Mitotic crossover promotes leukemogenesis in children born with TEL-AML1 via the generation of loss of heterozygosity at 12p
TEL-AML1 (ETV6-RUNX1) fusion gene which is formed prenatally in 1% of the newborns, is a common genetic abnormality in childhood Bcell precursor acute lymphoblastic leukemia. But only one child out of a hundred children born with this fusion gene develops leukemia (bottleneck phenomenon) later in its life, if contracts the second mutation. In other words, out of a hundred children born with TEL-AML1 only one child is at risk for leukemia development, which means that TEL-AML1 fusion gene is not sufficient for overt leukemia. There is a stringent requirement for a second genetic abnormality for leukemia development and this is the real or the ultimate cause of the leukemia <em>bottleneck</em> phenomenon. In most cases of TEL-AML1<sup>+</sup> leukemia, the translocation t(12;21) is complemented with the loss of the normal TEL gene, not involved in the translocation, on the contralateral 12p. The loss of the normal TEL gene, <em>i.e.</em> loss of heterozygosity at 12p, occurs postnatally during the mitotic proliferation of TEL-AML1<sup>+</sup> cell in the mitotic crossing over process. Mitotic crossing over is a very rare event with a frequency rate of 10<sup>–6</sup> in a 10 kb region. The exploration and identification of the environmental exposure(s) that cause(s) proliferation of the TELAML1<sup>+</sup> cell in which approximately 10<sup>6</sup> mitoses are generated to cause 12p loss of heterozygosity, <em>i.e.</em> TEL gene deletion, may contribute to the introduction of preventive measures for leukemia.