Bax Degradation Activity Is a Novel Poor Prognosis Marker of Chronic Lymphocytic Leukemia.

B-cell chronic lymphocytic leukemia (CLL) is a highly heterogeneous disease, with prognosis varying from a normal lifespan to death early after diagnosis. A large number of prognostic markers are now available in CLL. Better understanding of the mechanisms underlying the association of these markers with poor outcomes may guide their use in the clinic, as well as identify new therapeutic targets. Bax is an essential pro-apoptotic protein and decreased levels in malignant cells lead to resistance to apoptosis and chemotherapy. Using a Bax degradation activity (BDA) assay, CLL cells were found to show variable Bax instability. However, BDA did not correlate with Bax protein levels, BDA positive and negative cases each showing high and low baseline Bax levels. We looked at a cohort of patients with CLL and compared Bax degradation activity with established prognostic markers: Ig VH mutation status, Zap-70, CD38, cytogenetics, as well as clinical stage and lymphocyte doubling time (LDT). BDA positive cases showed a marked accumulation of poor prognostic markers - unmutated immunoglobulin heavy chain variable genes (VH), ZAP-70 and CD38 positivity, 11q22 and 17p13 deletion, and short lymphocyte doubling time. Patients with BDA positive cells had a shorter median overall survival (OS) (126 months v. not reached, p = 0.011) and shorter time to first treatment (16 v. 156 months, p = 0.029) than BDA negative cases; while patients with dual BDA and ZAP-70 positivity had a median OS of 84 months (p = 0.012). The BDA assay reflects the differential intrinsic ubiquitin/proteasome activity of CLL cells and measures dynamic changes in Bax protein levels, as opposed to single time point measurements. Mechanistically, Bax protein instability may represent a final common pathway for disparate prognostic markers, as well as being itself an indicator of poor prognosis.