Unusual forms of T cell gamma mRNA in a human T cell leukemia cell line: implications for gamma gene expression

T-cell receptor gamma: a microsatellite marker for colorectal cancer

BACKGROUND

T-cell receptor gamma (TCR-gamma) is involved in maintaining host cell integrity and homeostasis of the human immune system. We hypothesize that polymorphism of the TCR-gamma complex may be involved in the pathogenesis of colorectal cancer.

METHODS

The microsatellite markers D7S1818 and D7S2206 located within the TCR-gamma antigen locus on chromosome 7p were amplified by polymerase chain reaction, and genotypes were determined for 22 patients with early onset of colorectal cancer (<60 years old) and for 38 population-based control subjects.

RESULTS

Genotype BC of D7S1818 (P = .049) and haplotype AC of D7S1818/D7S2206 (P < or = .003) were associated with colorectal cancer as compared with the control population (extended Fisher's exact test).

CONCLUSIONS

This study identifies a novel genetic and clinical association between TCR-gamma and early-onset colorectal cancer. Many young patients do not fulfill the criteria for hereditary colorectal cancer syndromes and are therefore not identified by established screening programs. Markers such as D7S1818 and D7S2206 may become useful in the identification of patients at risk of developing colorectal cancer and permit earlier therapeutic intervention.

Human T-cell receptor variable gene segment families

Multiple DNA and protein sequence alignments have been constructed for the human T-cell receptor alpha/delta, beta, and gamma (TCRA/D, B, and G) variable (V) gene segments. The traditional classification into subfamilies was confirmed using a much larger pool of sequences. For each sequence, a name was derived which complies with the standard nomenclature. The traditional numbering of V gene segments in the order of their discovery was continued and changed when in conflict with names of other segments. By discriminating between alleles at the same locus versus genes from different loci, we were able to reduce the number of more than 150 different TCRBV sequences in the database to a repertoire of only 47 functional TCRBV gene segments. An extension of this analysis to the over 100 TCRAV sequences results in a predicted repertoire of 42 functional TCRAV gene segments. Our alignment revealed two residues that distinguish between the highly homologous V delta and V alpha, one at a site that in VH contacts the constant region, the other at the interface between immunoglobulin VH and VL. This site may be responsible for restricted pairing between certain V delta and V gamma chains. On the other hand, V beta and V gamma appear to be related by the fact that their CDR2 length is increased by four residues as compared with that of V alpha/delta peptides.

Consistent presence of isochromosome 7q in hepatosplenic T gamma/delta lymphoma: a new cytogenetic-clinicopathologic entity

Peripheral T-cell lymphoma (PTL), which is characterized by hepatosplenic presentation and the gamma/delta T-cell receptor (TCR) phenotype on the malignant cells, is a rare but distinct subtype of non-Hodgkin's lymphomas. Little is known about the chromosomal changes in these lymphomas. We report the cytogenetic analysis of three patients who had neoplastic proliferation of T gamma/delta cells in the spleen, bone marrow, and liver, but not in lymph nodes or skin. Isochromosome 7q and trisomy 8 were observed in all three patients. Isochromosome 7q as the sole abnormality has been previously reported in one patient with similar clinicopathologic features. It is suggested that i(7q) is a primary, nonrandom chromosomal abnormality in hepatosplenic T gamma/delta PTL.

gamma delta lineage-specific transcription of human T cell receptor gamma genes by a combination of a non-lineage-specific enhancer and silencers

The expression of the T cell receptor (TcR) gamma genes is restricted to TcR gamma delta + T lymphocytes. Transgenic and somatic cell hybrid experiments had suggested that the expression of a functionally rearranged TcR gamma gene was extinguished in TcR alpha beta + T cells, possibly by putative cis-acting transcriptional silencers. We have identified such negative cis-acting sequences in the 3' non-coding region of the human TcR gamma (TRG) locus, upstream of an enhancer located at 6.5 kb of the TcR C gamma 2 gene (TRGC2). These silencers were capable of repressing the transcription from a minimal heterologous promoter in a position- and orientation-independent fashion. When analyzed individually, the silencers and the enhancer were equally active in the TcR alpha beta + and TcR gamma delta + T cell lines studied. In contrast, the association of the enhancer with either silencer was shown to restrict transcription to the TcR gamma delta + T cell lines.

