Structural variants of human and murine immunoglobulins

Immunoproliferative small intestinal disease: Mediterranean lymphoma and alpha heavy chain disease

Gastrointestinal lymphoma, uncommon in the West, is far more prevalent in developing countries where it falls into two groups: 'Western'-type lymphomas, similar to those seen in developed countries, and the so-called Mediterranean-type lymphoma. It is now accepted that Mediterranean lymphoma represents, in the majority if not in all cases, the late stage of alpha heavy chain disease (alpha-HCD). This disease is characterized by abnormal secretion of an immunoglobulin fragment; alpha-HCD and Mediterranean lymphoma constitute two ends of a spectrum of pathology now classified as immunoproliferative small intestinal disease (IPSID). IPSID is associated predominantly with poor socioeconomic conditions; patients present with progressive malabsorption in the second and third decades of life. Diagnosis is established by small bowel biopsy, with or without high serum levels of the alpha heavy chain protein. Treatment consists of an initial staging laparotomy, with debulking of lymphomatous deposits if appropriate, followed by chemotherapy or radiotherapy. Overall prognosis is poor but the recent use of doxorubicin-based chemotherapy offers some hope for the future.

Alpha heavy chain disease alpha mRNA contain nucleotide sequences of unknown origins

Human alpha heavy chain disease is characterized by the production of abnormally short alpha IgH chains. In previously published cases it has been found that the malignant cells produce abnormal alpha mRNA, lacking VH and CH1 sequences and composed of a leader sequence peptide, sequences of variable length (69 to 84 bp) and of unknown origin, followed by normal CH2 and CH3 sequences. In this study we established the nucleotide sequence of alpha mRNA for six cases of alpha heavy chain disease. We observed that all six alpha mRNA lack the VH and CH1 sequences as do those previously described. They also contain in-frame inserts of unknown origin between the leader peptide and the normal CH2 and CH3 coding sequences. These inserts are of variable length (42 to 105 bp) and they are unrelated. These results suggest the existence of a common mechanism defect leading to deletions/insertions in alpha heavy chain disease rather than a specific interaction between alpha 1 IgH gene with a unique defined molecular species.

Differential mRNA stabilities affect mRNA levels in mutant mouse myeloma cells

A series of mouse myeloma cell lines producing mutant gamma 2b immunoglobin heavy chains, which resemble heavy chain disease proteins, were analyzed for messenger RNA abundance as a function of mRNA alterations. A mutation effectively deleting the gamma 2b-CH1 domain of the mRNA had little or no effect on Ig heavy chain mRNA abundance on half-life (mutant 10.1). A mutation in the gamma 2b-CH2 and CH3 domain, causing premature termination of translation, had more deleterious effects on Ig heavy chain mRNA abundance and half-life (mutant I17). Substitution of the deleted portions of the gamma 2b mRNA with gamma 2a sequences by subclass switching in the cells (mutants K23 and K25) resulted in increased heavy chain abundance and half-life relative to the parent I17. In contrast, kappa light chain mRNA levels and half-lives remain constant among the mutants. The wild-type and mutant cell lines transcribed the Ig heavy chain gamma 2b locus equally when compared with an internal beta-actin standard by transcription run on studies. Therefore, half-life of the Ig heavy chain mRNA seems to be the principal determinant in cytoplasmic mRNA abundance in this system.

