19Background: Inverse association between premorbid body mass index (BMI) and 20 amyotrophic lateral sclerosis (ALS) has been discovered in observational studies; 21 however, whether this association is causal remains largely unknown. 22 Methods: We employed a two-sample Mendelian randomization approach to evaluate 23 the causal relationship of genetically increased BMI with the risk of ALS. The 24 analyses were implemented using summary statistics obtained for the independent 25 instruments identified from large-scale genome-wide association studies of BMI (up 26 to ~770,000 individuals) and ALS (up to ~81,000 individuals). The causal relationship 27 between BMI and ALS was estimated using inverse-variance weighted methods and 28 was further validated through extensive complementary and sensitivity analyses.
29Findings: Using 1,031 instruments strongly related to BMI, the causal effect of per 30 one standard deviation increase of BMI was estimated to be 1.04 (95% CI 0.97~1.11, 31 p=0.275) in the European population. The null association between BMI and ALS 32 discovered in the European population also held in the East Asian population and was 33 robust against various modeling assumptions and outlier biases. Additionally, the 34 Egger-regression and MR-PRESSO ruled out the possibility of horizontal pleiotropic 35 effects of instruments. 36 Interpretation: Our results do not support the causal role of genetically increased or 37 decreased BMI on the risk of ALS. 38 Key words 39 Body mass index, Amyotrophic lateral sclerosis, Mendelian randomization, 40 Instrumental variable, Genome-wide association studies 41 3/23 1. Introduction 42 Amyotrophic lateral sclerosis (ALS) is a frequent adult-onset fatal neurodegenerative 43 disease clinically characterized by rapidly progressive motor neurons degeneration 44 and death because of respiratory failure [1]. Although great advance has been made 45for the understanding of ALS in the past decades, the pathogenic mechanism 46 underlying ALS remains largely unknown and only few therapeutic options can be 47 50 development of ALS [5, 6, 7, 8, 9, 10, 11]. However, no replicable and definitive 51 environmental risk factors are currently well established for ALS. In addition, due to 52 the quickly growing ageing of the population in the upcoming years, it is evaluated 53 that the number of ALS cases across globe will increase by about 70% [12], which is 54 anticipated to result in rather serious socioeconomic and health burden. Therefore, 55 understanding the risk factors of ALS for improving the medical intervention and 56 quality of life for ALS patients is considerably important from both disease treatment 57 and public health perspectives. 58 Among extensive epidemiological researches of ALS, an interesting and surprising 59 observation is that ALS patients often encounter a loss of weight or a decrease of body 60 mass index (BMI) at the early phase of diagnosis with many possible explanations [1, 61 13, 14, 15, 16, 17, 18, 19, 20]. Indeed, substan...