Classically, blood arises from stem cells through a series of oligopotent progenitors that become increasingly restricted to unipotent progenitors, each slotted into a hierarchical layer based on their differentiation potential. The presence of oligopotent cells is critical to the standard model of blood differentiation as they define the path from stem cells to unipotent progenitors. We developed a new cell-sorting scheme to resolve myeloid (My), erythroid (Er) and megakaryocytic (Mk) fates from single CD34+ cells and then mapped the progenitor hierarchy across human development. Fetal liver contained large numbers of distinct oligopotent progenitors with entangled My, Er and Mk fates. Unexpectedly in adult bone marrow, few oligopotent progenitor intermediates were present with multipotent and unipotent progenitors predominating, and now Er-Mk lineages emerged from multipotent cells. The developmental shift to an adult 'two-tier' hierarchy challenges current dogma and provides a new framework to understand normal and disease states of human hematopoiesis. ‡

Intratumoral heterogeneity arises through the evolution of genetically diverse subclones during tumor progression. However, it remains unknown whether cells within single genetic clones are functionally equivalent. By combining DNA copy number alteration (CNA) profiling, sequencing, and lentiviral lineage tracking, we followed the repopulation dynamics of 150 single lentivirus-marked lineages from 10 human colorectal cancers through serial xenograft passages in mice. CNA and mutational analysis distinguished individual clones and showed that clones remained stable upon serial transplantation. Despite this stability, the proliferation, persistence, and chemotherapy tolerance of lentivirally marked lineages were variable within each clone. Chemotherapy promoted the dominance of previously minor or dormant lineages. Thus, apart from genetic diversity, tumor cells display inherent functional variability in tumor propagation potential, which contributes to both cancer growth and therapy tolerance.

Graft failure in the transplantation of hematopoietic stem cells occurs despite donor-host genetic identity of human leukocyte antigens, suggesting that additional factors modulate engraftment. With the nobese diabetic (NOD)-severe combined immunodeficiency (SCID) xenotransplantation model, we found that the NOD background allowed better hematopoietic engraftment than did other strains with equivalent immunodeficiency-related mutations. We used positional genetics to characterize the molecular basis for this strain specificity and found that the NOD Sirpa allele conferred support for human hematopoiesis. NOD SIRP-alpha showed enhanced binding to the human CD47 ligand, and its expression on mouse macrophages was required for support of human hematopoiesis. Thus, we have identified Sirpa polymorphism as a potent genetic determinant of the engraftment of human hematopoietic stem cells.

The detection of primitive hematopoietic cells based on repopulation of immune-deficient mice is a powerful tool to characterize the human stem-cell compartment. Here, we identify a newly discovered human repopulating cell, distinct from previously identified repopulating cells, that initiates multilineage hematopoiesis in NOD/SCID mice. We call such cells CD34neg-SCID repopulating cells, or CD34neg-SRC. CD34neg-SRC are restricted to a Lin-CD34-CD38- population without detectable surface markers for multiple lineages and CD38 or those previously associated with stem cells (HLA-DR, Thy-1 and CD34). In contrast to CD34+ subfractions, Lin-CD34-CD38- cells have low clonogenicity in short-and long-term in vitro assays. The number of CD34neg-SRC increased in short-term suspension cultures in conditions that did not maintain SRC derived from CD34+ populations, providing independent biological evidence of their distinctiveness. The identification of this newly discovered cell demonstrates complexity of the organization of the human stem-cell compartment and has important implications for clinical applications involving stem-cell transplantation.

Highly regenerative tissues such as blood must possess effective DNA damage responses (DDR) that balance long-term regeneration with protection from leukemogenesis. Hematopoietic stem cells (HSCs) sustain life-long blood production, yet their response to DNA damage remains largely unexplored. We report that human HSCs exhibit delayed DNA double-strand break rejoining, persistent gammaH2AX foci, and enhanced p53- and ASPP1-dependent apoptosis after gamma-radiation compared to progenitors. p53 inactivation or Bcl-2 overexpression reduced radiation-induced apoptosis and preserved in vivo repopulating HSC function. Despite similar protection from irradiation-induced apoptosis, only Bcl-2-overexpressing HSCs showed higher self-renewal capacity, establishing that intact p53 positively regulates self-renewal independently from apoptosis. The reduced self-renewal of HSCs with inactivated p53 was associated with increased spontaneous gammaH2AX foci in secondary transplants of HSCs. Our data reveal distinct physiological roles of p53 that together ensure optimal HSC function: apoptosis regulation and prevention of gammaH2AX foci accumulation upon HSC self-renewal.

The composition of the human hematopoietic stem cell compartment is poorly understood due to the absence of experimental tools with which to characterize the developmental program of individual stem cells. We report here that human stem cells differ markedly in their repopulation capacity and self-renewal potential, as determined using nonobese diabetic-severe combined immunodeficiency (NOD-SCID) mice transplanted with retrovirally transduced cord blood stem cells, called SCID-repopulating cells (SRCs). Clonal stem cell analysis based on the identification of unique retroviral integration sites within serial bone marrow aspirates showed that repopulation was generally oligoclonal with extensive variability in the lifespan and proliferative capacity of individual SRCs. Most clones contributed to human cell engraftment for several weeks after transplantation and then disappeared but others appeared later and persisted. Further evidence for stem cell heterogeneity was found in the secondary transplantation capacity of SRCs. These data point to the existence of different classes of human stem cells with variable self-renewal potential and short- or long-term repopulating capacity.

A major problem hampering effective stem cell-based therapies is the absence of a clear understanding of the human hematopoietic stem cell (HSC) pool composition. The severe combined immunodeficiency (SCID) repopulating cell (SRC) xenotransplant assay system provides a powerful tool for characterizing the frequency, cell surface markers, cell cycle status, homing and response to cytokine stimulation of human HSCs. Clonal tracking of retrovirally transduced SRCs and transplantation of specific subpopulations revealed SRC classes with distinct repopulation potentials. However, all HSC repopulation assays are based on intravenous injection, a complex process that requires circulation through blood, recognition and extravasation through bone marrow vasculature, and migration to a supportive microenvironment. Thus, some classes of HSCs may remain undetected. By direct intrafemoral injection, we identified rapid SRCs (R-SRCs) within the Lin-CD34+CD38loCD36- subpopulation. R-SRCs rapidly generate high levels of human myeloid and erythroid cells within the injected femur, migrate to the blood and colonize individual bones of non-obese diabetic (NOD)-SCID mice within 2 weeks after transplantation. Lentivector-mediated clonal analysis of individual R-SRCs revealed heterogeneity in their proliferative and migratory properties. The identification of a new HSC class and an effective intrafemoral assay provide the tools required to develop more effective stem cell-based therapies that rely on rapid reconstitution.

Fanconi anaemia (FA) is an autosomal recessive disease characterized by bone marrow failure, variable congenital malformations and predisposition to malignancies. Cells derived from FA patients show elevated levels of chromosomal breakage and an increased sensitivity to bifunctional alkylating agents such as mitomycin C (MMC) and diepoxybutane (DEB). Five complementation groups have been identified by somatic cell methods, and we have cloned the gene defective in group C (FAC)(7). To understand the in vivo role of this gene, we have disrupted murine Fac and generated mice homozygous for the targeted allele. The -/- mice did not exhibit developmental abnormalities nor haematologic defects up to 9 months of age. However, their spleen cells had dramatically increased numbers of chromosomal aberrations in response to MMC and DEB. Homozygous male and female mice also had compromised gametogenesis, leading to markedly impaired fertility, a characteristic of FA patients. Thus, inactivation of Fac replicates some of the features of the human disease.