Gene selection in hemoglobin and in antibody-synthesizing cells

POU2AF1, an amplification target at 11q23, promotes growth of multiple myeloma cells by directly regulating expression of a B-cell maturation factor, TNFRSF17

Multiple myeloma (MM), a progressive hematological neoplasm, is thought to result from multiple genetic events affecting the terminal plasma cell. However, genetic aberrations related to MM are seldom reported. Using our in-house array-based comparative genomic hybridization system to locate candidate target genes with following their expression analysis, we identified POU2AF1 at 11q23.1 as a probable amplification target in MM cell lines. POU2AF1 is a B-cell-specific transcriptional co-activator, which interacts with octamer-binding transcription factors Oct-1 and Oct-2, and augments their function. Downregulation of POU2AF1 expression by specific small-interfering RNA (siRNA) inhibited MM cell growth, whereas ectopic expression of POU2AF1 promoted growth of MM cells. Among putative transcriptional targets for POU2AF1, B-cell maturation factor, TNFRSF17, enhanced its transcription by POU2AF1, and POU2AF1 directly bound to an octamer site within the 5' region of TNFRSF17. Expression level of TNFRSF17 was closely correlated with that of POU2AF1 in cell lines and primary samples of MM, and decreasing TNFRSF17 expression by means of TNFRSF17 siRNA inhibited MM cell growth. Taken together, our results suggest that POU2AF1, when activated by amplification or other mechanisms, may contribute to progression of MM by accelerating growth of MM cells through direct transactivation of one of its target genes, TNFRSF17.

Mechanism and control of V(D)J recombination versus class switch recombination: similarities and differences

V(D)J recombination is the process by which the variable region exons encoding the antigen recognition sites of receptors expressed on B and T lymphocytes are generated during early development via somatic assembly of component gene segments. In response to antigen, somatic hypermutation (SHM) and class switch recombination (CSR) induce further modifications of immunoglobulin genes in B cells. CSR changes the IgH constant region for an alternate set that confers distinct antibody effector functions. SHM introduces mutations, at a high rate, into variable region exons, ultimately allowing affinity maturation. All of these genomic alteration processes require tight regulatory control mechanisms, both to ensure development of a normal immune system and to prevent potentially oncogenic processes, such as translocations, caused by errors in the recombination/mutation processes. In this regard, transcription of substrate sequences plays a significant role in target specificity, and transcription is mechanistically coupled to CSR and SHM. However, there are many mechanistic differences in these reactions. V(D)J recombination proceeds via precise DNA cleavage initiated by the RAG proteins at short conserved signal sequences, whereas CSR and SHM are initiated over large target regions via activation-induced cytidine deaminase (AID)-mediated DNA deamination of transcribed target DNA. Yet, new evidence suggests that AID cofactors may help provide an additional layer of specificity for both SHM and CSR. Whereas repair of RAG-induced double-strand breaks (DSBs) involves the general nonhomologous end-joining DNA repair pathway, and CSR also depends on at least some of these factors, CSR requires induction of certain general DSB response factors, whereas V(D)J recombination does not. In this review, we compare and contrast V(D)J recombination and CSR, with particular emphasis on the role of the initiating enzymes and DNA repair proteins in these processes.

Expression of mu and gamma immunoglobulin heavy chains in different cells of a cloned mouse lymphoid line

A cloned cell line derived from mouse bone marrow and transformed by Abelson virus is shown to synthesize two different heavy chains, mu and gamma 2B, in vitro. This characteristic is stable because it persists upon subcloning. Although most of the immunoglobulin-synthesizing cells produce either mu or gamma 2B heavy chains, a few cells contain both heavy chains, suggesting immunoglobulin class switching. Karyotypes show a complement of 41 chromosomes. Two copies of chromosome 12, to which immunoglobulin heavy chain structural genes have been assigned, were found. No light chain was found in either the mu- or the gamma 2B-producing cells. However, fusion of the cell line with a myeloma that synthesizes neither heavy nor light chains caused expression of kappa light chain in the hybridoma synthesizing mu chain. No light chain could be detected in the hybridomas synthesizing gamma 2B heavy chain.

