Nondisjunction of chromosomes leading to hyperdiploid childhood B-cell precursor acute lymphoblastic leukemia is an early event during leukemogenesis.

A hyperdiploid karyotype is found in 30% of B-cell precursor acute lymphoblastic leukemias in childhood. The time of nondisjunction of chromosomes leading to hyperdiploidy during leukemogenesis is unknown. We used the 3 clonotypic immunoglobulin heavy chain (IgH) gene rearrangements as molecular markers for each of the 3 chromosomes 14 in a case with hyperdiploid acute lymphoblastic leukemia to define the order of events-namely, somatic recombination and nondisjunction of chromosomes-during leukemia development. A partial sequence homology of the incomplete DJ(H) rearrangement with 1 of the 2 nonfunctional VDJ(H) rearrangements suggests that the doubling of chromosomes had occurred after this DJ(H) rearrangement and thus during early B-cell differentiation. The occurrence of the nondisjunction of chromosomes as well as ongoing rearrangement processes in utero were confirmed by the presence of all 3 IgH rearrangements in neonatal blood spots, providing the first evidence that hyperdiploidy formation is an early event in leukemogenesis in these leukemias.