. One was a transcript where the proximal part of intron 15 was retained in mRNA. The second was a transcript where exon 15 was spliced to nucleotide 4186 of exon 18. Thus, this transcript was generated using the normal donor splice site in intron 15, and a cryptic AG acceptor splice site in exon 18. Translation of the two mutant transcripts is predicted to give nonfunctional proteins, as both the membrane-spanning domain and the cytoplasmic domain of the receptor are deleted. Scanning of the autoradiograms showed that the amounts of each of the two mutant transcripts were approximately 10 times higher than that of the normal transcript in our heterozygous patient. The finding of higher levels of mutant transcripts was confirmed by an allele-specific transcript quantitation method, in which the amount of the two mutant transcripts together was approximately 5 times higher than the amount of the normal transcript. Deletion of destabilizing elements (AU-rich elements) by FH Helsinki are proposed to cause the increased levels of mutant transcripts.

Analysis of LDL receptor gene mutations in Italian patients with homozygous familial hypercholesterolemia

The aim of this study was the characterization of mutations of the LDL receptor gene in 39 Italian patients with homozygous familial hypercholesterolemia, who were examined during the period 1994 to 1996. The age of the patients ranged from 1 to 64 years; one third of them were older than 30. Plasma LDL cholesterol level ranged from 10.8 to 25.1 mmol/L. The residual LDL receptor activity, measured in cultured fibroblasts of 32 patients, varied from <2% to 30% of normal and was inversely correlated with the plasma LDL cholesterol level (r=-0.665; P<0.003). The most severe coronary atherosclerosis was observed in those patients with the lowest residual LDL receptor activity (</=5% of normal) and the highest plasma LDL cholesterol levels. Twenty-nine patients (23 of whom were unrelated) were found to be homozygotes at the LDL receptor locus. In this group we discovered 2 major rearrangements and 12 different point mutations (9 in the coding region and 3 in splice sites). Some mutations (D200G, C358R, V502M, G528D, and P664L) were found in 3 or more unrelated patients. Patients with the same mutation shared the same haplotype at the LDL receptor gene locus and came from the same geographic area. Ten patients (9 of whom were unrelated) were found to be compound heterozygotes. The mutations found in this group consisted of one large deletion and 12 point mutations (11 in the coding sequence and one in a splice site). In 3 compound heterozygotes we failed to identify the second mutant allele at the LDL receptor locus. These observations confirm the allelic heterogeneity underlying familial hypercholesterolemia in the Italian population and indicate that the variability of phenotypic expression of homozygous familial hypercholesterolemia is, to a large extent, related to the type of mutation of the LDL receptor gene.

Identification of a common low density lipoprotein receptor mutation (C163Y) in the west of Scotland

Familial hypercholesterolaemia (FH) is an autosomal codominant disorder characterised by high levels of LDL cholesterol and a high incidence of coronary artery disease. Our aims were to track the low density lipoprotein receptor (LDLR) gene in individual families with phenotypic FH and to identify and characterise any mutations of the LDLR gene that may be common in the west of Scotland FH population using single strand conformational polymorphism analysis (SSCP). Patient samples consisted of 80 heterozygous probands with FH, 200 subjects who were related to the probands, and a further 50 normal, unrelated control subjects. Tracking of the LDLR gene was accomplished by amplification of a 19 allele tetranucleotide microsatellite that is tightly linked to the LDLR gene locus. Primers specific for exon 4 of the LDLR gene were used to amplify genomic DNA and used for SSCP analysis. Any PCR products with different migration patterns as assessed by SSCP were then sequenced directly. In addition to identifying probands with a common mutation, family members were screened using a forced restriction site assay and analysed using microplate array diagonal gel electrophoresis (MADGE). Microsatellite D19S394 analysis was informative in 20 of 23 families studied. In these families there was no inconsistency with segregation of the FH phenotype with the LDLR locus. Of the FH probands, 15/80 had a mutant allele as assessed by SSCP using three pairs of primers covering the whole of exon 4 of the LDLR gene. Direct DNA sequencing showed that 7/15 of the probands had a C163Y mutation. Using a PCR induced restriction site assay for the enzyme RsaI and MADGE, it was determined that the C163Y mutation cosegregated with the FH phenotype in family members of the FH probands. This mutant allele was not present in any of the control subjects. Microsatellite analysis has proven useful in tracking the LDLR gene and could be used in conjunction with LDL cholesterol levels to diagnose FH, especially in children and young adults where phenotypic diagnosis can be difficult.

Evaluation of a clinically applicable mutation screening technique for genetic diagnosis of familial hypercholesterolemia and familial defective apolipoprotein B

We have recently developed a simple mutation screening assay based on the denaturing gradient gel electrophoresis (DGGE) technique for detection of mutations in the coding and regulatory regions of the low density lipoprotein receptor (LDLR) gene and the codon 3500 region of the apolipoprotein (apo) B-100 gene leading to familial hypercholesterolemia (FH) and familial defective apo B-100 (FDB), respectively. To evaluate the assay, 14 Danish families suspected of FH were studied. In ten families, the DGGE assay detected seven different point mutations, including mutations undescribed prior to establishing the assay. In addition, in one of these ten families and in one of the remaining four families, Southern blotting detected the FH-DK3 exon 5 deletion. Based on segregation analysis and clinical data, the FH diagnosis was dubious in the remaining three families without DGGE or Southern blotting detectable mutations. In conclusion, a simple DGGE based mutation screening assay may detect underlying mutations in most FH/FDB families, thus allowing its routine use in genetic counselling of FH-families.

Update on low density lipoprotein receptor mutations

Recent research has focused on the rapid detection of new LDL receptor gene variants and large scale screening for known mutations. Whether the nature of the mutation in the LDL receptor gene in familial hypercholesterolaemia determines clinical variability has been examined, as well as the potential value of detecting mutation carriers for clinical practice. There is also evidence that some patients with clinical familial hypercholesterolaemia do not have detectable defects in the LDL receptor or apolipoprotein B.