Simultaneous expression of mouse immunoglobulins M and D is determined by the same homolog of chromosome 12

A hamster-mouse hybrid cell line expressiong both murine IgM and murine IgD on the membrane was shown to have only one copy of mouse chromosome 12. This chromosome is known to carry the structural genes for the immunoglobulin heavy chains. Cloning of populations selected for loss of mouse membrane IgM yielded cells that had also lost expression of membrane IgD, but not the expression of hamster immunoglobulin heavy chain. Karyotype analysis of these subclones demonstrated the concurrent loss of the chromosome 12 present in the parental hybrid. Absence of this chromosome was confirmed by use of the isozyme acid phosphatase 1. The results of the genetic analysis prove that the coexpression of mu and delta immunoglobulin heavy chains is not due to long-lived immunoglobulin mRNA nor to the transcription of genes on homologous chromosomes. We conclude that the genetic information for IgM and IgD expressed by a single cell lies on the same chromosome.

Immunoglobulin V/J recombination is accompanied by deletion of joining site and variable region segments

A site-specific recombination event is responsible for the somatic activation of immunoglobulin genes and for generating a major share of immunoglobulin gene diversity. Although several possible mechanisms can be proposed to account for this process, recombinatio accompanied by deletion is a particularly attractive mechanism because it might utilize inverted repeat sequences noted on the 3' side of all variable regions and on the 5' side of all joining site segments thus far studied. Testing this model is complicated by the fact that antibody cells are at least diploid and gene segments on the inactive chromosome can obscure deletions occurring within the active gene. Accordingly, we have screened several immunoglobulin-producing plasmacytoma lines to select those in which both chromosomes are rearranged. By using appropriate cell lines and variable and joining region probes in in situ hybridization experiments, we show that recombination is accompanied by the deletion of both variable and joining region genes. These experiments also allow us to map the site of V/J recombination of several active immunoglobulin genes and suggest an order and orientation for variable, joining, and constant region sequences.

Deletion of immunoglobulin heavy chain genes from expressed allelic chromosome

We have studied the organization of immunoglobulin heavy-chain genes in a gamma 2b-chain (BALB/c allotype)-producing myeloma BKC F1 # 15 induced in a F1 mouse between C57BL and BALB/c. Southern blot hybridization studies using cloned mu, gamma 1 and gamma 2b-chain genes as probes demonstrate that the mu- and gamma 1-chain genes of the expressed chromosome are deleted while these genes of the unexpressed chromosome are retained. The gamma 2b-chain gene of the expressed allele is rearranged while that gene of the unexpressed allele seems unchanged, as do the gamma 2a-chain genes. These results support the allelic deletion mechanism in heavy-chain class switch and the order of H chain genes.

Deletions in the constant region locus can account for switches in immunoglobulin heavy chain expression

Deletions spanning portions of the heavy chain constant region (CH) locus in murine plasmacytomas are revealed by hybridization with cloned immunoglobulin DNA molecules. The pattern of deletions suggests that a lymphocyte clone switches expression of CH genes by successive deletions and that the probable CH gene order is 5' mu-gamma 3-gamma 1-gamma 2b-gamma 2a-alpha 3'.

Evolution of two major chorion multigene families as inferred from cloned cDNA and protein sequences

Complete or partial sequences are reported from six chorion cDNA clones of the silkmoth Antheraea polyphemus. The proteins encoded belong to the two major chorion protein classes, A and B, each of which is encoded by a multigene family. The sequence comparisons define some major features of the families and suggest how these genes may be evolving. Deletions and insertions might be involved in expanding or contracting internally repetitive regions. Sequence divergence is localized, thus defining sequence domains of distinct evolutionary properties and presumably distinct functions.

Chromatin conformation during cell differentiation of human myeloid leukemia cells

A novel human promyelocytic leukemia cell line (HL-60) has been shown to form terminally differentiated granulocytes in the presence of dimethyl-sulfoxide (DMSO), some other chemicals, or colony stimulating factor. Compared to chromatin from HL-60 cells, chromatin from DMSO treated HL-60 cells showed an enrichment in low temperature melting material. The decrease in thermostability of chromatin from HL-60 cells after DMSO treatment is similar to the shift in thermostability of chromatin from human lymphocytes after stimulation with phytohemagglutinin (PHA). These results suggest that changes in the thermostability of chromatin may not be specific for cell differentiation or PHA stimulation.

