Delta-chain Locus Research Articles

Loss of heterozygosity on chromosome 14 in nasopharyngeal carcinoma.

The main objective of this study was to determine the precise frequency of chromosome 14q loss of heterozygosity in nasopharyngeal carcinomas and to define its minimal deletion regions. Thirty-nine tumors were selected for PCR-based deletion mapping using 19 microsatellite polymorphic markers spanning the long arm of this chromosome. Loss of heterozygosity for at least one marker was observed in 29 (74.4%) tumors, while 24 of these tumors displayed partial loss and provided an informative basis for detailed deletion mapping. Three minimal regions of loss were delineated, the first defined by markers D14S278 and D14S288, the second being between D14S51 and the telomere. These data confirmed 2 potential tumor-suppressor-gene loci at 14q12-13 and 14q32. Interestingly, the third region of loss was located at the T-cell-receptor delta-chain locus. This may reflect another tumor-suppressor-gene locus at 14q11.2, or may be the consequence of a specific genomic rearrangement of this region. In addition, these allelic losses occurred with high frequency in all tumor grades and stages and in all histological sub-types. These findings suggest that the genetic alteration of chromosome 14 is common and crucial during nasopharyngeal-carcinoma development.

Normal D-JH rearranged products of the IgH gene in SCID mouse bone marrow.

SCID mice are profoundly immunodeficient, resulting from an inability to carry out the V(D)J recombination reaction during both B cell and T cell development. Recently, however, it was revealed that normal rearrangement frequently did occur in the TCR delta and gamma chain loci in the SCID thymus. To evaluate whether the normal rearrangement occurring in SCID is a T-cell-specific phenomenon, we directly cloned using PCR the DQ52-JH2 and DFL16.1-JH2 rearranged segments of the IgH gene from SCID bone marrow. The subsequent analysis revealed that normal V(D)J recombination occurred in a significant number of the analyzed clones. By quantitative Southern hybridization it was shown that the quantity of normal DQ52-JH2 joints existing in the SCID bone marrow is approximately 4-7% that in normal bone marrow. D-JH rearrangement in SCID mice and normal mice differs in the frequency of nucleotide insertion (N insertion). Although most of the normal mouse clones exhibited N insertion in the D-JH rearrangement, in SCID mouse clones N insertion was identified in only a few D-JH rearrangements. Furthermore, in several normal rearranged clones, the recombination occurred at the short homologous sequence. These observations suggest that the V(D)J recombination of IgH normally occurs at the early stage of SCID B cell development, just as TCR gene rearrangement occurs during SCID T cell development. Furthermore, the features of rearranged products isolated from SCID bone marrow cells were remarkably similar to those from leaky SCID mice.

A δ T-Cell Receptor Deleting Element Transgenic Reporter Construct Is Rearranged in αβ but not γδ T-Cell Lineages

T cells can be divided into two groups on the basis of the expression of either alpha beta or gamma delta T-cell receptors (TCRs). Because the TCR delta chain locus lies within the larger TCR alpha chain locus, control of the utilization of these two receptors is important in T-cell development, specifically for determination of T-cell type: rearrangement of the alpha locus results in deletion of the delta coding segments and commitment to the alpha beta lineage. In the developing thymus, a relative site-specific recombination occurs by which the TCR delta chain gene segments are deleted. This deletion removes all D delta, J delta, and C delta genes and occurs on both alleles. This delta deletional mechanism is evolutionarily conserved between mice and humans. Transgenic mice which contain the human delta deleting elements and as much internal TCR delta chain coding sequence as possible without allowing the formation of a complete delta chain gene were developed. Several transgenic lines showing recombinations between deleting elements within the transgene were developed. These lines demonstrate that utilization of the delta deleting elements occurs in alpha beta T cells of the spleen and thymus. These recombinations are rare in the gamma delta population, indicating that the machinery for utilization of delta deleting elements is functional in alpha beta T cells but absent in gamma delta T cells. Furthermore, a discrete population of early thymocytes containing delta deleting element recombinations but not V alpha-to-J alpha rearrangements has been identified. These data are consistent with a model in which delta deletion contributes to the implementation of a signal by which the TCR alpha chain locus is rearranged and expressed and thus becomes an alpha beta T cell.

