Tracking single cell evolution via clock-like chromatin accessibility

Single cell chromatin accessibility sequencing (scATAC) reconstructs developmental trajectory by phenotypic similarity. However, inferring the exact developmental trajectory is challenging. Here, we show a simple, accurate and phenotypic-neutral measure of cell developmental hierarchy - the fraction of accessible clock-like loci. As cells undergo mitosis, the heterogeneity of chromatin accessibility on clock-like loci is reduced, providing a measure of mitotic age. We developed a method, EpiTrace, that counts the fraction of opened clock-like loci from scATAC data to determine cell age and perform lineage tracing. EpiTrace works in various cell lineages and animal species, shows concordance with known developmental hierarchies, correlates well with DNA methylation-based clocks, and is complementary with mutation-based lineage tracing, RNA velocity, and stemness predictions. Applying EpiTrace to scATAC data revealed a multitude of novel biological insights with clinically relevant implications, ranging from hematopoiesis, organ development, tumor biology and immunity to cortical gyrification. Our work discovered a universal epigenomic hallmark during cellular development, which facilitates the study of cellular hierarchies and organismal aging.