Sustained NF-kappaB activity in chronic lymphocytic leukemia is independent of genetic and epigenetic alterations in the TNFAIP3 (A20) locus

Inappropriate nuclear factor (NF) kappa B activity is one major hallmark of B-cell malignancies and chronic lymphocytic leukemia (CLL). NF kappa B-dependent genes are involved in antiapoptosis, cell proliferation and metastasis and are responsible for survival and proliferation of tumors. However, the mechanisms of NF kappa B activity in CLL still need to be elucidated. Previously, we identified translocations in a region on chromosome 6q that encodes tumor necrosis factor alpha-induced protein 3, which is a key player in negative feedback loop regulation of NF kappa B. Inactivation of this ubiquitin-editing enzyme is involved in immunopathologies and in tumorigenesis. Frequent mutations in the A20 locus-leading to sustained NF kappa B activity-could be shown to play a dominant role in development of different B-cell malignancies. To check if A20 is involved in upregulation of NF kappa B activity in CLL, we sequenced Exons 2-9 of the A20 gene in 55 CLL DNA samples. Furthermore, we determined the methylation status of the promoter region in 63 CLL DNA samples and compared to 10 control DNAs of B cells from healthy donors. Contrary to reports from other B-cell malignancies, the A20 region showed neither mutations nor aberrant DNA methylation. Moreover, its expression could be confirmed by immunoblotting and showing comparable results to healthy B cells. These results indicate that malignant development in CLL differs from most of other B-cell malignancies, which show frequent inactivation of A20.