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

Abstract

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.

Extensive molecular mapping of TCRα/δ- and TCRβ-involved chromosomal translocations reveals distinct mechanisms of oncogene activation in T-ALL

“…5,6 Specific mechanistic differences in V(D)Jmediated translocation mechanisms have been shown to guide break location and clustering in T-ALL. 7 Two main types of oncogenic translocations involving the TCR␤ and TCR␣/␦ have been described. 8 In the so-called type 1 translocations (supplemental Figure 1, available on the Blood Web site; see the Supplemental Materials link at the top of the online article), a cryptic but functional recombination signal sequence (cRSS) is present near the oncogene and is mistakenly targeted by the RAG recombinase as a partner for a recombining TCR gene segment.…”

Breakpoint sites disclose the role of the V(D)J recombination machinery in the formation of T-cell receptor (TCR) and non-TCR associated aberrations in T-cell acute lymphoblastic leukemia

“…5 Non TCRassociated chromosomal aberrations such as deletional aberrations (SIL-TAL1) and insertions (HPRT1) have also been appointed as V(D)J recombination-mediated events based on the presence of cRSSs at BP sites. [28][29][30][31] Until now, TCR-associated translocation mechanisms have mainly been evaluated for only a few BP sites by means of ex vivo experiments, 4,6,31,32 basically confirming the concept of RAG mistargeting to cRSSs. These oncogenes and their respective BP sites were usually chosen for their high frequency in T-ALL and also because of the probability that they would function as a cRSS based on structural criteria.…”

“…Analysis of BP sites of SIL-TAL1 deletions (del(1p)) showed that approximately 99% (84 of 85) of all TAL1 BP sites are associated with a cRSS whilst, in contrast to earlier suggestions, 31,32 none of the SIL BP sites tested had a cRSS in their vicinity (Table 3). Although TAL1 also frequently translocates to TCR loci, interestingly none of the TAL1 BP sites involved in SIL-TAL1 deletions were in the direct vicinity of the TAL1 BP sites involved in TCR translocations.…”

“…This reinforces previous notions based on smaller series that TCR translocations predominantly occur via the 'Type 2' translocation pathway. 5,32 Furthermore, this suggests that the V(D)J recombination machinery per se is not the driving force in the induction of DSBs at the majority of the BP sites that lead N.S.D. Larmonie et al to TCR translocation formation.…”

“…These oncogenes and their respective BP sites were usually chosen for their high frequency in T-ALL and also because of the probability that they would function as a cRSS based on structural criteria. 32 Despite the postulated role of V(D)J recombination, it is still not clear to what extent the V(D)J recombination machinery is mechanistically involved in the formation of TCR and non-TCR aberrations in T-ALL. Here we examined 117 molecularly defined BP sites and their sequences from 22 different TCR translocation partners as well as 118 BP sites from non-TCR aberrations in our T-ALL cohort and T-ALL cases described in literature.…”