Somatic mutation in genes for the variable portion of the immunoglobulin heavy chain

Large-scale inference of the point mutational spectrum in human segmental duplications

BACKGROUND

Recent segmental duplications are relatively large (> or = 1 kb) genomic regions of high sequence identity (> or = 90%). They cover approximately 4-5% of the human genome and play important roles in gene evolution and genomic disease. The DNA sequence differences between copies of a segmental duplication represent the result of various mutational events over time, since any two duplication copies originated from the same ancestral DNA sequence. Based on this fact, we have developed a computational scheme for inference of point mutational events in human segmental duplications, which we collectively term duplication-inferred mutations (DIMs). We have characterized these nucleotide substitutions by comparing them with high-quality SNPs from dbSNP, both in terms of sequence context and frequency of substitution types.

RESULTS

Overall, DIMs show a lower ratio of transitions relative to transversions than SNPs, although this ratio approaches that of SNPs when considering DIMs within most recent duplications. Our findings indicate that DIMs and SNPs in general are caused by similar mutational mechanisms, with some deviances at the CpG dinucleotide. Furthermore, we discover a large number of reference SNPs that coincide with computationally inferred DIMs. The latter reflects how sequence variation in duplicated sequences can be misinterpreted as ordinary allelic variation.

CONCLUSION

In summary, we show how DNA sequence analysis of segmental duplications can provide a genome-wide mutational spectrum that mirrors recent genome evolution. The inferred set of nucleotide substitutions represents a valuable complement to SNPs for the analysis of genetic variation and point mutagenesis.

Amino acid insertions and deletions contribute to diversify the human Ig repertoire

The sequence analysis of Ig variable region genes transcribed within different B-cell subpopulations from human tonsil led us to identify a rare DNA sequence modification event consisting of bp insertions and/or deletions (I/D). Although these events were previously reported, they had never been formally associated with the somatic hypermutation process. I/D events share with more conventional somatic hypermutation events their localization within hypervariable regions and, most particularly, within DNA motifs known to be mutational hot spots. Repetitive DNA tracts or DNA elements capable of forming DNA loop intermediates seem to be the preferred substrate for I/D to occur. These characteristics suggest a model for somatic hypermutation reminiscent of the "polymerase slippage" model involved in replication and repair mutations in prokaryotes, yeast, and mammals.

Staphylococcal protein A binding to VH3 encoded immunoglobulins

Staphylococcal protein A (SPA) is a B-cell superantigen which binds specifically to the variable region of human VH3 encoded antibodies. We undertook to identify the VH3 regions involved in the interaction with SPA by producing mutant antibodies in the baculovirus expression system. We had previously shown that a single amino acid change at position 57 in the CDR2 of a human SPA nonbinding VH3 encoded rheumatoid factor converted it to an SPA binder, implicating CDR2 in SPA binding. When regions of the mutated binder were exchanged with those from a mouse nonbinding antibody, the pattern of SPA binding indicated that residues in FR1, CDR2 and FR3 are involved in the interaction between VH3 encoded antibodies and SPA. In addition, all three regions are simultaneously required for SPA binding to occur. When any one of the three regions was altered, SPA binding was severely disrupted.

Mechanism of allergic cross-reactions--III. cDNA cloning and variable-region sequence analysis of two IgE antibodies specific for trinitrophenyl

As a first step toward defining the molecular interactions between ligands and the IgE antigen-combining site, we report here the cDNA cloning and variable (V) region nucleic acid sequences of the heavy (H) and light (L) chains of 2 monoclonal mouse IgE antibodies to trinitrophenyl (ATCC-TIB142 = IGELa2 and ATCC-TIB141 = IGELb4). In all instances, full-length cDNA clones were obtained to facilitate future expression studies. The H chains were encoded by VH genes from the VH3660 and J558 gene families in context with DQ52 and DSP2.2 diversity (D) mini genes, and JH3 and JH4 joining (J) gene segments, respectively. Vk8/Jk2 and Vk1/Jk5 rearrangements encoded the respective L chain V-regions. Both antibodies exhibited considerable conservation of complementarity determining region (CDR) sequences, which will facilitate template-based computer modeling of the three-dimensional structures of complexes formed between various ligands and these antibodies. From sequence comparison between the dinitrophenyl (DNP)-binding myeloma protein MOPC-315 and these IgE antibodies likely candidates for hapten-contact residues within the binding sites of IGELa2 and IGELb4 have been suggested.

