Chronic lymphocytic leukemia (CLL) is a unique malignancy characterized by the accumulation of quiescent lymphocytes in the peripheral blood. Clinical outcomes in CLL are very heterogenous. Multiple efforts have been undertaken to better predict prognosis in each individual case of the disease. In this respect, assessment of the IgV^sub H^ mutational status has become the gold standard. Recently, it has been shown that high ZAP-70 (Zeta-chain associated protein kinase - 70 kDa) expression level correlates with low IgV^sub H^ mutational status and predicts poor outcome in B-CLL. A variety of molecular pathways (such as bcl-2 and p53) involved in cell cycle control have been investigated in an effort to explain this phenomenon. Dipeptidyl Peptidase 2 (DPP2) is a serine protease known to maintain cells in the quiescent state. In this study we investigated if DPP2 is involved in cell cycle control in B-CLL.
The study included 38 patients with B-CLL and 20 healthy controls. Median age of CLL patients was 67 years. Median time from diagnosis to enrollment in the study was 102 months. 21 patients (55.3%) received treatment before enrollment in the study. Standard Ficoll-Hypaque techniques were used to isolate peripheral blood mononuclear cells from healthy donors and CLL patients. Cells were treated with Val-boro-Pro, an inhibitor of DPP2, harvested after 16-24 hours and resuspended in propidium iodide (PI) buffer. PI uptake was immediately assessed by flow cytometry to evaluate apoptosis. Expression of CD38 in CLL was assessed by flow cytometry, ZAP-70 - real-time reverse-transcription polymerase chain reaction, bcl-2 and p53 - western blot analysis.
In this study specific inhibition of DPP2 resulted in caspase-dependent apoptosis of lymphocytes from all healthy subjects. In individuals with CLL, death of B-lymphocytes (and rescue by caspase inhibitors) was observed in 22 cases (57.9%). In the remaining 16 cases (42.1%) malignant B-cells did not undergo apoptosis upon inhibition of DPP2. We found that CLL cells resistant to DPP2 inhibition-induced apoptosis demonstrated higher levels of expression of ZAP-70. Those patients exhibited worse disease prognosis such as shorter treatment-free period (p
Thus, resistance vs. susceptibility to DPP2 inhibition-induced apoptosis can be employed as a novel prognostic factor in CLL.
ALEX V. DANILOV, MD; ANDREAS K. KLEIN, MD
Memorial Hospital; New England Medical Center
Copyright Rhode Island Medical Society Nov 2005
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