Molecular involvement of the pvt-1 locus in a gamma/delta T-cell leukemia bearing a variant t(8;14)(q24;q11) translocation.

A highly malignant human T-cell receptor (TCR) gamma/delta+ T-cell leukemia was shown to have a productive rearrangement of the TCR delta locus on one chromosome 14 and a novel t(8;14)(q24;q11) rearrangement involving the J delta 1 gene segment on the other chromosome 14. Chromosome walking coupled with pulsed-field gel electrophoretic (PFGE) analysis determined that the TCR J delta 1 gene fragment of the involved chromosome was relocated approximately 280 kb downstream of the c-myc proto-oncogene locus found on chromosome band 8q24. This rearrangement was reminiscent of the Burkitt's lymphoma variants that translocate to a region identified as the pvt-1 locus. Sequence comparison of the breakpoint junctions of interchromosomal rearrangements in T-cell leukemias involving the TCR delta-chain locus revealed novel signal-like sequence motifs, GCAGA(A/T)C and CCCA(C/G)GAC. These sequences were found on chromosome 8 at the 5' flanking site of the breakpoint junction of chromosome 8 in the TCR gamma/delta leukemic cells reported here and also on chromosome 1 in T-cell acute lymphocytic leukemia patients carrying the t(1;14)(p32;q11) rearrangement. These results suggest that (i) during early stages of gamma delta T-cell ontogeny, the region 280 kb 3' of the c-myc proto-oncogene on chromosome 8 is fragile and accessible to the lymphoid recombination machinery and (ii) rearrangements to both 8q24 and 1p32 may be governed by novel sequence motifs and be subject to common enzymatic mechanisms.

Molecular Involvement of the pvt-1 Locus in a γ/δ T-Cell Leukemia Bearing a Variant t(8;14)(q24;qll) Translocation

A highly malignant human T-cell receptor (TCR) gamma/delta+ T-cell leukemia was shown to have a productive rearrangement of the TCR delta locus on one chromosome 14 and a novel t(8;14)(q24;q11) rearrangement involving the J delta 1 gene segment on the other chromosome 14. Chromosome walking coupled with pulsed-field gel electrophoretic (PFGE) analysis determined that the TCR J delta 1 gene fragment of the involved chromosome was relocated approximately 280 kb downstream of the c-myc proto-oncogene locus found on chromosome band 8q24. This rearrangement was reminiscent of the Burkitt's lymphoma variants that translocate to a region identified as the pvt-1 locus. Sequence comparison of the breakpoint junctions of interchromosomal rearrangements in T-cell leukemias involving the TCR delta-chain locus revealed novel signal-like sequence motifs, GCAGA(A/T)C and CCCA(C/G)GAC. These sequences were found on chromosome 8 at the 5' flanking site of the breakpoint junction of chromosome 8 in the TCR gamma/delta leukemic cells reported here and also on chromosome 1 in T-cell acute lymphocytic leukemia patients carrying the t(1;14)(p32;q11) rearrangement. These results suggest that (i) during early stages of gamma delta T-cell ontogeny, the region 280 kb 3' of the c-myc proto-oncogene on chromosome 8 is fragile and accessible to the lymphoid recombination machinery and (ii) rearrangements to both 8q24 and 1p32 may be governed by novel sequence motifs and be subject to common enzymatic mechanisms.