Detection of T-cell receptor beta chain mRNA in frozen and paraffin-embedded biopsy tissue using digoxigenin-labelled oligonucleotide probes in situ

In situ hybridization techniques using a cocktail of digoxigenin-labelled T-cell receptor (TcR) constant (C) region beta oligonucleotide probes were used to detect TcR beta mRNA in frozen and paraffin-embedded tissue sections. The specificity of the C beta cocktail was confirmed by Northern blot analysis. The TcR C beta cocktail successfully hybridized to T cells in frozen and paraffin-embedded tissue obtained from patients with inflammatory arthropathies, B- and T-cell non-Hodgkin's lymphoma (NHL), and reactive tonsillitis, and showed staining patterns comparable to those obtained by conventional immunohistological detection of T cells. This is the first report of in situ studies using labelled TcR C beta oligonucleotide probes and may indicate the feasibility of investigating clonal T-cell populations using digoxigenin-labelled clonospecific probes in clinical samples in situ.

Identification of a T cell-specific transcriptional enhancer 3' of the human T cell receptor gamma locus

Positive and negative transcriptional regulatory mechanisms are thought to play a major role in the expression of T cell antigen receptor (TCR) genes. Since the alpha beta and gamma delta T cell receptor heterodimers are expressed in a mutually exclusive fashion and since TCR genes are sequentially activated during T cell ontogeny, transcriptional activation and repression must at least in part determine T lineage-specific and developmental-specific expression of these genes. We have identified a transcriptional enhancer located 6.5 kb downstream from the human T cell receptor gamma (TRG) locus. The nucleotide sequence of the enhancer core element shows strong sequence homology to the recently identified murine C gamma 1 enhancer. The enhancer demonstrates T cell-specific activity, but not gamma delta sublineage-specificity in combination with either a heterologous or gene-specific promoter. Thus, additional regulatory elements may be required to repress the expression of rearranged TRG genes in non-gamma delta T cells.

T cell receptor gamma cDNA in human fetal liver and thymus: variable regions of gamma chains are restricted to V gamma I or V9, due to the absence of splicing of the V10 and V11 leader intron

Although complete in-frame transcripts of the human T cell receptor gamma V10 and V11 genes have been described, the corresponding gamma chains have never been found in gamma delta T cell receptors. In this study, we show that the leader intron of all V10 and V11 cDNA isolated from fetal thymus, fetal liver and adult peripheral blood lymphocytes are unspliced. We demonstrate that, due to the absence of splicing, V10 and V11 are pseudogenes and cannot be expressed in gamma chains. They are the first pseudogenes of this type described in a rearranging T cell receptor/immunoglobulin locus. Therefore the gamma repertoire at the protein level is limited to subgroup V gamma I and to V9. By analysis of the gamma polymerase chain reaction products from total cDNA, we find that the gamma locus is active in early ontogeny (8 weeks), as shown by the presence of rearranged V9 and V10 gene transcripts in the liver. At 13 weeks, the V gamma I genes as well as V9 and V10 have undergone productive rearrangements in the liver, and in the thymus. Most rearrangements, if not all, involve the T cell receptor gamma C1 region (JP1, JP, J1 segments) in both tissues, confirming the accessibility of the C1 region in early stages of development.

Preferential rearrangements of the V gamma I subgroup of the gamma-chain of the T-cell antigen receptor to J gamma 2C gamma 2 gene segments in peripheral blood lymphocyte transcripts from normal donors

