Constitutive 53BP1/γH2AX foci are increased in cells of ALL patients dependent on BCR-ABL and TEL-AML1 preleukemic gene fusions.

Childhood leukemia arises from hematopoietic stem cells by induction of mutations. Quite often chromosomal translocations arise prenatally as first key event in multistage process of leukemogenesis. These translocations result in so called preleukemic gene fusions (PGFs), such as BCR-ABL and TEL-AML1, which generate hybrid proteins with altered properties. Critical DNA damage resulting in translocations are DNA double-strand breaks (DSBs). BCR-ABL and TEL-AML1 were shown to be associated with increased constitutive DSBs in various model systems. We analyzed cells from peripheral blood and CD34-/CD34+ cells from bone marrow of pediatric acute lymphoblastic leukemia (ALL) patients harboring BCR-ABL or TEL-AML1. We used sensitive technique that is based on automated enumeration of DSB co-localizing proteins γH2AX and 53BP1, which form so called DNA repair foci. We found that level of constitutive γH2AX/53BP1 foci is significantly higher in cells of ALL pediatric patients than in healthy subjects. There was also significant increased level of constitutive γH2AX/53BP1 foci in cells from ALL patients harboring BCR-ABL or TEL-AML1 compared to patients without PGFs. The same increase was observed regardless cell type for both PGFs. Our data on increased DSB levels in the BCR-ABL/TEL-AML1 patient's cells support a model where BCR-ABL/TEL-AML1 induces DNA instability through facilitating mutagenesis and appearance of additional genetic alterations driving leukemogenesis.