TCR Loci Research Articles

Identification of New Loci Controlling Collagen-induced Arthritis in Mouse Using a Partial Advanced Intercross and Congenic Strains

Collagen-induced arthritis (CIA) is a well studied mouse model of the human disease rheumatoid arthritis (RA). Both CIA and RA are complex diseases affected by multiple genes as well as environmental factors. Identifying the genes that determine susceptibility to arthritis would give invaluable clues to the largely unknown aetiology of RA. In this study, we dissected a known locus, Cia6, as well as a genomic region on chromosome 14 with no previously known arthritis loci, using a partial advanced intercross and a collection of congenic strains. The chromosome 14 congenic fragment, containing the T-cell receptor alpha (Tcra) locus, was included based on the hypothesis that the Cia6 locus is caused by a polymorphism in the Tcr beta (Tcrb) locus and that the two loci interact. Splitting up the congenic fragments revealed multiple loci affecting arthritis traits as well as production of collagen-specific autoantibodies. In total seven new loci were identified of which four were in the previously unlinked chromosome 14 region. Both Tcr loci were within CIA loci making them candidate susceptibility genes. The results demonstrate the importance of breaking up genetic regions in smaller fragments to identify the underlying complex set of loci.

Germline transcription from T-cell receptor Vβ gene is uncoupled from allelic exclusion

Allelic exclusion operates in B and T lymphocytes to ensure clonal expression of antigen receptors after V(D)J recombination. Germline transcription, which proceeds V(D)J recombination, has been postulated to provide an instructive signal for allelic exclusion. Here, we use a genetic marker to track germline transcription from a Vbeta gene within the TCRbeta locus. We find that developing thymocytes exhibit uniformed, bi-allelic activation of the Vbeta gene before V-DJ recombination, a process subject to allelic exclusion. We further show that V-DJ rearrangement promotes activation rather than silencing of germline transcription from the remaining Vbeta genes on either the functionally or non-functionally rearranged chromosome. Results presented here suggest that germline transcription, although necessary for V(D)J recombination, is not sufficient to instruct allelic exclusion.

PVRL2 Is Translocated to the TCRA Locus in t(14;19)(q11;13) Positive Peripheral T-Cell Lymphoma.

T-cell non-Hodgkin lymphomas (NHLs) are uncommon malignancies that represent approximately 15% of aggressive lymphomas in Western countries. Contrasting with B-cell lymphomas, very few recurrent chromosomal abnormalities have been identified. Nevertheless, the TCR loci and preferentially the TCRAD at chromosome band 14q11 could be involved in up to 15% of cases. We recently reported a novel and recurrent t(14;19)(q11;q13) chromosomal translocation in peripheral T-cell lymphoma (PTCL). FISH analysis performed in three cases suggested an implication of the TCRAD locus at 14q11 and of BCL3 at 19q13. We now report the molecular cloning of the genomic junctions for these three patients. Sequence analysis confirmed the involvement of the TCRAD on 14q11 and assessed the breakpoint's localisation to the TCRA variable region for all cases. In the first, the chromosomal break occurred between the J30 and the J31 segments while in the second it was in the J58 segment. Thus, both translocations probably occurred after an illegitimate V(D)J recombination. Remarkably, for the third patient, junctions with the deltaRec and psi(J)alpha elements were identified on the der(14) and der(19) respectively. Consequently this translocation probably results from an illegitimate deltarec-Psi(J)alpha rearrangement (TCRD deletion) during thymopoiesis before Valpha-Jalpha rearrangements. On chromosome 19, our results indicated a new clustering of breakpoints outside the region documented in t(14;19)(q32;q13) in chronic lymphocytic leukemia. Remarkably, all three breaks occurred ~130kb downstream from BCL3, in the PVRL2 gene. Consequently, the TCRA locus enhancer is juxtaposed to PVRL2 and BCL3 on the derivative chromosome 19. For two patients, mRNA expression levels were investigated by real time RT-PCR experiments, which confirmed a high expression of both PVRL2 and BCL3. In conclusion, we identified PVRL2 as the recurrent translocation partner of the TCRA locus in PTCL. Whether this clustering results from an important instability of the PVRL2 gene during thymopoiesis remains to be investigated. The PVRL2 overexpression itself may be an important step for T-cell lymphomagenesis.

