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V(D)J-mediated Translocations in Lymphoid Neoplasms: A Functional Assessment of Genomic Instability by Cryptic Sites

Most lymphoid malignancies are initiated by specific chromosomal translocations between immunoglobulin (Ig)/T cell receptor (TCR) gene segments and cellular proto-oncogenes. In many cases, illegitimate V(D)J recombination has been proposed to be involved in the translocation process, but this has never been functionally established. Using extra-chromosomal recombination assays, we determined the ability of several proto-oncogenes to target V(D)J recombination, and assessed the impact of their recombinogenic potential on translocation rates in vivo. Our data support the involvement of 2 distinct mechanisms: translocations involving LMO2, TAL2, and TAL1 in T cell acute lymphoblastic leukemia (T-ALL), are compatible with illegitimate V(D)J recombination between a TCR locus and a proto-oncogene locus bearing a fortuitous but functional recombination site (type 1); in contrast, translocations involving BCL1 and BCL2 in B cell non-Hodgkins lymphomas (B-NHL), are compatible with a process in which only the IgH locus breaks are mediated by V(D)J recombination (type 2). Most importantly, we show that the t(11;14)(p13;q32) translocation involving LMO2 is present at strikingly high frequency in normal human thymus, and that the recombinogenic potential conferred by the LMO2 cryptic site is directly predictive of the in vivo level of translocation at that locus. These findings provide new insights into the regulation forces acting upon genomic instability in B and T cell tumorigenesis.

High expression of lipoprotein lipase in poor risk B-cell chronic lymphocytic leukemia.

We investigated the pattern of lipoprotein lipase (LPL) expression in B-cell chronic lymphocytic leukemia (B-CLL) and assessed its prognostic relevance. Expression of LPL mRNA as well as protein was highly restricted to leukemic B cells. The intensity of intracellular immunoreactivity of LPL was higher in samples of patients with unmutated immunoglobulin heavy-chain variable region genes (IGVH) compared to those with mutated IGVH genes. LPL mRNA levels in peripheral blood mononuclear cells (PBMNC) from 104 CLL patients differed by 1.5 orders of magnitude between cases with mutated (N=51) or unmutated (N=53) IGVH (median: 1.33 vs 45.22 compared to normal PBMNC). LPL expression correlated strongly with IGVH mutational status (R=0.614; P<0.0001). High LPL expression predicted unmutated IGVH status with an odds ratio of 25.90 (P<0.0001) and discriminated between mutated and unmutated cases in 87 of 104 patients (84%). LPL expression was higher in patients with poor risk cytogenetics. High LPL expression was associated with a shorter treatment-free survival (median 40 vs 96 months, P=0.001) and a trend for a shorter median overall survival (105 months vs not reached). Our data establish LPL as a prognostic marker and suggest functional consequences of LPL overexpression in patients with B-CLL.

High expression of activation-induced cytidine deaminase (AID) mRNA is associated with unmutated IGVH gene status and unfavourable cytogenetic aberrations in patients with chronic lymphocytic leukaemia

Activation-induced cytidine deaminase (AID) is essential for somatic hypermutation of B-cells. We investigated the expression of AID mRNA by real-time polymerase chain reaction (PCR) in peripheral blood mononuclear cells of 80 patients with B-CLL. AID expression was detected in 45 of 80 patients (56%) at various levels, but was undetectable in 35 patients (44%). AID PCR positivity was associated with unmutated IGVH gene status (22 of 25 patients; P=0.002) and unfavourable cytogenetics (18 of 23 patients with deletion in 11q or loss of p53; P=0.040). Using a threshold level of 0.01-fold expression compared to Ramos control cells, even more significant associations were observed (P=0.001 for IGVH; P=0.002 for cytogenetics). A correlation was observed between individual AID levels and the percentage of VH homology (R=0.41; P=0.001). AID positivity predicted unmutated IGVH status with an odds ratio of 8.31 (P=0.003) and poor risk cytogenetics with an odds ratio of 3.46 (P=0.032). Significance was retained after adjustment for Binet or Rai stages. AID mRNA levels were stable over time. These data suggest a potential role of AID as a prognostic marker in B-CLL.

