Oncogene database

8115

TCL1A

T-cell leukemia/lymphoma 1A;TCL1A;T-cell leukemia/lymphoma 1A

T-cell leukemia/lymphoma protein 1A|T-cell lymphoma-1|oncogene TCL-1|oncogene TCL1|protein p14 TCL1

14q32.1

protein-coding

A ser891ala RET proto-oncogene mutation has been previously discovered in a single kindred with familial medullary thyroid carcinoma (FMTC). An additional two probands with this mutation and with medullary thyroid carcinoma (MTC) without any other manifestations of MEN 2 have been identified. In one of thse families, two other individuals also had the mutant sequence and FMTC. Analysis of both cases showed cosegregation of the mutation and MTC. To facilitate detection of this mutation, a primer was engineered which creates a Hha I recognition site in the presence of the mutant sequence. As a result, this codon 891 exon 15 mutation can be identified with a restriction enzyme digestion.

AIDS-related non-Hodgkin s lymphoma (AIDS NHL) comprises a diverse and heterogeneous group of high-grade B cell tumors. Certain classes of AIDS NHL are associated with alterations in oncogenes or tumor-suppressor genes or infections by oncogenic herpesviruses. However, the clinically significant class of AIDS NHL designated immunoblastic lymphoma plasmacytoid (AIDS IBLP) lacks any consistent genetic alterations. We identified the TCL1 oncogene from a set of AIDS IBLP-associated cDNA fragments generated by subtractive hybridization with non-AIDS IBLP. Aberrant TCL1 expression has been implicated in T cell leukemia/lymphoma development, and its expression also has been seen in many established B cell tumor lines. However, TCL1 expression has not been reported in AIDS NHL. We find that TCL1 is expressed in the majority of AIDS IBLP tumors examined. TCL1 protein expression is restricted to tumor cells in AIDS IBLP tissue samples analyzed with immunohistochemical staining. Hyperplastic lymph node and tonsil also exhibit strong TCL1 protein expression in mantle zone B cells and in rare interfollicular zone cells, whereas follicle-center B cells (centroblasts and centrocytes) show weaker expression. These results establish TCL1 as the most prevalent of ALL of the surveyed oncogenes associated with AIDS IBLP. They also indicate that abundant TCL1 expression in quiescent mantle zone B cells is down-regulated in activated germinal center follicular B cells in parallel to the known expression pattern of BCL-2. High-level expression in nonproliferating B cells suggests that TCL1 may function in protecting naive preactivated B cells from apoptosis.

Activation of the TCL1 oncogene has been implicated in T cell leukemias/lymphomas and recently was associated with AIDS diffuse large B cell lymphomas (AIDS-DLBCL). Also, in nonmalignant lymphoid tissues, antibody staining has shown that mantle zone B cells expressed abundant Tcl1 protein, whereas germinal center (GC; centrocytes and centroblasts) B cells showed markedly reduced expression. Here, we analyze isolated B cell subsets from hyperplastic tonsil to determine a more precise pattern of Tcl1 expression with development. We also examine multiple B cell lines and B lymphoma patient samples to determine whether different tumor classes retain or alter the developmental pattern of expression. We show that TCL1 expression is not affected by Epstein-Barr virus (EBV) infection and is high in naive B cells, reduced in GC B cells, and absent in memory B cells and plasma cells. Human herpesvirus-8 infected primary effusion lymphomas (PEL) and multiple myelomas are uniformly TCL1 negative, whereas ALL other transformed B cell lines tested express moderate to abundant TCL1. This observation supports the hypothesis that PEL, like myeloma, usually arise from post-GC stages of B cell development. Tcl1 protein is also detected in most naive/GC-derived B lymphoma patient samples (23 of 27 [85%] positive), whereas most post-GC-derived B lymphomas lack expression (10 of 41 [24%] positive). These data indicate that the pattern of Tcl1 expression is distinct between naive/GC and post-GC-derived B lymphomas (P < 0.001) and that the developmental pattern of expression is largely retained. However, post-GC-derived AIDS-DLBCL express TCL1 at a frequency equivalent to naive/GC-derived B lymphomas in immune-competent individuals (7 of 9 [78%] positive), suggesting that TCL1 down-regulation is adversely affected by severe immune system dysfunction. These findings demonstrate that TCL1 expression in B cell lymphoma usually reflects the stage of B cell development from which they derive, except in AIDS-related lymphomas.

