Intraluminal crystalloids are highly associated with prostatic adenocarcinoma on concurrent biopsy specimens

Prostatic crystalloids are intraluminal eosinophilic structures with variable size and shape. Their presence has been described in conjunction with the occurrence of prostatic adenocarcinoma (pCA). We herein report the association of crystalloids and pCA in a prospective trial utilizing an extended multi-site transrectal ultrasound-guided (TRUS) prostate biopsy protocol. Three hundred and forty-four consecutive patients were prospectively enrolled at the Dallas Veterans Administration Hospital from November 2002 to September 2003. Indications for biopsy included a prostate-specific antigen (PSA) > or =4 ng/ml and/or abnormal digital rectal exam. A single pathologist evaluated all biopsy cores and documented the presence or absence of significant histopathologic features. Univariate and multivariate logistic regression analysis were applied to test the association of these features with the presence of pCA on concurrent biopsy. Median number of core biopsies per patient was 12 (range 3-36). Overall cancer detection rate was 42.7%. pCA was diagnosed in 66 (81.5%) of 81 patients with crystalloids, 70 (69.3%) of 101 patients with high-grade prostatic intraepithelial neoplasia (HGPIN), and 32 (84.2%) of 38 patients with both HGPIN and crystalloids on biopsy. Multivariate analysis identified crystalloids (RR 4.53, 95% CI 2.30-8.88) and HGPIN (RR 3.20, 95% CI 1.84-5.57) as independent predictors of the presence of cancer on concurrent biopsy (P<0.001). In this prospective analysis, crystalloids were significantly associated with pCA on concurrent biopsy and more predictive of the presence of pCA than HGPIN. These findings suggest that the presence of crystalloids alone or in combination with HGPIN in prostate biopsies may be a more compelling indication for repeat biopsy than HGPIN alone.

Impact of training level of urology residents on the detection of prostate cancer on TRUS biopsy

The objective of this study is to evaluate the performance of urology residents at each training level in detecting prostate cancer with transrectal ultrasound-guided (TRUS) biopsy. The inclusion criteria were: (1) prostate-specific antigen (PSA) 4-10 ng/ml; and (2) 10-12 cores per biopsy session. Data from repeat biopsy sessions were excluded. Overall prostate cancer detection rate for 170 patients was 39.4%. PSA, digital rectal examination (DRE), and prostate volume were predictors of cancer detection. There were no significant differences in overall cancer detection rates, PSA, DRE, or prostate volume between resident levels. In conclusion, urology residents at all levels of training perform equally well at detecting cancer using TRUS prostate biopsy technology.

The IgA/IgM receptor expressed on a murine B cell lymphoma is poly-Ig receptor

T560, a mouse B lymphoma that originated in gut-associated lymphoid tissue, expresses receptors that bind dimeric IgA and IgM in a mutually inhibitory manner but have little affinity for monomeric IgA. Evidence presented in this paper indicates that the receptor is poly-Ig receptor (pIgR) known in humans and domestic cattle to bind both IgA and IgM. The evidence includes the demonstration that binding of IgM is J chain dependent, and that pIg-precipitated receptor has an appropriate Mr of 116-120 kDa and can be detected on immunoblots with specific rabbit anti-mouse pIgR. Overlapping RT-PCR performed using template mRNA from T560 cells and oligonucleotide primer pairs designed from the published sequence of mouse liver pIgR indicate that T560 cells express mRNA virtually identical with that of the epithelial cell pIgR throughout its external, transmembrane, and intracytoplasmic coding regions. Studies using mutant IgAs suggest that the Calpha2 domain of dimeric IgA is not involved in high-affinity binding to the T560 pIgR. Inasmuch as this mouse B cell pIgR binds IgM better than IgA, it is similar to human pIgR and differs from rat, mouse, and rabbit epithelial cell pIgRs that bind IgA but not IgM. Possible explanations for this difference are discussed. All clones of T560 contain some cells that spontaneously secrete both IgG2a and IgA, but all of the IgA recoverable from the medium and from cell lysates is monomeric; it cannot be converted to secretory IgA by T560 cells.

