Immunoglobulin gene alterations in human heavy chain diseases

Exploiting Nanobodies' Singular Traits

The unique class of heavy chain-only antibodies, present in Camelidae, can be shrunk to just the variable region of the heavy chain to yield VHHs, also called nanobodies. About one-tenth the size of their full-size counterparts, nanobodies can serve in applications similar to those for conventional antibodies, but they come with a number of signature advantages that find increasing application in biology. They not only function as crystallization chaperones but also can be expressed inside cells as such, or fused to other proteins to perturb the function of their targets, for example, by enforcing their localization or degradation. Their small size also affords advantages when applied in vivo, for example, in imaging applications. Here we review such applications, with particular emphasis on those areas where conventional antibodies would face a more challenging environment.

Recent advances in the selection and identification of antigen-specific nanobodies

Nanobodies represent the next-generation antibody-derived biologics with significant advances over conventional antibodies. Several rapid and robust techniques for isolating highly specific nanobodies have been developed. Antigen specific nanobodies are selected from constructed nanobody libraries, which can be classified into 3 main types: immune library, naïve library, and semisynthetic/synthetic library. The immune library is the most widely used strategy for nanobody screening. Target specific nanobodies are highly enriched in immune libraries than in non-immune libraries; however, it is largely limited by the natural antigenicity of antigens. The naïve library is thus developed. Despite the lack of somatic maturation, protein engineering can be employed to significantly increase the affinities of selected binders. However, a substantial amount of blood samples collected from a large number of individual animals is a prerequisite to ensure the diversity of the naïve library. With this issue considered, the semisynthetic/synthetic library may be a promising path toward obtaining a limitless source of nanobodies against a variety of antigens without the need of animals. In this review, we summarize the state-of-the-art screening technologies with different libraries. The approaches presented here can further boost the diverse applications of nanobodies in biomedicine and biotechnology.

Nanobodies: natural single-domain antibodies

Sera of camelids contain both conventional heterotetrameric antibodies and unique functional heavy (H)-chain antibodies (HCAbs). The H chain of these homodimeric antibodies consists of one antigen-binding domain, the VHH, and two constant domains. HCAbs fail to incorporate light (L) chains owing to the deletion of the first constant domain and a reshaped surface at the VHH side, which normally associates with L chains in conventional antibodies. The genetic elements composing HCAbs have been identified, but the in vivo generation of these antibodies from their dedicated genes into antigen-specific and affinity-matured bona fide antibodies remains largely underinvestigated. However, the facile identification of antigen-specific VHHs and their beneficial biochemical and economic properties (size, affinity, specificity, stability, production cost) supported by multiple crystal structures have encouraged antibody engineering of these single-domain antibodies for use as a research tool and in biotechnology and medicine.

Aggregates, crystals, gels, and amyloids: intracellular and extracellular phenotypes at the crossroads of immunoglobulin physicochemical property and cell physiology

Recombinant immunoglobulins comprise an important class of human therapeutics. Although specific immunoglobulins can be purposefully raised against desired antigen targets by various methods, identifying an immunoglobulin clone that simultaneously possesses potent therapeutic activities and desirable manufacturing-related attributes often turns out to be challenging. The variable domains of individual immunoglobulins primarily define the unique antigen specificities and binding affinities inherent to each clone. The primary sequence of the variable domains also specifies the unique physicochemical properties that modulate various aspects of individual immunoglobulin life cycle, starting from the biosynthetic steps in the endoplasmic reticulum, secretory pathway trafficking, secretion, and the fate in the extracellular space and in the endosome-lysosome system. Because of the diverse repertoire of immunoglobulin physicochemical properties, some immunoglobulin clones' intrinsic properties may manifest as intriguing cellular phenotypes, unusual solution behaviors, and serious pathologic outcomes that are of scientific and clinical importance. To gain renewed insights into identifying manufacturable therapeutic antibodies, this paper catalogs important intracellular and extracellular phenotypes induced by various subsets of immunoglobulin clones occupying different niches of diverse physicochemical repertoire space. Both intrinsic and extrinsic factors that make certain immunoglobulin clones desirable or undesirable for large-scale manufacturing and therapeutic use are summarized.