Tcl1 and Mtcp1, members of the Tcl1 family, are implicated in T-cell prolymphocytic leukemia. The crystal structure of a dimer of murine Tcl1 has been determined at 2.5 A resolution with an R factor of 0.225. Murine Tcl1, human Tcl1 and Mtcp1 share very similar subunit structures, with RMS differences of 0.6 and 1.4 A for C(alpha) atoms, respectively, while the sequences share 50 and 36% identity, respectively. These structures fold into an eight-stranded beta-barrel of unique topology and high internal symmetry of 1.1-1.3 A for the two halves of human and murine Tcl1 and 1.7 A for Mtcp1, despite the low 12-13% sequence identity. The molecular surfaces of ALL three structures showed a common planar region which is likely to be involved in protein-protein interactions.

The members of the TCL1 proto-oncogene family (TCL1, MTCP1, and TCL1b) bind to Akt1, increasing its phosphorylation status and kinase activity. This is thought to be secondary to the formation of TCL1-Akt oligomers within which Akt is preferentially phosphorylated. Here we show that, in contrast to Akt1 and Akt2, which bind to ALL members of the TCL1 family, Akt3 specifically interacts with TCL1 but not with MTCP1 or TCL1b. This association is functional, as the presence of TCL1 but not MTCP1 or TCL1b increased Akt3 kinase activity in in vitro kinase assays. Functional specificity is determined by the Akt pleckstrin homology domain as chimeric Akt1, where Akt1 PH domain was replaced by that of Akt3 was no longer able to interact with MTCP1 or TCL1b and its kinase activity was solely enhanced by TCL1. Moreover, we show that, in TCL1-overexpressing SUPT-11 T-cell leukemia and P3HR-1 Burkitt s lymphoma cell lines, TCL1 interacts with endogenous Akt1, Akt2, and Akt3. TCL1 enhanced hetero-oligomerization of Akt1 with Akt3 and as a consequence facilitated transphosphorylation of Akt molecules, which may contribute to Akt activation and TCL1-induced leukemogenesis in vivo.

AKT has a critical role in relaying cell survival and proliferation signals initiated by ligand binding to surface receptors in mammalian cells. Induction of AKT serine/threonine kinase activity is augmented by the T-cell leukemia-1 (TCL1) oncoprotein through a physical association requiring the AKT pleckstrin homology domain. Here, we used molecular modeling and identified an exposed hydrophobic patch composed of two discontinuous amino acid stretches near one end of the TCL1 beta-barrel that was required for a TCL1-AKT association. Site-directed mutations of this region did not affect TCL1 secondary structure, yet they disrupted interactions with AKT. This region was found in other members of the TCL1 oncoprotein family, such as TCL1b and MTCP1, and suggested a conserved, novel AKT binding domain. Interestingly, TCL1 and AKT co-localize in multiple cell compartments, but only extracts from the plasma membrane stimulate optimal complex formation in vitro. Identification of an AKT binding domain on TCL1 is an important step in deciphering the complex interactions that regulate AKT kinase activity in lymphocyte development and neoplasia within the immune system.

The TCL1 protooncogene is overexpressed in many mature B cell lymphomas, especially from AIDS patients. To determine whether aberrant expression promotes B cell transformation, we generated a murine model in which a TCL1 transgene was overexpressed at similar levels in both B and T cells. Strikingly, transgenic mice developed Burkitt-like lymphoma (BLL) and diffuse large B cell lymphoma (DLBCL) with attendant Bcl-6 expression and mutated J(H) gene segments at a very high penetrance beginning at 4 months of age. In contrast, only one mouse developed a T cell malignancy at 15 months, consistent with a longer latency for transformation of T cells by TCL1. Activation of premalignant splenic B cells by means of B cell antigen receptor (BCR) engagement resulted in significantly increased proliferation and augmented AKT-dependent signaling, including increased S6 ribosomal protein phosphorylation. Transgenic spleen cells also survived longer than wild-type spleen cells in long-term culture. Together these data demonstrate that TCL1 is a powerful oncogene that, when overexpressed in both B and T cells, predominantly yields mature B cell lymphomas.