Genetic control of quantitative variation in carbonic anhydrase isozymes of mammals. I. The concentration within single erythrocytes of the mouse

The levels of the carbonic anhydrase isozymes (CA and CA II) in single erythrocytes of DBA/2J mice were assayed by measuring the specific immunofluorescence of CA I and CA II with a microspectrofluorometric technique. Measurements of 100 randomly selected cells showed a range (in relative fluorescence units) of 21-52 (mean 31.3 +/- 7.5) for CA I and 30-80 (mean 45.6 +/- 10.7) for CA II. The CA II/CA I ratio of the means obtained by the single-cell fluorescence assay was similar to the ratios obtained for the two isozymes from hemolysates of DBA/2J mice using an immunodiffusion assay. The influence of cell age on the variation in carbonic anhydrase levels was determined by separating red cells into several fractions by a gravity sedimentation procedure. The younger cells showed higher levels of CA I and CA II than the older cells; however, the extensive overlap in the variability between the cells from the different fractions indicated that although cell age was contributing to the overall heterogeneity, its influence was not pronounced.

Genes for gamma-globin in human adult erythroid DNA

Purified gamma-globin specific complementary DNA has been used to demonstrate the presence of the gene for gamma-globin in DNA from human adult red blood cells. This finding sheds doubt on any theory involving looping-out excision of genes to explain the switch over from synthesis of gamma-globin to beta-globin at birth.

Hemoglobin ontogenesis: test of the gene excision hypothesis

The gene excision hypothesis of hemoglobin ontogenesis was tested in persons with HbSC disease, with the use of monospecific fluorescent antibodies for the identification of hemoglobins S, C, and F in individual erythrocytes. The results are incompatible with the prediction that only one gamma- or beta-globin gene may be active in any single chromosome and provide further evidence for incomplete repression of gamma-globin genes lying cis to active beta-globin genes.

Stability of the individual globin genes during erythroid differentiation

The genes of sheep betaA, betaC, and gamma globin were all present in DNA from erythroid cells which synthesized only betaC globin. Similarly, selective excision of non-expressed genes was shown not to occur during human erythroid differentiation. In contrast, evolutionary deletion of the betaC gene accounts for the inability of many sheep to make this globin.

Evidence for somatic rearrangement of immunoglobulin genes coding for variable and constant regions

A high-molecular-weight DNA from Balb/c mouse early embryo or from MOPC 321 plasmacytoma (a k-chain producer) was digested to completion with Bacillus amyloliquefaciens strain H restriction enzyme (BamH I). The resulting DNA fragments were fractionated according to size in preparative agarose gel electrophoresis. DNA fragments carrying gene sequences coding for the variable or constant region of k chains were detected by hybridization with purified, 125I-labeled, whole MOPC 321 K MRNA and with its 3'-end half. The pattern of hybridization was completely different in the genomes of embryo cells and of the plasmacytoma. The pattern of embryo DNA showed two components, one of which (molecular weight=6.0 million) hybridized with C-gene sequences and the other (molecular weight=3.9 million) with V-gene sequences. The pattern of the tumor DNA showed a single component that hybridized with both V-gene and C-gene sequences and that is smaller (molecular weight=2.4 million) than either of the components in embryo DNA. The results were interpreted to mean that the Vk and Ck genes, which are some distance away from each other in the embryo cells, are joined to form a contiguous polynucleotide stretch during differentiation of lymphocytes. Such joining occurs in both of the homologous chromosomes. Relevance of these findings with respect to models for V-C gene joining, activation of a specific V k gene, and allelic exclusion in immunoglobulin gene loci is discussed.

Close linkage between genes coding for allotypic markers on chicken IgG and IgM

The allotypic markers Gla and Gld (present on chicken IgG molecules) and Mla and Mlc (present on IgM) have previously been defined. The inheritance of these markers was studied in the F2 progeny of a cross between birds of the inbred lines CB (Gla+,d-,Mla+,c-) and WA (Gla-,d+,Mla-,c+). The results suggest that antigens Gla and Gld, and Mla and Mlc, are genetically controlled by alleles at two loci (Gl and Ml, respectively) and also show that these loci are closely linked.

Stimulation of fetal hemoglobin synthesis in bone marrow cultures from adult individuals

The regulation of fetal hemoglobin in adult erythroid cells was investigated with bone marrow cultures. Fetal hemoglobin (Hb F) was identified in individual erythroid colonies with fluorescent antibodies against Hb F and synthesis of gamma chains was determined with analyses of radioactive globins. The appearance of fetal hemoglobin in erythroid colonies was clonal. All the cells of the Hb F synthesizing colonies contained fetal hemoglobin. The frequency of erythroid colonies showing Hb F was higher than expected compared to the frequency of Hb F containing cells in the blood. Production of Hb F in culture, as shown by analysis of the radioactive globins, was 5 to 14 times higher than baseline Hb F synthesis. These results suggest that the ability for gamma chain synthesis in erythroid cells is determined at or above the level of the precursor cell from which the erythroid colonies, in vitro, derive (probably an erythropoietin responsive stem cell), and that stimulation of fetal hemoglobin synthesis in adult erythroid cells is possible.

