An apparent inconsistency in parent to offspring transmission of point mutations of LDLR gene in familial hypercholesterolemia.

BACKGROUND: Familial Hypercholesterolemia (FH), the most common form of autosomal co-dominant hypercholesterolemia, is due to mutations in the LDLR gene, mostly minute or point mutations in the coding sequence. METHODS: Analysis of LDLR gene was performed by direct resequencing and multiplex ligation-dependent probe amplification (MLPA). RESULTS: LDLR gene resequencing showed that proband I.G., with the clinical diagnosis of homozygous FH, was homozygous for a mutation in exon 12 (c.1775 G>A, G571E) known to be pathogenic, and heterozygous for a mutation in intron 14 (c.2140 +5G>A). Proband's daughter with heterozygous FH carried only the intron 14 mutation. To explain this inconsistency we assumed that the proband was a carrier of a gene deletion. MLPA showed that the proband and her daughter were heterozygous for a deletion of exons 11 and 12. This explains the apparent homozygosity of the c.1175 G>A mutation in the proband. Ex 11-12 deletion was linked to the c.2140 +5G>A mutation. Other FH patients, heterozygotes for c.2140 +5G>A, were found to carry the Ex 11-12 deletion found in the proband or other pathogenic mutations. CONCLUSIONS: Inconsistencies in the parent to offspring transmission of point mutations in LDLR gene may be due to a large deletion not detected by resequencing.

Functional analysis of two novel splice site mutations of APOB gene in familial hypobetalipoproteinemia.

Familial hypobetalipoproteinemia (FHBL) is a co-dominant disorder characterized by reduced plasma levels of low density lipoprotein cholesterol (LDL-C) and its protein constituent apolipoprotein B (apoB), which may be due to mutations in APOB gene, mostly located in the coding region of this gene. We report two novel APOB gene mutations involving the acceptor splice site of intron 11 (c.1471-1G>A) and of intron 23 (c.3697-1G>C), respectively, which were identified in two patients with heterozygous FHBL associated with severe fatty liver disease. The effects of these mutations on APOB pre-mRNA splicing were assessed in COS-1 cells expressing the mutant APOB minigenes. The c.1471-1G>A APOB minigene generated two abnormal mRNAs. In one mRNA the entire intron 11 was retained; in the other mRNA exon 11 joined to exon 12, in which the first nucleotide was deleted due to the activation of a novel acceptor splice site. The predicted products of these mRNAs are truncated proteins of 546 and 474 amino acids, designated apoB-12.03 and apoB-10.45, respectively. The c.3697-1G>C APOB minigene generated a single abnormal mRNA in which exon 23 joined to exon 25, with the complete skipping of exon 24. This abnormal mRNA is predicted to encode a truncated protein of 1220 amino acids, designated apoB-26.89. These splice site mutations cause the formation of short truncated apoBs, which are not secreted into the plasma as lipoprotein constituents. This secretion defect is the major cause of severe fatty liver observed in carriers of these mutations.