Gene Expression and Proteomic Profiling Predict Therapeutic Response to ABT-737 in Human and Mouse Models of Mantle Cell Lymphoma

Currently, there is no curative therapy for mantle cell lymphoma (MCL), an entity characterized by aberrant CCND1/Cyclin D1 activation followed by sequential genetic abnormalities that condition the neoplastic phenotype and therapeutic responses. To improve outcome of patients with MCL, novel drugs targeting molecules aberrantly expressed in MCL have been developed. However, initial clinical trials are showing that these compounds are only effective in subsets of patients, underlying the need for selective markers to predict specific drug-sensitivity on an individual basis. Evaluation of these drugs is also hampered by the absence of experimental mouse models that mimic human MCL. Our goal in this study was to search for such predictive therapeutic markers in an in vitro system with human cell lines and in a novel human-like MCL transgenic mouse model. We treated ten extensively characterized human MCL cell lines with a variety of compounds currently used in MCL therapy. Individual cytotoxicity to the drugs was measured and correlated with genomic, gene expression and proteomic tumor profiles. We have initially focused on the BH3-only mimetic ABT-737, which selectively killed four MCL cell lines while six remained resistant in vitro (p<0,0001). The ABT-737-sensitive subgroup did not show different cytotoxicity profiles to CDK-inhibitors (flavopiridol, roscovitine), histone deacetylase inhibitors (TSA and valproic acid) and proteasome inhibitors (bortezomib) compared to the resistant cell line subgroup. Genetic profiling revealed that genomic amplification of chromosome 18q21, causing gene and protein over-expression of Bcl2, was exclusively present in the four sensitive cell lines (p<0,001). Additional proteomic studies revealed that response to ABT-737 also correlated with lower levels of Mcl1 but not with alterations of other proteins commonly involved in MCL pathogenesis, such as Bim, Bcl-XL, Cdk4/Cdk6, p53, p16/Arf, Rb, Atm and Myc, or with CCND1/cyclin D1 expression levels. Ectopic expression of BCL2 in refractory MCL cell lines significantly reversed ABT-737 resistance, as shown by increased apoptosis determined by annexin V staining and cytochrome C release. Similar data were obtained when ABT-737 therapy was applied to MCL cell line xenogratfs with different Bcl2 expression levels injected into immunodeficient mice. To further evaluate these data in vivo, we generated a MCL model in mice by integrating a CombiTA-Cyclin D1 vector into murine pro-B cells. To induce additional genetic changes trying to mimic human disease, isolated single-cell transgenic clones were irradiated ex vivo (1 to 10 Gy) and then injected into RAG2−/− IL2gc−/− immunocompromised mice. After 3 to 5 weeks, several clones generated cyclin D1-expressing lymphomas. In these tumors, sensitivity to ABT-737 was related to Bcl2 expression levels, validating our previous data. Analysis of the genomic status and expression level of Bcl2 was measured by FISH, array-CGH and immunohistochemistry on clinical samples. Among 198 MCL biopsies evaluated, 22 (11%) presented genomic gain/amplification of BCL2, in most cases correlated with Bcl2 over-expression. Our data indicate that genomic amplification and over-expression of Bcl2 can predict sensitivity to ABT-737 in MCL. Consequently, ABT-737 may be selectively effective in a subset of patients with MCL