Immunoglobulin gene rearrangements and the pathogenesis of multiple myeloma

The ability to rearrange the germ-line DNA to generate antibody diversity is an essential prerequisite for the production of a functional repertoire. While this is essential to prevent infections, it also represents the “Achilles heal” of the B-cell lineage, occasionally leading to malignant transformation of these cells by translocation of protooncogenes into the immunoglobulin (Ig) loci. However, in evolutionary terms this is a small price to pay for a functional immune system. The study of the configuration and rearrangements of the Ig gene loci has contributed extensively to our understanding of the natural history of development of myeloma. In addition to this, the analysis of Ig gene rearrangements in B-cell neoplasms provides information about the clonal origin of the disease, prognosis, as well as providing a clinical useful tool for clonality detection and minimal residual disease monitoring. Herein, we review the data currently available on both Ig gene rearrangements and protein patterns seen in myeloma with the aim of illustrating how this knowledge has contributed to our understanding of the pathobiology of myeloma.

Block in development at the pre-B-II to immature B cell stage in mice without Ig kappa and Ig lambda light chain

Silencing individual C (constant region) lambda genes in a kappa(-/-) background reduces mature B cell levels, and L chain-deficient (lambda(-/-)kappa(-/-)) mice attain a complete block in B cell development at the stage when L chain rearrangement, resulting in surface IgM expression, should be completed. L chain deficiency prevents B cell receptor association, and L chain function cannot be substituted (e.g., by surrogate L chain). Nevertheless, precursor cell levels, controlled by developmental progression and checkpoint apoptosis, are maintained, and B cell development in the bone marrow is fully retained up to the immature stage. L chain deficiency allows H chain retention in the cytoplasm, but prevents H chain release from the cell, and as a result secondary lymphoid organs are B cell depleted while T cell levels remain normal.

Emergence and evolution of functional heavy-chain antibodies in Camelidae

Antibodies of jawed-vertebrates are composed of paired heavy (H) and light (L) polypeptide chains. Surprisingly, the sera of camelids, nurse shark and wobbegong shark, and possibly ratfish contain antibodies that lack L-chains. In camelids, these Heavy-chain antibodies (HCAbs) are gamma-isotypes, and are functional in antigen binding. In this review we focus on the dedicated immunoglobulin (Ig) genes that encode the HCAb in Camelidae (camels, dromedaries and llamas), about their origin, and how these camel immunoglobulins evolved and acquire a large and diverse repertoire of antigen binding sites in absence of the H-L combinatorial diversity.

Gamma heavy chain disease in man: independent structural abnormalities and reduced transcription of a functionally rearranged lambda L-chain gene result in the absence of L-chains

Human Ig heavy chain diseases of the alpha and gamma classes are characterized by the absence of light chain production as well as the disease-defining abnormalities in the heavy chain protein. Prior studies have suggested concomitant structural defects in productively rearranged L-chain genes as the reason for the absent L-chain proteins. We have found that the single rearranged lambda L-chain gene in the OMM heavy chain disease (HCD) cell line has a mutation in the splice donor site at the 3' end of the J exon, resulting in direct splicing of the 3' end of the leader to the acceptor site of the constant region. The cells contain an mRNA consisting of the leader-coding region joined directly to the constant region. The V-region exon is skipped and the shortened mRNA is translated into a truncated protein containing no V-region amino acids. We have also noted that, in contrast to most normal and neoplastic Ig-producing cells, the OMM cells produce an excess of heavy to light chain mRNA as well as protein. The excess is independent of the structural gene abnormality and is due to a low level of L-chain transcription, which can be increased by fusing the HCD cell to the murine myeloma cell line NS-1 or transfecting the defective OMM L-chain gene into a murine plasma cell. The latter data suggest that the OMM cells either lack a transcription factor present in mature plasma cells or have a functional repressor of L-chain transcription.

Loss of splice consensus signal is responsible for the removal of the entire C(H)1 domain of the functional camel IGG2A heavy-chain antibodies

The molecular basis for the absence of the C(H)1 domain in naturally occurring heavy-chain antibodies of the camelids was assessed by determining the entire Camelus dromedarius gamma2a heavy-chain constant gene. The organization of the camel gamma2a constant heavy-chain gene obtained from a liver genomic library appears to be typical of all other mammalian gamma genes sequenced to date. It contains the switch, CH1, hinge, CH2, CH3, M1 and M2 exons. In contrast to the case in mouse and human heavy chain diseases, the camel gamma2a gene shows no major structural defect, and its equivalent CHI exon is intact. However, sequence analysis has revealed that the splicing site, immediately after the CH1 exon, is defective due to point mutations, especially the G(+1) to A(+1) transversion seems to be detrimental. It is concluded that the loss of the splice consensus signal is responsible for the removal of the entire CH1 domain in camel gamma2a heavy-chain immunoglobulins. Additionally, a closer analysis of the hinge exon suggests the possible involvement of transposons in the genetic variation of mammalian Cgamma hinges.

