Apolipoprotein E affects serial changes in total and low-density lipoprotein cholesterol in adolescent girls: Project HeartBeat!

Effects of apolipoprotein E genotype on blood cholesterol in adolescent girls

BACKGROUND

Few investigations have examined whether associations between the apolipoprotein E genotype (apo E) and total cholesterol or LDL-C are modified or explained by other characteristics. The objective of this study was to explore effects of behavioral characteristics, physical growth, body composition, sexual maturation, and endocrine function on age trajectories of total cholesterol and LDL-C by apo E in adolescent girls.

METHODS

Participants were 247 Caucasian adolescent girls followed for 4 years. Apo E genotyping and plasma lipid concentrations were determined from fasting blood samples using standard enzymatic methods. Age; gender; fat-free mass (FFM); BMI; percent body fat (PBF); sexual maturation (pubic hair, Tanner Stages 1-5); estradiol concentration (EST); energy intake; and physical activity were collected or calculated with standard methods.

RESULTS

In models including the proposed explanatory variables, apo E genotype remained strongly associated with total cholesterol and LDL-C. Girls with the epsilon (epsilon)3/3 and epsilon3/4 genotypes (where epsilon is the protein isoform of the apo E gene), relative to those with epsilon2/3, had total cholesterol and LDL-C values 16-23 mg/dL higher throughout adolescence. Age-apo E interaction terms remained significant. FFM, BMI, PBF, pubic-hair stage, and EST showed a significant effect on total cholesterol and LDL-C. When the combination of pubic-hair stage, EST, and one of FFM, BMI, and PBF was included in total cholesterol or LDL-C models, only EST was significant.

CONCLUSIONS

Adolescent girls with epsilon3/3 and epsilon3/4 genotypes had higher total cholesterol and LDL-C and showed different patterns of change, compared to those with epsilon2/3 genotype. These apo E effects were independent of behavioral characteristics, physical growth, body composition, sexual maturation, and endocrine function. Girls with epsilon3/3 or epsilon3/4 genotypes may be at risk for elevated total cholesterol and LDL-C later in life.

Genetic influences on blood lipids and cardiovascular disease risk: tools for primary prevention

Genetic polymorphism in human populations is part of the evolutionary process that results from the interaction between the environment and the human genome. Recent changes in diet have upset this equilibrium, potentially influencing the risk of most common morbidities such as cardiovascular diseases, obesity, diabetes, and cancer. Reduction of these conditions is a major public health concern, and such a reduction could be achieved by improving our ability to detect disease predisposition early in life and by providing more personalized behavioral recommendations for successful primary prevention. In terms of cardiovascular diseases, polymorphisms at multiple genes have been associated with differential effects in terms of lipid metabolism; however, the connection with cardiovascular disease has been more elusive, and considerable heterogeneity exists among studies regarding the predictive value of genetic markers. This may be because of experimental limitations, the intrinsic complexity of the phenotypes, and the aforementioned interactions with environmental factors. The integration of genetic and environmental complexity into current and future research will drive the field toward the implementation of clinical tools aimed at providing dietary advice optimized for the individual's genome. This may imply that dietary changes are implemented early in life to gain maximum benefit. However, it is important to highlight that most reported studies have focused on adult populations and to extrapolate these findings to children and adolescents may not be justified until proper studies have been carried out in these populations and until the ethical and legal issues associated with this new field are adequately addressed.

Testing informative missingness in genetic studies using case-parent triads

In genetic studies, the transmission/disequilibrium test (TDT) using case-parent triads has gained popularity attributable to its robustness to population admixture. Several extensions have been proposed to accommodate incomplete triads. Some strategies assume that parental genotypes are missing completely at random (MCAR) to insure an unbiased conclusion and some methods allow parental genotypes to be missing informatively, resulting in reduced power when the missing data pattern is indeed MCAR. However, these tests assumed that offspring genotypes were MCAR. Recently, Guo indicated that when offspring genotypes were missing informatively, an occurrence that can be considered as ascertainment bias, inflated type-I error and/or reduced power may occur using the TDT when incomplete triads are excluded. In an effort to avoid an erroneous conclusion, we propose a strategy called testing informative missingness (TIM) that compares conditional distributions of parental genotypes among complete triads and incomplete data with only one parent to examine the missing data pattern. Through computer simulations, TIM has decent power to detect informative missingness and is robust to population admixture. In addition, we illustrate TIM with an application to the Framingham Heart Study.

