Inhibition of the apoptosis cascade is an important cause of therapy resistance in Diffuse Large B-cell Lymphomas (DLBCL). In this study we determined expression levels of apoptosis related genes in isolated and selected lymphoma cells of DLBCL biopsies. Moreover, we investigated whether expression levels of apoptosis regulating genes correlate with sensitivity to therapy and whether neutralizing the effect of apoptosis inhibitors would result in induction of apoptosis.Using multiplex quantitative RT-PCR analysis, two DLBCL groups were identified; one with relatively low expression levels of both pro- and anti-apoptotic genes and one group with high expression levels of these genes. Clinical response to chemotherapy in cases with low expression of apoptosis regulating genes was significantly more favorable than cases with high expression levels. Functional analysis in the latter DLBCL samples and DLBCL cell lines with matching expression profiles revealed high levels of spontaneous caspase 9 activity. Cell lines showing high expression levels of pro-apoptotic genes were resistant to Etoposide induced caspase 3/7 activation and apoptosis, whereas cell lines with low expression levels of these genes where sensitive to Etoposide induced caspase 9-dependent apoptosis. Inhibition of the effect of XIAP resulted in induction of caspase 9 mediated apoptosis and caspase 3/7 activation in cells with constitutive caspase 9 activity.We conclude that a subset of DLBCL demonstrate constitutive activation of the intrinsic apoptosis pathway with concomitant downstream inhibition of this pathway. In these DLBCL neutralizing the function of XIAP restores caspase 9 dependent apoptosis. Thus, inhibition of downstream apoptosis inhibitory proteins may be an important alternative therapy for chemotherapy refractory DLBCL and apoptosis profiling can predict the response to apoptosis sensitizing therapy.

[First AACR International Conference on Molecular Diagnostics in Cancer Therapeutic Development, Sep 12-15, 2006]