T-cell receptor delta gene recombination in common acute lymphoblastic leukemia: preferential usage of V delta 2 and frequent involvement of the J alpha cluster

A high frequency (greater than 80%) of acute lymphoblastic leukemias (ALL) exhibit a recombination of the T-cell receptor (TCR) delta chain locus. Interestingly, distinct TCR delta elements are preferentially used in immunologic subtypes. In a recent series of 201 children with common ALL (cALL) we observed a TCR delta rearrangement in 162 patients, 57% of the latter showing a hybridization pattern in Southern blots suggestive of a V delta 2 to D delta 3 recombination. To verify this interpretation and to elucidate in more detail the diversity of this common type of TCR delta recombination we amplified and sequenced the junctional region of nine cALL patients and cell line REH-6 by polymerase chain reaction (PCR). A V delta 2 D delta 3 recombination was confirmed in all cases; convincing evidence for the participation of D delta 1 or D delta 2 elements was not obtained. Eight of nine patients and REH-6 showed complete 5' D delta 3 boundaries within V delta 2 D delta 3 segments, a limitation of junctional diversity also detected in 50% of peripheral blood cell clones derived from two healthy probands. Notably, sequence identity at the V delta 2 D delta 3 junction was demonstrated for a cALL and one of the control clones. Another group of 35 of 162 cALL patients was characterized by V delta 2 rearrangements and biallelic deletion of J delta and C delta sequences. Using a J alpha consensus primer, PCR-directed sequence analysis demonstrated V delta 2 D delta 3 J alpha recombinations in all four cases analyzed by this approach. The J alpha segments of these patients differed, but were identical or homologous to published J alpha elements. Our data suggest a recombination pathway of the TCR delta/alpha locus leading to chimeric TCR alpha molecules, containing V delta and, remarkably, also D delta sequences.

T-Cell Receptor δ Gene Recombination in Common Acute Lymphoblastic Leukemia: Preferential Usage of Vδ2 and Frequent Involvement of the Jα Cluster

Abstract A high frequency (greater than 80%) of acute lymphoblastic leukemias (ALL) exhibit a recombination of the T-cell receptor (TCR) delta chain locus. Interestingly, distinct TCR delta elements are preferentially used in immunologic subtypes. In a recent series of 201 children with common ALL (cALL) we observed a TCR delta rearrangement in 162 patients, 57% of the latter showing a hybridization pattern in Southern blots suggestive of a V delta 2 to D delta 3 recombination. To verify this interpretation and to elucidate in more detail the diversity of this common type of TCR delta recombination we amplified and sequenced the junctional region of nine cALL patients and cell line REH-6 by polymerase chain reaction (PCR). A V delta 2 D delta 3 recombination was confirmed in all cases; convincing evidence for the participation of D delta 1 or D delta 2 elements was not obtained. Eight of nine patients and REH-6 showed complete 5′ D delta 3 boundaries within V delta 2 D delta 3 segments, a limitation of junctional diversity also detected in 50% of peripheral blood cell clones derived from two healthy probands. Notably, sequence identity at the V delta 2 D delta 3 junction was demonstrated for a cALL and one of the control clones. Another group of 35 of 162 cALL patients was characterized by V delta 2 rearrangements and biallelic deletion of J delta and C delta sequences. Using a J alpha consensus primer, PCR-directed sequence analysis demonstrated V delta 2 D delta 3 J alpha recombinations in all four cases analyzed by this approach. The J alpha segments of these patients differed, but were identical or homologous to published J alpha elements. Our data suggest a recombination pathway of the TCR delta/alpha locus leading to chimeric TCR alpha molecules, containing V delta and, remarkably, also D delta sequences.

The human T cell receptor genes are targets for chromosomal abnormalities in T cell tumors.