Cyclin D2 Dysregulation by Chromosomal Translocations to TCR Loci in T-Cell Acute Lymphoblastic Leukemia (T-ALL).

D-type cyclins are key regulators of progression through G1 phase of the cell cycle. Strong expression of at least one of the three D cyclins is common in human cancers. However, while the cyclin D1 and D3 genes (CCND1 and CCND3) are recurrently involved in genomic rearrangements, especially in mantle-cell lymphoma and multiple myeloma, no clear involvement of the cyclin D2 gene (CCND2) has been reported to date in human malignancies. In T-cell acute lymphoblastic leukemia (T-ALL), the T-cell receptor genes TCRA/D and TCRB are frequently involved in chromosomal rearrangements. In order to identify new genomic rearrangements and oncogenes in human T-ALL, we performed an interphasic FISH screening of T-ALL cases using TCR flanking probes. Using this approach, we identified two new chromosomal translocations: t(7;12)(q34;p13) and t(12;14)(p13;q11), involving the TCRB and TCRA/D loci, respectively. Molecular analysis of the breakpoint sequences demonstrated the involvement of the CCND2 locus at 12p13. Expression analysis using RQ-PCR and immunoblotting demonstrated dramatic cyclin D2 overexpression in the translocated cases (n=3) compared to other T-ALLs (total, n=86), whereas other genes located near the translocation breakpoints were not deregulated on microarray analysis. To further evaluate expression in T-ALL with respect to normal T-cell differentiation, we analyzed CCND2 expression in purified subpopulations from normal human thymus. CCND2 levels were downregulated through progression from the early stages of normal human T-cell differentiation and transition to beta-selection. In the most immature T-ALLs, a moderate CCND2 expression was observed, consistent with their differentiation stage, while low expression was found in other T-ALL. By contrast, the massive and sustained expression in the CCND2-rearranged T-ALL cases strongly suggested an oncogenic role due to the TCR translocation. T-ALL oncogenesis is a multi-step process; we here found that the TCR-CCND2 translocations were associated with other oncogene expression (TAL1, HOXA, or TLX3/HOX11L2), NOTCH1 activating mutations, and/or CDKN2A/p16/ARF deletion, showing that cyclin D2 dysregulation could contribute to multi-event oncogenesis in various T-ALL groups. In conclusion, this report is the first clear evidence of a direct involvement of cyclin D2 in human cancer due to recurrent somatic genetic alterations. This reinforces the view that the strong expression of cyclin D2 which is detected in various types of cancer can contribute to oncogenesis, and points to cyclin D2 as a potential target for therapy in these tumors.

Cytogenetic Classification of T Lineage Acute Lymphoblastic Leukaemia: Multiple Partners of BCL11B and Other Novel Rearrangements.

