T-cell acute lymphoblastic leukemia (T-ALL) is associated with chromosomal aberrations characterized by juxtaposition of proto-oncogenes to T-cell receptor gene loci (TCR), resulting in the deregulated transcription of these proto-oncogenes. Here, we describe the molecular characterization of a novel chromosomal aberration, inv(14)(q11.2q32.31), in a T-ALL sample, involving the recently described BCL11B gene and the TCRD locus. The inversion joined the 5 0 part of BCL11B, including exons 1-3, to the TRDD3 gene segment of the TCRD locus, whereas the reciprocal breakpoint fused the TRDV1 gene segment to the fourth exon of BCL11B. The TRDV1-BCL11B joining region was 1344 bp long and contained fragments derived from 20q11.22, 3p21.33 and from 11p12, indicating the complex character of this aberration. A strong expression of inframe transcripts with truncated BCL11B and TCRD constant region (TRDC) were observed, but in contrast to normal T cells and other T-ALL samples, no wild-type BCL11B transcripts were detected in the T-ALL sample. Screening of 37 other T-ALLs revealed one additional case with expression of the BCL11B-TRDC fusion transcript. As BCL11B appears to play a key role in T-cell differentiation, BCL11B disruption and disturbed expression may contribute to the development of T-cell malignancies in man.

The B-cell chronic lymphocytic leukemia (CLL)/lymphoma 11B gene (BCL11B) encodes a Kru¨ppel-like zincfinger protein, which plays a crucial role in thymopoiesis and has been associated with hematopoietic malignancies. It was hypothesized that BCL11B may act as a tumorsuppressor gene, but its precise function has not yet been elucidated. Here, we demonstrate that the survival of human T-cell leukemia and lymphoma cell lines is critically dependent on Bcl11b. Suppression of Bcl11b by RNA interference selectively induced apoptosis in transformed T cells whereas normal mature T cells remained unaffected. The apoptosis was effected by simultaneous activation of death receptor-mediated and intrinsic apoptotic pathways, most likely as a result of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) upregulation and suppression of the Bcl-xL antiapoptotic protein. Our data indicate an antiapoptotic function of Bcl11b. The resistance of normal mature T lymphocytes to Bcl11b suppression-induced apoptosis and restricted expression pattern make it an attractive therapeutic target in T-cell malignancies.

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...

The NK-like family of homeobox genes includes TLX1, TLX3 and NKX2-5, which are ectopically activated in distinct subsets of T-cell acute lymphoblastic leukemia (T-ALL) cells. Here we analysed their effect on the miR-17-92 cluster overexpressed in several types of cancer, including T-ALL. The pri-miR-17-92 polycistron encodes micro-RNAs (miRNAs), which decrease E2F1 protein expression, regulating proliferation and/or apoptosis. Quantification of pri-miR-17-92 in T-ALL cell lines suggested an implication of the NK-like homeodomain proteins in transcriptional regulation. Lentiviral-mediated overexpression of NKX2-5 in the T-ALL cell line MOLT-4 consistently resulted in increased miR-17-92 pri-miRNA levels and decreased amounts of E2F1 protein. Induction of apoptosis by treating miR17-92 or E2F1 transduced T-ALL cells with etoposide led to reduced or enhanced cell viability, respectively. Furthermore, analysis of pri-miR-17-92 in T-ALL patients indicated elevated expression in those bearing TLX1/3 positive cells. These data support an activatory effect of NK-like homeodomain proteins on pri-miR-17-92 expression and concomitantly reduced E2F1 protein levels, thereby enhancing survival of leukemic T-cells.