Minimal residual disease in acute lymphoblastic leukemia: technical aspects and implications for clinical interpretation

Minimal residual disease (MRD) monitoring has proven to be one of the fundamental independent prognostic factors for patients with acute lymphoblastic leukemia (ALL). Sequential monitoring of MRD using sensitive and specific methods, such as real-time quantitative polymerase chain reaction (qPCR) or flow cytometry (FCM), has improved the assessment of treatment response and is currently used for therapeutic stratification and early detection. Although both FCM and qPCR yield highly consistent results with sensitivities of 10(-4), each method has several limitations. For example, qPCR is time-consuming and laborious: designing primers that correspond to the immunoglobulin (IG) and T-cell receptor (TCR) gene rearrangements at diagnosis can take 3-4 weeks. In addition, the evolution of additional clones beyond the first or index clone during therapy cannot be detected, which might lead to false-negative results. FCM requires experienced technicians and sometimes does not achieve a sensitivity of 10(-4). Accordingly, a next generation sequencing (NGS)-based method has been developed in an attempt to overcome these limitations. With the advent of high-throughput NGS technologies, a more in-depth analysis of IG and/or TCR gene rearrangements is now within reach, which impacts all applications of IG/ TR analysis. However, standardization, quality control, and validation of this new technology are warranted prior to its incorporation into routine practice.