Conservation of sequence in recombination signal sequence spacers

The role of components of recombination signal sequences in immunoglobulin gene segment usage: a V81x model

It has long been appreciated that some immunoglobulin (and T-cell receptor) gene segments are used much more frequently than others. The VHsegment V81x is a particularly striking case of overusage. Its usage varies with the stage of B-cell development and with the strain of mice, but it is always high in B cell progenitors. We have found that the coding sequence and the recombination signal sequences (RSS) are identical in five mouse strains, including CAST/Ei, a strain derived from the species Mus castaneus. Thus, the strain differences cannot be attributed to sequences within V81x itself. V81x RSS mediated recombination at rates significantly higher than another VHRSS. Although the V81x nonamer differs at one base pair from the consensus sequence, an RSS with this nonamer and a consensus heptamer recombines as well as the consensus RSS. When the V81x spacer is replaced by that of VA1, the frequency of recombination decreases by approximately 5-fold; thus, the contribution of variation in natural spacers to variability in VHusage in vivo is likely to be more than has been previously appreciated. Furthermore, the contribution of the heptamer and nonamer to differential VHusage in our assay is correlated inversely with their conservation throughout the VHlocus.

IgM Heavy Chain Complementarity-Determining Region 3 Diversity Is Constrained by Genetic and Somatic Mechanisms Until Two Months After Birth

Due to the greater range of lengths available to the third complementarity determining region of the heavy chain (HCDR3), the Ab repertoire of normal adults includes larger Ag binding site structures than those seen in first and second trimester fetal tissues. Transition to a steady state range of HCDR3 lengths is not complete until the infant reaches 2 mo of age. Fetal constraints on length begin with a genetic predilection for use of short DH (D7-27 or DQ52) gene segments and against use of long DH (e.g., D3 or DXP) and JH (JH6) gene segments in both fetal liver and fetal bone marrow. Further control of length is achieved through DH-specific limitations in N addition, with D7-27 DJ joins including extensive N addition and D3-containing DJ joins showing a paucity of N addition. DH-specific constraints on N addition are no longer apparent in adult bone marrow. Superimposed upon these genetic mechanisms to control length is a process of somatic selection that appears to ensure expression of a restricted range of HCDR3 lengths in both fetus and adult. B cells that express Abs of an “inappropriate” length appear to be eliminated when they first display IgM on their cell surface. Control of N addition appears aberrant in X-linked agammaglobulinemia, which may exacerbate the block in B cell development seen in this disease. Restriction of the fetal repertoire appears to be an active process, forcing limits on the diversity, and hence range of Ab specificities, available to the young.

Detection of RAG protein-V(D)J recombination signal interactions near the site of DNA cleavage by UV cross-linking

V(D)J recombination is initiated by double-strand cleavage at recombination signal sequences (RSSs). DNA cleavage is mediated by the RAG1 and RAG2 proteins. Recent experiments describing RAG protein-RSS complexes, while defining the interaction of RAG1 with the nonamer, have not assigned contacts immediately adjacent to the site of DNA cleavage to either RAG polypeptide. Here we use UV cross-linking to define sequence- and site-specific interactions between RAG1 protein and both the heptamer element of the RSS and the coding flank DNA. Hence, RAG1-DNA contacts span the site of cleavage. We also detect cross-linking of RAG2 protein to some of the same nucleotides that cross-link to RAG1, indicating that, in the binding complex, both RAG proteins are in close proximity to the site of cleavage. These results suggest how the heptamer element, the recognition surface essential for DNA cleavage, is recognized by the RAG proteins and have implications for the stoichiometry and active site organization of the RAG1-RAG2-RSS complex.

Omenn syndrome: a disorder of Rag1 and Rag2 genes

In vertebrates, generation of the T- and B-cell repertoire relies on genomic rearrangement of T-cell receptor and immunoglobulin gene coding segments. This process, known as V(D)J recombination, is initiated by the lymphoid specific proteins Rag1 and Rag2. Both in humans and in animal models, mutations that abrogate expression of either the Rag1 or Rag2 proteins result in severe combined immune deficiency with a complete lack of circulating T and B cells due to an early block in lymphoid development. We have recently shown that mutations that impair, but do not completely abolish the function of Rag1 and Rag2 in humans result in Omenn syndrome, an enigmatic form of combined immune deficiency characterized by oligoclonal, activated T lymphocytes with a skewed Th2 profile.

