Molecular discrimination between relapsed and secondary acute lymphoblastic leukemia: proposal for an easy strategy

Discrimination between late relapse of acute lymphoblastic leukemia (ALL) and secondary ALL might be clinically important, because the former might still respond favorably to chemotherapy and/or bone marrow transplantation, whereas secondary ALL is associated with poor prognosis.

We present a pre-B-ALL patient in whom disease recurred 2 years after completion of treatment. Differences in cytomorphology and immunophenotyping raised a suspicion of secondary ALL. We performed detailed molecular studies of immunoglobulin and T-cell receptor genes for discrimination between relapsed and secondary ALL.

Southern blot analysis showed an oligoclonal immunoglobulin heavy chain (IGH) gene configuration at diagnosis and a monoclonal configuration at relapse. The size of one of the rearranged bands at relapse was identical to one of the faint rearranged bands at diagnosis. However, heteroduplex PCR analysis demonstrated that none of the clonal IGH gene rearrangements at diagnosis and at relapse was fully identical. Sequencing of several clonal PCR products revealed an identical DH6-13<-->JH6b junction shared by two different rearrangements at diagnosis and one rearrangement at relapse, thereby proving the clonal relationship between diagnosis and late relapse in this patient.

We propose a stepwise molecular approach for discrimination between relapsed and secondary ALL based on the rapid and cheap heteroduplex PCR technique, including mixing of clonal (homoduplex) PCR products identified at diagnosis and at relapse. Direct sequencing and comparative sequence analysis of IGH gene rearrangements at diagnosis and at relapse should be regarded as an ultimate standard, but can be limited to the rare cases, in which no identical clonal PCR products at diagnosis and at relapse were detected with the mixed heteroduplex PCR analyses.