A routine method for storing lymphocytes for repeated isolation of DNA

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

Using a simple, standardized denaturing gradient gel electrophoresis (DGGE) based mutation screening technique, a novel G-to-A mutation in the last base of the intron 12 splice acceptor site of the LDL receptor gene was found in 2 Danish families with familial hypercholesterolemia (FH). The mutation is shown to result in 2 mRNA splice variants, both leading to truncated LDLR proteins, containing only the first 594 of the normal 839 amino acids. In one of the FH-families harbouring the mutation, a striking difference in the clinical picture amongst biochemically diagnosed FH patients was clarified when genetic analysis showed that 2 hypercholesterolemic family members, who despite advanced age had no atherosclerotic disease, had not inherited the family LDLR mutation. DGGE analyses of the LDLR exons, LDLR promoter, and apolipoprotein B codon 3456-3553 as well as Southern blotting of the LDLR gene were without signs of other mutations in the non-atherosclerotic hypercholesterolemics of the family. Availability of the clinically applicable mutation screening assay for FH may thus aid in defining reasons for phenotypic differences in FH families and potentially supply information allowing a more differentiated therapeutic approach to individual members of FH families.

Allele-specific measurement of low-density lipoprotein receptor transcript levels

We have developed an assay for allele-specific determination of low-density lipoprotein receptor (LDLR) mRNAs. Transcript levels are measured by reverse transcription (RT), PCR, and electrophoresis on an automatic DNA sequencer using fluorescence-labeled primers and direct quantitation of the allele-specific RT-PCR products. The discrimination between the allelic products is based on the use of DNA polymorphisms located in the coding regions of the gene as markers for the individual alleles. Using this method on LDLR mRNA from heterozygous patients with familial hypercholesterolemia (FH) due to a defective LDLR protein, it is possible to relate the expression of the mutant allele directly to the expressed amounts of the normal allele, thus overcoming the problems of using artificial internal standards in the PCR. To validate the method we have measured (1) the range of normal LDLR allele transcript levels, and (2) the transcript levels in patients heterozygous for different types of mutant LDLR alleles associated with FH. The method is general in principle and can be applied in the allele-specific analysis of transcripts from all genes harbouring DNA polymorphisms in their coding regions.

Characterization of a disease-causing Glu119-Lys mutation in the low-density lipoprotein receptor gene in two Danish families with heterozygous familial hypercholesterolemia

Mutations in the gene for the low-density lipoprotein receptor (LDL receptor) cause the autosomal dominant inherited disease familial hypercholesterolemia (FH). In 15 Danish patients with heterozygous FH we have screened exon 4 of the LDL receptor gene for point mutations and small rearrangements employing genomic DNA amplification and bidirectional solid-phase sequencing. Two subjects were found to be heterozygous for a guanine to adenine base substitution at nucleotide position 418 of the LDL receptor cDNA. This point mutation results in an amino acid change from glutamic acid to lysine at amino acid residue 119 in the third repeat of the cysteine-rich ligand binding domain of the mature LDL receptor. Disruption of LDL receptor function by the Glu119-Lys mutation was confirmed by site-directed mutagenesis and expression in COS-7 cells. By Western blotting the mutation was found to affect the processing of the LDL receptor protein. Using flow cytometric analysis of the transfected cells a decreased binding and internalization of LDL by the mutant receptor was documented. By means of a mutation-specific PCR-based assay the Glu119-Lys mutation was not detected in another 85 apparently unrelated Danish heterozygous FH patients. We identified six persons in the index families with the Glu119-Lys mutation cosegregating with the clinical syndrome of FH in these families. Furthermore, haplotype analysis revealed that the haplotype [SfaNI+, StuI+, AvaII-, (dTA)7] of the mutation carrying allele was the same in the two apparently unrelated patients. This indicates that the mutation has been inherited from a common ancestor.