Molecular Mechanisms and Selection Influence the Generation of the Human VλJλ Repertoire

To define the λ light chain repertoire in humans, a single-cell PCR technique using genomic DNA obtained from individual peripheral B cells was employed. Of the 30 known functional Vλ genes, 23 were detected in either the nonproductive or productive repertoires. Specific Vλ genes, including 2A2, 2B2, 1G, and 4B, were overexpressed in the nonproductive repertoire, whereas some Vλ genes, such as 3R, 2A2, 2B2, 1C, 1G, and 1B, were overexpressed in the productive repertoire. Comparison of the nonproductive and productive repertoires indicated that no Vλ genes were positively selected, whereas a number of Vλ genes, including 4C, 1G, 5B, and 4B, were negatively regulated. All four of the functional Jλ segments were found in both repertoires, with Jλ7 observed most often. Evidence of terminal deoxynucleotidyltransferase activity was noted in nearly 80% of nonproductive VλJλ rearrangements, and exonuclease activity was apparent in the majority. Despite this, the mean CDR3 length was 30 base pairs in both productive and nonproductive repertoires, suggesting that it was tightly regulated at the molecular level. These results have provided new insights into the dimensions of the human Vλ repertoire and the influences that shape it.

V(D)J recombination signal recognition: distinct, overlapping DNA-protein contacts in complexes containing RAG1 with and without RAG2

Protein interactions with V(D)J recombination signal sequences (RSSs) were mapped in complexes containing RAG1 with (M1/2) or without (M1) RAG2. In both complexes, RAG interactions with the DNA backbone are biased toward one side of the helix; nonamer contacts resemble those of Hin with hixL. In the M1 complex, DNA contacts are centered on the nonamer. In the M1/2 complex, protein-RSS interactions extend through the spacer and into the nonamer-proximal portion of the heptamer. Chemical modifications near the heptamer-coding junction are overrepresented in the M1/2 complex, providing evidence for perturbation of DNA structure in this region. Thus, while RAG1 alone can bind the nonamer, RAG2 is required for heptamer occupancy.

The effect of Me2+ cofactors at the initial stages of V(D)J recombination

V(D)J site-specific recombination mediates the somatic assembly of the antigen receptor gene segments. This process is initiated by the recombination activating proteins RAG1 and RAG2, which recognize the recombination signal sequences (RSS) and cleave the DNA at the coding/RSS junction. In this study, we show that RAG1 and RAG2 have the ability to directly interact in solution before binding to the DNA. RAG1 forms a homodimer, which leads to the appearance of two distinct RAG1.RAG2 complexes bound to DNA. To investigate the properties of the two RAG1.RAG2 complexes in the presence of different Me2+ cofactors, we established an in vitro Mg2+-based cleavage reaction on a single RSS. Using this system, we found that Mg2+ confers a specific pattern of DNA binding and cleavage. In contrast, Mn2+ allows aberrant binding of RAG1.RAG2 to single-stranded RSS and permits cleavage independent of binding to the nonamer. To determine the contribution of Me2+ ions at the early stages of V(D)J recombination, we analyzed specific DNA recognition and cleavage by RAG1.RAG2 on phosphorothioated substrates. These experiments revealed that Me2+ ions directly coordinate the binding of RAG1.RAG2 to the RSS DNA.

Partial V(D)J recombination activity leads to Omenn syndrome

Genomic rearrangement of the antigen receptor loci is initiated by the two lymphoid-specific proteins Rag-1 and Rag-2. Null mutations in either of the two proteins abrogate initiation of V(D)J recombination and cause severe combined immunodeficiency with complete absence of mature B and T lymphocytes. We report here that patients with Omenn syndrome, a severe immunodeficiency characterized by the presence of activated, anergic, oligoclonal T cells, hypereosinophilia, and high IgE levels, bear missense mutations in either the Rag-1 or Rag-2 genes that result in partial activity of the two proteins. Two of the amino acid substitutions map within the Rag-1 homeodomain and decrease DNA binding activity, while three others lower the efficiency of Rag-1/Rag-2 interaction. These findings provide evidence to indicate that the immunodeficiency manifested in patients with Omenn syndrome arises from mutations that decrease the efficiency of V(D)J recombination.