Increasing numbers of genetic changes are being described in T lineage acute lymphoblastic leukaemia (T ALL), which may be used to classify patients into subgroups, defining multi-step oncogenic pathways. We have integrated the significant abnormalities into a comprehensive genetic classification of T ALL, using appropriate probes for fluorescence in situ hybridization (FISH). Break-apart probes were designed, which detected rearrangements of the TCR loci. Metaphase FISH, confirmed by informative break-apart probes for the significant oncogenes, were used to identify partner genes, as shown in the table. This approach revealed new recurrent translocation partners, as well as determining the incidence and simultaneous occurrence of the different abnormalities. The series included 295 patients, children 0–14 years (n=206) and adults ≥ 15 years (n=89), with a diagnosis of T ALL, entered to one of the UK MRC/NCRI ALL treatment trials. The incidences of the common cryptic abnormalities, SIL-TAL1 fusion and TLX3 were more prevalent in children (20% and 17%, respectively) compared to adults (9% each). There was no difference in event free survival between the childhood patients with SIL-TAL1 fusion and TLX3 rearrangements. CALM-AF10 fusion and MLL rearrangements accounted for 4% each. A single patient was found with a BCR-ABL fusion, but the same probe identified nine (3%) with NUP214-ABL1 amplification. Deletions involving CDKN2A were present in 49% of patients, in association with all abnormalities. Among the patients with NUP214-ABL1 amplification, associated abnormalities were: CDKN2A deletions (n=9), TRA@-TLX1 (n=2), BCL11B-TLX3 (n=2), TRB@-TLX3 (n=1). Concurrent rearrangements were found between the TCR genes, as well as associations between MYC, IGH and the other oncogenes. For example, (1) complex abnormalities between (a) TRA@, TRG@, BCL11B (n=2) and (b) HOXA@ (n=1); (2) deletions of 3′TRB@ in association with (a) complex ring chromosomes (n=2) and (b) cytogenetically visible deletions (n=2). FISH detected several novel, recurrent rearrangements, in particular a t(6;14) involving BCL11B and the 6q26 region (n=5) and a t(9;14)(p24;q31.1) involving JAK2 (n=2), the partners of which are currently being defined. BCL11B was also involved with (a) LMO2 and (b) the 2q23 region; LMO2 was rearranged with an unidentified partner in a complex translocation with chromosomes 16 and 18; TLX1 was involved in a translocation with 3q; new partners of TRB@ were found at (a) 1q11, (b) on 12p (n=2), (c) on 21q. These findings demonstrate the valuable role of FISH analysis, with a panel of carefully selected probes, to classify T ALL patients into genetic subgroups, including rare variants, and provide information on the relationship between them. A metaphase FISH approach has facilitated the identification of potential new target genes. In particular, multiple partners of TRB@ and BCL11B, other than the known TLX3 and HOXA@ genes, have emerged, highlighting the importance of these genes in the pathogenesis of T ALL.Promotor and Oncogenes in T ALLPromotor GenesOncogenesBCL11BTRA@TRB@TRG@CDK6Novel***/UnknownTLX338*312TLX11231HOXA@151LMO11LMO2114314**LYL12TAL26NOTCH11MYC231MYB1IGH@4*includes 3 telomeric deletions, **includes 3 centromeric deletions, ***listed in text

A new type of radiosensitive T-B-NK+ severe combined immunodeficiency caused by a LIG4 mutation

V(D)J recombination of Ig and TCR loci is a stepwise process during which site-specific DNA double-strand breaks (DSBs) are made by RAG1/RAG2, followed by DSB repair by nonhomologous end joining. Defects in V(D)J recombination result in SCID characterized by absence of mature B and T cells. A subset of T-B-NK+ SCID patients is sensitive to ionizing radiation, and the majority of these patients have mutations in Artemis. We present a patient with a new type of radiosensitive T-B-NK+ SCID with a defect in DNA ligase IV (LIG4). To date, LIG4 mutations have only been described in a radiosensitive leukemia patient and in 4 patients with a designated LIG4 syndrome, which is associated with chromosomal instability, pancytopenia, and developmental and growth delay. The patient described here shows that a LIG4 mutation can also cause T-B-NK+ SCID without developmental defects. The LIG4-deficient SCID patient had an incomplete but severe block in precursor B cell differentiation, resulting in extremely low levels of blood B cells. The residual D(H)-J(H) junctions showed extensive nucleotide deletions, apparently caused by prolonged exonuclease activity during the delayed D(H)-J(H) ligation process. In conclusion, different LIG4 mutations can result in either a developmental defect with minor immunological abnormalities or a SCID picture with normal development.