To investigate whether there are preferential VJC gene rearrangements of the gamma-chain of the human T-cell antigen receptor (TCR), we amplified and sequenced gamma-chain TCR transcripts from peripheral blood lymphocytes from adult normal donors. cDNA was synthesized from total RNA and amplified by the polymerase chain reaction (PCR) using 5' primers specific for either the V gamma I or the V gamma II subgroups of the gamma-chain of the TCR. The amplified cDNAs were then cloned and sequenced. The majority (approximately 83%) of the cDNAs employing V-I subgroup gene segments rearranged to J gamma 2 (J gamma 2.1 or J gamma 2.3) C gamma 2 gene segments. This was in contrast to the predominant rearrangement of the V gamma II subgroup (V gamma 9) to J gamma 1.2C gamma 1. The remaining 13% of the cDNAs employing V gamma I subgroup gene segments rearranged to J gamma 1.1C gamma 1 or J gamma 1.3C gamma 1. There was significant N diversity as well as imprecise joining at the VJ junction. gamma delta TCR utilizing the C gamma 1 gene segment are disulfide-linked, whereas those utilizing the C gamma 2 gene segment are non-disulfide-linked. These results demonstrate that peripheral blood gamma-chain transcripts exhibit preferential rearrangements of V gamma I subgroup gene segments to J gamma 2(2.1,2.3)C gamma 2 gene segments. By contrast, V gamma II subgroup (V gamma 9) transcripts exhibit rearrangements to J gamma 1.2C gamma 1.

Characterization of T cell receptor assembly and expression in a Ti gamma delta-positive cell line

T cell antigen receptor (TcR) heterodimers of both the Ti-alpha beta and Ti-gamma delta types are expressed at the surface of T cells noncovalently associated with the CD3 complex composed of the monomorphic chains gamma, delta, epsilon and zeta. The structural relationship and assembly of the various components of this multimeric protein complex is still not fully understood. In this report, the human leukemic T cell line Lyon which expresses a Ti-gamma delta/CD3 complex, was characterized and compared to another human leukemic T cell line Jurkat (Ti-alpha beta/CD3). Membrane TCR-/CD3- variants of the T cell Lyon were induced and found to produce all of the Ti/CD3 components, with the exception of Ti-delta. Biochemical analysis indicated that: (1) Ti-gamma/CD3 gamma, delta, epsilon complexes were formed in the endoplasmic reticulum in the absence of Ti-delta; (2) the CD3-zeta chain did not associate with the Ti-gamma/CD3 gamma delta epsilon complex and (3) the Ti-delta chain was required for cell surface expression of the Ti-gamma delta/CD3 complex. Introduction of Jurkat wild-type Ti-alpha cDNA into Lyon T cells resulted in Ti-alpha beta/CD3 expression and abrogated Ti-gamma delta/CD3 expression. In contrast, the expression of the Ti-gamma delta/CD3 complex was not affected by transfection of a mutated Ti-alpha cDNA into Lyon cells. The mutated Ti-alpha chain formed complexes with Ti-beta and CD3 gamma delta epsilon, but the CD3-zeta chain did not associate with these complexes. Taken together analysis of Lyon cells transfected with either wild-type or mutated Ti-alpha suggested that the CD3-zeta chain may have higher affinity for Ti-alpha beta/CD3 complexes than for Ti-gamma delta/CD3 complexes.

The human gamma/delta + and alpha/beta + T cells: a branched pathway of differentiation

In human peripheral blood, most of the CD3+ cells express the alpha/beta T cell receptor. A smaller fraction of CD3+ T cells express the gamma/delta T cell receptor (from 1 to 10% depending the individuals, with an average of 3-5%). Interestingly, although the alpha/beta + T cells never express the gamma chain at the cell surface, most of them (about 98%) rearrange the gamma locus on both alleles, the remaining 2% alpha/beta + T cells have one rearranged TRG locus. We previously proposed that V-J joinings in the human TRG locus occurred sequentially and we recently demonstrated that two successive rearrangements may occur on the same chromosome [Alexandre et al. (Int. Immunol, 3, 973-982, 1991)]. In this paper, we discuss the implications of these sequential rearrangements on the relatedness of the human gamma/delta + and alpha/beta + T cell lineages.

