A DE-NOVO POINT MUTATION OF THE LOW-DENSITY-LIPOPROTEIN RECEPTOR GENEIN AN ITALIAN SUBJECT WITH PRIMARY HYPERCHOLESTEROLEMIA

Severe hypercholesterolemia was found in an Ii-year-old boy with no fa mily history of familial hypercholesterolemia. The reduced LDL-recepto r activity in cultured skill fibroblasts (40% I-125-LDL degradation as compared with a control cell line) indicated the presence of an LDL-r eceptor defect. The analysis of the promoter region and the exons of L DL-receptor gene by single strand conformation polymorphism revealed a n abnormal migration pattern in exon 1, which was due to a T-->A trans version at nucleotide 28 of the cDNA. This novel mutation causes an ar ginine for tryptophane substitution at position - 12 of the signal pep tide (W-12R) and introduces an AviII restriction site in exon 1. Scree ning of the mutation by polymerase chain reaction (PCR) amplification of exon 1 and AviII digestion revealed that none of the proband's fami ly members carried the mutation. Non-paternity was excluded after the analysis of a battery of 14 short tandem repeats located in 13 differe nt chromosomes. These results are consistent with the hypothesis that the proband is heterozygous for a 'de novo' mutation of the LDL-recept or gene producing a non-conservative amino acid substitution. We sugge st that the change in the net charge of the signal peptide, caused by the addition of a positively charged amino acid, impairs the co-transl ational translocation of the nascent receptor protein across the endop lasmic reticulum membrane.