Autoantibodies and autoantigens: a conserved system that may shape a primary immunoglobulin gene pool

Formation of certain autoantibodies is associated with a variety of autoimmune diseases, but the production of small amounts of autoantibodies also occurs in the normal immune system. Germline-encoded IgM antibodies that are autoreactive and bind to diverse antigen structures with low affinity are prominent in the primary antibody repertoire. Many IgG disease-related autoantibodies differ in structure and binding properties from these normally occurring IgM autoantibodies. The two sets may arise independently, but some properties, such as shared idiotypes, link some members of the two populations. Many autoantigen targets of both sets of autoantibodies are structurally conserved among species, as are certain features of the autoantibodies themselves. These elements, interacting before exposure of the system to foreign antigens, may constitute a conserved system that contributes to shaping and maintaining a primary immunoglobulin gene pool.

Organization of the murine immunoglobulin VH complex: placement of two new VH families (VH10 and VH11) and analysis of VH family clustering and interdigitation

During B cell development, there is an ordered expression of heavy chain variable region (VH) genes during ontogeny such that JH proximal VH genes are rearranged and expressed before the more JH distal VH genes. Thus, the relative chromosomal position of VH genes is biologically significant. We have previously employed deletion mapping to order the nine described murine VH gene families as follows: 3609-J558-(J606/VGAM3-8/S107)-3660-(X24/Q52/7183 ). (Families within parentheses were not mapped relative to each other.) In this report we continue this analysis by mapping two recently described heavy chain variable region gene families (VH10 and VH11). VH10 is located at the JH proximal end of the major cluster of J558 VH gene segments. VH11 (a very small family) is intermingled with the 3660 family. Although in general VH genes are thought to be clustered, we and others have reported some interspersion between families. To further address this issue, we have analyzed 80 recombinant phage clones containing J558 VH gene segments for the presence of other VH family genes. Our data indicate that the J558 and 3609 VH families are extensively intermingled as has recently been described for the most JH proximal Q52 and 7183 families.

High-level production of a functional immunoglobulin heterodimer in a baculovirus expression system

A murine immunoglobulin heterodimer has been expressed in a baculovirus expression system. This was achieved by using both double infection of insect cells with separate heavy- and light-chain-expressing viruses and infection with a double-recombinant virus containing both the immunoglobulin heavy- and light-chain cDNAs. In both cases, the polypeptide chains were correctly processed, glycosylated, and assembled into normal H2L2 (H = heavy, L = light) immunoglobulin monomers. These molecules bound antigen and expressed both polyclonal idiotype and monoclonal idiotopes. Furthermore, the transfer vectors described have been modified to contain the F1 origin of replication for the production of single-stranded DNA, which facilitates site-specific mutations of either the polyhedrin promoter or the inserted foreign gene. Use of this system should significantly advance the analysis of the structural bases for both idiotype expression and antigen binding by immunoglobulin. More importantly, it provides a generic method for the high-level expression of antibodies of diverse interest.

Expression in non-lymphoid cells of mouse recombinant immunoglobulin directed against the tumour marker human placental alkaline phosphatase

From a mouse hybridoma cell line secreting a monoclonal antibody directed against the tumour marker human placental alkaline phosphatase, mRNA coding for the H and L chains of this antibody was isolated and cloned as cDNA. Sequence analysis of the H and L chain cDNAs confirmed the IgG2b,kappa subtype previously established. Recloning the H and L chain cDNA information into SV40-based vectors enabled us to obtain expression of functional immunoglobulin upon cotransfection into COS or CHO dhfr- cells. This illustrates that non-lymphoid cells also have the capacity to assemble active immunoglobulins.

