Do maternal intrauterine environmental influences that lower offspring birthweight causally increase offspring cardiometabolic risk factors in later life? A Mendelian randomization study of 45,849 genotyped parent offspring pairs in the HUNT study

Introduction: There is a robust and well-documented observational relationship between lower birthweight and higher risk of cardiometabolic disease in later life. The Developmental Origins of Health and Disease (DOHaD) hypothesis posits that adverse environmental factors in utero or in the early years of life result in increased future risk of cardiometabolic disease. Our aim was to investigate whether there was evidence for causal effects of the intrauterine environment, as proxied by maternal single nucleotide polymorphisms (SNPs) that influence offspring birthweight independent of offspring genotype, on offspring cardiometabolic risk factors such as blood pressure, non-fasting glucose, body mass index (BMI), and lipid levels. Methods: We investigated whether a genetic risk score of maternal SNPs associated with offspring birthweight was also associated with offspring cardiometabolic risk factors, after controlling for offspring genotypes at the same loci, in up to 26,057 mother-offspring pairs from the Nord-Troendelag Health (HUNT) Study. We also conducted similar analyses in 19,792 father-offspring pairs from the same study to investigate whether there was evidence that any such causal effects operated through the postnatal, rather than the intrauterine environment. To take account of the considerable cryptic relatedness in HUNT, we implemented a computationally efficient genetic linear mixed model using the OpenMx software package to perform our analyses. Results: We found little evidence for a maternal genetic effect of birthweight associated variants on offspring cardiometabolic risk factors after adjusting for offspring genotypes at the same loci. Likewise, we found little evidence for paternal genetic effects on offspring cardiometabolic risk factors performing similar analyses in father-offspring pairs. In contrast, offspring genetic risk scores of birthweight associated variants were strongly related to many cardiometabolic risk factors, even after conditioning on maternal genotypes at the same loci. Conclusion: Our results suggest that the maternal intrauterine environment, as proxied by maternal SNPs that influence offspring birthweight, is unlikely to be a major determinant of adverse cardiometabolic outcomes in population based samples of individuals. In contrast, genetic pleiotropy appears to explain some of the observational relationship between offspring birthweight and future cardiometabolic risk.