Sequence of the spacer in the recombination signal sequence affects V(D)J rearrangement frequency and correlates with nonrandom Vkappa usage in vivo

Functional variable (V), diversity (D), and joining (J) gene segments contribute unequally to the primary repertoire. One factor contributing to this nonrandom usage is the relative frequency with which the different gene segments rearrange. Variation from the consensus sequence in the heptamer and nonamer of the recombination signal sequence (RSS) is therefore considered a major factor affecting the relative representation of gene segments in the primary repertoire. In this study, we show that the sequence of the spacer is also a determinant factor contributing to the frequency of rearrangement. Moreover, the effect of the spacer on recombination rates of various human Vkappa gene segments in vitro correlates with their frequency of rearrangement in vivo in pre-B cells and with their representation in the peripheral repertoire.

The specific variable domain of camel heavy-chain antibodies is encoded in the germline

The variable domains of the functional heavy-chain antibodies (VHHs) discovered in camels are related to the human VH subgroup III. They are nevertheless clearly distinguishable from the VHs of conventional four-chain immunoglobulins by the presence of important amino acid substitutions, located in the solvent-exposed surface normally covered by the variable domain of the light chain. The analysis of an unrearranged dromedary DNA library revealed that the specific VHH gene with its characteristic amino acid substitutions is encoded in the germline. Therefore, it is concluded that the VHHs do not arise through an ontogenic process of somatic hypermutation. The presence of putative DNA recombination signals that are more prevalent in the camel VHH, compared to the VH germline gene, might play a role in the formation and efficient expansion of the VHH repertoire.

Nicking is asynchronous and stimulated by synapsis in 12/23 rule-regulated V(D)J cleavage

The first step in DNA cleavage at V(D)J recombination signals by RAG1 and RAG2 is creation of a nick at the heptamer/coding flank border. Under proper conditions in vitro the second step, hairpin formation, requires two signals with spacers of 12 and 23 bp, a restriction referred to as the 12/23 rule. Under these conditions hairpin formation occurs at the two signals at or near the same time. In contrast, we find that under the same conditions nicking occurs at isolated signals and hence is not subject to the 12/23 rule. With two signals the nicking events are not concerted and the signal with a 12 bp spacer is usually nicked first. However, the extent and rate of nicking at a given signal are diminished by mutations of the other signal. The appearance of DNA nicked at both signals is stimulated by more than an order of magnitude by the ability of the signals to synapse, indicating that synapsis accelerates nicking and often precedes it. These observations allow formulation of a more complete model of catalysis of DNA cleavage and how the 12/23 rule is enforced.

The T cell receptor beta genes of Xenopus

cDNA of the T cell receptor beta (TCRB) have been isolated from the anuran amphibian Xenopus and they show strong structural homology to TCRB sequences of other vertebrates. Ten BV families, two D segments, ten J segments, and a single C region have been defined so far. Each V family consists of one to two members per haploid genome. A unique feature of the Xenopus TCRB constant region is the lack of N-linked carbohydrate glycosylation sites. The recombination signal sequences suggest that the mechanism of rearrangements are identical to those of mammals. The locus is inherited in a diploid manner despite the pseudotetraploidy of the Xenopus laevis and X. gilli used in this study.

RAG1 mediates signal sequence recognition and recruitment of RAG2 in V(D)J recombination

Recent studies have demonstrated that DNA cleavage during V(D)J recombination is mediated by the RAG1 and RAG2 proteins. These proteins must therefore bind to the recombination signals, but the specific binding interaction has been difficult to study in vitro. Here, we use an in vivo one-hybrid DNA binding assay to demonstrate that RAG1, in the absence of RAG2, can mediate signal recognition via the nonamer, with the heptamer acting to enhance its binding. A region of RAG1 with sequence similarity to bacterial invertases is essential for DNA binding. Localization of RAG2 to the signal is dependent upon the presence of RAG1 and is substantially more efficient with a 12 bp spacer signal than with a 23 bp spacer signal.