Treatment of rheumatoid arthritis with a DR4/1 peptide

OBJECTIVE

To determine the safety and potential clinical efficacy of primary and booster injections of a DR4/1 peptide in patients with active rheumatoid arthritis (RA) despite methotrexate therapy. METHODS. Subjects with active RA were enrolled in a randomized, placebo controlled, double blind, dose-escalating clinical trial of synthetic DR4/1 peptide containing the shared epitope. The primary injection of the DR4/1 peptide in alum adjuvant was administered at one of 3 doses, 1.3, 4.0, and 13 mg, followed by up to 3 or 4 booster injections every 6 or 8 weeks at the same dose. The primary outcomes were the occurrence of adverse effects and changes in measures of immune function. Clinical efficacy was assessed using the American College of Rheumatology 20% criteria for clinical improvement.

RESULTS

Fifty-three patients were entered into the trial, including 44 who completed the study. In the absence of any observations of a dose response to the DR4/1 peptide injections, the 3 dosage groups were combined for subsequent analysis into 3 groups: patients receiving DR4/1 peptide injections every 6 weeks, patients receiving DR4/1 peptide injections every 8 weeks, and a placebo group. At all doses and each dosing interval the primary and booster injections of synthetic DR4/1 peptide were well tolerated and did not produce any significant changes in lymphocyte counts or evidence of generalized immunosuppression. Analysis of clinical efficacy showed that the 6 week group had trends toward improvement in disease measures.

CONCLUSION

Primary and booster injections of the DR4/1 peptide containing the shared epitope were safe and did not broadly suppress immune function.

Hypothesis: genes which function in a stochastic lineage commitment process are subject to monoallelic expression

The collection of genes which are now known to be monoallelically expressed in mammals is a diverse set. In the case of the genes which encode transducing receptors, such as immunoglobulins or odoront receptors, monoallelic expression ensures that cell activity is related to encountering a unique ligand. However, some monoallelically expressed genes do not encode receptors, and in these cases the physiological purpose of monoallelic expression is uncertain. Even more puzzling are the cases of imprinted genes, where only the maternal or only the paternal allele is expressed. In this article we consider the hypothesis that some of these cases of monoallelic expression reflect the unusual instances in development in which lineage commitment results from a selective rather than an instructive mechanism. These mechanisms are distinguished by their reliance on either external signals (instructive) or internal, cell autonomous events (selective) to cause the changes in gene expression which correspond to lineage commitment. While the instructive mechanism predicts that lineage commitment genes will be expressed or silenced biallelically, the selective mechanism predicts that commitment genes will be subject to monoallelic expression. Specifically, for the cases in which lineage commitment results from activating gene expression, the selective mechanism predicts that commitment genes will be monoallelically expressed following commitment, such as observed recently for some cytokine and transcription factor genes. For the cases in which extinction of gene expression causes commitment, the selective mechanism predicts that the commitment genes will be monoallelically expressed prior to commitment, as for X-linked and imprinted genes.

Variegated expression of the endogenous immunoglobulin heavy-chain gene in the absence of the intronic locus control region

The expression of chromosomally integrated transgenes usually varies greatly among independent transfectants. This variability in transgene expression has led to the definition of locus control regions (LCRs) as elements which render expression consistent. Analyses of expression in single cells revealed that the expression of transgenes which lack an LCR is often variegated, i.e., on in some cells and off in others. In many cases, transgenes which show variegated expression were found to have inserted near the centromere. These observations have suggested that the LCR prevents variegation by blocking the inhibitory effect of heterochromatin and other repetitive-DNA-containing structures at the insertion site and have raised the question of whether the LCR plays a similar role in endogenous genes. To address this question, we have examined the effects of deleting the LCR from the immunoglobulin heavy-chain locus of a mouse hybridoma cell line in which expression of the immunoglobulin mu heavy-chain gene is normally highly stable. Our analysis of mu expression in single cells shows that deletion of this LCR resulted in variegated expression of the mu gene. That is, in the absence of the LCR, expression of the mu gene in the recombinant locus could be found in either of two epigenetically maintained, metastable states, in which transcription occurred either at the normal rate or not at all. In the absence of the LCR, the on state had a half-life of approximately 100 cell divisions, while the half-life of the off state was approximately 40,000 cell divisions. For recombinants with an intact LCR, the half-life of the on state exceeded 50,000 cell divisions. Our results thus indicate that the LCR increased the stability of the on state by at least 500-fold.