T cells express either of the two forms of antigen-specific receptors, the alpha/beta and gamma/delta heterodimers. Their structure closely resembles that of immunoglobulins, and the variable part of the receptor molecule is created by somatic assembly of variable, diversity, and joining regions. The genetic structure of T cell receptor (TCR) genes and their rearrangement in T cell development have been elucidated in great detail in recent years. The human genes for the gamma and beta subunits are located on the short and long arms of chromosome 7, respectively, whereas the delta- and alpha-chain genes are located in tandem on the centromeric half of the long arm of chromosome 14. Expression of either alpha/beta or gamma/delta TCR complexes on T cells in the developing thymus is likely to proceed in an ordered fashion and results in the appearance of distinct T cell subpopulations. The process of DNA rearrangements required for the generation of functional variable region genes also predisposes lymphoid cells to aberrant DNA rearrangements, which can be detected as chromosomal abnormalities such as translocations and inversions. Molecular analysis of such aberrant rearrangements has shown that rearranging loci are fused to loci unrelated to antigen receptor genes. Furthermore, the breakpoint structures represent nonproductive intermediates in the hierarchy of physiological rearrangements. Accordingly, T cell tumors arising early in T cell development often carry chromosomal abnormalities involving the delta-chain locus, whereas tumors generated later in T cell development tend to show aberrations in the alpha-chain gene. This pattern seems to reflect the stage-specific accessibility of TCR loci for rearrangement by the recombinase machinery. This enzyme is guided by specific recombination signals that can sometimes also be found at the site of breakage on the participating locus in chromosomal abnormalities. Although some features of the mechanism of aberrant rearrangements are known, their biological consequences are less well understood. However, molecular analysis of the mechanism of chromosomal aberrations in T cell tumors suggests that their biological consequences may vary. Firm evidence for the pathogenic significance is missing for most of these lesions. This provides a challenge to molecular immunology to determine how chromosomal abnormalities are involved in tumor pathogenesis.

Clustering of breakpoints on chromosome 10 in acute T-cell leukemias with the t(10;14) chromosome translocation.

The T-cell receptor (TCR) alpha/delta chain locus on chromosome 14q11 is nonrandomly involved in translocations and inversions in human T-cell neoplasms. We have analyzed three acute T-lymphoblastic leukemia samples carrying a t(10;14)(q24;q11) chromosome translocation by means of somatic cell hybrids and molecular cloning. In all cases studied the translocation splits the TCR delta chain locus. Somatic cell hybrids containing the human 10q+ chromosome resulting from the translocation retain the human terminal deoxynucleotidyltransferase gene mapped at 10q23-q24 and the diversity and joining, D delta 2-J delta 1, regions of the TCR delta chain, but not the V alpha region (variable region of the TCR alpha chain), demonstrating that the split occurred within the V alpha-D delta 2 region. Molecular cloning of the breakpoint junctions revealed that the TCR delta chain sequences involved are made from the D delta 2 segment. The chromosome breakpoints are clustered within a region of approximately 263 base pairs of chromosome 10. The results suggest that the translocation of the TCR delta chain locus to a locus on 10q, which we have designated TCL3, results in deregulation of this putative oncogene, leading to acute T-cell leukemia.

Chromosomal translocation in a human leukemic stem-cell line disrupts the T-cell antigen receptor delta-chain diversity region and results in a previously unreported fusion transcript.

We have studied a leukemic stem-cell line, DU.528, that is able to differentiate into myeloid and lymphoid cells. The leukemic cells have a translocation between chromosomes 1 and 14, t(1;14)(p33;q11), which we have molecularly cloned and sequenced. Initial screening used joining (J)-segment probes from the T-cell receptor (TCR) alpha- and delta-chain loci. In apparent concert with the translocation, a deletion has occurred between delta-chain diversity (D)-region genes D delta 1 and D delta 2. D delta 2 was observed on derivative chromosome 1 [der(1)] and D delta 1 on der(14) with a deletion of the intervening 10 kilobases of germ-line DNA. The nature of the D delta 1-D delta 2 deletional event implicates a lymphoid recombinase in the mechanism of the translocation. As a consequence of the translocation, an unusual fusion transcript was generated. Probes from chromosome 1 detected a previously unreported transcript in RNA from both the cell line and the patient. A chromosome 14 probe identified the same transcript, thus confirming a fusion transcript derived from both chromosomes 1 and 14. This translocation may identify a gene for which we propose the name SCL (stem-cell leukemia) that is important for hemopoietic development and oncogenesis and that has been disrupted or altered in this stem-cell line.