Genomic alterations in a case of alpha heavy chain disease leading to the generation of composite exons from the JH region

Human alpha heavy chain disease (HCD) is characterized by the presence in patient's serum of a short Ig alpha chain devoid of light chains. We analyzed the serum protein, the alpha HCD mRNA and the productive rearranged H chain gene from the leukemic cells of a new case (YAO) of alpha HCD. The abnormal YAO alpha 1 Ig was devoid of VH and CH1 domains and started at the beginning of the hinge region. The alpha HCD mRNA was shorter than normal alpha mRNA and the cDNA prepared from YAO mRNA encoded a leader sequence, an insert of 70 nucleotides and the CH2 and CH3 exons. The origin of the inserted sequence was assessed by cloning and sequence analysis of the alpha 1 productive gene. It started with a leader exon, a leader-VH intron and the first 11 bp of a VH exon. Then the VH region was deleted and replaced by a 19-nucleotide sequence that turned out to correspond to the 3' part of a modified JH5 exon. It was followed by a 221-bp sequence homologous to the JH5-psi JH3 intron and by an inserted sequence of unknown origin. The 3' part of this insertion and the remnant of a JH6 exon delineated a third exon that was followed by a relatively conserved JH6-C alpha intron. These two composite exons were flanked by splicing sites and accounted for the 70-nucleotide insert of the cDNA. The genomic nucleotide sequence also revealed a large deletion in the switch CH1 region which eliminated normal splicing sites and resulted in splicing of the third exon directly to the CH2 exon.

Gamma heavy chain disease in man. Genomic sequence reveals two noncontiguous deletions in a single gene

A genomic clone was isolated from a human lymphoid cell line which synthesized an NH2-terminally deleted gamma 3 heavy chain disease protein. Nucleotide sequence analysis revealed a normal sequence from 310 bp 5' to the initiator ATG through the codon for VH amino acid 14. Amino acid 15 was derived from the codon for the last J4 amino acid. Thus, the clone contained a deletion of the codons for the VH region beyond amino acid 14, as well as those for the entire D region and most of the J coding region. Some sequence abnormalities were observed in the 400 bp after the deletion. Beyond this, there was excellent homology to published J and intervening sequences, including those containing the enhancer elements. The 1,200-bp switch region was abruptly interrupted by a sequence corresponding to the 3' one-third of CH1. Thus, a second deletion eliminated the acceptor splice site at the 5' end of CH1. When splicing of the primary RNA transcript occurred, the truncated VH region was joined via the J4 donor splice site to the next available acceptor site 5' to the first hinge exon. Hence, the aberrant serum protein was the product of two deletions and a splice correction as well as postsynthetic NH2-terminal proteolysis.

An immunoglobulin heavy chain gene deletion at direct repeats: nucleotide sequence and effect on mRNA accumulation

The DNA from the mouse myeloma cell, I17, which produces aberrant gamma 2b heavy chain mRNAs, was cloned and sequenced. The I17 mutant, and its parent line 10.1, share a small deletion at the splice junction of the CH1 domain which results in the absence of CH1 sequences from the mRNA. In addition, the genomic DNA of I17 has a deletion of 253 nucleotides which fuses the CH2 and CH3 exons, causes a frameshift of the next 43 amino acids and results in a truncated protein. The deleted nucleotides are flanked by two direct repeats of the CAGCA pentamer in the normal gene. One copy of the repeat and the interposed DNA is removed in the mutant. The DNA deletion is colinear with the mRNA. Both I17 and 10.1 cells have decreased accumulation of the secretory-specific gamma 2b mRNA. The amounts of membrane-specific gamma 2b mRNA are also affected in the mutants.

Assembly and secretion of heavy chains that do not associate posttranslationally with immunoglobulin heavy chain-binding protein

Heavy chain-binding protein (BiP) associates posttranslationally with nascent Ig heavy chains in the endoplasmic reticulum (ER) and remains associated with these heavy chains until they assemble with light chains. The heavy chain-BiP complex can be precipitated by antibody reagents against either component. To identify sites on heavy chain molecules that are important for association with BiP, we have examined 30 mouse myelomas and hybridomas that synthesize Ig heavy chains with well characterized deletions. Mutant Ig heavy chains that lack the CH1 domain could not be demonstrated to associate with BiP, whereas mutant Ig heavy chains with deletions of the CH2 or CH3 domain were still able to associate with BiP. In two light chain negative cell lines that produced heavy chains with deletions of the CH1 domain, free heavy chains were secreted. When Ig assembly and secretion were examined in mutants that did not associate with BiP, and were compared with normal parental lines, it was found that the rate of Ig secretion was increased in the mutant lines and that the Ig molecules were secreted in various stages of assembly. In one mutant line (CH1-) approximately one-third of the secreted Ig molecules were incompletely assembled, whereas the Ig molecules secreted by the parental line were completely assembled. Our data show the CH1 domain to be important for association with BiP and that when this association does not occur, incompletely assembled heavy chains can be secreted. This implies a role for BiP in preventing the transport of unassembled Ig molecules from the ER.

