The Molecular Basis for BCL-2 Oncogene Addiction in CLL.

CLL cells consistently express BCL-2 at a high level. Using a compound demonstrated to antagonize BCL-2 function, ABT-737, we show that CLL cells in short-term primary culture are uniformly sensitive to single-agent BCL-2 antagonism, with EC50’s in the 10 nM range. To understand the mechanism of CLL sensitivity to this compound, we studied mitochondria from these primary CLL cells. We employed a panel of peptides derived from the BH3 domain of pro-death BH3-only proteins, certain of which selectively inhibit BCL-2 function in vitro. We demonstrate that those with activity against BCL-2 in vitro induce cytochrome c release, a hallmark of the mitochondrial dysfunction during apoptosis. A non-malignant cellular model of BCL-2 dependence revealed that binding of the pro-apoptotic protein BIM to BCL-2 correlated with sensitivity to BCL-2 inhibitors. We likewise discovered that BCL-2 binds and sequesters BIM in CLL cells. In contrast to the BCL-2 dependent CLL cells, we examined myeloma cell lines with defined MCL-1 dependence and relative BCL-2 independence. These cell lines were 100 fold less sensitive to ABT-737. Furthermore, in mitochondrial assays only BH3 peptides which interact with MCL-1 induce cytochrome c release. These experiments reveal that cellular requirement for an anti-apoptotic protein function can be deduced from a functional mitochondrial requirement determined using our panel of BH3 peptides. In CLL, and perhaps other malignancies, it appears that BCL-2 maintains survival by sequestering pro-apoptotic proteins like BIM. Inhibition of this sequestration provokes cell death ex vivo, even with a single agent BCL-2 inhibitor. Reliance on BCL-2 for survival may be an Achilles heel for CLL and other cancers.