Two novel frameshift mutations associated with the presence of direct repeats of the LDL receptor gene in familial hypercholesterolemia

Genetic Analysis of Japanese Children Clinically Diagnosed with Familial Hypercholesterolemia

Aim: This study aimed to elucidate the gene and lipid profiles of children clinically diagnosed with familial hypercholesterolemia (FH).

Methods: A total of 21 dyslipidemia-related Mendelian genes, including FH causative genes (LDLR,APOB, andPCSK9) and LDL-altering genes (APOE,LDLRAP1, andABCG5/8), were sequenced in 33 Japanese children (mean age, 9.7±4.2 years) with FH from 29 families.

Results: Fifteen children (45.5%) with pathogenic variants inLDLR (eight different heterozygous variants) and one child (3.0%) with thePCSK9 variant were found. Among 17 patients without FH causative gene variants, 3 children had variants in LDL-altering genes, anAPOE variant and twoABCG8 variants. The mean serum total cholesterol (280 vs 246 mg/dL), LDL-cholesterol (LDL-C, 217 vs 177 mg/dL), and non-HDL cholesterol (228 vs 188 mg/dL) levels were significantly higher in the pathogenic variant-positive group than in the variant-negative group. In the variant-positive group, 81.3% of patients had LDL-C levels ≥ 180 mg/dL but 35.3% in the variant-negative group. The mean LDL-C level was significantly lower in children with missense variants, especially with the p.Leu568Val variant, than in children with other variants inLDLR, whereas the LDL-altering variants had similar effects on the increase in serum LDL-C toLDLR p.Leu568Val.

Conclusion: Approximately half of the children clinically diagnosed with FH had pathogenic variants in FH causative genes. The serum LDL-C levels tend to be high in FH children with pathogenic variations, and the levels are by the types of variants. Genetic analysis is useful; however, further study on FH without any variants is required.

Coronary Artery Plaque Regression by a PCSK9 Antibody and Rosuvastatin in Double-heterozygous Familial Hypercholesterolemia with an LDL Receptor Mutation and a PCSK9 V4I Mutation

The low-density lipoprotein-cholesterol (LDL-C) level of a 38-year-old man diagnosed with acute coronary syndrome was 257 mg/dL. The administration of a proprotein convertase subtilisin-kexin type 9 (PCSK9) antibody in addition to rosuvastatin plus ezetimibe was initiated, reducing his LDL-C level to 37 mg/dL. A genetic analysis revealed both an LDL receptor (LDLR) mutation and a PCSK9 V4I mutation. Nine months after revascularization, intravascular ultrasound revealed plaque regression in the coronary arteries. LDLR/PCSK9 mutation carriers are prone to coronary artery disease. Intensive LDL-C lowering by including PCSK9 antibody was associated with coronary plaque regression, suggesting the expectation of prognosis improvement.

Molecular genetic analysis of familial hypercholesterolemia: spectrum and regional difference of LDL receptor gene mutations in Japanese population

To determine the molecular basis of familial hypercholesterolemia (FH) in Japan, 200 unrelated patients with clinically diagnosed heterozygous FH were screened for mutations in coding and promoter region of the low density lipoprotein (LDL) receptor gene using denaturing gradient-gel electrophoresis (DGGE), DNA sequencing and Southern blotting analysis. About 37 different mutations in the LDL receptor gene were identified in 125 (62.5%) of the patients, 22 of these mutations have not been described before. The most common mutations were K790X (19.5%), P664L (6.0%), FH-Tonami-1 (6.0%), IVS15-3C>A (5.5%) and FH-Tonami-2 (4.5%), whereas the other mutations were rare. No apolipoprotein B (apoB) mutations responsible for familial ligand-defective apoB-100 (FDB) were identified. Polymorphisms of apolipoprotein E (apoE) and scavenger receptor class B type I (SR-BI) were observed to have minor effects on the lipid and lipoprotein profile. In 75 (32.5%) of the FH patients, LDL receptor gene mutations could not be identified. These patients had significantly lower total cholesterol (7.71+/-1.64 vs. 8.68+/-1.47 mmol/l, P<0.001) and LDL-cholesterol (6.02+/-1.51 vs. 6.87+/-1.47 mmol/l, P<0.001) in plasma, also a lower incidence of coronary heart disease (CHD) (22 vs. 29%, P=0.05) compared with patients with a LDL receptor gene mutation, suggesting that besides LDL receptor, defect of other genes involved in LDL metabolism may be a cause of FH with a milder phenotypic expression in Japanese population.