Cyclin D2 dysregulation by chromosomal translocations to TCR loci in T-cell acute lymphoblastic leukemias

Strong expression of at least one of the three D-type cyclins is common in human cancers. While the cyclin D1 and D3 genes (CCND1 and CCND3) are recurrently involved in genomic rearrangements, especially in B-cell lymphoid neoplasias, no clear involvement of the cyclin D2 gene (CCND2) has been reported to date. Here, we identified chromosomal translocations targeting the CCND2 locus at 12p13, and the T-cell receptor beta (TCRB) or the TCRA/D loci in T-cell acute lymphoblastic leukemias (T-ALLs). Expression analysis demonstrated dramatic cyclin D2 overexpression in the translocated cases (n=3) compared to other T-ALLs (total, n=89). In order to evaluate dysregulation in T-ALL with respect to normal T-cell differentiation, we analyzed CCND2 expression in normal purified human thymic subpopulations. CCND2 levels were downregulated through progression from the early stages of human T-cell differentiation, further suggesting that the massive and sustained expression in the CCND2-rearranged T-ALL cases was oncogenic. Association with other oncogene expression (TAL1, HOXAs, or TLX3/HOX11L2), NOTCH1 activating mutations, and/or CDKN2A/p16/ARF deletion, showed that cyclin D2 dysregulation could contribute to multi-event oncogenesis in various T-ALL groups. This report is the first clear evidence of a direct involvement of cyclin D2 in human cancer due to recurrent somatic genetic alterations.

Genomic organization of the sheep TRG1@ locus and comparative analyses of Bovidae and human variable genes

γδ T cells commonly account for 0.5%–5% of human (γδ low species) circulating T cells, whereas they are very common in chickens, and they may account for > 70% of peripheral cells in ruminants (γδ high species). We have previously reported the ovine TRG2@ locus structure, the first complete physical map of any ruminant animal TCR locus. Here we determined the TRG1@ locus organization in sheep, reported all variable ( V) gamma gene segments in their germline configuration and included human and cattle sequences in a three species comparison. The TRG1@ locus spans about 140 kb and consists of three clusters named TRG5, TRG3, and TRG1 according to the constant ( C) genes. The predicted tertiary structure of cattle and sheep V proteins showed a remarkably high degree of conservation between the experimentally determined human Vγ9 and the proteins belonging to TRG5 Vγ subgroup. However systematic comparison of primary and tertiary structure highligthed that in Bovidae the overall conformation of the γδ TCR, is more similar to the Fab fragment of an antibody than any TCR heterodimer. Phylogenetic analysis showed that the evolution of cattle and sheep V genes is related to the rearrangement process of V segments with the relevant C, and consequentely to the appartenence of the V genes to a given cluster. The TRG cluster evolution in cattle and sheep pointed out the existence of a TRG5 ancient cluster and the occurrence of duplications of its minimal structural scheme of one V, two joining ( J), and one C.

Generation of Cloned Mice by Direct Nuclear Transfer from Natural Killer T Cells

Cloning mammals by nuclear transfer (NT) remains inefficient. One fundamental question is whether clones have really been derived from differentiated cells rather than from rare stem cells present in donor-cell samples. To date, cells, such as mature lymphocytes, with genetic differentiation markers have been cloned to generate mice only via a two-step NT involving embryonic stem (ES) cell generation and tetraploid complementation [1, 2 and 3]. Here, we show that the genome of a unique T-cell population, natural killer T (NKT) cells, can be fully reprogrammed by a single-step NT. The pups and their placentas possessed the rearranged TCR loci specific for NKT cells. The NKT-cell-cloned embryos had a high developmental potential in vitro: Most (71%) developed to the morula/blastocyst stage, in marked contrast to embryos from peripheral blood T cells (12%; p < 1 x 10(-25)). Furthermore, ES cell lines were efficiently established from these NKT-cell blastocysts. These findings clearly indicate a high level of plasticity in the NKT-cell genome. Thus, differentiation of the genome is not always a barrier to NT cloning for either reproductive or therapeutic purposes, so we can now postulate that at least some mammals cloned to date have indeed been derived from differentiated donor cells.