Human T-cell antigen receptor (TCR) delta-chain locus and elements responsible for its deletion are within the TCR alpha-chain locus.

Individual T cells express the CD3 molecule in association with alternative gamma delta or alpha beta heterodimeric T-cell receptors (TCRs). T-cell precursors and occasional gamma delta-expressing T cells in humans possess an unexpected 2.0-kilobase (kb) mRNA in which a tandemly repeated motif, TEA (T early alpha), has been spliced to the constant (C alpha) region. Long-range pulsed-field gel mapping as well as molecular cloning showed that TEA is located immediately 5' to the most upstream joining (J alpha) segment of the TCR alpha-chain locus. The TCR delta-chain locus is immediately 5' to TEA, and diversity (D delta) gene segments, J delta, C delta, and TEA are linked within 35 kb. The human TCR delta locus conserves a 12/23-base-pair (bp) spacer paradigm in which J delta possesses a 12-bp and V delta a 23-bp spacer, while the D delta segments have a 12 bp-D delta-23 bp spacer motif. Considerable TCR delta diversity can be generated despite the predominant use of one V delta and one J delta segment. Two D delta segments, D delta 1 and D delta 2, are 9 and 13 bp long, are frequently recombined as D delta 1-D delta 2, and reveal exonucleolytic trimming with extensive N-segment addition. A gamma delta clonal T cell possessed an effective VDDJ delta rearrangement and an intermediate DDJ delta rearrangement, arguing that the TCR delta locus displays allelic exclusion. Specific rearranging elements that delete the delta locus, delta Rec and psi J alpha, were mapped and found to separate the delta locus from the alpha locus. The delta locus including D delta 1-D delta 2-J delta 1-C delta-TEA was deleted in mature, alpha beta-expressing T cells, whereas V delta 1 was frequently retained. The location of the delta locus within the alpha locus may necessitate an exclusive choice between delta or alpha expression.

Expression of T-cell receptor alpha-chain genes in transgenic mice.

To examine the influences responsible for shaping the T-cell repertoire in vivo, we have introduced T-cell receptors of defined specificity into mice. In this report, we analyze transgenic mice carrying a T-cell receptor alpha-chain gene from a pigeon cytochrome c-reactive T-cell line. A variant of this construct, which has the immunoglobulin heavy-chain enhancer inserted into the JC intron, was also introduced into mice. Addition of the enhancer increased the steady-state level of transgene-encoded mRNA three- to fivefold in cultured T cells, leading to a two- to threefold increase in surface expression. In vivo, the difference between these two constructs was even more significant, increasing the number of transgene-positive cells from approximately 5 to 70% and the T-cell receptor surface density two- to threefold. Surprisingly, while surface expression of either type of transgene was limited to T cells, we found little tissue specificity with respect to transcription. In T cells expressing the alpha chain from the enhancer-containing construct, immunoprecipitation with a 2B4 alpha-specific monoclonal antibody revealed the expected disulfide-linked dimer. Costaining of these T cells with the 2B4 alpha-specific monoclonal antibody versus anti-CD3 indicated that expression of the transgene-encoded alpha chain precludes expression of endogenous alpha chains on the majority of cells; in contrast, 2B4 alpha-chain expression from the construct lacking the enhancer is inefficient at suppressing endogenous alpha-chain expression. In mice of the enhancer lineage, Southern blot analysis indicated suppression of endogenous alpha-chain rearrangements in T-cell populations, consistent with the observed allelic exclusion at the cellular level. Interestingly, newborn, but not adult, mice of this lineage also showed an increase in retention of unrearranged delta-chain loci in thymocyte DNA, presumably resulting from the suppression of alpha-chain rearrangements. This observation indicates that at least a fraction of alpha:beta-positive T cells have never attempted to produce functional delta rearrangements, thus suggesting that alpha:beta and gamma:delta T cells may be derived from different T-cell compartments (at least during the early phases of T-cell differentiation).

Expression of T-cell receptor alpha-chain genes in transgenic mice.

