Person:

Weinreb, Caleb

Bone marrow transplantation therapy relies on the life-long regenerative capacity of haematopoietic stem cells (HSCs). HSCs present a complex variety of regenerative behaviours at the clonal level, but the mechanisms underlying this diversity are still undetermined. Recent advances in single cell RNA sequencing (scRNAseq) have revealed transcriptional differences amongst HSCs, providing a possible explanation for their functional heterogeneity. However, the destructive nature of sequencing assays prevents simultaneous observation of stem cell state and function. To solve this challenge, we implemented expressible lentiviral barcoding, which enabled simultaneous analysis of lineages and transcriptomes from single adult HSCs and their clonal trajectories during long-term bone marrow reconstitution. Differential gene expression analysis between clones with distinct behaviour unveiled an intrinsic molecular signature that characterizes functional long-term repopulating HSCs. Probing this signature through in vivo CRISPR screening, we found the transcription factor Tcf15 to be required, and sufficient, to drive HSC quiescence and long-term self-renewal. In situ, Tcf15 expression labels the most primitive subset of true multipotent HSCs. In conclusion, our work elucidates clone-intrinsic molecular programs associated with functional stem cell heterogeneity, and identifies a mechanism for the maintenance of the self-renewing haematopoietic stem cell state.