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Two point mutations were discovered in the low-density lipoprotein genes of patients with familial hypercholesterolaemia (FH). Defective genes were cloned and/or amplified by the polymerase chain reaction (PCR) method and the DNA sequences determined. A guanine to adenine base transition in exon 4 was found to be the molecular defect in 20% of cases of FH in the Afrikaner population. A second mutation, a guanine to adenine base substitution in exon 9, was identified in two homozygous FH individuals. Restriction enzyme analysis of PCR-amplified DNA from blood and tissue samples now permits accurate diagnosis of these mutations.

A new restriction fragment length polymorphism (RFLP) in the low-density lipoprotein receptor gene is described using the Stu I restriction endonuclease and a cDNA probe. The frequency of the two RFLP alleles was determined in 60 unrelated white subjects and 11.7% of them were found to be heterozygous for the polymorphism. Mendelian segregation of the RFLP was found in 3 informative families. The possible use of the RFLP in the diagnosis of familial hypercholesterolaemia in South Africa is discussed.

The prevalence of familial hypercholesterolaemia (FH), an autosomal dominant disease characterised by raised low-density lipoprotein (LDL) cholesterol levels, is at least five times higher in the white Afrikaner population than in most other population groups in the world. A founder gene effect has been suggested to explain this abnormally high frequency. Detection of a polymorphic Stu I site in the 5' region of the LDL receptor gene and association of both restriction fragment length polymorphism alleles with FH in Afrikaners, indicated the existence of at least two founder members for the disease in this population. DNA from a hetero-allelic FH homozygote from this South African group has been analysed through genomic cloning and sequencing. The DNA polymorphic site is caused by a single guanine to adenine transition within exon 8 of the LDL receptor gene and can be used in the determination of haplotype-associated defects.

Ten restriction fragment length polymorphisms of the LDL receptor gene were used for haplotype analysis in 12 unrelated patients with homozygous familial hypercholesterolemia. These patients were drawn from the Black, Coloured, and White population groups and collectively represent 24 mutant alleles underlying the FH phenotype. Five distinct haplotypes were detected. Hybridization analysis using DNA codigested with EcoRI and PstI revealed that haplotype IV was associated with two distinct mutations. When coupled to the recent demonstration by other workers of two receptor defects in South African Afrikaners homozygous for FH and haplotype I, these data are suggestive of at least seven distinct LDL receptor mutations in the FH patients examined and thus in the general South African population.