Molecular characterization of the A/J J558 family of heavy chain variable region gene segments

The nucleotide sequences of the coding as well as the flanking regions of 11 A/J J558 heavy chain variable region (VH) gene segments are presented. Among these J558 VH segments was the unrearranged germline VH gene segment recruited in the predominant A strain-specific anti-arsonate response. Three other VH gene segments that are greater than 92% related to the p-azophenylarsenate (Ars) A VH gene segment were also isolated. Detailed analysis of the nucleotide sequences of these as well as the remaining seven J558 VH gene segments reveal that the J558 VH gene family is composed of distinct, but related, J558 VH subfamilies. Deletion mapping analyses were used to position the Ars A VH gene segment proximally with respect to the DH-JH clusters within the J558 VH gene family and distally with respect to its own J558 subfamily. The documentation of J558 VH subfamilies is discussed in the context of J558 VH family evolution and diversification.

The genetic origin of murine lupus-associated autoantibodies

Systemic lupus erythematosus and rheumatoid arthritis in human and murine systems are characterized by circulating autoantibodies and immune complex deposition in various organs causing tissue damage and disease. To define the molecular and clonotypic origin of these anti-self responses, and to determine whether abnormalities in Ig genes or somatic mechanisms generating autoantibody diversity may contribute to lupus etiology, we performed molecular analyses of the Ig germline gene organization and the Ig gene segments expressed in monoclonal autoantibodies from autoimmune mice. Comparative restriction fragment length polymorphism analysis of a large number of Ig gene loci from autoimmune and normal mice indicated that (a) lupus can develop in different Ig heavy and kappa light chain variable region gene haplotypes, and (b) the Ig germline genes in lupus mice might be normal. To determine whether autoantibodies are encoded by unique Ig gene segments present in the normal germline repertoire, but not expressed in exogenous responses, we compared nucleic acid sequences encoding lupus autoantibodies and antibodies against foreign antigens. Similar, and in some instances even identical, gene segments were expressed in both types of antibodies, indicating that anti-self and anti-foreign responses use the same, or at least an overlapping, germline gene repertoire. A large variety of Ig variable, diversity, and joining gene segments encoded these autoantibodies with different specificities. Hence, the overall murine lupus-associated anti-self response may be essentially unrestricted. Furthermore, limited evidence has been obtained that both germline genes and somatically mutated genes encode autospecificity, making gross abnormalities in mechanisms for somatic mutation of Ig variable genes unlikely.(ABSTRACT TRUNCATED AT 250 WORDS)

High-frequency expression of a conserved kappa light-chain variable-region gene in chronic lymphocytic leukemia

Malignant B lymphocytes from several patients with chronic lymphocytic leukemia (CLL) were examined for reactivity with murine monoclonal antibody 17.109. This antibody, prepared against the rheumatoid factor (RF) paraprotein Sie, recognizes a crossreactive idiotype on 48% of human IgM RF paraproteins, but does not react with IgM paraproteins without RF activity or substantially with normal pooled immunoglobulin. The 17.109-reactive idiotype is a marker for a kappa III variable-region gene, designated V kappa RF, that is conserved in outbred human populations. In a limited study of 31 CLL patients, the leukemic cells from 5 of 20 patients with kappa light chain-expressing CLL were recognized by the 17.109 monoclonal antibody. Despite having malignant cells specifically reactive with this antibody, patients with 17.109-positive CLL did not have elevated serum levels of circulating antibody bearing 17.109-reactive determinants. Total RNAs isolated from the CLL B lymphocytes, or from hybridomas produced by fusing the CLL cells with the WI-L2-729-HF2 cell line, were fractionated electrophoretically and examined by blot hybridization. Under stringent hybridization conditions capable of discerning a single base-pair mismatch, RNA from the 17.109-idiotype-positive CLL cells hybridized to synthetic oligonucleotide probes corresponding to framework and complementary-determining regions in the V kappa RF gene. The high frequency of the 17.109-associated idiotype and the V kappa RF gene in CLL suggests that the disease may arise from B lymphocytes that express a restricted set of inherited immunoglobulin variable-region genes with little or no somatic mutation.

