Several sensitivity analyses were used to check and correct for the presence of pleiotropy in the causal estimates. Cochran’s
Q was computed to quantify heterogeneity across the individual causal effects, with a
P-value ≤ 0.05 indicating the presence of pleiotropy, and that consequently, a random effects IVW MR analysis should be used
43 (link),48 (link). We also assessed the potential presence of horizontal pleiotropy using MR-Egger regression based on its intercept term, where deviation from zero denotes the presence of directional pleiotropy. Additionally, the slope of the MR-Egger regression provides valid MR estimates in the presence of horizontal pleiotropy when the pleiotropic effects of the genetic variants are independent from the genetic associations with the exposure
49 (link),50 (link). We also computed OR estimates using the complementary weighted-median method that can give valid MR estimates under the presence of horizontal pleiotropy when up to 50% of the included instruments are invalid
44 (link). The presence of pleiotropy was also assessed using the MR-PRESSO. In this, outlying SNPs are excluded from the accelerometer-measured physical activity instrument and the effect estimates are reassessed
51 (link). For all of the aforementioned sensitivity analyses to identify possible pleiotropy, we considered the estimates from the extended 10 SNP instrument as the primary results due to unstable estimates from the 5 SNP instrument. A leave-one-SNP out analysis was also conducted to assess the influence of individual variants on the observed associations. We also examined the selected genetic instruments and their proxies (
r2 > 0.8) and their associations with secondary phenotypes (
P-value < 5 × 10
−8) in Phenoscanner
(http://www.phenoscanner.medschl.cam.ac.uk/) and GWAS catalog (date checked April 2019).
For the extended 10 SNP instrument, we also conducted multivariable MR analyses to adjust for potential pleiotropy due to BMI because the initial GWAS on physical activity reported several strong associations (
P-value < 10
−5) between the identified SNPs and BMI
52 (link). The new estimates correspond to the direct causal effect of physical activity with the BMI being fixed. The genetic data on BMI were obtained from a GWAS study published by The Genetic Investigation of ANthropometric Traits (GIANT) consortium
53 (link) (Supplementary Table
9). Additionally, for the extended 10 SNP instrument, we also conducted analyses with adiposity-related SNPs (i.e. those previously associated with BMI, waist circumference, weight, or body/trunk fat percentage in GWAS studies at
P-value < 10
−8) excluded (
n = 5; rs34517439, rs6775319, rs11012732, rs1550435, rs59499656). Finally, we conducted two-sample MR analyses using BMI adjusted GWAS estimates for the 5 SNP accelerometer-measured physical activity instrument
11 (link). However, the MR results using the BMI adjusted GWAS estimates should be interpreted cautiously due to the potential for collider bias
11 (link).
All the analyses were conducted using the MendelianRandomisation
54 (link) and TwoSampleMR
55 (link) packages, and the R programming language.
Papadimitriou N., Dimou N., Tsilidis K.K., Banbury B., Martin R.M., Lewis S.J., Kazmi N., Robinson T.M., Albanes D., Aleksandrova K., Berndt S.I., Timothy Bishop D., Brenner H., Buchanan D.D., Bueno-de-Mesquita B., Campbell P.T., Castellví-Bel S., Chan A.T., Chang-Claude J., Ellingjord-Dale M., Figueiredo J.C., Gallinger S.J., Giles G.G., Giovannucci E., Gruber S.B., Gsur A., Hampe J., Hampel H., Harlid S., Harrison T.A., Hoffmeister M., Hopper J.L., Hsu L., María Huerta J., Huyghe J.R., Jenkins M.A., Keku T.O., Kühn T., La Vecchia C., Le Marchand L., Li C.I., Li L., Lindblom A., Lindor N.M., Lynch B., Markowitz S.D., Masala G., May A.M., Milne R., Monninkhof E., Moreno L., Moreno V., Newcomb P.A., Offit K., Perduca V., Pharoah P.D., Platz E.A., Potter J.D., Rennert G., Riboli E., Sánchez M.J., Schmit S.L., Schoen R.E., Severi G., Sieri S., Slattery M.L., Song M., Tangen C.M., Thibodeau S.N., Travis R.C., Trichopoulou A., Ulrich C.M., van Duijnhoven F.J., Van Guelpen B., Vodicka P., White E., Wolk A., Woods M.O., Wu A.H., Peters U., Gunter M.J, & Murphy N. (2020). Physical activity and risks of breast and colorectal cancer: a Mendelian randomisation analysis. Nature Communications, 11, 597.