Murine SHP-1 splice variants with altered Src homology 2 (SH2) domains. Implications for the SH2-mediated intramolecular regulation of SHP-1

SHP-1 is a protein-tyrosine phosphatase with two Src homology 2 (SH2) domains. These SH2 domains determine which proteins SHP-1 associates with, but they also autoregulate the activity of the catalytic domain. In this report, we find that the murine SHP-1 transcript is processed to yield a series of alternatively spliced in-frame transcripts, the majority of which exclude exons encoding one or the other SH2 domain. We have examined the corresponding protein isoforms in several ways. First, our measurements of V(max) and K(m) under different conditions indicate that the SH2 variants have elevated activity because of lessened autoregulation. Second, to ascertain whether regulation by the SH2 domains reflects intra- or intermolecular effects, we analyzed the state of SHP-1 by high performance liquid chromatography and sucrose density gradient centrifugation. Our results showed that SHP-1 is a monomer and, thus, is regulated in an intramolecular manner. Third, our analyses detected shape differences between SHP-1 and the active splice variant protein deleted of the amino-terminal SH2 domain; i.e. SHP-1 was globular and resistant to proteolytic digestion, while the splice variant protein was "rod-shaped" and more susceptible to proteolytic digestion.

Infrequent translation of a nonsense codon is sufficient to decrease mRNA level

In many organisms nonsense mutations decrease the level of mRNA. In the case of mammalian cells, it is still controversial whether translation is required for this nonsense-mediated RNA decrease (NMD). Although previous analyzes have shown that conditions that impede translation termination at nonsense codons also prevent NMD, the residual level of termination was unknown in these experiments. Moreover, the conditions used to impede termination might also have interfered with NMD in other ways. Because of these uncertainties, we have tested the effects of limiting translation of a nonsense codon in a different way, using two mutations in the immunoglobulin mu heavy chain gene. For this purpose we exploited an exceptional nonsense mutation at codon 3, which efficiently terminates translation but nonetheless maintains a high level of mu mRNA. We have shown 1) that translation of Ter462 in the double mutant occurs at only approximately 4% the normal frequency, and 2) that Ter462 in cis with Ter3 can induce NMD. That is, translation of Ter462 at this low (4%) frequency is sufficient to induce NMD.

Role of the intronic elements in the endogenous immunoglobulin heavy chain locus. Either the matrix attachment regions or the core enhancer is sufficient to maintain expression

High level expression in mice of transgenes derived from the immunoglobulin heavy chain (IgH) locus requires both the core enhancer (Emu) and the matrix attachment regions (MARs) that flank Emu. The need for both elements implies that they each perform a different function in transcription. While it is generally assumed that expression of the endogenous IgH locus has similar requirements, it has been difficult to assess the role of these elements in expression of the endogenous heavy chain gene, because B cell development and IgH expression are strongly interdependent and also because the locus contains other redundant activating elements. We have previously described a gene-targeting approach in hybridoma cells that overcomes the redundancy problem to yield a stable cell line in which expression of the IgH locus depends strongly on elements in the MAR-Emu-MAR segment. Using this system, we have found that expression of the endogenous mu gene persists at substantially (approximately 50%) normal levels in recombinants which retain either the MARs or Emu. That is, despite the dissimilar biochemical activities of these two elements, either one is sufficient to maintain high level expression of the endogenous locus. These findings suggest new models for how the enhancer and MARs might collaborate in the initiation or maintenance of transcription.

Structural and functional analysis of J chain-deficient IgM

Previous studies have discerned two forms of polymeric mouse IgM: moderately cytolytic (complement-activating) pentamer, which contains J chain, and highly cytolytic hexamer, which lacks J chain. To investigate the relationships among polymeric structure, J chain content, and cytolytic activity, we produced IgM in J chain-deficient and J chain-proficient mouse hybridoma cell lines. Both hexamer and pentamer were produced in the absence as well as the presence of J chain. Hexameric IgM activated (guinea pig) complement approximately 100-fold more efficiently than did J chain-deficient pentamer, which, in turn, was more active than J chain-containing pentamer. These results are consistent with the hypothesis that J chain-containing pentamer cannot activate complement. We also analyzed the structure of IgM-S337, in which the mu-chain bears the C337S substitution. Like normal IgM, IgM-S337 was formed as a hexamer and as both J chain deficient- and J chain-containing pentamers. Unlike normal IgM, IgM-S337 dissociated in SDS into various subunits. For IgM-S337 pentamer, the predominant subunits migrated as mu2kappa2 and mu4kappa4, and the subunit distribution was unaltered by J chain. However, J chain was found only in the mu2kappa2 species, suggesting that some arrangement of inter-mu bonds directs incorporation of J chain. IgM-S337 hexamer also dissociated to mu2kappa2 and mu4kappa4, but also yielded several species migrating much more slowly in SDS-PAGE than wild-type mu12kappa12. To account for these forms, we propose that each mu-chain can interact with three other mu-chains and that some hexameric molecules contain two catenated mu6kappa6 circles.