V kappa and J kappa gene segments of A/J Ars-A antibodies: somatic recombination generates the essential arginine at the junction of the variable and joining regions

The germ-line elements for the variable (V) and joining (J) regions of the kappa chains (V kappa-Ars, J kappa 1) that give rise to the productive allele for the A/J mouse light chains of Ars-A monoclonal antibodies (directed against the hapten p-azophenylarsonate) have been cloned and sequenced as well as a rearranged Ars-A light chain gene. Using the V kappa-Ars gene as a hybridization probe, we provide evidence that the V kappa 10 family is relatively small and that only one member gives rise to the productive allele in all Ars-A antibodies. An unusual feature of Ars-A light chains is that all contain an arginine at position 96, the V-J junctional position. We had shown by chain recombination that arginine-96 was essential for antigen binding. In the present study, an arginine codon was not found either at the 3' end of the V kappa-Ars gene segment or at the 5' end of J kappa 1. However, an arginine codon (CGG) can easily be generated by recombination between these two germ-line elements. Thus, we document that junctional diversity through intracodonic recombination can be crucial to the antibody binding function of the resulting molecule.

Recombination between an expressed immunoglobulin heavy-chain gene and a germline variable gene segment in a Ly 1+ B-cell lymphoma

The early stages of murine B-cell differentiation are characterized by a series of immunoglobulin gene rearrangements which are required for the assembly of heavy(H) and light(L)-chain variable regions from germline gene segments. Rearrangement at the heavy-chain locus is initiated first and consists of the joining of a diversity (DH) gene segment to a joining (JH) gene segment. This forms a DJH intermediate to which a variable (VH) gene segment is subsequently added. Light-chain gene rearrangement follows and consists of the joining of a VL gene segment to a JL gene segment: once a productive light-chain gene has been formed the cell initiates synthesis of surface immunoglobulin M (sIgM) receptors (reviewed in ref. 1). These receptors are clonally distributed and may undergo further diversification either by somatic mutation or possibly by continued recombinational events. Such recombinational events have been detected in the Ly 1+ B-cell lymphoma NFS-5, which has been shown to rearrange both lambda and H-chain genes subsequent to the formation of sIgM (mu kappa) molecules. Here we have analysed a rearrangement of the productive allele of NFS-5 and found that it is due to a novel recombination event between VH genes which results in the replacement of most or all of the coding sequence of the initial VHQ52 rearrangement by a germline VH7183 gene. Embedded in the VH coding sequence close to the site of the cross-over is the sequence 5' TACTGTG 3', which is identical to the signal heptamer found 5' of many DH gene segments. This embedded heptamer is conserved in over 70% of known VH genes. We suggest that this heptamer mediates VH gene replacement and may play an important part in the development of the antibody repertoire.

Genetic basis for expression of the idiotypic network. One unique Ig VH germline gene accounts for the major family of Ab1 and Ab3 (Ab1') antibodies of the GAT system

Ig germline genes have been isolated from recombinant clones prepared in separate libraries constructed from adult BALB/c liver DNA either in pBR328 plasmid or in EMBL 3 phage. Three clones that gave a very strong positive hybridization signal with a VH anti-GAT-specific probe were completely characterized and sequenced. All three were greater than 95% homologous, with the exception of the 5' noncoding region, which was only 85% homologous but contained characteristic regulatory signals. One of these genes, H10, had a sequence that was completely identical to that of a cDNA derived from a GAT-specific BALB/c hybridoma. Southern blot analysis using Eco RI-digested DNA from rearranged GAT-specific hybridomas revealed that the same gene was used for other GAT-specific VH regions, including one differing from the H10 sequence by 12 nucleotides, which must have been generated by a somatic mechanism. The same H10 germline gene was also used, in most cases without any nucleotide substitution, in hybridomas of the Ab1' set of the GAT idiotypic cascade, suggesting that immunization with Ab2 (antiidiotypic) antibodies preferentially stimulates the direct expression of VH germline genes. Finally, the previous hypothesis that NPa and GAT VH genes were derived from the same germline gene was definitively confirmed, both from sequence data and Southern blot analysis.