To examine the influences responsible for shaping the T-cell repertoire in vivo, we have introduced T-cell receptors of defined specificity into mice. In this report, we analyze transgenic mice carrying a T-cell receptor alpha-chain gene from a pigeon cytochrome c-reactive T-cell line. A variant of this construct, which has the immunoglobulin heavy-chain enhancer inserted into the JC intron, was also introduced into mice. Addition of the enhancer increased the steady-state level of transgene-encoded mRNA three- to fivefold in cultured T cells, leading to a two- to threefold increase in surface expression. In vivo, the difference between these two constructs was even more significant, increasing the number of transgene-positive cells from approximately 5 to 70% and the T-cell receptor surface density two- to threefold. Surprisingly, while surface expression of either type of transgene was limited to T cells, we found little tissue specificity with respect to transcription. In T cells expressing the alpha chain from the enhancer-containing construct, immunoprecipitation with a 2B4 alpha-specific monoclonal antibody revealed the expected disulfide-linked dimer. Costaining of these T cells with the 2B4 alpha-specific monoclonal antibody versus anti-CD3 indicated that expression of the transgene-encoded alpha chain precludes expression of endogenous alpha chains on the majority of cells; in contrast, 2B4 alpha-chain expression from the construct lacking the enhancer is inefficient at suppressing endogenous alpha-chain expression. In mice of the enhancer lineage, Southern blot analysis indicated suppression of endogenous alpha-chain rearrangements in T-cell populations, consistent with the observed allelic exclusion at the cellular level. Interestingly, newborn, but not adult, mice of this lineage also showed an increase in retention of unrearranged delta-chain loci in thymocyte DNA, presumably resulting from the suppression of alpha-chain rearrangements. This observation indicates that at least a fraction of alpha:beta-positive T cells have never attempted to produce functional delta rearrangements, thus suggesting that alpha:beta and gamma:delta T cells may be derived from different T-cell compartments (at least during the early phases of T-cell differentiation).

Sequence and organization of the diversity, joining, and constant region genes of the human T-cell delta-chain locus.

In this paper we describe the genomic organization and sequence of the human T-cell receptor delta-chain diversity, joining, and constant genes. There is one delta-chain constant region gene (C delta) located approximately equal to 85 kilobases (kb) upstream of the alpha-chain constant region. The delta-chain constant region consists of four exons, whose organization is very similar to that of the C alpha exons, suggesting that C alpha and C delta may have arisen from a gene duplication event. The first exon encodes most of the extracellular constant domain, the second encodes a hinge-like region, and the third encodes the entire transmembrane segment and intracytoplasmic portion, whereas the last exon contains exclusively 3' untranslated sequences. Three joining segments, J delta 1, J delta 2, and J delta 3, are found approximately equal to 12, approximately equal to 5.7, and approximately equal to 3.4 kb upstream of the first exon of C delta. Two functional diversity gene segments, D delta 1 and D delta 2, which can be productively translated in all three reading frames, are found 1 and 9.6 kb upstream of J delta 1. The presence of two D delta with such potential for diversity may offset the limited repertoire of the J delta and V delta genes. The spacer distribution in the recombinational signals flanking D delta and J delta segments allows recombination with V alpha gene segments; however, examination of delta-chain messages does not indicate that this is the case, suggesting that the delta chain uses unique variable gene segments and raising the question as to the reasons for this phenomenon.