Domain-switched mouse IgM/IgG2b hybrids indicate individual roles for C mu 2, C mu 3, and C mu 4 domains in the regulation of the interaction of IgM with complement C1q

Although polymeric IgM and monomeric IgG are potent activators of the classical complement pathway, previous studies have indicated that monomeric IgM is inactive. To understand this and to examine the roles of the individual mu domains in complement activation, we created a set of IgM/IgG2b mouse chimeric Abs in which homologous domains of both Abs have been interchanged, either singly or together with adjacent domains. The monomer subunits (H2L2) of the resulting chimeras were analyzed for their capacities to bind C1q and to initiate complement-mediated lysis (CML) of haptenated erythrocytes. When C gamma 2 was flanked by C mu 4, the inherent C1q-binding activity of the C gamma 2 domain was lost. This demonstrates that C mu 4 can suppress the C1q-binding activity of the adjacent C gamma 2 domain, and suggests that C mu 4 may exert a similar effect on the C mu 3 domain in the IgM monomer subunit. When C mu 3 was located in an IgG2b background and potentially freed from the constraints imposed by the IgM background, the monomer was not able to bind C1q or initiate CML. This suggests that these activities are not expressed inherently in the C mu 3 domain. The transplantation of C mu 3 together with C mu 4 into the IgG background permitted polymer formation. This polymer was able to bind C1q, although neither the monomer nor the polymer forms were active in CML; conversely, all IgM polymers with a transplanted C gamma 2 domain were active in both C1q binding and CML, and demonstrated apparent Kd values similar to that of wild-type IgM.

Domain-switched mouse IgM/IgG2b hybrids indicate individual roles for C mu 2, C mu 3, and C mu 4 domains in the regulation of the interaction of IgM with complement C1q

Although polymeric IgM and monomeric IgG are potent activators of the classical complement pathway, previous studies have indicated that monomeric IgM is inactive. To understand this and to examine the roles of the individual mu domains in complement activation, we created a set of IgM/IgG2b mouse chimeric Abs in which homologous domains of both Abs have been interchanged, either singly or together with adjacent domains. The monomer subunits (H2L2) of the resulting chimeras were analyzed for their capacities to bind C1q and to initiate complement-mediated lysis (CML) of haptenated erythrocytes. When C gamma 2 was flanked by C mu 4, the inherent C1q-binding activity of the C gamma 2 domain was lost. This demonstrates that C mu 4 can suppress the C1q-binding activity of the adjacent C gamma 2 domain, and suggests that C mu 4 may exert a similar effect on the C mu 3 domain in the IgM monomer subunit. When C mu 3 was located in an IgG2b background and potentially freed from the constraints imposed by the IgM background, the monomer was not able to bind C1q or initiate CML. This suggests that these activities are not expressed inherently in the C mu 3 domain. The transplantation of C mu 3 together with C mu 4 into the IgG background permitted polymer formation. This polymer was able to bind C1q, although neither the monomer nor the polymer forms were active in CML; conversely, all IgM polymers with a transplanted C gamma 2 domain were active in both C1q binding and CML, and demonstrated apparent Kd values similar to that of wild-type IgM.

