Different chromosomal breakpoints impact the level of LMO2 expression in T-ALL

Abstract

The t(11;14)(p13;q11) is presumed to arise from an erroneous T-cell receptor delta TCRD V(D)J recombination and to result in LMO2 activation. However, the mechanisms underlying this translocation and the resulting LMO2 activation are poorly defined. We performed combined in vivo, ex vivo, and in silico analyses on 9 new t(11;14)(p13;q11)-positive T-cell acute lymphoblastic leukemia (T-ALL) as well as normal thymocytes.Our data support the involvement of 2 distinct t(11;14)(p13;q11) V(D)J-related translocation mechanisms. We provide compelling evidence that removal of a negative regulatory element from the LMO2 locus, rather than juxtaposition to the TCRD enhancer, is the main determinant for LMO2 activation in the majority of t(11;14)(p13;q11) translocations. Furthermore, the position of the LMO2 breakpoints in T-ALL in the light of the occurrence of TCRD-LMO2 translocations in normal thymocytes points to a critical role for the exact breakpoint location in determining LMO2 activation levels and the consequent pressure for T-ALL development. IntroductionThe t(11,14)(p13;q11) (LMO2-TCRD) occurs frequently (7%) and is considered a paradigm for T-cell receptor (TCR)-associated translocations in human T-cell acute lymphoblastic leukemia (T-ALL). 1,2 Although the causative mechanism is still poorly understood, it is generally assumed to result from erroneous V(D)J recombination during T-cell development. 3 So far only 5 t(11;14)(p13;q11) junctions have been sequenced. [4][5][6][7][8][9] In 3 cases, cryptic sequences resembling TCR/Ig recombination signal sequences (RSSs) were located next to the LMO2 breakpoint. Two of these concerned the same cryptic RSS (cRSS). Consequently, RAG mistargeting of an LMO2 cRSS at the moment of TCRD recombination was proposed as the causal mechanism. [4][5][6][7] Recently, we and others demonstrated that this shared LMO2 cRSS could indeed function as target for V(D)J recombination in ex vivo recombination assays. 10,11 Although these data fit with illegitimate V(D)J recombination due to RAG mistargeting, they are clearly too limited to draw firm conclusions regarding recurrent involvement of this mechanism in t(11;14)(p13;q11). Dogmatically, TCR-associated translocations are believed to result in protooncogene activation due to juxtaposition to a TCR enhancer or other regulatory elements. 1,12 LMO2 consists of 6 exons and is transcribed from 2 promoters, a distal promoter upstream of exon 1 and a proximal promoter upstream of exon 3, with both transcripts encoding the same protein. 13 Strikingly, our retrospective analysis of published t(11;14)(p13;q11) translocation junctions 4-8 revealed no juxtaposition of LMO2-coding exons to the TCRD enhancer, suggesting alternative LMO2 activation mechanisms. Distal promoter/negative regulatory element (NRE) removal has been suggested as an LMO2 activation mechanism in t(11;14)(p13;q11) 13,14 but this has never been shown experimentally.Clearly, many issues regarding how t(11;14)(p13;q11) leads to LMO2 activation and T-ALL are so far poorly unde...

Monoallelic or biallelic LMO2 expression in relation to the LMO2 rearrangement status in pediatric T-cell acute lymphoblastic leukemia

“…3 It was recently proposed that for most LMO2 translocations, LMO2 activation is presumably caused by the loss of the NRE region during the translocation event rather than the juxtaposition of LMO2 in the vicinity of the TCR enhancers. 4 …”

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.…”

“…This further confirmed our previous findings of a high correlation between the RIC score obtained from in silico analysis and the translocation efficiency determined by means of ex vivo recombination substrate assay. 6 Only approximately 25% of the whole spectrum of TCR translocations in human T-ALL is driven by RAG mistargeting of cRSSs ('Type 1' translocations). Interestingly, a large fraction (approx.…”

“…These aberrant recombinations result in juxtaposition of oncogenes in the vicinity of TCR cis-acting regulatory elements such as enhancers, or in removal of negative regulatory elements (NRE) from the oncogene promoter. 6 As a result, expression of the involved oncogene, which often encodes for a transcription factor, becomes deregulated. The deregulation of the oncogene is considered to be an early key event in T-ALL leukemogenesis.…”

“…The deregulation of the oncogene is considered to be an early key event in T-ALL leukemogenesis. 6 An overview of the TCR translocations, translocation partners and occurrence in T-ALL is given in Table 1.…”