Somatic mutations reveal asymmetric cellular dynamics in the early human embryo

Somatic cells acquire mutations throughout the course of an individual’s life. Mutations occurring early in embryogenesis will often be present in a substantial proportion of, but not all, cells in the post-natal human and thus have particular characteristics and impact1. Depending upon their location in the genome and the proportion of cells they are present in, these mosaic mutations can cause a wide range of genetic disease syndromes2 and predispose to cancer3,4. They have a high chance of being transmitted to offspring as de novo germline mutations and, in principle, can provide insights into early human embryonic cell lineages and their contributions to adult tissues5. Although it is known that gross chromosomal abnormalities are remarkably common in early human embryos6 our understanding of early embryonic somatic mutations is very limited. Here, we use whole genome sequences of adult normal blood from 241 individuals to identify 163 early embryonic mutations. We estimate that approximately three base substitution mutations occur per cell per cell-doubling in early human embryogenesis and these are mainly attributable to two known mutational signatures7. We used the mutations to reconstruct developmental lineages of adult cells and demonstrate that the two daughter cells of many early embryonic cell doublings contribute asymmetrically to adult blood at an approximately 2:1 ratio. This study therefore provides insights into the mutation rates, the mutational processes and the developmental outcomes of cell dynamics operative during early human embryogenesis.