1. Reviewer #2 (Public Review):

    In this manuscript, Dahlen et al. aimed to agnostically investigate the association between ABO and RhD blood group and disease occurrence for a large number of disease phenotypes using large-scale population-based Swedish healthcare registries. Using 2 large subject cohorts, they convincingly demonstrate that beyond the known associations between ABO, infectious diseases and thrombosis, there are other associations with very different diseases. This paper is purely epidemiological with no biological data to explain the observed associations. The clinical phenotypes are derived from hospital coding and probably lack precision, especially in terms of diagnostic certainty.

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  2. Reviewer #1 (Public Review):

    The authors aimed to survey a large transfusion database in Sweden to catalog associations between ABO/RhD blood group antigens and a wide variety of clinical phenotypes in a systematic, unbiased and comprehensive manor. They succeed at surveying over 1200 phenotypes in over 5 million people and identify 49 statistically significant associations for ABO blood group and point out a couple novel associations. Their statistical methods are appropriate and help eliminate potential false positive associations. The strengths of this study are the unbiased survey of a large database and the appropriate corrections for multiple observations which allow the authors to explore a large number of associations without loosing site of what is really a significant association.

    This study sheds light on a topic of interest to many scientists. The ABO gene encodes a glycosyltransferase enzyme that has 4 major haplotypes in human populations and results in a specific pattern of posttranslational modification of plasma proteins and blood cells including erythrocytes. Proteins decorated with an H antigen can receive additional carbohydrate antigens from ABO transferase intracellularly. The common A allele transfers UDP-GalNAc while the B allele transfers UDP-Gal. The A2 allele is hypomorphic compared to the A allele and transfers lower amounts of UDP-GalNAc and the common O allele is a null resulting in no transferase activity.

    The allele frequencies of these common alleles varies by ancestry and has geographic differences. Variation at ABO is unconstrained with many rare variants contributing to the four common haplotypes at ABO. Interestingly, geographically specific selective pressures may have led to allele frequency differences. For example. ~40-50% of individuals are homozygous for the null (type O) allele. These null haplotypes are more common in individuals of Latino or African ancestry while 'A' haplotypes are slightly more common in individuals of European origin and 'B' alleles are more common in individuals of Asian and African ancestry. Overall, O is more common than A or B alleles. An unbiased survey of phenotype frequencies by blood type allows for confirmation of previous associations and discovery of novel associations. In this largely European ancestry cohort, blood type A is the most common (45-47%) while blood type O is second most common at 38-39%.

    Limitations of Phenome-wide Association Studies (PheWAS) like the one presented in this manuscript should be noted. Associations with complex phenotypes or those with small effect size will not be detected even in a large cohort such as the SCANDAT. This study is also biased toward associations with phenotypes more common in the Scandinavian population. This may present associations related to the population substructure and not a direct association with ABO. In genome-wide association studies this can be addressed through multiple methods but it is not clear how the authors correct for population structure in this study. Likewise, the insight into the mechanistic reasons for ABO associations is not a strength of this study and will await subsequent studies for many phenotypes. Mechanistic insight might be particularly interesting for the novel associations uncovered by this study.

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  3. Evaluation Summary:

    In this manuscript, Dahlen et al. agnostically survey a large transfusion database in Sweden to investigate the association between ABO and RhD blood group and disease occurrence for a large number of clinical phenotypes. The data reported are purely epidemiological associations, with no direct insight into biological mechanism. Nonetheless, these data are a valuable resource for the research community, and offer the potential for a number of important biologic hypotheses and insights for investigation in the future.

    (This preprint has been reviewed by eLife. We include the public reviews from the reviewers here; the authors also receive private feedback with suggested changes to the manuscript. Reviewer #1 agreed to share their name with the authors.)

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