Familial combined hyperlipidaemia: under - defined and under - diagnosed?

Familial combined hyperlipidaemia (FCH) was identified in early genetic studies of populations as a dominant condition associated with mixed hyperlipidaemia and early onset coronary heart disease. Later studies extended the phenotype and noted that this genetic hyperlipidaemia was sensitive to environmental effects. This article reviews the definitions, animal models and genetics of FCH. In contrast to familial hypercholesterolaemia, which is caused by mutations in a limited number of affected genes, the genetics of FCH have remained obscure and very few definite candidate genes have been identified. A strong role for the apoA-I, A-IV, A-V, C-III cluster on chromosome 11 was identified early on and multiple associations have been found to hyperlipidaemia in this region and more strongly to adjacent sections of the chromosome. More recently quantitative trait mapping has identified a number of candidate genes including upstream transcription factor -1 (USF-1) on 1 q21 and CD-36 on chromosome 4. Of these the strongest evidence, based on 4 analyses, links the lipid components of FCH to intronic variants in the USF-1 gene on chromosome 1q21-23. Unfortunately USF-1 yet fails to show clear associations with diabetes and the metabolic syndrome which co-map to this region and are also associated with mixed hyperlipidaemia. Large scale validation of USF-1 variants in other populations is still awaited. It is likely that FCH is a heterogeneous condition, that is subject to wide-scale environmental confounding from common traits such as obesity and the metabolic syndrome, and that the resolution of its genetics is going to prove a severe challenge.