CHARACTERIZATION OF A SPLICE-SITE MUTATION IN THE GENE FOR THE LDL RECEPTOR-ASSOCIATED WITH AN UNPREDICTABLY SEVERE CLINICAL PHENOTYPE IN ENGLISH PATIENTS WITH HETEROZYGOUS FH

We have identified a substitution of G to A in the first base pair of intron 3 in the LDL receptor gene of an English heterozygous familial hypercholesterolemia (FH) patient. Reverse transcription, amplificatio n, and nucleotide sequencing of the LDL receptor mRNA from mononuclear blood cells showed both the normal mRNA and one that lacked the nucle otides encoded by exon 3, which codes for repeat 2 of the ligand-bindi ng domain. The same mutant allele was identified in 2/200 unrelated FH patients from the London area and 4/77 from Manchester. Immunoblottin g of cultured lymphoblasts from the index patient revealed the normal receptor protein and smaller amounts of a receptor protein with electr ophoretic mobility consistent with a deletion of the 41 amino acid res idues encoded by exon 3. Normal amounts of a similar protein were obse rved when the mutant cDNA was expressed in heterologous cells; this pr otein showed reduced binding affinity for LDL but bound apoprotein E-c ontaining lipoproteins normally. Despite these and other observations that repeat 2 of the binding domain is relatively unimportant for rece ptor function in vitro, carriers of this allele exhibit a severe clini cal phenotype, typical of FH. Thus, the relationship between genotype and phenotype in heterozygous FH is not always predictable.