The homeodomain region of Rag-1 reveals the parallel mechanisms of bacterial and V(D)J recombination

The V(D)J recombinase subunits Rag-1 and Rag-2 mediate assembly of antigen receptor gene segments. We studied the mechanisms of DNA recognition by Rag-1/Rag-2 using surface plasmon resonance. The critical step for signal recognition is binding of Rag-1 to the nonamer. This is achieved by a region of Rag-1 homologous to the DNA-binding domain of the Hin family of bacterial invertases and to homeodomain proteins. Strikingly, the Hin homeodomain can functionally substitute for the Rag-1 homologous region. Rag-1 also interacts with the heptamer but with low affinity. Rag-2 shows no direct binding to DNA. Once the Rag-1/Rag-2 complex is engaged on the DNA, subsequent cleavage is directed by the heptamer sequence. This order of events remarkably parallels mechanisms that mediate transposition in bacteria and nematodes.

DNA sequence and structure requirements for cleavage of V(D)J recombination signal sequences

Purified RAG1 and RAG2 proteins can cleave DNA at V(D)J recombination signals. In dissecting the DNA sequence and structural requirements for cleavage, we find that the heptamer and nonamer motifs of the recombination signal sequence can independently direct both steps of the cleavage reaction. Proper helical spacing between these two elements greatly enhances the efficiency of cleavage, whereas improper spacing can lead to interference between the two elements. The signal sequences are surprisingly tolerant of structural variation and function efficiently when nicks, gaps, and mismatched bases are introduced or even when the signal sequence is completely single stranded. Sequence alterations that facilitate unpairing of the bases at the signal/coding border activate the cleavage reaction, suggesting that DNA distortion is critical for V(D)J recombination.

Distinct DNA sequence and structure requirements for the two steps of V(D)J recombination signal cleavage

Cleavage of V(D)J recombination signals by purified RAG1 and RAG2 proteins permits the dissection of DNA structure and sequence requirements. The two recognition elements of a signal (nonamer and heptamer) are used differently, and their cooperation depends on correct helical phasing. The nonamer is most important for initial binding, while efficient nicking and hairpin formation require the heptamer sequence. Both nicking and hairpin formation are remarkably tolerant of variations in DNA structure. Certain flanking sequences inhibit hairpin formation, but this can be bypassed by base unpairing, and even a completely single-stranded signal sequence is well utilized. We suggest that DNA unpairing around the signal-coding border is essential for the initiation of V(D)J combination.

A defective Vkappa A2 allele in Navajos which may play a role in increased susceptibility to haemophilus influenzae type b disease

The antibody response to H. influenzae type b (Hib) is pauciclonal, and is dominated by antibodies using the VkappaA2 gene. Navajos have a 5-10-fold increased incidence of Hib disease compared with control populations. We hypothesized that a polymorphism in one of the genes in this oligoclonal response may lead to increased disease susceptibility. Since the predominant A2+ anti-Hib antibodies have high avidity for Hib and can be unmutated, the A2 Vkappa gene was analyzed. Over half of the Navajos studied, but only one control individual, had a new allele of A2, termed A2b, with three changes from the published A2 germline sequence. One of the changes was in the recombination signal sequence, suggesting that the A2b allele might not undergo V-J rearrangement very frequently. This possibility was confirmed by analyzing the relative frequency of non-productive A2 rearrangements in A2a/b heterozygous Navajos. Many fewer A2b rearrangements were observed, showing that the A2b allele is defective in its ability to undergo rearrangement. The prevalence of this allele in Navajos may play a role in their increased susceptibility to invasive Hib disease. If so, it would underscore the importance of the germline Ig repertoire for protective antibody responses to pathogenic bacteria in unimmunized children.

Environmental and genetic factors shape the human T-cell receptor repertoire

Both environmental and genetic factors combine to shape the TCR repertoire as measured by V gene usage. These factors may result in dramatic shifts in normal subjects, which cannot be discounted when studies are performed in patients with disease. Future studies need to explore further examples of genetic and environmental factors that shape the TCR repertoire to understand the full extent of variation in a normal population and the mechanisms involved. Some of these mechanisms may also apply to TCRG, TCRD, and immunoglobulin loci. Certainly variations in the efficiency of V(D)J rearrangement could affect any rearranging multigene locus. Eventually such studies will lead to better designed clinical studies of the repertoire in disease, through the selection of control populations matched for environmental exposure and genetic background. In this respect, family studies will be most useful.