Studies on monozygotic twins with concordant leukemia and retrospective scrutiny of neonatal blood spots of patients with leukemia indicate that chromosomal translocations characteristic of pediatric leukemia often arise prenatally, probably as initiating events. The modest concordance rate for leukemia in identical twins (Ϸ5%), protracted latency, and transgenic modeling all suggest that additional postnatal exposure and͞or genetic events are required for clinically overt leukemia development. This notion leads to the prediction that chromosome translocations, functional fusion genes, and preleukemic clones should be present in the blood of healthy newborns at a rate that is significantly greater than the cumulative risk of the corresponding leukemia. Using parallel reverse transcriptase-PCR and real-time PCR (Taqman) screening, we find that the common leukemia fusion genes, TEL-AML1 or AML1-ETO, are present in cord bloods at a frequency that is 100-fold greater than the risk of the corresponding leukemia. Single-cell analysis by cell enrichment and immunophenotype͞ fluorescence in situ hybridization multicolor staining confirmed the presence of translocations in restricted cell types corresponding to the B lymphoid or myeloid lineage of the leukemias that normally harbor these fusion genes. The frequency of positive cells (10 ؊4 to 10 ؊3 ) indicates substantial clonal expansion of a progenitor population. These data have significant implications for the pathogenesis, natural history, and etiology of childhood leukemia.C himeric fusion genes generated by chromosomal translocations are consistent genetic abnormalities in pediatric acute leukemia (1-3). DNA breaks and fusions occur in introns, and each patient's leukemic cells have a unique or clonotypic breakpoint, providing a specific, sensitive, and stable marker for tracking leukemic clones (4). Using this approach in identical twins with concordant leukemia (5-7) and, retrospectively, with neonatal blood spots (8-10), it was demonstrated that common fusion genes-MLL fusions in infants with acute lymphoblastic leukemia (ALL) and TEL-AML1 and AML1-ETO in children with ALL or acute myeloblastic leukaemia (AML), respectively-arise predominantly in utero, probably as initiating events, and are present before and at birth in circulating blood.The evolutionary, clonal development of pediatric cancers involves a sequence of two or more independent genetic events (11), and it therefore is unlikely that fusion genes, initiating the disease, would be sufficient. This supposition is supported by the modest concordance rate for ALL in monozygotic twin children, 5-10% (12), by protracted postnatal latency (up to 14 years) (7) and by the absence of overt signs of leukemia in mice transgenic for AML1-ETO (13) or TEL-AML1 (14). This finding leads to the prediction that chromosomal translocations, functional fusion genes, and preleukemic clones should be generated in stem cells during fetal hemopoiesis, and present in blood at birth, at a rate that substantially exceeds the k...

Summary. In postnatal life, mesenchymal stem cells (MSC) self-replicate, proliferate and differentiate into mesenchymal tissues, including bone, fat, tendon, muscle and bone marrow (BM) stroma. Possible clinical applications for MSC in stem cell transplantation have been proposed. We have evaluated the frequency, phenotype and differentiation potential of MSC in adult BM, cord blood (CB) and peripheral blood stem cell collections (PBSC). During culture, BM MSC proliferated to confluence in 10-14 d, maintaining a stable non-haemopoietic phenotype, HLA class-1 + , CD29 + , CD44 + , CD90 + , CD45 -, CD34 -and CD14 through subsequent passages. Using the colony forming unit fibroblasts assay, the estimated frequency of MSC in the BM nucleated cell population was 1 in 3AE4 · 10 4 cells. Both adipogenic and osteogenic differentiation of BM MSC was demonstrated. In contrast, CB and PBSC mononuclear cells cultured in MSC conditions for two passages produced a population of adherent, non-confluent fibroblast-like cells with a haemopoietic phenotype, CD45 + , CD14 + , CD34 -, CD44 -, CD90 -and CD29 -. In paired experiments, cultured BM MSC and mature BM stroma were seeded with CB cells enriched for CD34 + . Similar numbers of colony-forming units of granulocytes-macrophages were produced by MSC-based and standard stroma cultures over 10 weeks. We conclude that adult BM is a reliable source of functional cultured MSC, but CB and PBSC are not.

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