Objective: Combined hyperlipidemia is a common disorder, characterized by a highly atherogenic lipoprotein profile and a substantially increased risk of coronary heart disease. The purpose of this study was to establish whether variations of apolipoprotein A5 (APOA5), a newly discovered gene of lipid metabolism located 30 kbp downstream of the APOA1/C3/A4 gene cluster, contributes to the transmission of familial combined hyperlipidemia (FCHL).
Methods and results: We performed linkage and association tests on 128 families. Two independent alleles, APOA5c.56G and APOC3c.386G, of the APOA1/C3/A4/A5 gene cluster were overtransmitted in FCHL (P=0.004 and 0.007, respectively). This was paired with reduced transmission of the common APOA1/C3/A4/A5 haplotype (frequency 0.4461) to affected subjects (P=0.012). The APOA5c.56G genotype accounted for 7.3% to 13.8% of the variance in plasma triglyceride levels in probands (P<0.004). The APOC3c.386G genotypes accounted for 4.4% to 5.1% of the variance in triglyceride levels in FCHL spouses (P<0.007), suggesting that this allele marks a FCHL quantitative trait as well as representing a susceptibility locus for the condition.
Conclusions: A combined linkage and association analysis establishes that variation at the APOA1/C3/A4/A5 gene cluster contributes to FCHL transmission in a substantial proportion of northern European families.