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Background: It is unclear whether there are causal associations between blood lipids, statin use and cancer risks. Under certain assumptions, Mendelian randomization analysis of a genetic marker for an exposure eliminates reverse causation and confounding.

Methods: We applied Mendelian randomization analysis to genetic scores, comprising 26-41 single-nucleotide polymorphisms (SNPs), as instrumental variables (IVs) for triglycerides and low- and high-density lipoprotein cholesterol (LDLC, HDLC), using a prospective cohort of 26 904 individuals in which there were 6607 incident cancers. We also investigated cancer risk for a SNP (rs12916) in the gene encoding hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR), the targeted enzyme in statin treatment. We used logistic regression and SNP pleiotropy-adjusted analyses to estimate the odds ratio per standard deviation (OR).

Results: The OR for the triglyceride IV as a predictor of any cancer was 0.91 [95% confidence interval (CI): 0.80-1.03] unadjusted, and 0.87 (95% CI: 0.78-0.95) from the pleiotropy-adjusted analysis. For the HMGCR rs12916 per LDLC-lowering T-allele, the OR was 1.09 (95% CI: 1.01-1.18) for prostate cancer and 0.89 (95% CI: 0.82-0.96) for breast cancer. The LDLC IV was not associated with prostate cancer or breast cancer. There were no associations between IVs and cancers of the lung, colon or bladder.

Conclusions: Under the assumptions of Mendelian randomization, there is a causal and negative association between serum triglycerides and risk of any cancer. Further, the HMGCR genetic variant might be associated with risks of prostate and breast cancers but the biological mechanisms behind these findings are unclear, as the LDLC IV was not associated with these cancers.