Loss of a consensus splice signal in a mutant immunoglobulin gene eliminates the CH1 domain exon from the mRNA

A series of mouse myeloma mutants, derived from a cell line of the murine MPC-11 tumor (gamma 2b, kappa), resemble human heavy-chain disease in their loss of an internal domain (exon). In these mutants, most of the gamma 2b CH1 exon was present in the nuclear RNA but was removed during splicing to form the mature cytoplasmic RNA. Amino acid sequence studies of one mutant (10.1) are consistent with the loss of the complete CH1 domain. A second mutant cell line (I17) derived from 10.1 and containing the same CH1 alteration was shown by S1 nuclease protection experiments to have an additional mRNA deletion spanning the CH2-CH3 domain boundary. This second deletion was shown to result from a genomic alteration that provided a marker for the isolation of the expressed H-chain allele. To determine the basis of the CH1 splicing defect, the 117 genome-expressed gamma 2b constant region DNA was cloned. Sequence studies showed a deletion of 99 nucleotides around the 3' end of the CH1 domain, which removed the splice site and flanking DNA, apparently causing the aberrant splicing of the RNA transcript. The sequence deleted in the mutant is flanked by short repeats of the octameric sequence CCAGCCAG in the wild-type gene. In the mutant, one copy of the repeat, in addition to the sequences between the repeats, has been lost.

Loss of a consensus splice signal in a mutant immunoglobulin gene eliminates the CH1 domain exon from the mRNA

A series of mouse myeloma mutants, derived from a cell line of the murine MPC-11 tumor (gamma 2b, kappa), resemble human heavy-chain disease in their loss of an internal domain (exon). In these mutants, most of the gamma 2b CH1 exon was present in the nuclear RNA but was removed during splicing to form the mature cytoplasmic RNA. Amino acid sequence studies of one mutant (10.1) are consistent with the loss of the complete CH1 domain. A second mutant cell line (I17) derived from 10.1 and containing the same CH1 alteration was shown by S1 nuclease protection experiments to have an additional mRNA deletion spanning the CH2-CH3 domain boundary. This second deletion was shown to result from a genomic alteration that provided a marker for the isolation of the expressed H-chain allele. To determine the basis of the CH1 splicing defect, the 117 genome-expressed gamma 2b constant region DNA was cloned. Sequence studies showed a deletion of 99 nucleotides around the 3' end of the CH1 domain, which removed the splice site and flanking DNA, apparently causing the aberrant splicing of the RNA transcript. The sequence deleted in the mutant is flanked by short repeats of the octameric sequence CCAGCCAG in the wild-type gene. In the mutant, one copy of the repeat, in addition to the sequences between the repeats, has been lost.

