Editor—One of the most common single site mutations in the human ApoB gene, R3500Q, results in mild to severe hypercholesterolaemia and an increased risk for early onset atherosclerosis.1 Intensive mutation screening studies of the ApoB gene in subjects with hypercholesterolaemia have identified a few less frequent variants, associated with an even milder phenotype and localised within a region coding for the ApoB-100 receptor binding domain.2 3

It is widely accepted that the average population frequency of the R3500Q mutation is about 1:500-1:700 among white populations.4 However, results from population studies, as well as data from studies of high risk groups in Europe, show that frequencies vary to a large extent among population groups, ranging between 1:715 and 1:1250.6 The simultaneous occurrence of the underlying G to A substitution and a rareApoB haplotype across different ethnic groups favours the hypothesis of a common origin of the R3500Q mutation and its further migration spread.7 Population groups with the highest frequencies are clustered in central Europe, and the mutation frequency decreases as one moves east, north, and south west,5 6 8-22 becoming extremely rare in Finland, southern Italy, and Spain.23-25 There is a lack of information about the mutation prevalence in the south east of central Europe, with the exception of one study on hypercholesterolaemic subjects from the fringes of Europe (Turkey) that failed to detect the R3500Q mutation.26 This lack of information about a sizeable region of Europe flanked by populations with high (Austria18 and Hungary19) and extremely low prevalence (Turkey26) leaves open the question of a frequency gradient to the south east.

This study of the occurrence of the R3500Q mutation and its associated haplotype(s) in subjects with hypercholesterolaemia from Bulgaria aimed to obtain data from a …