Cis-regions and trans-factors controlling TCL1 oncogene expression are not known. We identified the functional TCL1 promoter by mapping four transcriptional start sites 24-30 bp downstream of a TATA box. A 424-bp fragment upstream of the major start site showed robust promoter activity comparable with SV40 in both TCL1 expressing and non-expressing cell lines. Additional constructs spanning 10 kb upstream and 20 kb downstream of the start site showed only modest increases in reporter activity indicating that TCL1 expression is primarily controlled by the promoter. Ten putative Sp1-binding sites were identified within 300 bp of the start site, and three of these specifically bound Sp1. A dose-dependent transactivation of the TCL1 promoter with Sp1 addition in Sp1-negative Drosophila SL2 cells was observed, and mutation of the three identified Sp1-binding sites significantly repressed reporter gene expression in 293T cells, confirming a key role for Sp1 in activating the TCL1 promoter in vivo. In TCL1 silent cell lines, CpG DNA methylation was rarely observed at functional Sp1 sites, and methylation of a previously reported NotI restriction site was associated with dense CpG methylation rather than endogenous TCL1 gene silencing. Together, these results indicate that Sp1 mediates transactivation of the TCL1 core promoter and that TCL1 gene silencing is not dependent on mechanisms involving Sp1 and NotI site methylation.

Initially considered to be of natural killer (NK)-cell origin, CD4+ CD56+ blastic tumors (BTs) of skin have recently been proposed to be of dendritic cell lineage. We have previously described BTs with transformation to myelomonocytic leukemia. Here we report expression of the lymphoid proto-oncogene TCL1 in 10 (83%) of 12 BTs and in lymph node plasmacytoid dendritic cells (DC2s). TCL1 was also expressed in myelomonocytic blasts of 3 transformed BT cases but not in true NK-cell tumors (n = 18), de novo acute myelomonocytic leukemias (1 of 14, 7%), or mature T-cell malignancies (1 of 112, < 1%), with the exception of T-prolymphocytic leukemia (T-PLL). ALL BT cases were also positive for the DC2-associated marker CD123. These results further support derivation of BTs from DC2s, and demonstrate that TCL1 expression in this tumor is common to the immature blastoid, lymphoid-appearing, and subsequent myelomonocytic phases of this disease.

We used a modified subtractive suppression hybridization to identify cellular genes that show altered expression in Burkitt lymphomas (BLs) in the presence of Epstein-Barr virus (EBV). Comparison of the gene expression patterns of an EBV-negative clone of the originally EBV-positive BL line Akata, with its Neo(R)-EBV derivative, revealed a significant difference in the expression of the T cell leukemia 1 oncogene (TCL-1). Subsequent expression studies showed that the original EBV-positive Akata line and the EBV-reconstituted derivative expressed high levels of TCL-1, whereas the EBV-negative variant showed only a low level of expression. Two other independently established EBV-positive BLs (Mutu and OMA) that have also thrown off EBV showed a similar decrease in TCL-1 expression after virus loss. Reinfection with Neo(R)-EBV restored the TCL-1 expression levels in the EBV loss variants to as high a level as the originally EBV-positive lines. High-resolution immunostaining showed that TCL-1 was localized in both the cytoplasm and the nucleus. Our findings suggest that high expression of TCL-1 is necessary for the development of the BL phenotype. In view of the fact that germinal center B cells, regarded as the progenitors of BL, do not express TCL-1, we suggest that constitutive expression of this oncogene occurs by genetic or epigenetic changes in the EBV-negative BLs. In the originally EBV-positive BLs, the ability of the virus to switch on TCL-1 expression would obviate this need.

Epstein-Barr virus (EBV) is strongly implicated in the pathogenesis of several human malignancies, but notably in ALL endemic and some sporadic cases of Burkitt s lymphoma (BL). Although the virus s contribution to the development of BL remains unclear, evidence now suggests that a common feature of ALL BL tumours is the expression of the TCL-1 oncogene. A recent report shows that TCL-1 expression in virus-positive BL tumour cells is dependent on the presence of EBV. This finding suggests that the ability of EBV to induce TCL-1 would circumvent the need for the additional genetic or epigenetic changes that lead to the constitutive expression of this oncogene in EBV-negative BL tumours.

