A Highly Sensitive Method for Detecting Minimal Residual Disease in B-CellChronic Lymphocytic Leukemia: Interphase Fluorescence In Situ Hybridization on Flow-Sorted Cells

The introduction of new therapeutic strategies has resulted in increased complete remission rates in B-cell chronic lymphocytic leukemia (CLL). Preliminary data have suggested that the absence of minimal residual disease (MRD) is an endpoint of therapy that, if achieved, translates into an improved survival. We developed and validated a novel, combined method to assess MRD in CLL using fluorescence-activating cell sorting (FACS) and interphase fluorescence in situ hybridization (FISH) for the detection of numeric chromosomal aberrations. Cell sorting was performed on a FACS-Aria based on the CD19+CD5+ co-expression. Interphase FISH analysis was applied to purified cells with commercially available probes (Vysis/Abbott). Spiking experiments of CLL cells harboring a numeric chromosomal abnormality into normal blood with dilutions of 10−3 to 10−6 were performed; FISH detection of the specific chromosomal aberration in CD19+CD5+ purified cells allowed discrimination of CLL cells from normal precursor B-cells. Positive results were demonstrated in all dilutions up to 10−5 or even 10−6. This approach for the assessment of MRD in CLL reaches sensitivity at least as high as and even higher than other methods, such as 4-color flow cytometry or quantitative allele-specific polymerase chain reaction. It can be used for ≥ 80% of patients with CLL, including all patients with CLL with poor prognosis, as assessed by the presence of the deletion 11q (ataxia telangiectasia-mutated) or the deletion 17p (p53). Furthermore, it allows easy standardization among laboratories that apply FACS because it is based on 2-color labeling only and on FISH assays using commercially available probes.