Two alpha heavy chain disease proteins with different genomic deletions demonstrate that nonexpressed alpha heavy chain genes contain methylated bases

Two independently arising alpha heavy chain mutants have been found to synthesize heavy chains with CH1 deletions of approximately equal extent. Both were isolated from heavy chain-producing variants of the mouse myeloma W3129 and demonstrate that it is possible to arrive at the heavy chain disease phenotype by the pathway H + L leads to H leads to delta H. Analysis of genomic DNA by digestion with restriction endonucleases followed by molecular hybridization showed that one mutant (delta 37) had a deletion of approximately 0.2 kilobase and the second mutant (delta 15) had a deletion of approximately 0.5 kilobase. Mouse myeloma cells contain several alpha chain alleles but only one is expressed; the presence of the deletion in delta 37 and delta 15 made it possible to identify the restriction fragments from the expressed allele. Analysis of the fragments produced after cleavage with an isoschizomeric pair of restriction enzymes, Msp I and Hpa II, indicated that, in the W3129 cell line and its variants, the unexpressed alpha alleles contain methylated bases. The influence of methylation on gene expression remains to be elucidated.

Localization of the carbohydrate units in a human immunoglobulin light chain, protein Sm lambda

The carbohydrate structure and complete amino acid sequence of a human lambda-type immunoglobulin light chain, protein Sm lambda has been determined. The protein was isolated from the urine of a patient with a plasma cell dyscrasia resembling gamma-heavy-chain disease. 13 tryptic peptides covering the entire polypeptide chain of 135 residues were isolated from the aminoethylated protein, and 15 chymotryptic peptides, accounting for 131 residues, were recovered from the carboxymethylated protein. The sequence of 18 of these peptides was partially or completely determined by the Edman-dansyl technique or C-terminal analysis, permitting the establishment of the complete primary structure of the polypeptide chain. The sequences established that this light chain possessed an intramolecular deletion of 81 amino acid residues. The N-terminal 30 residues showed considerable homology with other lambda chains of subgroup II. The defect began at position 31, in the first hypervariable region, and encompassed the remainder of the variable region through position 109. The constant region was fully intact and normal synthesis recommenced with a glutaminyl residue at position 110, the first residue of the constant region. This light chain contained carbohydrate in the hypervariable region just preceding the deletion. The precise number and locations of the oligosaccharide chains were established by amino acid sequence analysis of glycopeptides isolated from proteolytic hydrolysates by chromatography on Bio-Gel P-6 columns. These studies showed that protein Sm lambda contains one N-glycosidically-linked chain attached to asparagine-25 and one O-glycosidically-linked oligosaccharide chain attached to serine-21. The structures of the oligosaccharide chains were determined by methylation analysis, gas chromatography and hydrolysis with specific glycosidases. The structure of the N-glycosidically-linked chain was NeuAc(alpha 2 leads to 6)Gal(beta 1 leads to 4)GlcNAc(beta 1 leads to 2)Man(alpha 1 leads to 6)[NeuAc(alpha 2 leads to 6)Gal(beta 1 leads to 4)GlcNAc(beta 1 leads to 2)Man(alpha 1 leads to 3)]Man(beta 1 leads to 4)GlcNAc(beta 1 leads to 4)[Fuc alpha 1 leads to 6]GlcNAc leads to Asn. The second O-glycosidically-linked chain was a disialylated tetrasaccharide with the structure, Neu(alpha 2 leads to 3)Gal(beta 1 leads to 3)[NeuAc(alpha 2 leads to 6)GalNAc leads to Ser. This mucin-type disialylated tetrasaccharide in close proximity to N-asparagine-linked chains has not been previously observed in the oligosaccharide chains of immunoglobulins.

Generation of immune specificity: a working hypothesis

A speculative model for epigenetic generation of immune specificity is presented. It is based on the idea that combinatorial splicing within the same pre-mRNA molecule could generate a variety o mRNAs leading to a diversity of immunoglobulins. The model postulates a special role for the nuclear envelope where translation of intron sequences into messenger maturation proteins would take place. The postulates are justified and experimental implications of the model are discussed.

RNA splicing generates a variant light chain from an aberrantly rearranged kappa gene

Both C kappa regions in MPC 11 cells are rearranged into active transcripion units, one producing a normal kappa chain and the other an internally deleted kappa fragment lacking a V region. The gene coding for the kappa fragment mRNA is aberrantly rearranged and lacks a site for V leads to C kappa splicing. An alternative splicing event which deletes the V region from the nuclear RNA precursor generates the kappa fragment mRNA.

