Immunoglobulin light chain gene rearrangements display hierarchy in absence of selection for functionality in precursor-B-ALL

The general order of the immunoglobulin (Ig) gene rearrangement process in human precursor-B cells is largely known. However, the exact Ig rearrangement patterns reflecting this process, especially those of the Ig light chain genes, are not well established. This requires detailed analysis of the gene configuration of all six IGH, IGK and IGL alleles at the single cell level. As such extensive analyses are difficult to perform in a reliable way within a single normal precursor-B cell, we used 169 precursor-B-ALL (ie six pro-B-ALL, 112 common ALL, and 51 pre-B-ALL) as clonal 'single cell' model system. The Ig gene recombinations show hierarchy starting with IGH gene rearrangements in all cases, followed by IGK rearrangements, 1GK deletions and/or IGL rearrangements in 71% of cases. 1GK deletions were found in the absence of IGL rearrangements in 34% of cases, which might be explained by the continuous recombinase activity in precursor-B-ALL, resulting in 'endstage' 1GK rearrangements, together with an apparently limited accessibility of the IGL locus. Remarkably, in 5% of cases IGL rearrangements took place in the absence of 1GK rearrangements. In addition we found that in-frame IGH rearrangements are not necessarily required for the induction of Ig light chain gene rearrangements and that IGL rearrangements can be induced irrespective of the frame of the accompanying 1GK rearrangements. In conclusion, precursor-B-ALL constitute a model system for studying Ig gene rearrangement processes without selection for functionality of the rearrangements or the influence of somatic hypermutations. Nevertheless, the hierarchy of IGH, 1GK and IGL rearrangements is apparent in precursor-B-ALL.