Splice-site mutation c.313+1, G>A in intron 3 of the LDL receptor gene results in transcripts with skipping of exon 3 and inclusion of intron 3.

Background: Familial hypercholesterolemia (FH) patients with the splice site mutation c.313+1, G>A in intron 3 of the low density lipoprotein receptor (LDLR) gene, present with a phenotype similar to that of FH patients in general. However, a mild phenotype would have been expected from the published data showing that the mutation only causes skipping of exon 3.
Methods: Epstein Barr virus-transformed lymphocytes from eight c.313+1, G>A heterozygotes and two c.313+1, G>A homozygotes were subjected to studies of the LDLR at the mRNA and protein levels.
Results: Mutation c.313+1, G>A not only causes skipping of exon 3, but also causes inclusion of intron 3. No functional LDLR was produced from the transcript with inclusion of intron 3. The transcript with skipping of exon 3 produced a receptor which had markedly reduced ability to internalize low density lipoprotein.
Conclusion: The findings that the mutation c.313+1, G>A in the LDLR gene also generates a mutant transcript with inclusion of intron 3, explains why the mutation c.313+1, G>A may result in a severe phenotype.