Pre-B-cell development in the absence of lambda 5 in transgenic mice expressing a heavy-chain disease protein

BACKGROUND

Heavy-chain diseases (HCDs) are human lymphoproliferative neoplasias that are characterized by the secretion of truncated immunoglobulin heavy chains devoid of light chains. We have previously proposed--by analogy to the process by which mutated growth factor receptors can be oncogenic--that because the genetic defects in HCDs result in the production of abnormal membrane-associated heavy chains lacking an antigen-binding domain, these abnormal B-cell antigen receptors might engage in ligand-independent signalling. Normal pre-B-cell development requires the presence of the pre-B-cell receptor, formed by the association of mu heavy chains with two polypeptides--so-called surrogate light chains, Vpre-B and lambda 5--that are homologous to the variable and constant portions of immunoglobulin light chains, respectively. To assess whether amino-terminal truncation of membrane-associated heavy chains results in their constitutive activation, we have examined the ability of a HCD-associated mu protein to promote pre-B-cell development in transgenic mice.

RESULTS

When the mu HCD transgene is introduced into SCID mice, CD43- pre-B cells develop normally. To determine whether this pre-B-cell development requires surrogate light chains, we backcrossed mice expressing full-length or truncated mu transgenes with lambda 5-deficient mice. Our results show that the truncated heavy chain, but not the normal chain, is able to promote pre-B-cell development in the absence of lambda 5. We also show that truncated mu chains spontaneously aggregate at the surface of bone marrow cells.

CONCLUSIONS

Expression of the truncated mu heavy chain overrides a tightly controlled step of pre-B-cell development, which strongly suggests that a constitutive signal is delivered by the truncated mu chain disease protein. The self-aggregation of mu chain disease proteins might account for this constitutive activation. We conclude that amino-terminal truncation of heavy chains could play a role in the genesis of HCD neoplasia if it occurs at an appropriate stage of B-cell differentiation, namely in a mature B cell.

An insertion-deletion event in murine immunoglobulin kappa gene resembles mutations at heavy-chain disease loci

The analysis of spontaneous somatic mutants gives insights into the regulation of gene expression. Human heavy-chain disease (HCD) is a monoclonal lymphoproliferative disorder characterized by the presence of truncated immunoglobulin (Ig) heavy chains without associated light chains. To better understand the molecular mechanisms leading to the loss of light-chain production, we have examined a murine cell line model of heavy-chain disease. R15, a spontaneous mutant of the IgA, kappa-producing myeloma cell line W3129, produces heavy chain but no light chain. The variant delta 15 derived from R15 resembles human HCD in that it secretes a shortened heavy chain with no associated light chain. Cloning and analysis of the R15 kappa light-chain gene revealed that a 358-nucleotide insertion of unknown origin replaced 22 bases of the wild-type leader-variable region (L-V) intron (IVS). Although this genomic change left the light-chain exons and known regulatory elements intact, it altered the mRNA processing pathway, yielding two alternative RNA products, neither of which encodes a functional protein. This mutant therefore provides new insights into how genomic changes can influence gene expression.

Unusual sequence of immunoglobulin L-chain rearrangements in a gamma heavy chain disease patient

Patients with gamma heavy chain disease (gamma-HCD) generally produce incomplete immunoglobulin (Ig) gamma-heavy chains (gamma-HCD protein) which cannot associate with light chains (IgL). In most patients Bence Jones proteins (BJP) are not observed. However, in the 61-year-old patient WIN we found gamma l-HCD proteins and lambda BJP in serum and urine. WIN gamma l-HCD protein does not carry the Ig Fd region, has a molecular weight of 33.5 kDa, and the seven N-terminal amino acid residues are not translated from any of the known immunoglobulin heavy chain (IgH) gene sequences. These residues are followed by the C gamma l-hinge region. In DNA from peripheral blood lymphocytes of patient WIN we found bands representing dominant rearrangements in one of the two alleles of the IgH, Ig kappa and Ig lambda locus. Taken together, the data from protein and DNA analysis strongly suggest, albeit do not formally prove, that one dominant B-cell clone which carries a rearranged and a non-rearranged allele of each Ig locus produces gamma-HCD protein and lambda BJP. The productive lambda-gene rearrangement in this clone thus has not been preceded by abortive rearrangements in both kappa-locus alleles. Lymphocytes with an unusual sequence of IgL-chain gene activation seem to be involved in the case of gamma-HCD described here.