Gamma/delta T cell antigen receptors expressed on tumor-infiltrating lymphocytes from patients with solid tumors

The expression of gamma/delta T cell antigen receptors (TcR) in T cell lines or clones derived from tumor-infiltrating lymphocytes (TIL) from patients with solid tumors was investigated. gamma/delta TcR T cell lines were derived from TIL from patients with Wilms tumor, sarcoma or metastatic melanoma by stimulation with autologous tumor cells alone and recombinant interleukin 2 and they exhibited nonspecific cytotoxicity against autologous and allogeneic tumor cells, or cells of the K562 or the MEL21 tumor cell lines. Two T cell lines were derived from a patient with Wilms tumor. One of them expressed a non-disulfide-linked gamma/delta TcR using the 60-kDa gamma chain, whereas, the other expressed a disulfide-linked gamma/delta TcR. A T cell line was derived from a patient with sarcoma and expressed a disulfide-linked gamma/delta TcR, whereas, a T cell line derived from a patient with melanoma expressed a non-disulfide-linked gamma chain of 62 kDa. Several T cell clones were developed from patients with metastatic melanoma or Wilms tumor and expressed either disulfide- or non-disulfide-linked gamma/delta TcR. Northern analysis of RNA from certain of these clones revealed a full-length gamma chain transcript, whereas, the alpha or beta chain transcripts were either absent or truncated. These T cell clones exhibited nonspecific cytotoxicity. Both disulfide- and non-disulfide-linked TIL T cell lines and clones expressed the delta TCS1 determinant. gamma/delta TcR+ cells in freshly prepared TIL from these patients were present in low proportions (less than 5%) and their delta TCS1/delta 1 ratios were within the range observed in the peripheral blood of normal donors. These results demonstrate that both disulfide- and non-disulfide-linked gamma/delta TcR are expressed on T cell lines and clones derived from TIL from solid tumors. Non-disulfide-linked gamma/delta TcR using the 56-66-kDa gamma chain are frequently found on TIL-derived T cell lines and clones. These 56-66-kDa gamma chains are rarely expressed on T cell lines or clones derived from peripheral blood lymphocytes of normal donors.

Direct involvement of CD7 (gp40) in activation of TcR gamma/delta+ T cells

In this study we reported that on T cell receptor (TcR) gamma/delta+ cells from three cell lines Peer, MOLT-13 and ICRF-1, the T cell antigen CD7 (gp40) can be directly involved in the activation process. This is shown by a rapid increase in cytoplasmic free calcium after stimulation of these cells with an anti-CD7 monoclonal antibody (mAb). Activation through CD7 was further confirmed by measuring the production of interleukin 2 in ICRF-1 cells stimulated with anti-CD7 mAb. In addition induction of mRNA for tumor necrosis factor (TNF)-alpha and TNF-beta in Peer and for granulocyte-macrophage-colony-stimulating factor in MOLT-13 was observed in these anti-CD7-stimulated cells. The same anti-CD7 antibody was unable to activate TcR alpha/beta+ Jurkat cells or normal resting peripheral blood T lymphocytes. We further showed that normal resting TcR gamma/delta+ cells were likewise activated via the CD7 molecule. TcR gamma/delta+ cells obtained from a patient with acute lymphoblastic leukemia 3 months after autologous bone marrow transplantation were induced to proliferate, as measured by [3H]thymidine incorporation after stimulation with anti-CD7 mAb but not with anti-CD3 mAb. Interestingly TcR alpha/beta+ cells from the same donor tested in parallel were not stimulated by anti-CD7 but by anti-CD3 mAb. In essence these findings contribute to the idea that on TcR gamma/delta+ cell, the CD7 antigen could play an important role during T cell differentiation.

Polymorphism of the T-cell receptor gamma variable and constant region genes in a Chinese population

The human T-cell receptor gamma gene region spans 160 kb genomic DNA. Restriction fragment length polymorphisms (RFLPs) have been previously documented for the constant region (TRGC) genes, the joining (TRGJ) segments and the variable (TRGV) genes. We have recently defined the alleles of the T-cell receptor gamma V, J and C genes and we have described seven haplotypes of the V gamma subgroup I genes characterized either by RFLPs or by deletion or insertion of V gamma genes. The number of V gamma I genes may vary from 7 to 10 per haploid genome, the 9-gene haplotype being the most frequent. Allelic fragments can unambiguously characterize the TRGC2 gene with duplication or triplication of the exon 2. These alleles and haplotypes have been analyzed in four different populations (French, Lebanese, Tunisian and Black African). In this paper, we compare these allele and haplotype frequencies with those found in a Chinese population and we describe new TRGV allelic restriction fragments found only in the Chinese samples. These results and the previous data demonstrate the flexibility of the human T cell receptor gamma locus and the importance of unequal crossing-overs in the evolution of that locus. Moreover, they underline the importance of studying these polymorphisms in population genetics.