Interplay of J chain and disulfide bonding in assembly of polymeric IgM

Normal mouse IgM is synthesized as hexamers in the absence of J chain and as pentamers in its presence. Previous work has suggested that polymer size is also closely related to formation of the inter-mu chain disulfide bond mediated by cysteine 414, one of three cysteines involved in inter-mu chain bonding. This correlation in turn suggested that formation of C414-C414 might be required for J chain to influence how IgM assembles and that formation of C414-C414 might affect the J chain/IgM stoichiometry. To test such hypotheses we have used cell lines which either expressed or did not express J chain to produce IgM in which serine was substituted for C414. In contrast to the case of IgM assembled from normal mu chains, IgM-S414 was secreted mostly as pentamers and tetramers but not as hexamers, irrespective of J chain synthesis. These results indicate that the role of J chain as modulator of IgM structure and function requires C414. Moreover, a more detailed analysis of the structure of J-plus and J-minus IgM-S414 revealed that J chain, in fact, influenced the nature of secreted IgM-S414: In the absence of J chain, some IgM-S414 was secreted as dimers and trimers, while in the presence of J chain, some IgM was secreted as non-covalently assembled pentamers. These results imply that disulfide bonding can occur differently from the pattern depicted in conventional models of IgM structure.

Expression of the (recombinant) endogenous immunoglobulin heavy-chain locus requires the intronic matrix attachment regions

The elements which regulate gene expression have traditionally been identified by their effects on reporter genes which have been transfected into cell lines or animals. It is generally assumed that these elements have a comparable role in expression of the corresponding endogenous locus. Nevertheless, several studies of immunoglobulin heavy-chain (IgH) gene expression have reported that the requirements for expressing IgH-derived transgenes differ from the requirements for expression of the endogenous IgH locus. Thus, although expression of transgenes requires multiple elements from the J(H)-C mu intron--the E mu core enhancer, the matrix attachment regions (MARs) which flank E mu, and several switch-associated elements--B-cell lines in which expression of the endogenous heavy-chain gene is maintained at the normal level in the absence of these intronic elements have occasionally been reported. Gene targeting offers an alternative method for assessing regulatory elements, one in which the role of defined segments of endogenous genes can be evaluated in situ. We have applied this approach to the IgH locus of a hybridoma cell line, generating recombinants which bear predetermined modifications in the functional, endogenous mu heavy-chain gene. Our analysis indicates the following. (i) Ninety-eight percent of the expression of the recombinant endogenous mu gene depends on elements in the MAR-E mu-MAR segment. (ii) Expression of the recombinant mu gene depends strongly on the MARs of the J(H)-C mu intron but not on the adjoining E mu core enhancer and switch regions; because our recombinant cell lines bear only a single copy of the mu gene, our results indicate that mu expression is activated by MAR elements lying within that same mu transcription unit. (iii) The MAR segment includes at least one activating element in addition to those defined previously by the binding of presumptive activating proteins in the nuclear matrix. (iv) Close association of the MARs with the E mu enhancer is not required for MAR-stimulated expression. (v) The other MARs in the IgH locus do not in their normal context provide the requisite MAR function.

Analysis of IgM structures involved in J chain incorporation

J chain is associated with pentameric IgM and polymeric IgA. In IgM, J chain is disulfide bonded to the C575 residue of the mu-chain, located in the mu tail piece (mu tp). Previous studies indicated that mu tp is not sufficient to mediate J chain incorporation into polymeric Ig. In this study, we analyzed which other C mu domains are involved in J chain incorporation. Three altered forms of mouse IgM were analyzed: IgM lacking the C mu 1 domain, IgM in which the C mu 2 and C mu 3 domains were replaced by the hinge region and the C gamma 2 domain of IgG2b, and IgM, in which the C mu 4 domain was replaced by C gamma 3. We found that neither C mu 1, C mu 2, nor C mu 3 was absolutely required for J chain incorporation. The importance of C mu 4 could not be fully analyzed because the C gamma 3 replacement mutant failed to form polymers. Also, we found that the glycosylation site at asparagine 563 of mu tp was important for J chain incorporation. Disruption of this site by replacement of either asparagine 563 by tyrosine or serine 565 by phenylalanine resulted in diminished J chain incorporation and increased production of hexameric IgM. These results demonstrate the importance of structural elements located close to mu tp in the incorporation of J chain into IgM.