Akt plays a central role in the regulation of cellular anti-apoptosis underlying various human neoplastic diseases. We have demonstrated previously that TCL1 (a proto-oncogene underlying human T cell prolymphocytic leukemia) interacts with Akt and functions as an Akt kinase co-activator. With the aim to develop an Akt kinase inhibitor, we hypothesized that a peptide, which spans the Akt-binding site, binds to Akt and modulates Akt kinase activity and its downstream biological responses. Indeed, we demonstrated that a peptide, named "Akt-in" (Akt inhibitor, NH(2)-AVTDHPDRLWAWEKF-COOH, encompassing the betaA strand of human TCL1), interacted with Akt and specifically inhibited its kinase activity. Nuclear magnetic resonance studies suggested that interaction of Akt-in with the pleckstrin homology domain (PH) of Akt caused conformational changes on the variable loop 1 of Akt, the locus mediating phosphoinositide binding. Consistently, interaction of Akt-in with the Akt PH domain prevented phosphoinositide binding and hence inhibited membrane translocation and activation of Akt. Moreover, Akt-in inhibited not only cellular proliferation and anti-apoptosis in vitro but also in vivo tumor growth without any adverse effect. The roles of Akt, which possesses a PH domain, in intracellular signaling were well established. Hence, Akt inhibitors create an attractive target for anticancer therapy. However, no effective inhibitors specific for Akt have been developed. Akt-in, which inhibits association of phosphatidylinositol with Akt, is the first molecule to demonstrate specific Akt kinase inhibition potency. This observation will facilitate the design of specific inhibitors for Akt, a core intracellular survival factor underlying various human neoplastic diseases.

TCL1 is an AKT kinase coactivator that, when dysregulated, initiates mature lymphocyte malignancies in humans and transgenic mice. While TCL1 augments AKT pathway signaling, additional TCL1 interacting proteins that may contribute to cellular homeostasis or transformation are lacking. Here, an exoribonuclease, PNPase, was identified in a complex with TCL1. The AKT interaction domain on TCL1 bound either RNase PH repeat domain of PNPase without influencing its RNA degrading activity, which was compatible with predicted docking models for a TCL1-PNPase complex. Our data provide a novel protein interaction for mammalian PNPase that may impact TCL1 mediated transformation.

Serine threonine kinase Akt, also called PKB (protein kinase B), plays a central role in regulating intracellular survival. Deregulation of this Akt signaling pathway underlies various human neoplastic diseases. Recently, the proto-oncogene TCL1 (T cell leukemia 1), with a previously unknown physiological function, was shown to interact with the Akt pleckstrin homology domain, enhancing Akt kinase activity; hence, it functions as an Akt kinase coactivator. In contrast to pathological conditions in which the TCL1 gene is highly activated in various human neoplasmic diseases, the physiological expression of TCL1 is tightly limited to early developmental cells as well as various developmental stages of immune cells. The NBRE (nerve growth factor-responsive element) of the proximal TCL1 promoter sequences can regulate the restricted physiological expression of TCL1 in a negative feedback mechanism. Further, based on the NMR structural studies of Akt-TCL1 protein complexes, an inhibitory peptide, "Akt-in," consisting of the betaA strand of TCL1, has been identified and has therapeutic potential. This review article summarizes and discusses recent advances in the understanding of TCL1-Akt functional interaction in order to clarify the biological action of the proto-oncogene TCL1 family and the development avenues for a suppressive drug specific for Akt, a core intracellular survival regulator.