Antibody differentiation: apparent sequence identity between variable regions shared by IgA and IgG immunoglobulins

We have analyzed a pair of human myeloma immunoglobulins (biclonal proteins) of the IgG and IgA classes from a single patient, GR. The light chains are identical in amino-acid sequence over 40 residues at their NH2-terminus, hwereas the heavy chains are identical throughout 45 residues of their NH2-terminus. Additional chemical and serological studies suggest the light chains and variable regions of the heavy chains (VH) are very similar, if not identical. The implications of these and of other published studies are discussed with regard to (i) the association of one VH region with multiple constant regions of the heavy chain (CH regions), (ii) two alternative types of V-C joining mechanisms, (iii) the differentiation of antibody-producing cells, and (iv) three categories of biclonal immunoglobulins.

The pattern of fetal haemoglobin disappearance after birth

Haemoglobin F (Hb F) levels were determined in 209 full-term newborn babies or infants of different ages ranging from birth to 11 months. A follow-up study of the disappearance of Hb F after birth was carried out on 25 premature babies; they were followed periodically from birth until 8 months. The results obtained in both samples show that Hb F levels remain constant after birth for periods varying from about 15 d in at term babies to about 40 d in premature babies; a linear decrease follows in both cases. The initial plateau is a finding contrary to what has been reported. A possible model for the phenomenon is discussed in terms of haemoglobin genes and ontogenetic differentiation.

The in vitro cellular synthesis of hemoglobin components in various media and its relevance to the concept of stenogenic and eurygenic states of differentiation

The increase above normal in the rate of synthesis of human fetal hemoglobin in incubated cells from adults, as observed previously, is only relative. The addition of serum to the incubation medium tends to reestablish the normal ratio of rates. In the absence of serum the decrease in rates of synthesis affects most strongly the hemoglobin component, whichever it be, that is synthesized more intensively at the cell stage under consideration. This is brought out by the comparison of the effects of different media on fetal and adult cells. The data lead to the definition of the concept of stenogenic and eurygenic states of differentiation.

Hemoglobin switching in sheep and goats: occurrence of hemoglobins A and C in the same red cell

Sheep and goats switch from the synthesis of hemoglobin A (alpha(2)beta(2)(A)) to hemoglobin C (alpha(2)beta(2)(C)) when made anemic. We have demonstrated the existence of the asymmetrical hybrid hemoglobin, alpha(2)beta(A)beta(C), in the circulating red cells of anemic sheep. These erythroid cells, therefore, synthesized both A and C hemoglobin simultaneously. Thus, the switch appears to be mediated by selective gene expression rather than by a clonal or cellular selective mechanism.