Structure of extrachromosomal circular DNAs excised from T-cell antigen receptor alpha and delta-chain loci

Small polydisperse circular (spc) DNA was isolated from mouse thymocytes, fragmented by HindIII digestion and cloned into the vector. Sixty DNA clones were randomly selected from the 10,400 phage library. The average size of insert was one-fifth of the original circular molecule. Twenty spc-DNA clones were homologous to DNA probes derived from T-cell antigen receptor (TCR) α-chain loci. We have characterized nine clones by DNA sequencing; they contain new germline sequences of the TCR α-chain variable (Vα) and joining (Jα) gene segments and the products out of the recombination of a Vα with a Jα gene segment. An additional four spc-DNA clones carried a new rearranging gene of the TCR δ-chain that is located between Vα and Jα genes. At least nine of 60 DNA clones carried the recombination junction of a heptamer-heptamer head-to-head structure expected from an excised product of V—J joining. This shows that most extrachromosomal circular DNAs in the thymus are formed by a sequence-dependent recombination mechanism. We suggest that a functional T-cell receptor Vα gene can be constructed by somatic random rearrangements through successive looping-out, excision and deletion.

A cluster of chromosome 11p13 translocations found via distinct D-D and D-D-J rearrangements of the human T cell receptor delta chain gene.

Human T cell tumours have few consistently occurring translocations which provide markers for this disease. The translocation t(11;14)(p13;q11), however, seems to be an exception, since it has been repeatedly observed in T-ALL. We have analysed a number of T-ALL samples carrying the t(11;14) with a view to assessing the nature of the translocated sequences on chromosomes 11 and 14. Three of the tumours studied have breakpoints, at 14q11, within the T cell receptor delta chain locus, while a fourth appears to break in the J alpha region. The TCR delta sequences involved in the translocation junctions are made from D delta-D delta-J delta joins or from D delta-D delta joins, allowing us to define distinct human D delta and J delta segments. These results allow us to make a comparison between the human and mouse TCR delta loci, both as regards sequence and rearrangement hierarchies. The disparate translocation breakpoints at chromosome 14q11 contrast with the marked clustering of breaks at chromosome 11p13; in all four cases, the breakpoint occurs within a region of less than 0.8 kb of chromosome 11. The analysis of junctional sequences at the 11p13 breakpoint cluster region only shows a consensus heptamer-like sequence in one out of four tumours analysed. Therefore, recombinase-mediated sequence specific recognition is not the only cause of chromosomal translocation.

Linkage relationships between beta- and delta-structural loci and African forms of beta thalassaemia.

Five families are described in which there have been matings between individuals doubly heterozygous for beta thalassaemia and the delta-chain variant haemoglobin A2' to normal persons. In all there were 24 informative offspring. There were no crossovers between the beta-thalassaemia and delta-chain loci; in three of the families the genes were linked in cis and in two families the genes were found in trans. Together with previously reported families there have now been 58 opportunities for crossing over between the beta-thalassaemia and delta-chain loci and there have been two possible and one highly probable crossovers. Of the total of 9 families reported to date 4 have had the genes for beta thalassaemia and Hb A2' in cis and 5 in trans. These findings are contrasted with the findings in families where a beta-chain structural variant and Hb A2' have been observed together and these genes have always been found in trans and never in cis. The reasons for linkage disequilibrium of this type are discussed. It is concluded tentatively that the distance between the delta-structural and beta-thalassaemia loci is greater than that between the delta-structural and beta-structural loci. To date this conclusion can only be applied to the beta+ -thalassaemia and beta-thalassaemia genes as found in the African population, since this is the only population with a high incidence of delta-chain mutants which allow linkage analysis of this type to be carried out.

Thalassaemia in northern Liberia. A survey in the Mount Nimba area.

A population survey in northern Liberia showed an average incidence of 9 percent of beta-thalassaemia trait. There was considerable tribal variation, the highest rates being mainly found in tribes having a low incidence of Hb S. A single example of deltabeta-thalassaemia trait was also found; the incidence of delta-chain variants was 1.8 percent. The effect of malaria on Hb A2 levels was investigated but appeared to be negligible. The problems of beta-thalassaemia and Hb S occurring in the same population are discussed. It is considered that, although made on a local sample, the results of this survey indicate a high incidence of beta-thalassaemia trait elsewhere in Liberia and that this is a suitable are for investigations of linkage at beta- and delta-chain loci. No conclusion is at present possible from the data on the presence of alpha-thalassaemia although 10 percent of neonates were found to have increased levels of Hb Barts.