GENETIC-VARIATION AT A SPLICING BRANCH POINT IN INTRON-9 OF THE LOW-DENSITY-LIPOPROTEIN (LDL)-RECEPTOR GENE - A RARE MUTATION THAT DISRUPTSMESSENGER-RNA SPLICING IN A PATIENT WITH FAMILIAL HYPERCHOLESTEROLEMIA AND A COMMON POLYMORPHISM

Mutations in the coding sequence, splice junctions or promoter of the gene for the low density lipoprotein (LDL) receptor are known to be th e underlying cause of familial hypercholesterolaemia (FH), but mutatio ns of this type cannot be identified in all patients with a clinical d iagnosis of FH, We show here that minor sequence changes elsewhere in introns can be deleterious, A minor rearrangement 30 bp upstream from the junction of intron 9 with exon 10 was detected as a heteroduplex i n amplified genomic DNA from one out of 300 heterozygous FH patients, The mutation destroys the only consensus sequence for a splicing branc h point in intron 9 and analysis of mRNA from cells from the patient s howed that it causes retention of intron 9 or, more rarely, in the use of cryptic splice sites in exon 10. The effect of the mutation on mRN A splicing was confirmed by analysis of mRNA in cells transfected with LDL-receptor mini-gene constructs expressing exons 9 and 10, together with the normal or mutant intron 9. A common C/T polymorphism within this branch point in intron 9 of the LDL-receptor gene does not affect mRNA splicing in vitro and is not associated with significant differe nces in mean plasma cholesterol concentration in a healthy population.