Aberrant expression of the TCL1 oncoprotein promotes malignant transformation of germinal center (GC) B cells. Repression of TCL1 in GC B cells facilitates FAS-mediated apoptosis and prevents lymphoma formation. However, the mechanism for this repression is unknown. Here we show that the CREB coactivator TORC2 directly regulates TCL1 expression independent of CREB Ser-133 phosphorylation and CBP/p300 recruitment. GC signaling through CD40 or the BCR, which activates pCREB-dependent genes, caused TORC2 phosphorylation, cytosolic emigration, and TCL1 repression. Signaling via cAMP-inducible pathways inhibited TCL1 repression and reduced apoptosis, consistent with a prosurvival role for TCL1 before GC selection and supporting an initiating role for aberrant TCL1 expression during GC lymphomagenesis. Our data indicate that a novel CREB/TORC2 regulatory mode controls the normal program of GC gene activation and repression that promotes B cell development and circumvents oncogenic progression. Our results also reconcile a paradox in which signals that activate pCREB/CBP/p300 genes concurrently repress TCL1 to initiate its silencing.

The TCL1/MTCP1 oncogenes were identified on the basis of their involvement in T-cell prolymphocytic leukemia (T-PLL). TCL1 and MTCP1 proteins directly interact with AKT and modulate the AKT signal-transduction pathway, but the relevance of this mechanism in leukemogenesis remains unclear. We investigate the biologic functions of TCL1 in the T-cell lineage using various cell lines, and primary malignant and normal lymphocytes. In the Jurkat cell line, expression of TCL1 had no effect in unstimulated cells, whereas it abrogated activation-induced cell death (AICD). These cellular effects were concomitant with a major inhibition by TCL1 of PKCtheta and ERK pathways. Secondly, the TCL1-driven T-cell leukemia cell line SUP-T11 was shown to have impaired PKCtheta and ERK phosphorylation upon stimulation, which were restored by TCL1 inhibition using RNA interference. Finally, defects in these pathways were also observed in primary malignant (T-PLL) and transduced normal T lymphocytes expressing TCL1. Altogether, our data demonstrated that TCL1 inhibits AICD in T cells by blocking PKCtheta and ERK activation, upon cellular activation.

Overexpression of the TCL1 gene family plays a role in the onset of T-cell leukemias in mice and in humans. The Tcl1 gene is tightly regulated during early embryogenesis in which it participates in embryonic stem (ES)-cells proliferation and during lymphoid differentiation. Here, we provide evidences that Tcl1 is also important in mouse hair follicle (HF) and skin homeostasis. We found that Tcl1(-/-) adult mice exhibit hair loss, leading to alopecia with extensive skin lesions. By analysing Tcl1 expression in the wild-type (wt) skin through different stages of hair differentiation, we observe high levels in the secondary hair germ (HG) cells and hair bulges, during early anagen and catagen-telogen transition phases. The loss of Tcl1 does not result in apparent skin morphological defects during embryonic development and at birth, but its absence causes a reduction of proliferation in anagen HFs. Importantly, we show the that absence of Tcl1 induces a significant loss of the stem-cell marker CD34 (but not alpha6-integrin) expression in the bulge cells, which is necessary to maintain stem-cell characteristics. Therefore, our findings indicate that Tcl1 gene(s) might have important roles in hair formation, by its involvement in cycling and self-renewal of transient amplifying (TA) and stem-cell (SC) populations.

PURPOSE: The oncogene TCL1 plays a key role in the development of B chronic lymphocytic leukemia (B-CLL), but it is not known whether TCL1 could be modulated by therapeutic approaches. EXPERIMENTAL DESIGN: B-CLL patient samples (n = 35) and B leukemic cell lines (EHEB, JVM2, JVM3, MEC1, MEC2, and BJAB) with different p53 status were exposed to Nutlin-3, a small-molecule inhibitor of the p53-MDM2 interaction. Modulations of the steady-state mRNA levels of TCL1 were analyzed by quantitative real-time PCR and Western blotting in both primary B-CLL samples and leukemic cell lines. In addition, transfection experiments with either p53 siRNA or with a TCL1 expression plasmid were carried out in the EHEB B-CLL cell line. RESULTS: Upon ex vivo treatment with Nutlin-3, TCL1 was significantly (P < 0.05) decreased in 23 of 28 B-CLL p53(wild-type). The functionality of the p53 pathway in the same leukemic cell samples was underscored by the concomitant ability of Nutlin-3 to significantly (P < 0.05) upregulate the p53 target gene MDM2 in the p53(wild-type) leukemic cells. The dependence of TCL1 downregulation by a functional p53 pathway was confirmed in a panel of B lymphoblastoid cell lines and by p53 knockdown experiments with p53 siRNA. The importance of TCL1 in promoting leukemic cell survival was underscored in transfection experiments, in which TCL1 overexpression significantly counteracted the Nutlin-3-mediated induction of apoptosis in EHEB. CONCLUSIONS: Our data indicate that the Nutlin-3 downregulates TCL1 mRNA and protein, which likely represents an important molecular determinant in the proapoptotic activity of Nutlin-3.