Regulated expression and structure of T cell receptor gamma/delta transcripts in human thymic ontogeny

Gamma delta (gamma delta) T cells have been found in all vertebrates examined, yet their function in vivo remains unknown. Because gamma delta T cell receptors are related to immunoglobulin, and because they are encoded by rearranging, multi-gene families, the receptors are thought to be antigen recognition molecules. However, a capacity to recognize naturally diverse antigens has not yet been shown. In this work, the expression and structure of human gamma delta transcripts have been examined in the fetal and early post-natal thymus. The data indicate that many gamma and delta genes are rearranged and expressed throughout ontogeny, but that as ontogeny proceeds, quite dramatic changes occur in the patterns of gene expression and rearrangement. In particular, receptors encoded by early to mid-gestation fetal thymic transcripts would be of quite restricted diversity. Only later in ontogeny can receptors of substantial diversity be generated. These properties are very similar to the patterns of gamma delta gene activation in the mouse, and they serve to reiterate similarities both in gene rearrangement and in gamma delta across vertebrate species.

Deletion, insertion, and restriction site polymorphism of the T-cell receptor gamma variable locus in French, Lebanese, Tunisian, and black African populations

The human T-cell receptor gamma region spans 160 kb of genomic DNA and is densely populated by coding sequences. Restriction fragment length polymorphisms have been previously documented for the constant region genes, the joining segments, and the variable genes belonging to subgroups I and IV. Here we further define the polymorphism of the V gamma I subgroup genes. based on complete mapping of the Eco RI and Taq I allelic restriction fragments. We describe seven haplotypes; five result from polymorphic restriction sites, the sixth corresponds to a deletion of about 10 kb encompassing V4 and V5, and the seventh results from an insertion of an additional gene, V3P, between V3 and V4. As a consequence of the deletion or insertion polymorphism, the number of V gamma I subgroup genes vary from seven in haplotype TRGVI*3 to ten in haplotype TRGVI*4, whereas the most common haplotype, TRGVI*1, has nine V genes, five of them being functional. Frequencies of the different TRGVI haplotypes in French, Lebanese, Tunisian, and Black African populations are given.

Disulfide-linked gamma delta T cell antigen receptors expressed on T cells derived from patients with primary immunodeficiency disorders

T cell lines or clones from two patients, one with a partial DiGeorge syndrome and one with severe common variable immunodeficiency expressed disulfide-linked gamma delta T cell antigen receptor (TCR) comprised of a gamma-chain polypeptide of 40-43 kD, and a delta-chain polypeptide of 37-40 kD. This gamma delta TCR appears to be similar to that found on T cell clones, and lines derived from peripheral blood lymphocytes from normal donors. Previous studies have shown that T cell lines derived from the peripheral blood of patients with immunodeficiency disorders express non-disulfide-linked gamma delta TCR. In contrast to the latter and coincident with findings in the present study, the vast majority of T cell lines and clones derived from the peripheral blood of normal donors express disulfide-linked gamma delta TCR.

The promoter regions of the T-cell receptor V9 gamma (TRGV9) and V2 delta (TRDV2) genes display short direct repeats but no TATA box

T lymphocytes expressing the T-cell gamma delta receptor have been shown to express preferentially the T-cell receptor V9 gamma (TRGV9) gene, in association with the T-cell receptor V2 delta (TRDV2) gene. In this paper, we report that the promoter regions of the TRDV2 and TRGV9 genes, which are preferentially expressed early in T-cell differentiation, display short direct repeats but no TATA box, in contrast to the V gamma genes belonging to subgroup I. The TCCTCAGT octanucleotide found 100 pb upstream of the ATG of the HD-Mar V alpha transcript, a TCR V alpha gene without a TATA box, is observed upstream of TRDV2 but not TRGV9. Of interest is the presence of a characteristic decanucleotide AGGTGGT(T)GAG in the promoter regions of both the TRDV2 and TRGV9 genes.