Quantitative representation of two germ-line V genes in the early antibody response to 2-phenyloxazolone

Early anti-phenyloxazolone antibodies of BALB/c mice include a subset that bears the nearly or totally unmutated VH-Ox1 sequence. A fraction of this subset also bears the nearly or totally unmutated V kappa-Ox1 sequence. The whole subset is recognized by an anti-idiotype serum 495 and the VH-Ox1/V kappa-Ox1 fraction also by another anti-idiotype serum 260. Frequencies of the VH-Ox1 subset and its V kappa-Ox1 fraction in early IgM and IgG antibodies were determined by typing hybridomas with anti-idiotype antisera. The whole subset appears to make up approximately one half of IgG antibodies, two thirds of this being 495+/260+ in both isotypes. IgM data are less certain but the same frequencies may be valid. Affinities of 495+/260+ antibodies ranged from 1.3 X 10(6) to 11 X 10(6), affinities of 495+/260- antibodies from 0.47 X 10(6) to 1.1 X 10(6), and affinities of doubly negative antibodies were less than 0.41 X 10(6). High affinity is probably an explanation for the high proportion (one third) of the 495+/260+ antibodies in the early response. Doubly negative (and low-affinity) hybridomas may not have been classified as producers of phenyloxazolone antibodies in earlier studies, and this could explain the still higher reported frequency (73%) of VH-Ox1/V kappa-Ox1 antibodies.

Heavy chain variable region. Multiple gene segments encode anti-4-(hydroxy-3-nitro-phenyl)acetyl idiotypic antibodies

The hapten (4-hydroxy-3-nitrophenyl)acetyl (NP), when conjugated to carrier proteins, elicits a characteristic idiotypic response (NPb) in C57BL/6 mice. The response can be divided serologically into two distinct NPb-positive groups of antibodies. The first group consists of four crossreacting subgroups (I-IV), the second of two subgroups (V, VI). Some antibodies of subgroups I and II have been shown to express the unmutated heavy chain variable region (VH) germline gene 186.2. Antibodies of subgroups V and VI crossreact extensively with the NPa-positive antibodies of BALB/c mice. We sequenced heavy chain complementary DNA from eight hybridomas producing anti-NP antibodies. Six of these belong to subgroups V and VI, and two were NPa-positive hybridomas of BALB/c origin. All sequences were homologous to each other, and differed by approximately 80 basepairs from the 186.2 C57BL/6 germline VH gene. From our sequence and Southern blot analyses we suggest: (a) the NPb idiotypic response is the product of several VH germline genes, (b) some of these genes are very homologous to the gene coding for the BALB/c NPa idiotype, and might represent the C57BL/6 allelic forms of this gene, (c) the diversity regions of NPb and NPa-positive antibodies are diverse in length and amino acid composition, except for the first residue, which is always tyrosine, (d) all four heavy chain joining region gene segments are expressed without mutation. We discuss our data in terms of diversity in the germline VH gene repertoire, as well as diversity created by gene segment-joining events and somatic mutation.

Analysis of the distribution of charged residues in the N-terminal region of signal sequences: implications for protein export in prokaryotic and eukaryotic cells

A statistical analysis of the distribution of charged residues in the N-terminal region of 39 prokaryotic and 134 eukaryotic signal sequences reveals a remarkable similarity between the two samples, both in terms of net charge and in terms of the position of charged residues within the N-terminal region, and suggests that the formyl group on Metf is not removed in prokaryotic signal sequences.

Complete amino acid sequence of heavy chain variable regions derived from two monoclonal anti-p-azophenylarsonate antibodies of BALB/c mice expressing the major cross-reactive idiotype of the A/J strain

The primary structure of A/J anti-p-azophenylarsonate (anti-Ars) antibodies expressing the major A-strain cross-reactive idiotype (CRIA) has provided important insights into issues of antibody diversity and the molecular basis of idiotypy in this important model system. Until recently, this idiotype was thought to be rarely, if ever, expressed in BALB/c mice. Indeed, it has been reported that BALB/c mice lack the heavy chain variable segment (VH) gene that is utilized by the entire family of anti-Ars antibodies expressing the A/J CRI. Recently, however, it has been possible to elicit CRIA+, Ars binding antibodies in the BALB/c strain by immunizing first with anti-CRI and then with antigen. Such BALB/c, CRIA+ anti-Ars antibodies can be induced occasionally with antigen alone. VH region amino acid sequences are described for two CRIA+ hybridoma products derived from BALB/c mice. While remarkably similar to each other, their VH segments (1-98) differ from the VH segments of A/J CRIA+, anti-Ars antibodies in over 40 positions. Rather than the usual JH2 gene segment used by most A/J CRIA+ anti-Ars antibodies, one BALB/c CRIA+ hybridoma utilizes a JH1 gene segment, while the other uses a JH4. However, the D segments of both of the BALB/c antibodies are remarkably homologous to the D segments of several A/J CRIA+ antibodies sequenced previously, as are the amino terminal amino acid sequences of their light chains. These data imply that BALB/c mice express the A/J CRIA by producing antibodies with very similar, if not identical, light chain and heavy chain D segments, but in the context of different VH and JH gene segments than their A/J counterparts. The results document that molecules that share serologic specificities can have vastly different primary structures.

