The relationship between circulating lipids and breast cancer risk: A Mendelian randomization study

Abstract

Background: A number of epidemiological and genetic studies have attempted to determine whether levels of circulating lipids are associated with risks of various cancers, including breast cancer (BC). However, it remains unclear whether a causal relationship exists between lipids and BC. If alteration of lipid levels also reduced risk of BC, this could present a target for disease prevention. This study aimed to assess a potential causal relationship between genetic variants associated with plasma lipid traits (high-density lipoprotein, HDL; low-density lipoprotein, LDL; triglycerides, TGs) with risk for BC using Mendelian randomization (MR).

Methods and findings: Data from genome-wide association studies in up to 215,551 participants from the Million Veteran Program (MVP) were used to construct genetic instruments for plasma lipid traits. The effect of these instruments on BC risk was evaluated using genetic data from the BCAC (Breast Cancer Association Consortium) based on 122,977 BC cases and 105,974 controls. Using MR, we observed that a 1-standard-deviation genetically determined increase in HDL levels is associated with an increased risk for all BCs (HDL: OR [odds ratio] = 1.08, 95% confidence interval [CI] = 1.04-1.13, P < 0.001). Multivariable MR analysis, which adjusted for the effects of LDL, TGs, body mass index (BMI), and age at menarche, corroborated this observation for HDL (OR = 1.06, 95% CI = 1.03-1.10, P = 4.9 × 10-4) and also found a relationship between LDL and BC risk (OR = 1.03, 95% CI = 1.01-1.07, P = 0.02). We did not observe a difference in these relationships when stratified by breast tumor estrogen receptor (ER) status. We repeated this analysis using genetic variants independent of the leading association at core HDL pathway genes and found that these variants were also associated with risk for BCs (OR = 1.11, 95% CI = 1.06-1.16, P = 1.5 × 10-6), including locus-specific associations at ABCA1 (ATP Binding Cassette Subfamily A Member 1), APOE-APOC1-APOC4-APOC2 (Apolipoproteins E, C1, C4, and C2), and CETP (Cholesteryl Ester Transfer Protein). In addition, we found evidence that genetic variation at the ABO locus is associated with both lipid levels and BC. Through multiple statistical approaches, we minimized and tested for the confounding effects of pleiotropy and population stratification on our analysis; however, the possible existence of residual pleiotropy and stratification remains a limitation of this study.

Conclusions: We observed that genetically elevated plasma HDL and LDL levels appear to be associated with increased BC risk. Future studies are required to understand the mechanism underlying this putative causal relationship, with the goal of developing potential therapeutic strategies aimed at altering the cholesterol-mediated effect on BC risk.

Fig 1. Results of MR analyses of the effects of lipids on BC risk.

Results plotted are after pruning for instrument heterogeneity. The forest plot on the right displays the OR of the effect of a 1-standard–deviation increase in genetically determined HDL-cholesterol on BC risk as a diamond, with error bars representing the 95% CI. The vertical dotted line delineates an OR of 1, i.e., no effect of the exposure on BC risk. BC, breast cancer; BMI, body mass index; CI, confidence interval; HDL, high-density lipoprotein; IVW, inverse-variance weighted MR; LDL, low-density lipoprotein; MR, Mendelian randomization; MVMR, multivariable MR; OR, odds ratio; TG, triglyceride.

Fig 2. MR results for HDL gene-specific instruments and meta-analysis of effect estimates across genes.

The forest plot on the right displays the OR of the effect of a 1-standard–deviation increase in genetically determined HDL-cholesterol for each locus on BC risk as a diamond, and the error bars represent the 95% CI of the effect estimate. The vertical dotted line delineates an OR of 1, i.e., no effect of the exposure on BC risk. For HDL gene-specific instruments, see S3 Table. ABCA1, ATP Binding Cassette Subfamily A Member 1; APOC, Apolipoprotein C; APOE, Apolipoprotein E; BC, breast cancer; CETP, Cholesteryl Ester Transfer Protein; CI, confidence interval; HDL, high-density lipoprotein; LIPC, Lipase C, Hepatic Type; LIPG, Lipase G, Endothelial Type; MR, Mendelian randomization; N SNPs, number of genetic instruments included in each locus’s MR analysis; OR, odds ratio; PLTP, Phospholipid Transfer Protein; SCARB1, Scavenger Receptor Class B Member 1.