Mutations provide an enduring record
Somatic mutations pepper our cells with change, but because they are not in the germline, they do not propagate to the next generation. Bizzotto et al. leveraged data on the distribution of somatic mutations in adults to take a backward look at the earliest moments of human development. Calculation of cellular lineages on the basis of shared somatic mutations shows the number of cells from which the body will develop when the human embryo gastrulates. The lineage for forebrain cells is identifiable, as are the asymmetrical fates spun out of many of the gastrula cells.
Science, this issue p. 1249
Although cell lineage information is fundamental to understanding organismal development, very little direct information is available for humans. We performed high-depth (250×) whole-genome sequencing of multiple tissues from three individuals to identify hundreds of somatic single-nucleotide variants (sSNVs). Using these variants as “endogenous barcodes” in single cells, we reconstructed early embryonic cell divisions. Targeted sequencing of clonal sSNVs in different organs (about 25,000×) and in more than 1000 cortical single cells, as well as single-nucleus RNA sequencing and single-nucleus assay for transposase-accessible chromatin sequencing of ~100,000 cortical single cells, demonstrated asymmetric contributions of early progenitors to extraembryonic tissues, distinct germ layers, and organs. Our data suggest onset of gastrulation at an effective progenitor pool of about 170 cells and about 50 to 100 founders for the forebrain. Thus, mosaic mutations provide a permanent record of human embryonic development at very high resolution.
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