The immunoglobulin heavy chain variable region (Igh-V) locus in the mouse. I. One hundred Igh-V genes comprise seven families of homologous genes

Seven families of Igh-V genes have been defined by Southern blot analysis of genomic DNA from eighteen inbred strains of mice. Each of twenty-four cloned Vh genes hybridized to one of seven nonoverlapping sets of Eco RI restriction fragments. These families contain from 2 to approximately 40 hybridizing fragments. From these data we estimate that the mouse Igh-V locus consists of one hundred Vh genes. Genes within a Vh family share greater than 80% sequence homology while the sequence homology between families is generally less than 70%. There is extensive restriction fragment length polymorphism among the strains analyzed allowing the assignment of complete (Igh-V + Igh-C) Igh haplotypes for eighteen inbred mouse strains.

Effect of somatic mutation within translocated c-myc genes in Burkitt's lymphoma

Our previous studies of a translocated c-myc gene in the Raji Burkitt's lymphoma cell showed somatic mutations in exons 1 and 2. We have extended these observations to two other translocated c-myc genes and find a common occurrence of mutation in the noncoding exon 1. We also found that in Raji cells, unlike other Burkitt's lymphoma cell lines, the normal allele of the c-myc gene is transcribed as well as the translocated gene. These results support a model in which c-myc oncogene activation in Burkitt's lymphoma occurs by disruption of a normal transcriptional control mechanism in which the c-myc protein is itself involved.

A serologic marker for the Ars-C family of anti-arsonate antibodies

Attempts are being made to define, at the serologic, structural and genetic levels, the anti-arsonate repertoire. We previously identified three families of anti-arsonate antibodies based on the amino acid sequence of the heavy chain V regions. The present report establishes the serologic correlates of one of these families, the Ars-C family. The determinants defining this family are expressed by anti-Ar molecules displaying the A/J minor idiotype or the BALB/c major idiotype; these determinants are also expressed on molecules of different specificity, notably by the DNP-binding myelomas, MOPC-460 and MOPC-315. All strains tested employed the Ars-C family in the response to arsonate.

Amino acid sequence diversity within the family of antibodies bearing the major antiarsonate cross-reactive idiotype of the A strain mouse

VH region amino acid sequences are described for five A/J anti-p-azophenylarsonate (anti-Ars) hybridoma antibodies for which the VL region sequences have previously been determined, thus completing the V domain sequences of these molecules. These antibodies all belong to the family designated Ars-A which bears the major anti-arsonate cross-reactive idiotype (CRI) of the A strain mouse. However, they differ in the degree to which they express the CRI in standard competition radioimmunoassays. Although the sequences are closely related, all are different from each other. Replacements are distributed throughout the VH region and occur in positions of the chain encoded by all three gene segments, VH, DH, and JH. It is likely that somatic diversification processes play a dominant role in producing the sequence variability in each of these segments. The number of differences from the sequence encoded by the germline is smallest for antibodies that express the CRI most strongly, suggesting that somatic diversification is responsible for loss of the CRI in members of the Ars-A antibody family. There is an unusual degree of clustering of differences in both CDR2 and CDR3 and many of the substitutions are located in "hot spots" of variation. The large number of differences between the chains prohibits the unambiguous identification of positions at which alterations play a major role in reducing the expression of the CRI. However, the data suggest that the loss of the CRI is associated with a definable repertoire of somatic changes at a restricted number of highly variable sites.