Analysis of IgM structures involved in J chain incorporation

J chain is associated with pentameric IgM and polymeric IgA. In IgM, J chain is disulfide bonded to the C575 residue of the mu-chain, located in the mu tail piece (mu tp). Previous studies indicated that mu tp is not sufficient to mediate J chain incorporation into polymeric Ig. In this study, we analyzed which other C mu domains are involved in J chain incorporation. Three altered forms of mouse IgM were analyzed: IgM lacking the C mu 1 domain, IgM in which the C mu 2 and C mu 3 domains were replaced by the hinge region and the C gamma 2 domain of IgG2b, and IgM, in which the C mu 4 domain was replaced by C gamma 3. We found that neither C mu 1, C mu 2, nor C mu 3 was absolutely required for J chain incorporation. The importance of C mu 4 could not be fully analyzed because the C gamma 3 replacement mutant failed to form polymers. Also, we found that the glycosylation site at asparagine 563 of mu tp was important for J chain incorporation. Disruption of this site by replacement of either asparagine 563 by tyrosine or serine 565 by phenylalanine resulted in diminished J chain incorporation and increased production of hexameric IgM. These results demonstrate the importance of structural elements located close to mu tp in the incorporation of J chain into IgM.

Analysis of a hot spot for DNA insertion suggests a mechanism for Ig switch recombination

We recently reported that transfected DNA inserts into the VDJ-Cmu intron much more frequently than into average DNA, and that insertion within this intron occurs preferentially into the switch region. To gain information about the mechanisms involved in DNA insertion, we sequenced the 5' and 3' junctions of typical transformants. Although the junction sequences did not indicate a preferred insertion motif within the switch region, our results suggest that joining of the transfected and chromosomal DNAs is facilitated by short regions of identity. Our analysis of the insertions into the non-switch part of the intron suggests that breakage of the chromosomal DNA occurs preferentially at sites that are flanked by short complementary sequences. This correlation suggests that the self-complementary DNA might form short stem-loops, which, in turn, are prone to enzymatic cleavage and thus facilitate the insertion of transfected DNA. A model is proposed in which this effect can account for both the higher than average frequency of insertion into the VDJ-Cmu intron and the preference for the switch region within this intron. An extension of this model is proposed to explain why the repetitive switch regions are the preferred breakage/rejoining sites for isotype switch rearrangements.

Analysis of a hot spot for DNA insertion suggests a mechanism for Ig switch recombination

We recently reported that transfected DNA inserts into the VDJ-Cmu intron much more frequently than into average DNA, and that insertion within this intron occurs preferentially into the switch region. To gain information about the mechanisms involved in DNA insertion, we sequenced the 5' and 3' junctions of typical transformants. Although the junction sequences did not indicate a preferred insertion motif within the switch region, our results suggest that joining of the transfected and chromosomal DNAs is facilitated by short regions of identity. Our analysis of the insertions into the non-switch part of the intron suggests that breakage of the chromosomal DNA occurs preferentially at sites that are flanked by short complementary sequences. This correlation suggests that the self-complementary DNA might form short stem-loops, which, in turn, are prone to enzymatic cleavage and thus facilitate the insertion of transfected DNA. A model is proposed in which this effect can account for both the higher than average frequency of insertion into the VDJ-Cmu intron and the preference for the switch region within this intron. An extension of this model is proposed to explain why the repetitive switch regions are the preferred breakage/rejoining sites for isotype switch rearrangements.

An element in the endogenous IgH locus stimulates gene targeting in hybridoma cells

Gene targeting of the immunoglobulin (Ig) heavy chain locus is the basis of improved methods of investigating gene expression and of antibody engineering. The VH-Cmu intron is a convenient region for mediating homologous recombination events which result in production of Ig bearing an altered heavy chain. Also, this segment includes several elements which are important for gene expression, replication and isotype switching: in some cases it will be advantageous to alter these processes by modifying this intron. Considering that multiple targeting steps might be needed to accomplish all the requisite changes, it is important to know whether any of the anticipated modifications also alter the recombinogenicity of the IgH locus. To test this possibility we have measured the frequency at which a mutation in the Cmu3 exon of the endogenous mu gene is corrected by homologous recombination with a transfected segment of Cmu DNA. Comparison of recombination frequencies in several engineered hybridomas indicates that deletion of a 7.1 kb segment from the VH-Cmu intron depresses recombination by approximately 10-fold.