A strategy for demonstrating the clonal origin of small numbers of T lymphocytes in histopathological specimens

A strategy is described for detecting large T-cell clones among small numbers of lymphoid cells in fresh or formalin-fixed tissue samples using the polymerase chain reaction (PCR) to amplify and identify rearrangements of the V and J genes of the T-cell gamma receptor. Following hybridization of primers with gene-specific sequences at judiciously selected locations on each of the eight potentially active V and five potentially active J genes, the PCR can theoretically amplify DNA segments which span the join between rearranged V and J genes and are of approximately 384 different sizes, each segment size reflecting different gamma gene clonal rearrangements. Large monoclonal populations of T lymphocytes, indicated by excessive amounts of particular PCR segment sizes, can be further characterized by direct nucleotide sequencing of the hypervariable N regions of these segments, and the presence of such clones can be confirmed directly in tissue sections by in situ hybridization with N region-specific oligonucleotide probes.

The human T-cell receptor gamma (TRG) genes

The human T-cell receptor gamma (TRG) chain genes, like those encoding the T-cell receptor alpha- and beta-polypeptides, undergo rearrangements specifically in T cells. The human TRG locus, which has been completely mapped, is composed of two constant region genes (TRGC), five joining segments (TRGJ) and at least 14 variable gamma-genes (TRGV). Eight variable genes are functional and belong to four different subgroups. The product of the rearranged TRG gene is the gamma-chain which is expressed, along with the delta-chain, at the surface of a subset of T lymphocytes. Although some gamma delta + cells display a cytolytic activity, their precise function remains to be elucidated.

Molecular mapping of the human T cell receptor gamma (TRG) genes and linkage of the variable and constant regions

In the human T cell receptor gamma (TRG) locus, fourteen variable (TRGV) genes belonging to four subgroups have been identified upstream of two constant region (TRGC) genes. Three joining segments, JP1, JP and J1, have been localized upstream of TRGC1, and two others, JP2 and J2, upstream of TRGC2. In this report, we demonstrate that a unique Xho I fragment of 120 kilobases (kb) contains the fourteen TRGV genes and that the hybridization of that fragment in pulsed-field gel electrophoresis (PFGE) allows linkage of the variable region to the constant region locus. We also show that the variable and the constant regions are remarkably close to each other since the distance between V11, the most 3' V gamma gene, and JP1, the most 5' J gamma segment, is only 16 kb. With its 14 V gamma genes, spanning 100 kb, the two C gamma genes and 5 joining segments covering less than 40 kb and only 16 kb separating the most 3' V gene from the most 5' J segment, the human TRG locus spans 160 kb of genomic DNA and represents a particularly condensed locus compared to the other rearranging gene loci.

Rearrangement of TCR gamma chain gene involving JP1 suggests early thymocyte origin of peripheral T-cell lymphoma

Peripheral T-cell lymphomas (PTL) are morphologically and immunophenotypically heterogeneous. We have examined a series of cases to determine whether this heterogeneity is reflected at the level of developmentally specific T-cell receptor (TCR) gene rearrangement. 4 of 5 cases had clonal rearrangements of TCR beta and gamma chain genes; one of these also had a probable DQ52-J immunoglobulin heavy chain gene rearrangement. 2 of the 4 TCR gamma gene rearrangements involved the most 5' J region, JP1, a characteristic of immature thymocytes. These 2 cases also had immunophenotypic features of immaturity. Taken together, our results suggest that TCR gene rearrangement is correlated with surface marker data and shows that in some cases PTL may arise from a very early stage of thymocyte maturation.