Amino acid sequence of the heavy chain variable region from the A/J mouse anti-arsonate monoclonal antibody 36-60 bearing a minor idiotype

In addition to the predominant idiotype family (IdCR) associated with the murine A/J anti-azophenylarsonate response, a second idiotype family (Id36-60) was defined on the basis of serologic cross-reactivity and amino acid sequence homology among monoclonal antibodies [Marshak-Rothstein, A., Margolies, M.N., Benedetto, J.D., & Gefter, M.L. (1981) Eur. J. Immunol. 11, 565-572]. The complete variable region amino acid sequence of the A/J IgG2a hybridoma protein 36-60 heavy chain was obtained by automated Edman degradation of the intact chain and fragments generated by cleavage with CNBr and by tryptic digestion of a succinylated CNBr peptide. A comparison of the Id36-60 heavy chain sequence to that of the IdCR heavy chain variable region reveals only 45% homology between them. The structural data indicate that different genes encode the VH, D, and JH gene-encoded sequences in the two idiotypes.

Three T cell hybridomas do not contain detectable heavy chain variable gene transcripts

We attempted to determine whether T cells express any VH gene segments. cDNA libraries were constructed from one suppressor and two helper T cell hybridomas. Both the library construction and screening were designed to maximize detection of a wide range of VH gene segments. One screening method should detect about half of the sequenced VH genes, while the second should detect most of these genes. The probability of detecting a VH gene homologous to the probes and present at 10 copies per cell was 77% for one helper cell cDNA library, 88% for the second helper cell library, and greater than 99% for the suppressor cell library. No cDNA clones with VH gene segments were detected. From this result, we conclude that VH gene segments are not likely to encode the antigen-specific receptor in the cells we tested.

Patterns of amino acids near signal-sequence cleavage sites

According to the signal hypothesis, a signal sequence, once having initiated export of a growing protein chain across the rough endoplasmic reticulum, is cleaved from the mature protein at a specific site. It has long been known that some part of the cleavage specificity resides in the last residue of the signal sequence, which invariably is one with a small, uncharged side-chain, but no further specific patterns of amino acids near the point of cleavage have been discovered so far. In this paper, some such patterns, based on a sample of 78 eukaryotic signal sequences, are presented and discussed, and a first attempt at formulating rules for the prediction of cleavage sites is made.

The genetic basis of antibody production: the dominant anti-arsonate idiotype response of the strain A mouse

Immunization of the A strain of mice with the hapten p-azophenylarsonate (Ars) results in an immune response in which approximately 50% of the anti-Ars antibodies share cross-reactive idiotypic determinants (IdCR). A gene or genes linked to the heavy chain constant region locus is required for the production of this idiotype. The expressed VH gene from a hybridoma cell line which expresses the IdCR has been cloned. DNA hybridization studies utilizing the VH gene have revealed that there are many related genes in both idiotype-producing and idiotype-nonproducing strains of mice. However, under stringent hybridization conditions, only a single band of 6.4 kb is present in Eco R1-digested A strain DNA. Strains of mice which are phenotypically idiotype-negative either lack this band completely or possess a much weaker one at this position. Utilizing DNA from Igh recombinant strains of mice, it has been shown that the VH locus controlling idiotype expression contains the structural gene information for the idiotype-positive heavy chains. It has also been shown that DNA at this locus appears to be sufficient for the production of the cross-reactive idiotype. Utilizing a DNA probe derived from regions flanking the structural gene has confirmed the relatedness of V genes in a variety of mouse strains and revealed a significant degree of polymorphism at the Igh locus.

Defective repair of 0(6)-methylguanine in autoimmune diseases

On the premise that somatic mutations may be involved in the pathogenesis of diseases such as systemic lupus erythematosus and rheumatoid arthritis, the proficiency of repair of 0(6)-methylguanine, a powerful, premutagenic, directly miscoding base lesion, was examined in the DNA of peripheral-blood mononuclear cells (mainly lymphocytes) from patients with such diseases. The capacity of lymphocytes to repair this lesion was impaired in many patients with autoimmune disease. In some healthy controls repair was also defective whereas in other, nonautoimmune diseases, it was not. These findings support the hypothesis that this defect of repair was not simply a result of the disease state. It is therefore postulated that defective repair of 0(6)-methylguanine could be one of the factors determining the susceptibility of autoimmune diseases in genetically predisposed individuals.