Autoimmune conditions and chronic infections in chronic lymphocytic leukemia patients at diagnosis are associated with unmutated IgVH genes

This study suggests that patients suffering from chronic conditions at the time of diagnosis of chronic lymphocytic leukemia are likely to have poor prognostic markers. Few data are available concerning the prevalence of autoimmune disease or chronic infections in chronic lymphocytic leukemia patients at diagnosis as well as their clinical outcome. We studied the frequency of such chronic conditions in relation to prognostic markers. A history of autoimmune disease or chronic infection was found in 21% of 186 chronic lymphocytic leukemia patients (12% in autoimmune diseases, 9% in chronic infections). Patients with a history of chronic stimulation were more likely to have unmutated IgVH genes (p<0.002), unfavorable or intermediate risk cytogenetics (11q, 17p deletions, trisomy 12) (p<0.001), and higher CD38 expression (p=0.004). Autoimmune conditions (n=22) were characterized by female predominance (55.0%) with a high frequency of unmutated IgVH (53,8%). Median time to first treatment was 83 months for the chronic stimulation group compared to 128 months for the non-chronic stimulation group (n.s.). Patients suffering from chronic conditions at chronic lymphocytic leukemia diagnosis are likely to have poor prognostic markers, particularly unmutated IgVH genes.

Overexpression of uridine diphospho glucuronosyltransferase 2B17 in high-risk chronic lymphocytic leukemia

Uridine diphospho glucuronosyltransferase 2B17 (UGT2B17) glucuronidates androgens and xenobiotics including certain drugs. The UGT2B17 gene shows a remarkable copy number variation (CNV), which predisposes for solid tumors and influences drug response. Here, we identify a yet undescribed UGT2B17 mRNA overexpression in poor-risk chronic lymphocytic leukemia (CLL). In total, 320 CLL patients and 449 healthy donors were analyzed. High (above median) UGT2B17 expression was associated with established CLL poor prognostic factors and resulted in shorter treatment-free and overall survival (hazard ratio ([death] 2.18; 95% CI 1.18-4.01; P = .013). The prognostic impact of mRNA expression was more significant than that of UGT2B17 CNV. UGT2B17 mRNA levels in primary CLL samples directly correlated with functional glucuronidation activity toward androgens and the anticancer drug vorinostat (R > 0.9, P < .001). After treatment with fludarabine containing regimens UGT2B17 was up-regulated particularly in poor responders (P = .030). We observed an exclusive involvement of the 2B17 isoform within the UGT protein family. Gene expression profiling of a stable UGT2B17 knockdown in the CLL cell line MEC-1 demonstrated a significant involvement in key cellular processes. These findings establish a relevant role of UGT2B17 in CLL with functional consequences and potential therapeutic implications.

Alternative end-joining in follicular lymphomas’ t(14;18) translocation

T(14;18) chromosomal translocation is assumed to result from illegitimate rearrangement between the BCL2 proto-oncogene and the IGH locus during the DH to JH joining phase of V(D)J recombination in early B cells. Analysis of the breakpoint junctions suggests that translocation derives from the fusion between normal V(D)J recombination intermediates at the IGH locus and non-V(D)J-mediated broken-ends at the BCL2 locus. So far, BCL2 broken-ends have only been observed fused to coding-ends, raising questions concerning the molecular constraints of the illegitimate joining process. Using a combination of genome walking and long-range PCR assays, we describe in this report that in 4.5% (2/44) of the t(14;18), one of the BCL2 broken-ends is fused to a signal-end. The formation of these JHRSS/BCL2 junctions provides direct evidence that BCL2 broken-ends are capable of joining to both products of V(D)J recombination, suggesting their presence in the RAG-mediated post-cleavage complex. In addition, junctions generated by this alternative end-joining do not involve deletion of the chromosome 14 intervening sequences generally lost in the standard translocation, providing a unique opportunity to investigate the rearrangement status of this region in the translocated IGH allele. In both cases, a DJH rearrangement could be detected 5′ of the JH-RSS/BCL2 junction. These findings, together with the previously reported bias towards the most external DH and JH segments in standard breakpoints, strongly suggest that t(14;18) preferentially occurs during an attempted secondary DH to JH rearrangement. This unusual and restricted window of differentiation opens intriguing questions concerning the etiology of the translocation.

Lipoprotein lipase in chronic lymphocytic leukaemia – Strong biomarker with lack of functional significance

In chronic lymphocytic leukaemia (CLL), lipoprotein lipase (LPL) mRNA overexpression is an established poor prognostic marker, its function, however, is poorly understood. Measuring extracellular LPL enzymatic activity and protein, we found no difference between levels in CLL patients and those of controls, both before and after heparin treatment in vivo and in vitro. Investigating LPL knock down effects, we determined five potential downstream targets, of which one gene, STXBP3, reportedly is involved in fatty acid metabolism. While possibly reflecting an epigenetic switch towards an incorrect transcriptional program, LPL overexpression by itself does not appear to significantly influence CLL cell survival.