Ordered DNA release and target capture in RAG transposition.

Following V(D)J cleavage, the newly liberated DNA signal ends can be either fused together into a signal joint or used as donor DNA in RAG-mediated transposition. We find that both V(D)J cleavage and release of flanking coding DNA occur before the target capture step of transposition can proceed; no coding DNA is ever detected in the target capture complex. Separately from its role in V(D)J cleavage, the DDE motif of the RAG1/2 active site is specifically required for target DNA capture. The requirement for cleavage and release of coding DNA prior to either physical target binding or functional target commitment suggests that the RAG1/2 transposase contains a single binding site for non-RSS DNA that can accommodate either target DNA or coding DNA, but not both together. Perhaps the presence of coding DNA may aid in preventing transpositional resolution of V(D)J recombination intermediates.

Transmission ratio distortion in mice.

The most studied example of transmission ratio distortion (TRD) in mice is that of the t-complex. This is a variant region of Chromosome 17 which exists as a polymorphism in wild mice. Males heterozygous for a t-haplotype and a normal Chr 17 transmit the t-haplotype to >50% of their young, up to 99%. Homozygous males are sterile. The TRD produced by the t-complex is due to the action of three or more distorter genes (Tcd) on a responder gene (Tcr). t-Haplotypes are maintained intact by crossover suppression induced by four neighboring inversions, the Tcd and Tcr loci lying in different inversions. Sperm formation is normal in t/t males, but sperm function is impaired through gross defects in sperm motility. The responder gene has been identified as a fusion gene formed from a sperm motility kinase and a ribosomal S6 kinase. Three candidate distorter genes have also been identified as genes coding for dynein chains, and thus possibly involved in sperm flagellar function.

The homeobox gene Hex induces T-cell-derived lymphomas when overexpressed in hematopoietic precursor cells.

Proviral insertions at the viral insertion site Lvis1 occur frequently in B- and T-cell leukemias and lymphomas in AKXD mice and activate two nearby genes, the divergent homeobox gene Hex and the kinesin-related spindle protein gene Eg5. To determine whether Hex misexpression results in the altered differentiation or neoplastic transformation of hematopoietic lineages, we have transplanted mice with bone marrow cells transduced with retrovirus containing the Hex coding region. High levels of Hex expression in hematopoietic precursor cells inhibit contribution to mature blood cell lineages by these precursors. Hex bone marrow transplant recipient mice also develop hematologic neoplasms that appear to originate in the bone marrow. The tumors have clonal rearrangements of the TCR locus, are Thy1+, and are CD4+CD8+, CD4-CD8-, or mixed, indicating tumor origin from a precursor T-cell population. Tumors in transplant mice contain clonal and transcriptionally active Hex proviral insertions, demonstrating a causal role for Hex misexpression in the onset of these neoplasms. Our results demonstrate that Hex can act as a T lineage oncogene when misexpressed in hematopoietic precursor cells.

T cell receptor revision does not solely target recent thymic emigrants.

CD4(+)Vbeta5(+) T cells enter one of two tolerance pathways after recognizing a peripherally expressed superantigen encoded by an endogenous retrovirus. One pathway leads to deletion, while the other, termed TCR revision, results in cellular rescue upon expression of an alternate TCR that no longer recognizes the tolerogen. TCR revision requires the rearrangement of novel TCR beta-chain genes and depends on recombinase-activating gene (RAG) expression in peripheral T cells. In line with recent findings that RAG(+) splenic B cells are immature cells that have maintained RAG expression, it has been hypothesized that TCR revision is limited to recent thymic emigrants that have maintained RAG expression and TCR loci in a recombination-permissive configuration. Using mice in which the expression of green fluorescent protein is driven by the RAG2 promoter, we now show that in vitro stimulation can drive reporter expression in noncycling, mature, peripheral CD4(+) T cells. In addition, thymectomized Vbeta5 transgenic RAG reporter mice are used to demonstrate that TCR revision can target peripheral T cells up to 2 mo after thymectomy. Both sets of experiments strongly suggest that reinduction of RAG genes triggers TCR revision. Approximately 3% of CD4(+)Vbeta5(+) T cells in thymectomized Vbeta5 transgenic reporter mice have undergone TCR revision within the previous 4-5 days. TCR revision can also occur in Vbeta5(+) T cells from nontransgenic mice, illustrating the relevance of this novel tolerance mechanism in unmanipulated animals.

