Two novel missense mutations in the LDL receptor gene causing familial hypercholesterolemia

Prevalence of cholesteryl ester storage disease among hypercholesterolemic subjects and functional characterization of mutations in the lysosomal acid lipase gene

Lysosomal acid lipase hydrolyzes cholesteryl esters and triglycerides contained in low density lipoprotein. Patients who are homozygous or compound heterozygous for mutations in the lysosomal acid lipase gene (LIPA), and have some residual enzymatic activity, have cholesteryl ester storage disease. One of the clinical features of this disease is hypercholesterolemia. Thus, patients with hypercholesterolemia who do not carry a mutation as a cause of autosomal dominant hypercholesterolemia, may actually have cholesteryl ester storage disease. In this study we have performed DNA sequencing of LIPA in 3027 hypercholesterolemic patients who did not carry a mutation as a cause of autosomal dominant hypercholesterolemia. Functional analyses of possibly pathogenic mutations and of all mutations in LIPA listed in The Human Genome Mutation Database were performed to determine the pathogenicity of these mutations. For these studies, HeLa T-REx cells were transiently transfected with mutant LIPA plasmids and Western blot analysis of cell lysates was performed to determine if the mutants were synthesized in a normal fashion. The enzymatic activity of the mutants was determined in lysates of the transfected cells using 4-methylumbelliferone-palmitate as the substrate. A total of 41 mutations in LIPA were studied, of which 32 mutations were considered pathogenic by having an enzymatic activity <10% of normal. However, none of the 3027 hypercholesterolemic patients were homozygous or compound heterozygous for a pathogenic mutation. Thus, cholesteryl ester storage disease must be a very rare cause of hypercholesterolemia in Norway.

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.

Two novel slavic point mutations in the low-density lipoprotein receptor gene in patients with familial hypercholesterolemia from St. Petersburg, Russia

Using PCR-single-strand conformation polymorphism analysis, followed by sequencing of the abnormal samples, two novel point mutations in the 5' end of the fourth exon of the low-density lipoprotein receptor gene were found in two Russian families with familial hypercholesterolemia. These missense mutations consist of C127W and C139G transitions and result in a loss of one of three disulfide bonds in the fourth cysteine-rich repeat of the ligand-binding domain of the low-density lipoprotein receptor. Hypercholesterolemia segregated with the identified mutations.

Software and database for the analysis of mutations in the human LDL receptor gene

The low-density lipoprotein receptor (LDLr) plays a pivotal role in cholesterol homeostasis. Mutations in the LDLr gene (LDLR), which is located on chromosome 19, cause familial hypercholesterolemia (FH), an autosomal dominant disorder characterized by severe hypercholesterolemia associated with premature coronary atherosclerosis. To date almost 300 mutations have been identified in the LDLR gene. To facilitate the mutational analysis of the LDLR gene, and promote the analysis of the relationship between genotype and phenotype, a software package along with a computerized database (currently listing 210 entries) have been created.