Molecular heterogeneity of gamma delta T-cell antigen receptors expressed by CD4- CD8- T-cell clones from normal donors: both disulfide- and non-disulfide-linked receptors are delta TCS1+

We investigated the molecular heterogeneity of gamma delta T-cell antigen receptors (TCR) expressed on T-cell clones generated from peripheral blood lymphocytes of normal donors. Extensive molecular heterogeneity was seen at the gamma-chain level and, to a lesser extent, at the delta-chain level. Both disulfide and non-disulfide gamma delta TCR were found and use different gamma chains with similar molecular masses (range, 41-43 kDa). In contrast, gamma chains of 55-60 kDa, which are expressed on T-cell lines derived from the peripheral blood of patients with immunodeficiency disorders, were not found on T-cell clones derived from the peripheral blood of normal donors. delta chains expressed on these T-cell clones had a molecular mass of 37 kDa and were either disulfide or nondisulfide linked. Significant delta-chain heterogeneity was identified in these clones using the anti-delta TCS1 and the anti-TCR delta 1 monoclonal antibodies. All clones tested were TCR delta 1+, whereas only 25% of the clones were delta TCS1+. The anti-delta TCS1 monoclonal antibody stained and immunoprecipitated both disulfide- and non-disulfide-linked gamma delta TCRs from different T-cell clones from normal donors.

Exon duplication and triplication in the human T-cell receptor gamma constant region genes and RFLP in French, Lebanese, Tunisian, and black African populations

The human T-cell receptor gamma (TCRG) locus comprises 14 variable genes (TRGV), five joining segments (TRGJ), and two constant region genes (TRGC). The constant gamma 1 gene, TRGC1, consists of three exons, whereas the TRGC2 gene contains four or five exons due to the duplication or triplication of exon 2 and spans 9.5 kb or 12 kb, respectively. In this paper, we define the alleles of the T-cell receptor gamma J2 and C2 genes, and we show that two Hind III allelic fragments, 5.4 kb and 8 kb, characterize unambiguously the C2 gene with duplication or triplication of exon 2. We show also that the cDNA of the HPB-MLT cell line results from the transcription of an allelic TRGC2 gene with duplicated exon 2. We propose a model involving unequal crossing-overs to explain the organization and the evolution of the TRGC locus. Moreover, we analyze the TCRG haplotypes in four different populations (French, Lebanese, Tunisian, and Black African) to underline their interest for population genetics.

The human T-cell rearranging gamma (TRG) genes and the gamma T-cell receptor

The human T-cell Rearranging Gamma genes or T-cell Receptor Gamma (TRG) chain genes, like those encoding the T-cell Receptor (TcR) alpha and beta polypeptides, undergo rearrangements specifically in T-cells. The human TRG locus which has been mapped to chromosome 7 (7p15) is composed of 2 constant region genes (TRGC), 5 joining segments (TRGJ) and at least 14 variable gamma genes (TRGV). 8 variable genes are functional and belong to 4 different subgroups. Based on restriction fragments, the TRG rearrangements can be assigned to given V and J segments, in normal T-cells, T leukemias and lymphomas. The product of the rearranged TRG gene is the gamma chain which is expressed at the surface of a subset of CD3+4-8- T lymphocytes lacking the conventional receptor alpha beta. Structural differences exist between the different 'gamma T-cell receptors', the gamma and delta polypeptides being disulfide or non-disulfide linked. Although the TRG+ cells display a cytolytic activity, their precise function remains to be elucidated.

Complex rearrangements within the human J delta-C delta/J alpha-C alpha locus and aberrant recombination between J alpha segments

We have examined DNA rearrangements within a 120 kb cloned region of the human T cell receptor J delta-C delta/J alpha-C alpha locus. Three types of pattern emerge from an analysis of T cell lines and clones. Firstly, cells with two rearrangements within J delta-C delta; secondly, cells with one rearrangement within J delta-C delta and one or more J alpha rearrangements, and finally, cells with rearrangements within J alpha and consequential deletion of the delta locus. Further analysis by cloning of rearrangements within the J alpha locus show that, in addition to V alpha-J alpha joins, J alpha-J alpha aberrant recombinations occur and rearrangement data indicate that such events are frequent. A model is presented to account for such recombinations.