The erythroid cells and haemoglobins of the chick embryo

The changes in the types of erythroid cells produced during embryogenesis of the chick have been correlated with the changes in the types of haemoglobins found in the embryo. Primitive erythroid cells constitute the only red blood cells of 2- to 5-day embryos. The first recognizable immature definitive erythroid cells appear in the embryonic circulation at 5 to 6 days and progressively replace the primitive cells, such that by 14 to 16 days the primitive cells constitute less than 1 % of the circulating erythroid cells. Primitive erythropoiesis is strikingly different from definitive erythropoiesis. At any one time point between 2 and 16 days, all of the isolated primitive cells appear, by morphological criteria, to be at the same stage of maturation, and, although variation in cell size is observed, for an individual maturation stage, the small cells are not more mature than the medium-size cells, nor are the large cells less mature than the medium or small cells. Maturing primitive erythroid cells undergo the progressive changes in cell structure characteristic of erythroid maturation in mammalian erythropoietic systems, but do so as a uniform cell population. Haemoglobin, isolated from primitive erythroid cells of 2- to 5-day embryos, shows two components on polyacrylamide gel electrophoresis, haemoglobin E and haemoglobin P. The haemoglobin E/P ratio is constant in lysates from 2- to 5-day embryos. A t 6 to 7 days when the first haemoglobinized immature definitive erythroid cells appear in the embryonic circulation, two new haemoglobin components are observed in lysates of erythroid cells. These two new haemoglobin components are electrophoretically and immunologically identical to the two haemoglobin components of adult chickens, haemoglobins A and D. As the definitive erythroid cells replace the primitive erythrocytes in the embryonic circulation, the haemoglobins A and D increase in amount and replace haemoglobin P. Haemoglobin P cannot be detected immunologically in erythroid cell lysates from 16-day embryos which contain less than 1 % primitive cells. In erythroid cell lysates from late embryos, which contained few, if any, primitive erythrocytes, a minor haemoglobin, electrophoretically similar to haemoglobin E on pH 10.3 polyacrylamide gels, is consistently observed. This component differs from haemoglobin E on pH 8.9 polyacrylmide gels, on Sephadex G-100 columns, on polyacrylamide gels of different porosities, and shows a reaction of only partial identity with haemoglobin E by two-dimensional immunodiffusion. This haemoglobin component, haemoglobin H, is detectable electrophoretically in lysates from 12-day embryos and immunologically in lysates from 8-day embryos. Haemoglobin H has not been observed in adult chickens. The switch from the production of primitive to definitive erythroid cells during development of the chick embryo is associated with the initiation of synthesis of three new haemoglobins, the two adult haemoglobins and haemoglobin H. The haemoglobin D /A ratio of adult chicken haemoglobin, determined from the ratio of gel scan peak masses, is 0.30. When haemoglobins D and A first appear in erythroid cell lysates from 6- to 7-day embryos, the haemoglobin D /A ratio is about 0.9. T he D/A ratio of lysates falls to 0.5 by 16 to 18 days, a time when 99 % of the erythroid cells of the embryo are mature definitive erythrocytes. However, the haemoglobin D /A ratio of lysates from late embryos and young chicks of 0.5 to 20 days of age is consistently greater than that of adult chicken haemoglobin. Definitive erythrocytes of chick embryos and young chicks appear to differ from definitive cells of adult chickens in at least two ways: the presence of haemoglobin H and the higher haemoglobin D/A ratio.

Hemogloblin switching in sheep and goats: erythropoietin-dependent synthesis of hemoglobin C in goat bone-marrow cultures

The anemia-induced switch from hemoglobin A (alpha(2)beta(2) (A)) to hemoglobin C (alpha(2)beta(2) (C)) synthesis occurring in vivo in sheep and goats has been reproduced in tissue culture of goat bone-marrow cells. Cultivation of primary cultures of goat bone marrow in the presence of erythropoietin results in the appearance of detectable amounts of beta(C) globin after 48-72 hr, as well as in a decrease in beta(A) globin. A population of proerythroblasts, as well as active heme and globin synthesis, are maintained for at least 3 days in erythropoietin-treated, but not in erythropoietin-deficient, cultures. These findings demonstrate (i) maintenance of erythropoietin-responsive cells from bone marrow in vitro, and (ii) switching in vitro from the synthesis of a globin chain coded by one gene to that coded by a different, nonallelic gene. Bone-marrow culture might be a useful model system for study of the mechanism of action of erythropoietin and for study of the activation (and inactivation) of specific genes in vitro.

Genetic determination of phenotypic variation in sickle cell trait

Two genetic sources of variation influence the percentage of sickle cell hemoglobin found in heterozygotes. One factor is strongly related to the percentage of hemoglobin S in the carrier parent and appears to be determined by sickle hemoglobin isoalleles, whereas the other is related to racial background and may well be polygenic.

Hemoglobin switching in sheep and goats: change in functional globin messenger RNA in reticulocytes and bone marrow cells

ANEMIA CAUSES A CHANGE IN THE TYPE OF CIRCULATING HEMOGLOBIN IN GOATS AND CERTAIN SHEEP: HbA (alpha(2)beta(2) (A)) is replaced by HbC (alpha(2)beta(2) (C)). We have isolated globin mRNA from erythroid cells of anemic and nonanemic animals to investigate the mechanism whereby anemia causes this switch. To study several stages in transition from beta(A) to beta(C) synthesis, active globin mRNA was isolated from bone marrow cells, as well as from reticulocytes. By assaying these globin mRNAs in a rabbit reticulocyte cell-free system, we have demonstrated that the switch from beta(A) to beta(C) globin synthesis is mediated via a change in functional globin mRNA.