Overexpression of multiple copies in T-cell lymphoma-1 (MCT-1) oncogene accompanies malignant phenotypic changes in human lymphoma cells. Specific disruption of MCT-1 results in reduced tumorigenesis, suggesting a potential for MCT-1-targeted therapeutic strategy. MCT-1 is known as a cap-binding protein and has a putative RNA-binding motif, the PUA-domain, at its C-terminus. We determined the crystal structure of apo MCT-1 at 1.7 A resolution using the surface entropy reduction method. Notwithstanding limited sequence identity to its homologs, the C-terminus of MCT-1 adopted a typical PUA-domain fold that includes secondary structural elements essential for RNA recognition. The surface of the N-terminal domain contained positively charged patches that are predicted to contribute to RNA-binding.

Dysregulated T-cell leukemia/lymphoma-1A (TCL1A), a modulator in B-cell receptor (BCR) signaling, is causally implicated in chronic lymphocytic leukemia (CLL). However, the mechanisms of the perturbed TCL1A regulation are largely unknown. To characterize TCL1A-upstream networks, we functionally screened for TCL1A-repressive micro-RNAs (miRs) and their transcriptional regulators. We identified the novel miR-484 to target TCL1A s 3 -UTR and to be downregulated in CLL. In chromatin immunoprecipitations and reporter assays, the oncogenic transcription factor of myeloid cells, EVI1, bound and activated the miR-484 promoter. Most common in CLL was a pan-EVI1 transcript variant. EVI1 protein expression revealed distinct normal-tissue and leukemia-associated patterns of EVI1/TCL1A co-regulation. EVI1 levels were particularly low in TCL1A-high CLL or such cellular subsets. Global gene expression profiles from a 337-patient set linked EVI1 networks to BCR signaling and cell survival via TCL1A, BTK and other molecules of relevance in CLL. Enforced EVI1, as did miR-484, repressed TCL1A. Furthermore, it reduced phospho-kinase levels, impaired cell survival, mitigated BCR-induced Ca-flux and diminished the in vitro ibrutinib response. Moreover, TCL1A and EVI1 showed a strongly interactive hazard prediction in prospectively treated patients. Overall, we present regressive EVI1 as a novel regulatory signature in CLL. Through enhanced TCL1A and other EVI1-targeted hallmarks of CLL, this contributes to an aggressive cellular and clinical phenotype.

The TCL1 oncogene on human chromosome 14q32.1 is involved in chromosome translocations [t(14;14)(q11;q32.1) and t(7;14)(q35;q32.1)] and inversions [inv14(q11;q32.1)] with TCR alpha/beta loci in T-cell leukemias, such as T-prolymphocytic (T-PLL). It is also involved in T-acute and -chronic leukemias arising in cases of ataxia-telangiectasia (AT), an immunodeficiency syndrome. Similar chromosomal rearrangements occur also in the clonally expanded T cells in AT patients before the appearance of the overt leukemia. We have analyzed the expression of TCL1 mRNA and protein in peripheral blood lymphocytes (PBLs) from four AT cases and from healthy controls. We found that the TCL1 gene was overexpressed in the PBLs of an AT patient with a large clonal T-cell population exhibiting the t(14;14) translocation but not in the lymphocytes of the other cases. Fluorescence in situ hybridization of the TCL1 genomic locus to lymphocyte metaphases from the AT patient with the T-cell clonal expansion showed that the breakpoint of the t(14;14) translocation lies within the TCL1 locus and is accompanied by an inverted duplication of the distal part of chromosome 14. These data indicate that TCL1 is activated in preleukemic clonal cells as a consequence of chromosome translocation involving sequences from the TCR locus at 14q11. Deregulation of TCL1 is the first event in the initiation of malignancy in these types of leukemias and represents a potential tool for clinical evaluation.

