Publication Date:
2013-11-15
Description:
Background Follicular lymphoma (FL) is a B-cell lymphoma whose cytogenetic hallmark is the translocation t(14;18)(q32;q21) which juxtaposes the BCL2 oncogene to the immunoglobulin heavy chain locus (IGHV). FLs maintain key features of normal germinal center reactions, such as ongoing somatic hypermutation (SHM) of IGHV genes and selection for a functional B-cell receptor. SHM is mediated by activation-induced cytidine deaminase (AID) leading to single nucleotide exchange in IGHV genes and to a much lesser extent in non-IG genes. It was our objective to investigate the clonal evolution of t(14,18) FL from primary to relapse tumors simultaneously on several genetic and epigenetic levels. Methods We studied paired primary and relapsed tumors from 33 patients with t(14;18)-positive FL (76 samples: 25 pairs; 6 trios and 2 quadruples). In a core set of 19 patients we performed Sanger and next generation sequencing of the clonal VHDHJH rearrangements of the IGHV locus. We performed deep sequencing of 9 genes (BCL2, BCL6, MYC, RHOH, PAX5, IRF4, C2TA, REL and PIM1) targeted by aberrant SHM (aSHM) in lymphoma for 69 FL samples including the complete core set. We furthermore analyzed mutations in the coding regions of 10 candidate driver genes of lymphomagenesis (BCL2, MLL2, CREBBP, TNFRSF14, EZH2, EP300, MEF2B, BCL6, MYC, TP53) by deep sequencing. In addition to genetic analyses, evolutionary patterns of DNA-methylation were addressed by Illumina 27k arrays. Results We found strong evidence for ongoing selection against replacement mutations in the IGHV genes both in complementarity determining regions and framework regions, consistent with ongoing dependence of FL on a functional B-cell receptor and stimulation by antigens. Using mean normalized Hamming distance as a quantitative measure for the evolutionary divergence of paired samples (IGHV-divergence) and analyzing phylogenetic trees we classified the patterns of evolution into 3 categories: “No Evolution” (shared IGHV sequences in primary and relapse tumors), “Sequential Evolution” (relapse sequences emerge out of primary ones), “Divergent Evolution” (sequences of primary and relapse sample appear disjoint). We observed a mutation frequency of 62.0 per 100 kb in aSHM targets. These mutations were strongly enriched at the WRCY/RGYW target motifs characteristic for the SHM/AID machinery (OR=3.4; p
Print ISSN:
0006-4971
Electronic ISSN:
1528-0020
Topics:
Biology
,
Medicine
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