An immunoglobulin deletion mutant with implications for the heavy-chain switch and RNA splicing

The IF2 immunoglobulin mutant is a DNA deletion of one coding segment and large sections of the flanking intervening sequences. The deletion results in a new splicing pattern and starts in a DNA region containing tandemly repeated sequences which may carry heavy-chain class switch signals.

A gamma l heavy-chain disease protein *EST) lacking the entire VH and CHl domains

A gamma l heavy-chain disease protein (EST) is described which lacks the entire VH and CHl domains and starts with the normal sequence of gamma l H-chains corresponding to the beginning of the hinger region (position 216). Although degradation cannot be excluded with certainty, it is probable that this protein is synthesized as an internally deleted gamma heavy-chain disease protein. Presumably a DNA recombination has occurred resulting in the deletion of the genes of coding for the VH and CHl domains with splicing of the precursor RNA to the sequence coding for the hinge region.

Immunoglobulin chain loss in hybridoma lines

Hybrid cells secreting one, two, or three different immunoglobulins were constructed. The loss of immunoglobulin heavy or light chain expression was monitored. Chain loss was random only in lines with an excess of active light chain genes over heavy chain genes. In all other combinations preferential heavy chain loss was observed. Variant cells altered in heavy or light chain synthesis exhibited an altered chain loss pattern. It is therefore proposed that free immunoglobulin heavy chain is toxic for the cells. The interdependence of the two gene products gives a possible molecular explanation of apparent directed chromosome loss in hybrid cells.

Correlation between fragmented immunoglobulin genes and heavy chain deletion mutants

It is generally accepted that the variable (V) and constant (C) regions of immunoglobulin (Ig) chains are under separate genetic control. The notion that the different domains and interdomain regions are also under the control of independent genetic units was initially based on the clearcut results obtained by studying the primary structure of deletion mutants and received definitive support from direct analysis of cloned heavy (H) and light (L) chain genes. Here we present additional studies carried out on two selected gamma 3 deletion mutants which indicate the genetic control of human H chains may be even more complex than previously believed.

A new case of gamma heavy chain disease: clinical, immunochemical and structural characterization

A new case of gamma heavy chain disease (gamma HCD) is described in a 77-year-old woman. The serum and urine contain an M-component and electrophoretic, antigenic and ultracentrifugal properties resembling those of the Fc fragment of IgG globulin. Analysis on SDS-polyacrylamide gel electrophoresis and ultracentrifugal studies show that gamma HCD portein is present in the serum as a dimer with a molecular weight of 58,000 daltons. Analysis of isotypic and allotypic markers along with the structural studies show that this HCD protein belongs to the IgG subclass and that deletion includes the total VH and CH1 regions with sequence starting at residue 225 in the middle of the hinge region.

A mouse myeloma variant with a defect in light chain synthesis

A spontaneous assembly variant, B 50, has been isolated from the MPC-11 mouse myeloma cell line. This variant synthesizes fewer light chains per cell than the parent resulting the production of a slight molar excess of heavy chains. These changes are associated with a delay and change in the pathway of assembly and a delay in secretion. Spontaneous revertants of B 50 have been obtained, all of which synthesize normal amounts of light chains and assemble and secrete the immunoglobulin molecule through the same pathways and with the same kinetics as the parental cells. A comparison of the tryptic-chymotryptic peptides of the parental, variant and revertant heavy and light chains did not reveal any differences. These studies indicate that variants in mouse myeloma cells can arise with defects in the quantitative expression of the immunoglobulin gene and suggest that the presence of excess light chains facilitates the assembly and secretion of some immunoglobulin molecules.

