*Leukaemia Research Fund Centre for Cell and Molecular Biology, Institute of Cancer Research, Chester Beatty Laboratories, London SW3 6JB, United Kingdom; t Division of Transplantation Sciences, Southmead Health Services, University of Bristol, Bristol BS10 SNB, United Kingdom;
and *North London Centre, National Blood Service, London NW9 SBD, United Kingdom

Communicated by Janet D. Rowley, University of Chicago Medical Center, Chicago, IL, April11, 2002 (received for review January 24,2002)

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 (=5%), protracted latency, and transgenic modeling all suggest that additional postnatal exposure and/or 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, TELAML1 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 immunophenotype/ 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 high-4 to 10 high-3) indicates substantial clonal expansion of a progenitor population. These data have significant implications for the pathogenesis, natural history, and etiology of childhood leukemia.

Fig. 5. Identification of TEL-AML 1 fusion gene-positive cells in cord blood (B128) by immunophenotype/FISH analysis. In each case the fluorescent signal corresponding to the TEL probe is green, the AML1 probe is red, and the fused red-green signals corresponding to the TEL-AML 1 fusion appear yellow. The cells positive for the corresponding immunophenotype are stained blue.CD 10-positive sorted cord blood (B 128) cells. (Right) The cell shows one green, two different-sized red, and one (yellow) fusion signal, the expected signal configuration for a TEL-AML 1-positive cell. The green (normal TEL) signal is present. (Left) The cell shows two green and two similar-sized red signals, corresponding to two normal copies of TEL and AML1. There is no fusion signal.