REARRANGEMENT STATUS OF THE MALIGNANT-CELL DETERMINES TYPE OF SECONDARY IGH REARRANGEMENT (V-REPLACEMENT OR V-JOINING TO DJ-JOINING) IN CHILDHOOD B-PRECURSOR ACUTE LYMPHOBLASTIC-LEUKEMIA

Immunoglobulin heavy chain (IgH) oligoclonality in childhood B precurs or acute lymphoblastic leukemia (ALL) as determined by Southern analys is is found in 30-50% of patients and has been shown to be the result of ongoing IgH rearrangement (mostly V-H-replacement and V-H to D-J(H) joining) after malignant transformation. It is unknown however, what determines the type of secondary rearrangement. Also the biological ba sis of the variable degree of oligoclonality observed in childhood ALL is poorly understood. We analyzed in detail the IgH rearrangement sta tus of the leukemic cells for a random panel of 18 childhood B precurs or ALL patients by polymerase chain reaction (PCR)/sequencing analysis and by Southern analysis. By Southern analysis 10/18 (55.6%) patients were considered oligoclonal and 8/18 (44.4%) monoclonal. In contrast, by PCR minor clonal rearrangements were detected in 14/18 (77.8%) pat ients. V-H-replacement was found in 7/14 patients, V-H to D-J(H) joini ng in 6/14 patients and an unusual type of secondary rearrangement, V- H-D to J(H) joining, in one patient. Only a single type of secondary r earrangement was detected in each patient. The type of secondary rearr angement (V-H-replacement or V-H to D-J(H) joining) depended on the re arrangement status (VDJ/VDJ or VDJ/DJ, respectively) of the dominant l eukemic clone as determined by Southern analysis. We found that in add ition to a more 'advanced' IgH rearrangement status patients with V-H- replacements also have a more 'advanced' TCR delta rearrangement statu s, which possibly reflects exposure of both the IgH locus and the TCR delta locus to recombinase activity in a preleukemic clone. Finally, w e investigated a putative relationship between oligoclonality by South ern analysis and S-phase fraction of the leukemic cell population. We found a significantly lower percentage cells in S-phase for oligoclona l patients as compared to monoclonal patients. Our data add to the und erstanding of ongoing rearrangement of antigen receptor loci in childh ood ALL and have implications for the monitoring of minimal residual d isease by PCR.