Host and environmental effects on plasma apolipoprotein B

Serum lipids, lipoproteins, and risk of breast cancer: a nested case-control study using multiple time points

BACKGROUND

There is strong evidence that breast cancer risk is influenced by environmental factors. Blood lipid and lipoprotein levels are also influenced by environmental factors and are associated with some breast cancer risk factors. We examined whether serial measures of serum lipids and lipoproteins were associated with breast cancer risk.

METHODS

We carried out a nested case-control study within a randomized long-term dietary intervention trial with 4690 women with extensive mammographic density followed for an average of 10 years for breast cancer incidence. We measured lipids in an average of 4.2 blood samples for 279 invasive breast cancer case subjects and 558 matched control subjects. We calculated subaverages of lipids for each subject based on menopausal status and use of hormone replacement therapy (HRT) at blood collection and analyzed their association with breast cancer using generalized estimating equations. All statistical tests were two-sided.

RESULTS

High-density lipoprotein-cholesterol (HDL-C) (P = .05) and apoA1 (P = .02) levels were positively associated with breast cancer risk (75(th) vs 25(th) percentile: HDL-C, 23% higher; apoA1, 28% higher) and non-HDL-C (P = .03) and apoB (P = .01) levels were negatively associated (75(th) vs 25(th) percentile: non-HDL-C, 19% lower; apoB, 22% lower). These associations were observed only when lipids were measured when HRT was not used. Total cholesterol and triglyceride levels were not statistically significantly associated with breast cancer risk.

CONCLUSIONS

These results demonstrate that serum lipids are associated with breast cancer risk in women with extensive mammographic density. The possibility that interventions for heart disease prevention, which aim to reduce non-HDL-C or raise HDL-C, may have effects on breast cancer risk merits examination.

Genome scan for quantitative trait loci influencing HDL levels: evidence for multilocus inheritance in familial combined hyperlipidemia

Several genome scans in search of high-density lipoprotein (HDL) quantitative trait loci (QTLs) have been performed. However, to date the actual identification of genes implicated in the regulation of common forms of HDL abnormalities remains unsuccessful. This may be due, in part, to the oligogenic and multivariate nature of HDL regulation, and potentially, pleiotropy affecting HDL and other lipid-related traits. Using a Bayesian Markov Chain Monte Carlo (MCMC) approach, we recently provided evidence of linkage of HDL level variation to the APOA1-C3-A4-A5 gene complex, in familial combined hyperlipidemia pedigrees, with an estimated number of two to three large QTLs remaining to be identified. We also presented results consistent with pleiotropy affecting HDL and triglycerides at the APOA1-C3-A4-A5 gene complex. Here we use the same MCMC analytic strategy, which allows for oligogenic trait models, as well as simultaneous incorporation of covariates, in the context of multipoint analysis. We now present results from a genome scan in search for the additional HDL QTLs in these pedigrees. We provide evidence of linkage for additional HDL QTLs on chromosomes 3p14 and 13q32, with results on chromosome 3 further supported by maximum parametric and variance component LOD scores of 3.0 and 2.6, respectively. Weaker evidence of linkage was also obtained for 7q32, 12q12, 14q31-32 and 16q23-24.

Evidence against linkage of familial combined hyperlipidemia to the apolipoprotein AI-CIII-AIV gene complex

Familial combined hyperlipidemia (FCHL) was originally described as a disorder characterized by elevated levels of either plasma cholesterol or triglyceride (TG) or both in members ofthe same family. More recent studies have indicated that apolipoprotein B levels (apoB) are also elevated in these individuals. Although a dominant mode of inheritance was originally proposed, recent studies have questioned this simple mode of inheritance, and the genetic basis of the disorder has eluded investigators. A study that reported evidence that FCHL is linked to the apolipoprotein AI-CIII-AIV region on chromosome 11 is therefore of interest. We have attempted to replicate this finding in three large, well-characterized FCHL kindreds by using a highly polymorphic marker in the apoCIII gene. Using the same definitions and parameters as were used in the initial report, we obtained strong evidence against linkage of FCHL to the apolipoprotein AI-CIII-AIV region on chromosome 11 (combined lod score of -7.87 at 0% recombination). Two other models, one based on total cholesterol (TC) levels alone and one based on the joint distribution of TC and apoB levels, also gave evidence against linkage of FCHL to this region (lod scores at 0% recombination of -8.95 and -2.58, respectively). An additional regression-based linkage analysis also gave no support for the existence of a locus in this region that influences these lipid levels in these pedigrees. Explanations for the differences in results between these studies include genetic heterogeneity, differences in clinical phenotype used to select the pedigrees, and ascertainment bias.

Genetic predictors of FCHL in four large pedigrees. Influence of ApoB level major locus predicted genotype and LDL subclass phenotype

The genetic basis of familial combined hyperlipidemia (FCHL) has eluded investigators for 20 years, despite the apparent segregation of FCHL as an autosomal dominant disorder affecting 1% to 2% of individuals. Etiologic heterogeneity and additive effects of traits controlled by other genetic loci have been suggested. Two traits have been implicated in FCHL. The first is the predominance of a small, dense low-density lipoprotein (LDL), LDL subclass phenotype B, which segregates as a mendelian trait. The second is a mendelian locus with large effects on apolipoprotein (apo) B levels that is defined by complex segregation analysis (predicted apoB level genotype). This study shows that these factors appear to be separate genetic effects, both of which aid in the prediction of FCHL in four large pedigrees. The results suggest that FCHL may be best predicted by a threshold model in which apoB level genotype and LDL subclass phenotype each act to increase the risk of FCHL. Heterogeneity in the transmission of apoB levels among families is suggested, supporting the etiologic heterogeneity of FCHL. These results emphasize the advantages inherent in the study of large pedigrees when disease heterogeneity is suspected.