Aberrations of Genes Regulating NF Kappa B Pathway in B-Cell Malignant Lymphoma.

Abstract 971

NFκB is a tightly regulated transcription factor of lymphocyte activation, proliferation and development. Controlled activity of NF κ B signaling pathway plays critical roles in coordination of immune and inflammatory response. Constitutive NFκB activation is recognized as a key pathological feature in several subsets of B-cell malignant lymphoma, and it is well known that lymphoma frequently occurred in association with chronic inflammation. Recently, our group showed frequent inactivation of A20, a negative regulator of NF κ B, in B-lineage malignant lymphomas. However, the molecular mechanism underlying the aberrant NF κ B activation in lymphomagenesis has not fully understood.

In this study, to clarify the genetic basis of the aberrant NFκB activation, we performed genome-wide analysis of copy number alterations as well as allelic imbalances of primary B-lineage lymphoma specimens using Affymetrix GeneChip 250K genomic microarray with the CNAG/AsCNAR algorithm. We also searched for possible mutations in CARD11, CYLD, IKK and TRAF family genes and IκB genes, to obtain comprehensive registry of lesions in genes regulating NFκB pathway. This study included 238 primary lymphoma samples, including 64 samples of diffuse large B-cell lymphomas (DLBCL), 52 of follicular lymphomas (FL), 35 of mantle cell lymphomas (MCL), and 87 of mucosa-associated tissue (MALT) lymphomas. Five Hodgkin lymphoma-derived cell line (KM-H2, L1236, HDLM2, RPMI1666, L540) was also analyzed.

Through a genome-wide analysis, we identified that each histology type had a unique genomic signature, suggesting a distinctive underlying molecular pathogenesis for different histology types. In contrast to the fact that A20 mutation was highly associated with loss of heterozygosity at 6q23.3, mutations of CARD11 (5 cases of DLBCL, 2 cases of MALT lymphoma) and IκB family genes (2 cases of DLBCL and 1 cases of MALT lymphoma) had no association with copy number abnormality at the locus of the genes. In total, mutations and copy number alterations in genes regulating NFκB pathway were found in more than 40% of B-cell lymphomas, which underpinned the importance of aberrant NFκB activation in lymphomagenesis.

To also assess the role of uncontrolled signaling of NFκB pathway in lymphomagenesis, we re-expressed wild-type A20 in two lymphoma-derived cell lines without normal functional A20 alleles (KM-H2 and L1236). In both cells, re-expression of wild-type A20 resulted in suppression of cell growth and induction of apoptosis, accompanied by down-regulation of NFκB activation. The A20-deficient KM-H2 stably generated tumors in immunodeficient mice, whereas the tumorigenicity was effectively suppressed by re-expression of A20. We further investigated the role of A20 inactivation during clonal expansion of lymphoma by competitive proliferation assays using A20-deficient lymphoma-derived cell lines with or without re-expression of A20. The proportion of A20-expressing cells gradually decreased during competitive cell culture, and A20-expressing cells outgrew control cells in NOG mice, indicating the importance of A20 inactivation in clonal evolution of lymphoma.

We demonstrated that uncontrolled NFκB signaling caused by alterations of multiple genes is a common feature of B-lineage lymphomas. Considering the physiological function of NFκB activation induced upon a variety of upstream stimuli, our observations provide an intriguing insight into and the pathogenesis of lymphoma. Our study also indicated that NFκB inhibition may have a role in lymphoma therapeutics.