Causal relevance of circulating adiponectin with cancer: a meta-analysis implementing Mendelian randomization

Some studies have observed a lower circulating level of adiponectin in cancer patients, but whether this observation is causal remains unresolved. We therefore undertook a meta-analysis implementing Mendelian randomization to exploit the causal relevance of circulating adiponectin with cancer by using multiple polymorphisms in adiponectin encoded gene ADIPOQ as instrumental variables. Eligible articles were identified from PubMed and Embase. Data and study quality were assessed in duplicate. Total 26 articles including 31 study groups were analyzed. Overall allelic association with cancer was significant for rs822396 (odds ratio (OR) = 0.91; P = 0.045) and rs1501299 (OR = 0.89; P = 0.051), with low or moderate heterogeneity. Carriers of rs2241766 GG genotype (weighted mean difference (WMD) = 0.86; P = 0.037) or G allele (WMD = 0.68; P = 0.047) had significantly higher circulating adiponectin than the TT genotype carriers, without heterogeneity. Using rs2241766 as an instrument in Mendelian randomization analysis, an increment of 1 mg/L in circulating adiponectin was significantly associated with a 43-50 % reduced risk for lung cancer, but with a 20-40 % increased risk of colorectal cancer, respectively. There was no observable publication bias. Genetically elevated circulating adiponectin might confer a protective effect against lung cancer, yet a risky effect for colorectal cancer. Further validation is urgently required.