NOTCH2 links protein kinase C delta to the expression of CD23 in chronic lymphocytic leukaemia (CLL) cells

One characteristic of chronic lymphocytic leukaemia (CLL) lymphocytes is high expression of CD23, which has previously been identified as a downstream target for NOTCH2 signalling. The mechanisms regulating NOTCH2‐dependent CD23 expression, however, are largely unknown. This study showed that peripheral CLL cells overexpressed transcriptionally active NOTCH2 (N2IC), irrespective of their prognostic marker profile. When placed in culture, NOTCH2 activity was spontaneously decreased in 25 out of 31 CLL cases (81%) within 24 h. DNA‐bound N2IC complexes could be maintained by the protein kinase C (PKC) activator phorbol 12‐myristate 13‐acetate (PMA) or by γ‐interferon (IFN‐γ), two CLL characteristic inducers of CD23 expression. Inhibition of PKC‐δ by RNA interference or by rottlerin antagonised PMA‐induced NOTCH2 activation and also suppressed NOTCH2 activity in CLL cases with constitutively activated NOTCH2 signalling. In 23 out of 29 CLL cases tested (79%), DNA‐bound N2IC complexes were found to be resistant to the γ‐secretase inhibitor (GSI) DAPT, suggesting that GSIs will be only effective in a subset of CLL cases. These data suggest that deregulation of NOTCH2 signalling is critically involved in maintaining the malignant phenotype of CLL lymphocytes and point to a link between PKC‐δ and NOTCH2 signalling in the leukemic cells.

V(D)J targeting mistakes occur at low frequency in acute lymphoblastic leukemia

Translocations of proto‐oncogenes to the B‐cell or T‐cell antigen receptor loci in acute T‐ or B‐cell leukemia and lymphoma have been, in most cases, accredited to V(D)J or switch recombination depending on the location of the breakpoint at the receptor locus. Only in rare instances, the reports take into account mechanistic characteristics of the translocation mechanism. To assess the functional ability of several sites implicated in supposedly V(D)J‐mediated translocations, we tested five sites at four proto‐oncogene loci in an ex vivo recombination substrate assay for their potential to act as direct target for V(D)J recombination. Our results show that the LMO2/RBTN2/TTG2 site and one LCK/P56 site readily engage in recombination with a genuine TCR element with the majority of breakpoint junctions showing the characteristics of V(D)J recombination, which strongly supports the involvement of this mechanism in the pathogenesis of the corresponding translocations in vivo. The site at the TLX1/HOX11 locus yielded 0.8% V(D)J‐specific junctions. Sites at the LCK/P56 and TCF3/E2A proto‐oncogenes resulted in exclusively unspecific breakpoints scattered over part of or the entire proto‐oncogene region tested, marking them as unlikely V(D)J recombination targets. Our data suggest that, while being a potentially dangerous mechanism due to the introduction of DNA breaks, V(D)J recombination is a tightly controlled mechanism allowing for only few direct mistakes.

Chronic Lymphocytic Leukemia Patients Have a Preserved Cytomegalovirus-Specific Antibody Response despite Progressive Hypogammaglobulinemia

Chronic lymphocytic leukemia (CLL) is characterized by progressive hypogammaglobulinemia predisposing affected patients to a variety of infectious diseases but paradoxically not to cytomegalovirus (CMV) disease. Moreover, we found reactivity of a panel of CLL recombinant antibodies (CLL-rAbs) encoded by a germ-line allele with a single CMV protein, pUL32, despite differing antibody binding motifs. To put these findings into perspective, we studied prospectively relative frequency of viremia, kinetics of total and virus-specific IgG over time, and UL32 genetic variation in a cohort of therapy-naive patients (n=200). CMV-DNA was detected in 3% (6/200) of patients. The decay of total IgG was uniform (mean, 0.03; SD, 0.03) and correlated with that of IgG subclasses 1-4 in the paired samples available (n=64; p<0.001). Total CMV-specific IgG kinetics were more variable (mean, 0,02; SD, 0,06) and mean decay values differed significantly from those of total IgG (p=0.034). Boosts of CMV-specific antibody levels were observed in 49% (22/45) of CMV-seropositive patients. In contrast, VZV- and EBV-specific IgG levels decayed in parallel with total IgG levels (p=0.003 and p=0.001, respectively). VZV-specific IgG even became undetectable in 18% (9/50) of patients whereas CMV-specific ones remained detectable in all seropositive patients. The observed CMV-specific IgG kinetics were predicated upon the highly divergent kinetics of IgG specific for individual antigens - glycoprotein B-specific IgG were boosted in 51% and pUL32-specific IgG in 32% of patients. In conclusion, CLL patients have a preserved CMV-specific antibody response despite progressive decay of total IgG and IgG subclasses. CMV-specific IgG levels are frequently boosted in contrast to that of other herpesviruses indicative of a higher rate of CMV reactivation and antigen-presentation. In contrast to the reactivity of multiple different CLL-rAbs with pUL32, boosts of humoral immunity are triggered apparently by other CMV antigens than pUL32, like glycoprotein B.