Date of Award

2012

Degree Type

Dissertation

Department

Biological, Geological and Environmental Sciences

First Advisor

Almasan, Alexandru

Subject Headings

Apoptosis, B cells -- Tumors -- Genetic aspects, Chronic lymphocytic leukemia, Apoptosis, Bcl-2 family, chronic lymphocytic leukemia, ABT-737

Abstract

The anti-apoptotic Bcl-2 proteins regulate lymphocyte survival and are over-expressed in lymphoid malignancies, including chronic lymphocytic leukemia (CLL). The small molecule inhibitor ABT-737 binds with high affinity to Bcl-2, Bcl-xl, and Bcl-w but with low affinity to Mcl-1, Bfl-1, and Bcl-b. The active analog of ABT-737, navitoclax, has shown a high therapeutic index in lymphoid malignancies developing a predictive marker for it would be clinically valuable for patient selection or choice of drug combinations. We compared expression of anti-apoptotic Bcl-2 genes that are known to be targeted by ABT-737. Our findings reveal that the relative ratio of Mcl-1 and Bfl-1 to Bcl-2 expression provides a highly significant linear correlation with ABT-737 sensitivity (r = 0.6, P < .001). The (Mcl-1 + Bfl-1)/Bcl-2 ratio was validated in a panel of leukemic cell lines subjected to genetic and pharmacologic manipulations. We also tested whether anti-apoptotic Bcl-2 proteins can be used as prognostic marker in CLL, since the current prognostic markers, such as CD38 and ZAP70 fail to predict the clinical outcome in a substantial number of CLL patients. Our findings indicate that only high Bcl-xl expression is strongly correlated (P=0.002) with short treatment-free survival. Strikingly, Bcl-xl was able to identify patients with high risk in the ZAP70 negative and CD38 negative groups as well as in patients with normal/unknown p53 deletion, thus providing a very powerful prognostic value for CLL. Finally, we investigated the potential mechanism responsible for ABT-737 resistance in leukemic cell lines. Compared with parental cells, cells that have developed acquired resistance to ABT-737 showed increased expression of Mcl-1 due to increased protein stability. Interestingly, increased Mcl-1 levels sequester the BH3-only protein Bim which mediates resistance to ABT-737. These data therefore reveal novel insights into the role of anti-apoptotic Bcl-2 proteins in clinical response, acquired resistance, and tumor progression in

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