Allelic exclusion in transgenic mice expressing a heavy chain disease-like human mu protein

Heavy chain diseases (HCD) are neoplastic proliferations of B cells which secrete truncated immunoglobulin heavy chains without associated light chains. These proteins are encoded by mutated genes which may also give rise to truncated membrane immunoglobulins. The neoplastic cells proliferate in vivo although they cannot bind any antigen, due to deletions in the variable domain of their antigen receptors. The reason for the clonal proliferation of HCD cells and the biological effects of the truncated membrane-bound chains are presently unknown. We wanted to determine whether the expression of HCD proteins would interfere with B cell development. To this end we made transgenic mice with a human mu gene, lacking the VDJ exon, that encodes a protein similar to that produced in two cases of HCD. Transgenic mice were also produced with a similar construct but encoding only the membrane-bound form of the truncated mu chain. Transgene encoded C mu proteins are expressed on the cell surface without associated light chains and are responsible for allelic exclusion of murine heavy chains.

Alpha heavy chain disease of patient MAL: structure of the non-functional rearranged alpha gene translocated on chromosome 9

Alpha heavy chain diseases (HCD) are lymphoproliferative disorders characterized by the production of truncated alpha immunoglobulin heavy chain without associated light chains, alpha HCD MAL is featured by multiple structural alterations of the alpha 1 productive gene and on original t(9;14)(p11;q32) translocation involving the other rearranged alpha 1 allele. We present here the structure of the der(9) chromosome. Sequence analysis provides evidence that the translocation occurred after local pairing of the two chromosomes mediated by an almost perfect nonameric sequence, followed by a staggered double-strand break of chromosome 14. This translocation occurred on a V(D)J rearranged locus; unexpectedly, there were a deletion of the 3' part of the VH gene, several insertions of non-immunoglobulin-related genes and multiple mutations, i.e. alterations reminiscent of those occurring on the HCD productive genes.

Immunoglobulin light chain transcripts with altered V regions in Burkitt's lymphoma cell lines producing short mu chains

The Burkitt lymphoma cell lines Ly91, Ly47 and Ly66 produce short mu chains in the absence of associated light (L) chains. mu mRNA lack two-thirds of the variable (V) region due to an alternate splicing event that takes place within V genes. L chain genes were found to be rearranged in these lines and to lead to transcripts with altered V regions. Although these abnormal transcripts displayed specific abnormalities in each cell line, they shared some features, including large deletions involving the V-J junction. In one cell line, L chain transcripts made up of either V kappa I subgroup gene J kappa 3 or V kappa IV gene-J kappa 4 recombination products bore a similar alteration: the donor splice site of the rearranged J kappa segment was modified, leading to the alternate use of the donor site of either the leader peptide exon or the next 3' unrearranged J kappa segment.

Production of an abnormal mu chain with a shortened VHIV subgroup variable region in a Burkitt's lymphoma cell line

The Burkitt's lymphoma cell line Ly66 produces a short mu chain which lacks 4 kDa in apparent molecular mass. Study of the corresponding messenger RNA showed it to be 0.3 kb shorter than normal mu transcripts. The cDNA sequence of the mu transcripts began by a short VH region consisting of the first one-third of a VHIV subgroup gene segment. It was followed by a normal mu constant region. This VH region coded for 38 amino acids, thus differing from two truncated VHIV regions previously reported in other Burkitt's lymphoma cell lines, which were interrupted at codon +26 by an alternate splicing event. In addition, the Ly66 variable sequence bore several point mutations and contained two potential N-glycosylation sites.

A new extra sequence at the amino terminal of a mu heavy chain disease protein (DAG)

The primary structure of a human mu heavy chain (DAG) protein is described. The native protein is a circular decamer with a molecular weight (Mr) of 500 kDa, each decamer being constituted of the constant domains C mu 2, C mu 3 and C mu4 and interlinked by 15 disulfide bridges. At its NH2-terminal each monomeric chain starts with an "extra sequence". The amino acid sequence of this segment is Arg-Gln-Ser-Asp-Asp-Pro-Val-Leu-Arg-Gly-Thr-Thr-Val-Pro-Val-Thr-Glu and its reinitiation point is located at Val223 (Gal numbering), at the beginning of C mu 2. This sequence has no homology with any other protein included in the present databases.

Oncogenic potential of the B-cell antigen receptor and its relevance to heavy chain diseases and other B-cell neoplasias: a new model

Some growth factor receptor genes can be activated to become oncogenic as a result of various mutations. The receptors that are encoded by these genes may deliver constitutive signals independent of ligand binding. The model which is presented here assumes that alterations in the B-cell antigen receptor could have similar effects. Evidence that membrane immunoglobulin alterations contribute to oncogenesis in vivo comes from studies of heavy chain diseases. Moreover, this model could have also implications for other B-cell neoplasias.