SNP-Arrays Provide New Insights Into the Pathogenesis of Richter Syndrome

RS represents the development of a diffuse large B-cell lymphoma (DLBCL) in the context of chronic lymphocytic leukemia (CLL). The pathogenesis of RS is still largely unknown. Analysis of RS has been often focused on the study of lesions previously identified in de novo DLBCL. However, we have previously shown that RS lacks many of the typical genetic lesions shown in DLBCL (Rossi et al, Blood 2011). Here, we have applied an unbiased, genome-wide approach, searching for DNA copy number alterations in a large series of RS, comparing them with de novo DLBCL, CLL-phase of RS and untransformed CLL. Methods. The study included 50 RS, 30 CLL-phases preceding RS, 100 DLBCL, 318 CLL. All RS were classified as DLBCL. DNA was extracted from frozen biopsies and was processed using the Affymetrix GeneChip Human Mapping SNP6 arrays. Differences in frequencies between subgroups were evaluated using Fisher's exact test. Clinical data were available in half of the cases and genomic lesions were evaluated for their impact on clinical outcome with the log-rank test. Results. In RS, the most common gains were +12 (40%), +4q12 (14%), +2p25.3-25.2 (13%), +8q23.3-qter (12%), +1q32.1, +2p16.1-13.2 (BCL11A/REL), +15q22.31-26.3, +18q21.33 (BCL2) (10%), and +3q24.1-q26.3 (9%). Amplifications were observed at the BCL2 locus and at 1q32.1. The most common losses were at 17p13.1 (TP53; 29%), 8p21.3 (TNFRSF10A/B; 15%), 8p23.3 (16%), 11q22.3 (ATM) (15%), 14q23.3 (13%), 7q33-35 (11%), 14q32.11 (10%), 14q32.31-32.33 (TRAF3), and 9p21.3 (CDKN2A) (9%). The latter locus was also the target of recurrent homozygous deletions. RS appeared intermediate between CLL-phase and de novo DLBCL in terms of number of recurrent lesions. When compared with de novo DLBCL, RS presented less -6q23 (TNFAIP3; P=2.54E-05), -15q15.1-q21.1 (B2M) (P=5.64E-04), +7/7q (P=5.71E-04), -6q21 (PRDM1; P=6.53E-04), +2p16.1 (BCL11A; P=3.22E-03), -1p36 (TNFRSF14; P 9.57E-03), copy-neutral LOH at 6p25.3-21.32 (P =1.01E-02), +5p (P=1.39E-02), +11q14.1-q25 (P=1.63E-02), +1q22-q41 (P 2.13E-02), +6p21 (P 2.48E-02). Conversely, RS presented more often +4q12 (P=8.96E-03), -7q33-35 (P=2.04E-02), -11q22.3 (ATM; P=2.14E-02), -14q23.3 (P=2.69E-02). No statistical differences were observed for lesions such as +3/+3q, +8q, -8p, - 9p21.3 (CDKN2A), +12q, +15q, -17p (TP53). Compared to CLL-phase, RS had significantly more gains at 1q32.1 (P=2.28E-02), 11q23.3 (P=5.38E-03), 11q24 (P=1.10E-02) and 9p21.3 losses (CDKN2A; P=1.10E-02). Compared to the large series of CLL with no history of transformation, CLL-phase samples presented less frequently losses at 13q14.3 (DLEU2, MIR15A, MIR16-1a; 6.58E-03), more commonly gains at 12 (P=2.66E-02), 4q12 (P=1.103E-02), +2p (P=2.97E-02), and loss at TP53 locus (P=2.23E-03). The presence of losses of CDKN2A (P=4.6E-03), TNFRSF10A/B (P=3.2E-02) and BCL2 gains (P=4.0E-02) determined a poor overall survival in RS. Conclusions. DNA copy number changes in RS are intermediate between de novo DLBCL and CLL-phase. Recurrent lesions were identified that appeared more common in RS than in DLBCL, and that seem to be involved in the progression from CLL to an aggressive lymphoma. Also, the acquisition of mono- and bi-allelic losses of CDKN2A appears as one of the most important event in the transition from CLL-phase to RS. CDKN2A and TNFRSF10A/B losses and BCL2 gains were associated with a poor outcome.