Domains and the hinge region of an immunoglobulin heavy chain are encoded in separate DNA segments

A 6.8-kilobase DNA fragment containing the sequence coding for the constant region of the mouse immunoglobulin gamma1 heavy chain was cloned from total cellular DNA. Electron microscopic and nucleotide sequencing studies showed that the three protein domains and the hinge region are encoded in separate DNA segments.

Monoclonal IgM cryoglobulinemia associated with gamma-3 heavy chain disease: immunochemical and biochemical studies

A patient (Mia) with a monoclonal IgM(kappa) cryoglobulin (cryo IgM) developed additional heavy chain disease proteins of the gamma3 subclass 8 years later. Biochemical studies of the cryo IgM indicated that the heavy chain was VHI, but the NH2-terminal amino acid sequence of the light chain did not permit a definite assignment of its Vkappa subgroup. Two major fragments of the gamma3 chain were distinguishable by electrophoresis in sodium dodecyl sulfate polyacrylamide gel. The smaller component (designated Mia F) had a molecular weight of approximately 30 000 and the larger component (designated Mia S) 35 000. Both fragments had G3m(21) and G3m(27) allotypic determinants. These data and the NH2-terminal amino acid sequence of the gamma chain fragments suggested that Mia S consists of the major part of the gamma3 hinge region plus the CH2 and CH3 domains of the gamma3 chain, whereas Mia F may be derived from the former as a result of postsynthetic cleavage. The partial amino acid sequence of the Mia S fragment is homologous to the hinge region amino acid sequence of human gamma3 chains reported in the literature, with only one amino acid difference out of the 11 residues compared. This difference may represent an allotypic difference within the gamma3 subclass. Alternatively, the production of Mia S may have resulted from the accidental derepression of a "silent" constant region gene not expressed in normal individuals.

Murine heavy chain disease

Using cultured mouse myeloma cells, it has been possible to derive cells which are now synthesizing products similar to human heavy chain disease proteins. An initial mutant was isolated which snythesized a heavy chain with an internal deletion and a normal light chain. In a subsequent step, a variant was identified in which the synthesis of the heavy chain with the deletion persisted in the absence of light chain synthesis. These experiments also demonstrate that in the MPC-11-derived cultured cell line, the continued synthesis of heavy chain does not require the synthesis of light chain.

Immunobiology and pathogenesis of alpha chain disease

alpha chain disease, the most frequent of the heavy chain diseases, is a proliferative disorder of B lymphoid cells involving primarily the small intestine and mesenteric nodes. The characteristic immunoglobulin, whose detection by immunochemical techniques may present some difficulties, consists of incomplete alpha chains devoid of light chains. The deleted portion of the alpha chain is located in the Fd segment and involves both the variable and first constant domains. In both of two proteins for which structural data are available, normal sequence resumes at the beginning of the hinge region. The absence of L chains is due to a failure of synthesis. alpha chain disease appears to proceed in two stages. The early stage is characterized by a possibly non-malignant diffuse and extensive plasma cell infiltration which may be reversible after administration of antibiotics. The later stage is characterized by overt neoplasia (immunoblastic lymphoma). The socio-geographic distribution of the digestive form of alpha chain disease shows a clear predilection for underpriviliged populations living in areas with a high degree of infestation by intestinal pathogens which play presumably a crucial role in the pathogenesis of the disease.

Molecular analysis of spontaneous somatic mutants

Eukaryotic structural gene mutations occurring spontaneously in a mouse myeloma cell line offer the opportunity to study somatic mutation in animal cells at the molecular level. Studies on the myeloma protein and on mRNA have enabled us to characterise four such mutants representing four different mutation mechanisms. The results may have some bearing on the origin of antibody diversity.

Structure and function of the constant regions of immunoglobulins

Immunoglobulins are somewhat unusual in that they are biologically active proteins capable of carrying out several different functions. Thus, they can bind specifically with antigen, activate the complement system, mediate many cytotropic reactions, and act as antigen receptors on lymphocyte membranes.