The TCL-1 gene maps at chromosome 14q32.1 and is activated in T cell leukemias and lymphomas by either chromosome translocations or inversions that juxtapose the TCL-1 gene to the alpha/delta or the beta locus of the T cell receptor. The open reading frame of the TCL-1 gene, coding for a protein of 114 amino acids, was expressed in bacteria and antisera were raised against it. The antibodies recognized the predicted TCL-1 M(r) 14,000 protein product in cells expressing TCL-1 mRNA. Cell fractionation experiments indicated that the TCL-1 protein is present in the microsomal fraction. These results were confirmed by confocal microscopy. The TCL-1 protein has considerable sequence similarities to the product of the MTCP-1 gene on chromosome Xq28, which is involved in T cell lympho-proliferative diseases. Thus, TCL-1 may represent a member of a novel family of genes involved in lymphoid proliferation and/or survival and in T cell malignancies.

In human leukemias and lymphomas nonrandom chromosomal rearrangements cause changes in cell growth and/or survival in such a way as to promote malignancy. The detailed study of the biochemical and genetic pathways altered in human cancer requires the identification or development of models to allow the study and manipulation of cancer gene function. Recently, the breakpoint gene TCL1, involved in chromosome translocations observed mostly in mature T-cell proliferations and chronic lymphocytic leukemias (CLL), was isolated and characterized, and showed to be part of a new gene family of proteins involved in these tumors. The murine Tcl1 gene, is similar in sequence to the murine and human MTCP1 gene also involved in T cell leukemias. The murine Tcl1 gene was shown to reside on mouse chromosome 12 in a region syntenic to human chromosome 14. Furthermore, we show that the murine Tcl1 gene is expressed early in mouse embryonic development and demonstrates expression in fetal hematopoietic organs as well as in immature T and B cells. Characterization of the murine Tcl1 gene will help in developing a mouse model of CLL and would provide the best opportunity to study and decipher the role of TCL1 in malignant transformation.

BACKGROUND: Chromosome rearrangements are frequently involved in the generation of hematopoietic tumors. One type of T-cell leukemia, T-cell prolymphocytic leukemia, is consistently associated with chromosome rearrangements characterized by the juxtaposition of the TCRA locus on chromosome 14q11 and either the TCL1 gene on 14q32.1 or the MTCP1 gene on Xq28. The TCL1 gene is preferentially expressed in cells of early lymphoid lineage; its product is a 14 kDa protein (p14TCL1), expressed in the cytoplasm. p14TCL1 has strong sequence similarity with one product of the MTCP1 gene, p13MTCP1 (41% identical and 61% similar). The functions of the TCL1 and MTCP1 genes are not known yet. They have no sequence similarity to any other published sequence, including those of well-documented oncogene families responsible for leukemia. In order to gain a more fundamental insight into the role of this particular class of oncogenes, we have determined the three-dimensional structure of p14TCL1. RESULTS: The crystal structure of p14TCL1 has been determined at 2.5 A resolution. The structure was solved by molecular replacement using the solution structure of p13MTCP1, revealing p14TCL1 to be an all-beta protein consisting of an eight-stranded antiparallel beta barrel with a novel topology. The barrel consists of two four-stranded beta-meander motifs, related by a twofold axis and connected by a long loop. This internal pseudo-twofold symmetry was not expected on basis of the sequence alone, but structure-based sequence analysis of the two motifs shows that they are related. The structures of p13MTCP1 and p14TCL1 are very similar, diverging only in regions that are either flexible and/or involved in crystal packing. p14TCL1 forms a tight crystallographic dimer, probably corresponding to the 28 kDa species identified in solution by gel filtration experiments. CONCLUSIONS: Structural similarities between p14TCL1 and p13MTCP1 suggest that their (unknown) function may be analogous. This is confirmed by the fact that these proteins are implicated in analogous diseases. Their structure does not show similarity to other oncoproteins of known structure, confirming their classification as a novel class of oncoproteins.