B and T cells are not required for the viable motheaten phenotype

Hematopoietic cell phosphatase (HCP), encoded by the hcph gene, (also called PTP1C, SHP, SH-PTP1, and PTPN6) is deficient in motheaten (me/me), and the allelic viable motheaten (me(v)/me(v)) mice. Since HCP is expressed in many cell types and protein phosphorylation is a major mechanism of regulating protein function, it is not surprising that the motheaten phenotype is pleiotropic. It is commonly thought that immune system involvement causes this disease. If so, the motheaten disease ought to be alleviated when the recombination activation gene-1 (RAG-1) is disrupted because there will be no V(D)J rearrangement and thus impaired development of B and T cells. We bred homozygous, double-mutant me(v)/me(v) x RAG 1 -/- mice and found that, in fact, inflamed paws, and splenomegaly with elevated myelopoiesis. Thus, except for autoantibodies, the motheaten phenotype does not depend on the presence of B and T cells. This observation cautions the use of motheaten mice as a model of autoimmune disease.

Targeted removal of the mu switch region from mouse hybridoma cells. A test of its role in gene expression in the endogenous IgH locus

The switch regions adjoining the DNA encoding the Ig heavy chain constant regions have been implicated in gene expression as well as isotype switching, in that transgenic mice express switch-containing transgenes at a level 100- to 1000-fold higher than the corresponding switch-deleted transgenes. To test whether the switch region of the natural IgH locus is also required for high level expression we have used homologous recombination to generate targeted recombinant hybridoma cell lines that lack the switch region sequences from the major intron of the mu gene. The expression pattern of these switch knock-out cell lines was compared with that of the parental cell line as well as to that of control recombinants using both steady-state mRNA level and nuclear run-on activity to assess heavy chain gene expression. In striking contrast with the results reported for transgenic animals, we have found only a minimal effect, if any, of deleting the switch element from the natural chromosomal location.

Targeted removal of the mu switch region from mouse hybridoma cells. A test of its role in gene expression in the endogenous IgH locus

The switch regions adjoining the DNA encoding the Ig heavy chain constant regions have been implicated in gene expression as well as isotype switching, in that transgenic mice express switch-containing transgenes at a level 100- to 1000-fold higher than the corresponding switch-deleted transgenes. To test whether the switch region of the natural IgH locus is also required for high level expression we have used homologous recombination to generate targeted recombinant hybridoma cell lines that lack the switch region sequences from the major intron of the mu gene. The expression pattern of these switch knock-out cell lines was compared with that of the parental cell line as well as to that of control recombinants using both steady-state mRNA level and nuclear run-on activity to assess heavy chain gene expression. In striking contrast with the results reported for transgenic animals, we have found only a minimal effect, if any, of deleting the switch element from the natural chromosomal location.

Epitope studies indicate that histidyl-tRNA synthetase is a stimulating antigen in idiopathic myositis

The most frequently found myositis-specific antibody, the anti-Jo-1 antibody (anti-HRS), binds to histidyl-tRNA synthetase (HRS). Although this antibody reacts with HRS, it is unclear whether HRS is the stimulating antigen or is merely a protein that cross-reacts with a yet undefined antigen. Because antibody directed against an unrelated antigen would not be expected to cross-react with HRS at multiple sites, we mapped the epitopes on HRS to resolve this issue. We found by Western blot analyses that immunoglobulins G (IgG) from 18 of 19 anti-HRS positive patient sera react with amino acids 2-44 and 286-509 of HRS. Patient IgG specific for these two epitopes were found not to inhibit HRS enzyme activity. Instead, the inhibitory property of anti-HRS was found to be associated with antibodies that do not react to HRS in immunoblots, indicating the presence of other epitopes. In addition, antibodies that react in immunoblots were found to represent only a small fraction of total anti-HRS antibody. Our finding that patient IgG recognized at least three distinct epitopes on HRS strongly suggests that the immunological response at some point in the disease is directed against HRS and not against a cross-reactive antigen.

Interchromosomal recombination is suppressed in mammalian somatic cells

Homologous recombination occurs intrachromosomally as well as interchromosomally, both in mitotic (somatic) cells as well as meiotically in the germline. These different processes can serve very different purposes in maintaining the integrity of the organism and in enhancing diversity in the species. As shown here, comparison of the frequencies of intra- and interchromosomal recombination in meiotic and mitotic cells of both mouse and yeast argues that interchromosomal recombination is particularly low in mitotic cells of metazoan organisms. This result in turn suggests that the recombination machinery of metazoa might be organized to avoid the deleterious effects of homozygotization in somatic cells while still deriving the benefits of species diversification and of DNA repair.

