A genetic determinant of the phenotypic variance of the molecular weight of low density lipoprotein

Abstract

The molecular weight of monodisperse human plasma low densitylipoprotein has been measured in 69 individuals and found to vary over the range of 2.4 to 3.9 X 10-6. By contrast, the molecular weight of low density lipoprotein measured on two separate occasions for specific individuals shows a mean difference of 0.07 X 10-6 and a standard deviation of 0.08 X 10-6; hence low density lipoprotein differing in molecular weight by greater than 0.2 X 10-6 may be considered different macomolecules. The distribution of the molecular weight of low density lipoprotein does not differ as a function of age or sex. Hyperlipemic subjects having monodisperse low density lipoprotein show similar molecular weight distribution to normal subjects, as do subjects with premature coronary artery disease. Family studies reveal a correlation coefficient of 0.82 between average molecular weights of parents and offspring, with significance at 0.01. In order to assess the influence of environment on molecular weight of low density lipoprotein, the correlation coefficient between the fathers' and mothers' low density lipoprotein was measured and no statistically significant correlation was found. These data are interpreted as strong evidence for a genetic determination of molecular weight of low density lipoprotein. A study of individuals in five families yields molecular weight data consistent with a single gene locus genetic mode of inheritance without dominance. The regression coefficient of the mean low denisty lipoprotein parental molecular weight on the offspring molecular weight is 0.30. If the variability of molecular weight is considered as an expression of phenotypic variance, then the regression analysis identified 30% of this phenotypic variance as arising from additive gene action presumably at a single locus. Segregation in the family data is consistent. Since the differences in molecular weight of low density lipoprotein arise from differences in the amount of lipid bound to the apoprotein, it is likely that an additional portion of the phenotypic variance of the molecular weight results from individual variations in the metabolism of low density lipoprotein, which yield differences in lipid content. The individual variation in molecular weight is only approximately 5%; hence those metabolic sequences that influence molecular weight of low density lipoproteins must be precisely controlled.