VDJ recombination: Artemis and its in vivo role in hairpin opening.

VDJ recombination: Artemis and its in vivo role in hairpin opening.

Disruption of the lineage restriction of TCR beta gene rearrangements.

Despite a common lymphoid recombinase, assembly of Ig genes is restricted to B cells, whereas TCR loci rearrange in T cells. Transcriptional promoters and enhancers are critical for the regulation of the recombination process. However, the specific function of such elements in conferring the lineage-restriction of V(D)J recombination remains poorly understood. To gain further insights into the mechanism restricting TCR beta-chain rearrangements to T cells, we generated mice in which an 11 kb region--containing the beta-chain constant region 2 and the TCR beta enhancer (E beta)--was replaced with the B cell specific Ig heavy-chain enhancer (E mu). Unlike the simple E mu to E beta replacement, this mutation allowed significant levels of D beta to J beta as well as V beta to DJ beta rearrangements in both T and B cells. Although the lineage restriction was disrupted, TCR beta allelic exclusion was still efficient in mutated T cells. Together these results demonstrate that changes in the activity of regulatory elements located at the TCR beta constant regions are sufficient to redirect the recombination pattern of TCR beta variable gene segments.

Differential regulation between gene expression and histone H3 acetylation in the variable regions of the TCRβ locus

Histone acetylation is suggested to regulate gene expression in the TCR loci. Using R2CD3 mouse model, we previously showed that germline transcription of the TCRβ chain gene is discrete between 5 ′Vβ regions and Dβ–Jβ–Cβ clusters plus Vβ14 region. In this study, we investigated a role of histone H3 acetylation in transcriptional regulation of the TCRβ locus. Our results showed that DN–DP transition is accompanied by significant promotion of histone H3 acetylation in both Dβ–Jβ–Cβ cluster and Vβ14 region, correlating with up-regulation of germline transcription. Surprisingly, termination of germline transcription of the 5 ′Vβ regions was inversely correlated with histone H3 hyperacetylation. Moreover, histone H3 acetylation showed equivalent level in both functionally rearranged Vβ region with active transcription and unrearranged Vβ region without transcription in mature T cells. These results suggest that histone acetylation is not a sole limiting factor in both terminating germline Vβ transcription during DN–DP transition and maintaining functionally rearranged Vβ gene expression.

Distinct roles for c-Myb and core binding factor/polyoma enhancer-binding protein 2 in the assembly and function of a multiprotein complex on the TCR delta enhancer in vivo.

Enhancers and promoters within TCR loci functionally collaborate to modify chromatin structure and to confer accessibility to the transcription and V(D)J recombination machineries during T cell development in the thymus. Two enhancers at the TCRalphadelta locus, the TCR alpha enhancer and the TCR delta enhancer (Edelta), are responsible for orchestrating the distinct developmental programs for V(D)J recombination and transcription of the TCR alpha and delta genes, respectively. Edelta function depends critically on transcription factors core binding factor (CBF)/polyoma enhancer-binding protein 2 (PEBP2) and c-Myb as measured by transcriptional activation of transiently transfected substrates in Jurkat cells, and by activation of V(D)J recombination within chromatin-integrated substrates in transgenic mice. To understand the molecular mechanisms for synergy between these transcription factors in the context of chromatin, we used in vivo footprinting to study the requirements for protein binding to Edelta within wild-type and mutant versions of a human TCR delta minilocus in stably transfected Jurkat cells. Our data indicate that CBF/PEBP2 plays primarily a structural role as it induces a conformational change in the enhanceosome that is associated with augmented binding of c-Myb. In contrast, c-Myb has no apparent affect on CBF/PEBP2 binding, but is critical for transcriptional activation. Thus, our data reveal distinct functions for c-Myb and CBF/PEBP2 in the assembly and function of an Edelta enhanceosome in the context of chromatin in vivo.

