Lawrence Berkeley National Laboratory

Combined hyperlipidemia (CHL) is a common disorder of lipid metabolism that leads to an increased risk of cardiovascular disease. The lipid profile of CHL is characterised by high levels of atherogenic lipoproteins and low levels of high-density-lipoprotein-cholesterol. Apolipoprotein (APO) A5 is a newly discovered gene involved in lipid metabolism located within 30kbp of the APOA1/C3/A4 gene cluster. Previous studies have indicated that sequence variants in this cluster are associated with increased plasma lipid levels. To establish whether variation at the APOA5 gene contributes to the transmission of CHL, we performed linkage and linkage disequilibrium (LD) tests on a large cohort of families (n=128) with familial CHL (FCHL). The linkage data produced evidence for linkage of the APOA1/C3/A4/A5 genomic interval to FCHL (NPL = 1.7, P = 0.042). The LD studies substantiated these data. Two independent rare alleles, APOA5c.56G and APOC3c.386G of this gene cluster were over-transmitted in FCHL (P = 0.004 and 0.007, respectively), and this was associated with a reduced transmission of the most common APOA1/C3/A4/A5 haplotype (frequency 0.4425) to affected subjects (P = 0.013). The APOA5c.56G allele was associated with increased plasma triglyceride levels in FCHL probands, whereas the second, and independent, APOC3c.386G allele was associated with increased plasma triglyceride levels in FCHL pedigree founders. Thus, this allele (or an allele in LD) may mark a quantitative trait associated with FCHL, as well as representing a disease susceptibility locus for the condition. This study establishes that sequence variation in the APOA1/C3/A4/A5 gene cluster contributes to the transmission of FCHL in a substantial proportion of affected families, and that these sequence variants may also contribute to the lipid abnormalities of the metabolic syndrome, which is present in up to 40 percent of persons with cardiovascular disease.