Chromosome translocations and covert leukemic clones are generated during normal fetal development

Abstract

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...

Adenovirus DNA is detected at increased frequency in Guthrie cards from children who develop acute lymphoblastic leukaemia

“…For species C adenovirus to contribute to the earliest steps in development of childhood leukaemia, this virus must infect the fetus at least as frequently as the incidence of neonatal preleukaemic clones in the population, estimated at 1 -5% (Mori et al, 2002). In addition to being a likely fetal pathogen (Towbin et al, 1994;Van den Veyver et al, 1998;Oyer et al, 2000;Baschat et al, 2003;Reddy et al, 2005), adenovirus DNA is also detected in the amniotic fluid from apparently normal pregnancies.…”

Expression profile of wild‐type ETV6 in childhood acute leukaemia

“…The ETV6-CBFA2 fusion product was detected in neonatal blood spots obtained from children who have ETV6-CBFA2 ALL (Wiemels et al, 1999), suggesting that the events leading to the translocation occur in utero. The fusion product was also detected in anonymized cord blood samples at a frequency of a hundred times higher than that expected from the incidence of the corresponding leukaemia (Mori et al, 2002). This, the long postnatal latency period prior to the development of leukaemia, and murine models (Andreasson et al, 2001), suggest that subsequent to the creation of the fusion gene, secondary changes are required for leukaemogenesis.…”

A conditional model of MLL-AF4 B-cell tumourigenesis using invertor technology

“…However, characterization of sorted cells from leukemic bone marrow of childhood high-risk ALL/ t(4;11) suggested an origin in a primitive lymphoidrestricted progenitor cell (Hotfilder et al, 2005). Moreover, studies in monozygotic twins with leukaemia, carrying an identical MLL-AF4 chromosomal translocation, but different immunoglobulin rearrangements strongly supported the model that the primary MLL-AF4 target cell was a progenitor cell at a differentiation stage before efficient RAG expression (Mori et al, 2002;Greaves et al, 2003). This issue could be addressed using the Mll-AF4 invertor model and an inducible Cre driven by a promoter of a gene, such as Lmo2, expressed in bone marrow progenitor cells.…”