Continued expression of recombination-activating genes and TCR gene recombination in human peripheral T cells.

It has been reported that a small population of peripheral T lymphocytes are capable of expressing V(D)J recombinase and initiating secondary V(D)J rearrangements. To determine whether RAG-expression and secondary TCR gene recombination in peripheral T cells are an antigen-driven process in secondary lymphoid tissues, we examined naive CD4(+) T cells, activated/memory CD4(+) T cells, and germinal center T cells from human tonsils. Our results showed that low levels of RAG-1 and RAG-2 mRNA were present in all T cell subpopulations except CD3(+)/CD4(-) T cells. LM-PCR analyses for double-strand DNA breaks showed that all the T cell subsets expressing RAG genes contain double-strand signal break ends, indicating ongoing TCR gene recombination. Continued RAG gene expression, introducing and repairing of double-strand DNA breaks at the TCR loci in the periphery may have significant implications in the development of some T cell neoplasms.

Genomic sequencing of the bovine T cell receptor beta locus

The T cell, which plays an integral role in the coordination of the immune system, has a heterodimer receptor (TCR) that can exist in one of the two forms: alpha/beta or gamma/delta. Cells displaying the gamma/delta TCR comprise less than 5% of T cell populations in humans and mice. In the bovine system, however, gamma/delta populations can reach as high as 60%. Differences in T cell populations make the bovine system an excellent candidate for genomic TCR sequencing and multi-species comparisons. In an effort to characterize the bovine TCR loci, a genomic library was screened for the beta TCR gene. A shotgun sequencing library was constructed and preliminary analysis demonstrates that the organization of the bovine TCR beta constant regions is different from both humans and mice. The bovine beta locus appears to have a third constant region. Overall, the genomic characterization of the bovine TCR genes will provide insight into the evolution of T cell receptor.

A role for secondary V(D)J recombination in oncogenic chromosomal translocations?

Chromosomal translocations are hallmarks of certain lymphoproliferative disorders. Indeed, in many leukemias and lymphomas, translocations are the transforming event that brings about malignancy. Recurrence of the immunoglobulin (Ig) and T-cell receptor (Tcr) loci at the breakpoints of oncogenic chromosomal translocations has led to speculation that the lymphocyte-specific process of V(D)J rearrangement, which is necessary for the generation of functional Ig and TCR antigen receptors on B and T lymphocytes, mediates translocation. Recent studies have led to a fuller understanding of the molecular mechanisms of V(D)J rearrangement and have revealed that the V(D)J recombinase possesses latent transposase activity. These studies have led to plausible models of illegitimate V(D)J recombination producing chromosomal translocations consistent with those present in lymphomas and leukemias. Errors of V(D)J recombination may even generate lymphomas with the phenotypes of mature cells. For example, follicular and Burkitt's lymphomas have been classified by phenotype and somatic genotype as malignant germinal center (GC) B or post-GC B cells. The GC is a site of affinity maturation where B cells undergo V(D)J hypermutation and Ig class switch; in addition, much evidence has accumulated to suggest that GC B cells may also support secondary V(D)J recombination. Interestingly, all three of these elements, genomic plasticity, mutation, and translocation breakpoints near switch sites or recombinational elements, are characteristic of certain lymphomas. The high frequency of lymphomas carrying these GC markers suggests that the GC reaction may play a significant role in lymphomagenesis.