Informative-Transmission Disequilibrium Test (i-TDT): combined linkage and association mapping that includes unaffected offspring as well as affected offspring

To date, there is no test valid for the composite null hypothesis of no linkage or no association that utilizes transmission information from heterozygous parents to their unaffected offspring as well as the affected offspring from ascertained nuclear families. Since the unaffected siblings also provide information about linkage and association, we introduce a new strategy called the informative-transmission disequilibrium test (i-TDT), which uses transmission information from heterozygous parents to all of the affected and unaffected offspring in ascertained nuclear families and provides a valid chi-square test for both linkage and association. The i-TDT can be used in various study designs and can accommodate all types of independent nuclear families with at least one affected offspring. We show that the transmission/disequilibrium test (TDT) (Spielman et al. [1993] Am. J. Hum. Genet. 52:506-516) is a special case of the i-TDT, if the study sample contains only case-parent trios. If the sample contains only affected and unaffected offspring without parental genotypes, the i-TDT is equivalent to the sibship disequilibrium test (SDT) (Horvath and Laird [1998] Am. J. Hum. Genet. 63:1886-1897. In addition, the test statistic of i-TDT is simple, explicit and can be implemented easily without intensive computing. Through computer simulations, we demonstrate that power of the i-TDT can be higher in many circumstances compared to a method that uses affected offspring only. Applying the i-TDT to the Framingham Heart Study data, we found that the apolipoprotein E (APOE) gene is significantly linked and associated with cross-sectional measures and longitudinal changes in total cholesterol.

Impact of dietary intervention, sex, and apolipoprotein E phenotype on tracking of serum lipids and apolipoproteins in 1- to 5-year-old children: the Special Turku Coronary Risk Factor Intervention Project (STRIP)

The effects of dietary intervention, sex, and apolipoprotein E phenotype on tracking of serum lipid values in young children have remained poorly characterized. We investigated these associations in 1062 infants who were randomized into control and intervention groups (n=522 and n=540, respectively) at age 7 months; the intervention group received counseling aimed at maintaining a low-saturated fat, low-cholesterol diet. In 519 children in the control (n=254) and intervention (n=265) groups, serum lipid values were studied annually between 13 months and 5 years of age. In all children, tracking was strongest for the ratio of high density lipoprotein (HDL) cholesterol to total cholesterol; when a 13-month-old child belonged to the lowest quartile of the distribution, the odds ratio for belonging to the same quartile at older ages was 39.0 (95% CI 23.1 to 66.0). Dietary intervention did not influence the tracking of serum lipids. Tracking of HDL cholesterol was stronger in the boys than in the girls (P=0.018). Tracking of non-HDL cholesterol and apolipoprotein B in the children with phenotypes E2/3 or E3/3 was stronger than that in the other children (P=0.031 and P=0.014, respectively). In conclusion, the apolipoprotein E phenotype strongly influences tracking of non-HDL cholesterol and apolipoprotein B values in early childhood, whereas dietary intervention had no effect on tracking of any of the lipids. A child's sex influenced tracking only of HDL cholesterol, with boys showing stronger tracking.

Determinants of lipid level variability in French-Canadian children with familial hypercholesterolemia

The wide variability in the biochemical expression of familial hypercholesterolemia (FH) is only partly explained by mutational heterogeneity in the low density lipoprotein receptor (LDLR) gene. In the current study, we measured this biochemical variability in a group of children heterozygous for the >15-kb LDLR gene deletion (n=67) and examined the contribution of apolipoprotein (apo) E and B allelic variations to this phenotypic variability. Variances of total cholesterol (TC), LDL-C, and apoB concentrations and of the ratio of TC to high density lipoprotein cholesterol (HDL-C) were increased in FH subjects compared with controls. However, after taking the means into account, the coefficients of variation showed that the variability of LDL-C and apoB concentrations was smaller for FH than for controls and that the variability of TC and of the ratio TC to HDL-C was similar between both groups. The epsilon2/3 genotype was associated with lower mean TC, LDL-C, and apoB concentrations in FH. The magnitude of this effect was smaller in controls than in FH. Indeed, the percentages of total variance of TC, LDL-C, and apoB attributable to the apoE locus were 19.9%, 18.1%, and 11.8%, respectively, in FH cases and 5.9%, 7.4%, and 6.0%, respectively, in controls. We did not detect any effect of the apoB insertion/deletion polymorphism on lipid traits in FH children. However, in controls, we observed a strong interaction between apoE and apoB genotypes on apoB concentrations and on TC to HDL-C ratios. Our study reemphasizes the important role of apoE in lipid metabolism and illustrates that the effects of allelic variations on lipid traits are context dependent.