Three novel small deletion mutations of the LDL receptor gene in Korean patients with familial hypercholesterolemia

The low-density lipoprotein (LDL) receptor gene from 80 unrelated Korean patients with familial hypercholesterolemia (FH) was analyzed to screen for small structural rearrangements that could not be detected by Southern blot hybridization. Three different small deletions were detected in exon 11 of 3 FH patients and were characterized by DNA sequence analysis. Of them two mutations are in-frame 36-bp (FH 2) and 9-bp (FH 34) deletions that result in the loss of twelve amino acids (from Met510 to Ile521) and three amino acids (Thr513, Asp514 and Trp515), respectively. Both mutations are located in the third of the five YWTD motifs of the LDL receptor gene. The third mutation (FH 400) is a 2-bp deletion that shifts the translational reading frame and results in a prematurely terminated receptor protein. The generation of a 36-bp deletion can be explained by the formation of a hairpin-loop structure mediated by inverted repeat sequences. On the other hand, the mechanism responsible for the 9- and the 2-bp deletions is probably strand-slippage mispairing mediated by short direct repeats. All of these three deletions are novel mutations. Each of the three deletions was detected only in a single pedigree out of 80 FH families analyzed.

In situ detection of frameshift mutations in mouse cells

A reporter gene system that allows in situ detection of cells that have suffered a specific frameshift mutation was developed. To construct the reporter gene, the open reading frame of a human placental alkaline phosphatase (PLAP) gene was disrupted by insertion of either 5 or 7 G:C basepairs, which formed mutant alleles carrying 9 and 11 consecutive G residues, respectively. The mutant PLAP genes did not produce alkaline phosphatase activity in cultured mouse cells in transient transfection assays. Several cell lines that contained integrated copies of the mutant PLAP genes were made. Histochemical staining of fixed cells showed that these cell lines contained a small number of cells that expressed PLAP activity and bound antibodies directed against PLAP. Cells carrying the allele with 11 consecutive G residues (G11 allele) acquired PLAP activity at a rate between 2 x 10(-3) and 2 x 10(-4) events per cell per generation, depending on the cell line. Cells carrying the allele with 9 consecutive G residues (G09 allele) acquired PLAP activity at a rate between 2 x 10(-5) and 2 x 10(-6) events per cell per generation, depending on the cell line. Cultures of PLAP+ cells were derived from cell lines carrying PLAP mutant genes. All the cells in these cultures had PLAP activity and bound anti-PLAP antibody. PLAP mRNA levels were the same in cultures where all cells were PLAP+ and in cultures where less than 1% of the cells expressed PLAP activity. DNA sequence analysis of PLAP+ cells showed that the G11 allele reverted by losing one basepair, and the G09 allele reverted by gaining one basepair.

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.

Two novel partial deletions of LDL-receptor gene in Italian patients with familial hypercholesterolemia (FH Siracusa and FH Reggio Emilia)

In the present study we report two novel partial deletions of the LDL-R gene. The first (FH Siracusa), found in an FH-heterozygote, consists of a 20 kb deletion spanning from the 5' flanking region to the intron 2 of the LDL-receptor gene. The elimination of the promoter and the first two exons prevents the transcription of the deleted allele, as shown by Northern blot analysis of LDL-R mRNA isolated from the proband's fibroblasts. The second deletion (FH Reggio Emilia), which eliminates 11 nucleotides of exon 10, was also found in an FH heterozygote. The characterization of this deletion was made possible by a combination of techniques such as single strand conformation polymorphism (SSCP) analysis, direct sequence of exon 10 and cloning of the normal and deleted exon 10 from the proband's DNA. The 11 nt deletion occurs in a region of exon 10 which contains three triplets (CTG) and two four-nucleotides (CTGG) direct repeats. This structural feature might render this region more susceptible to a slipped mispairing during DNA duplication. Since this deletion causes a shift of the BamHI site at the 5' end of exon 10, a method has been devised for its rapid screening which is based on the PCR amplification of exon 10 followed by BamHI digestion. FH Reggio Emilia deletion produces a shift in the reading frame downstream from Lys458, leading to a sequence of 51 novel amino acids before the occurrence of a premature stop codon (truncated receptor). However, since RT-PCR failed to demonstrate the presence of the mutant LDL-R mRNA in proband fibroblasts, it is likely that the amount of truncated receptor produced in these cells is negligible.