Characterisation of the lipoprotein structure in the St. Thomas' Mixed Hyperlipidaemic (SMHL) rabbit.

Familial combined hyperlipidaemia (FCHL) is a complex genetic disorder of unknown aetiology. Study of this human condition over many decades has been hampered by likely genetic heterogeneity. In order to find better phenotypic markers, we have characterised the structures of VLDL, IDL and LDL in the St. Thomas' Mixed Hyperlipidaemic (SMHL) rabbit--an animal model of FCHL in which the hyperlipidaemia is caused primarily by an increased production rate of apolipoprotein B (apoB)--containing lipoproteins-and compared them with those in the Watanabe Heritable Hyperlipidaemic (WHHL) rabbit, in which hyperlipidaemia is caused mainly by a defect in lipoprotein clearance, and those in the normolipidaemic New Zealand White (NZW) animal. All three rabbit strains were fed a cholesterol-enriched (0.08%, w/w) diet for at least 3 months prior to blood sampling. Both SMHL and WHHL rabbits showed combined hyperlipidaemia as evidenced by significantly increased levels of plasma cholesterol and triglycerides. Raised plasma lipids in the SMHL rabbit were attributable mainly to an overabundance of lipoprotein particles with the same lipid composition as those in NZW rabbits. VLDL and IDL in the SMHL rabbit showed a significantly increased sphingomyelin to phosphatidyl choline ratio. In the WHHL rabbit there was a high concentration of particles that were significantly enriched in cholesteryl esters and depleted in triglycerides. Phospholipids in all lipoprotein fractions from WHHL rabbits contained significantly more sphingomyelin and less phosphatidyl choline resulting in a significantly increased sphingomyelin to phosphatidyl choline ratio. We found that the VLDL of SMHL rabbits could be distinguished from that of NZW rabbits on the basis of the cholesterol:apoB and the sphingomyelin:phosphatidylcholine ratios, and from that of WHHL rabbits by the sphingomyelin:triglyceride ratio. Extrapolating these findings to the human condition, an assessment of particle core composition, together with the proportion of sphingomyelin in phospholipids especially in VLDL might help in the differentiation of the combined hyperlipidaemia of FCHL into disorders of lipoprotein overproduction versus decreased clearance.