The Ig heavy chain switch region is a hotspot for insertion of transfected DNA

The Ig heavy chain class switch usually occurs by breaking and rejoining DNA in the switch (S) regions, which consist of tandemly repeated sequences 5' of the constant region exons. Various studies have suggested that S DNA can also recombine with non-S sequences. To measure the frequency of such recombination events, the hybridoma cell line igm692, a deletion mutant that lacks the C mu 1 and C mu 2 exons and the 3' end of the S mu region, was transfected with a fragment bearing the C mu 1-2 exons, but no S mu DNA. Insertion of this fragment into the residual VDJ-C mu intron of igm692 can restore a functional mu gene, yielding a transformant that is detected as a plaque-forming cell (PFC). PFC comprise approximately 8 x 10(-7) of the surviving transfected cells. In 10 of 12 PFCs, the C mu 1-2 fragment inserted into the 2.5-kb residual S mu region, whereas insertion in two cases occurred in the 3.5-kb segment 5' of S mu. Using a PCR assay to measure the frequency of insertion of the transferred fragment elsewhere in the hybridoma genome, we found that approximately 9% of the surviving transfected cells had stably acquired the C mu 1-2 fragment. These results indicate that the S mu region is approximately 100-fold more recombinogenic than the average genomic site, and approximately 7-fold more recombinogenic than the non-S mu segment of the residual VDJ-C mu, i.e., the S mu region is a hotspot for insertion of transfected DNA.

The Ig heavy chain switch region is a hotspot for insertion of transfected DNA

The Ig heavy chain class switch usually occurs by breaking and rejoining DNA in the switch (S) regions, which consist of tandemly repeated sequences 5' of the constant region exons. Various studies have suggested that S DNA can also recombine with non-S sequences. To measure the frequency of such recombination events, the hybridoma cell line igm692, a deletion mutant that lacks the C mu 1 and C mu 2 exons and the 3' end of the S mu region, was transfected with a fragment bearing the C mu 1-2 exons, but no S mu DNA. Insertion of this fragment into the residual VDJ-C mu intron of igm692 can restore a functional mu gene, yielding a transformant that is detected as a plaque-forming cell (PFC). PFC comprise approximately 8 x 10(-7) of the surviving transfected cells. In 10 of 12 PFCs, the C mu 1-2 fragment inserted into the 2.5-kb residual S mu region, whereas insertion in two cases occurred in the 3.5-kb segment 5' of S mu. Using a PCR assay to measure the frequency of insertion of the transferred fragment elsewhere in the hybridoma genome, we found that approximately 9% of the surviving transfected cells had stably acquired the C mu 1-2 fragment. These results indicate that the S mu region is approximately 100-fold more recombinogenic than the average genomic site, and approximately 7-fold more recombinogenic than the non-S mu segment of the residual VDJ-C mu, i.e., the S mu region is a hotspot for insertion of transfected DNA.

Assembly of IgM. Role of disulfide bonding and noncovalent interactions

Polymeric IgM is usually envisaged as an array of mu 2L2 monomers in which the mu heavy chains are held together by disulfide bonds involving cysteines at positions 337, 414, and 575. We have studied the importance of inter-mu-chain disulfide bonds for formation of IgM polymers and monomers by analyzing the effects of eliminating one or more of these disulfide bondings. Ablation of all inter-chain bonds by either chemical reduction and alkylation or by mutagenesis resulted in the exclusive production of halfmers (muL) molecules. IgM composed of mu-chains bearing each of the other six possible combinations of cysteine to serine replacements was produced as different mixtures of polymers, monomers, and halfmers. Cysteine 575 was both necessary and sufficient for efficient assembly of IgM polymers and sufficient but not necessary for efficient assembly of monomers. Cysteine 337 was sufficient but not necessary for efficient assembly of monomers, and was neither sufficient nor necessary for formation of polymers. Cysteine 414 was neither necessary nor sufficient for efficient formation of either monomers or polymers. Data also suggest that noncovalent interactions between C mu 2 domains are stronger than the interactions between C mu 4/tail domains.

Assembly of IgM. Role of disulfide bonding and noncovalent interactions

Polymeric IgM is usually envisaged as an array of mu 2L2 monomers in which the mu heavy chains are held together by disulfide bonds involving cysteines at positions 337, 414, and 575. We have studied the importance of inter-mu-chain disulfide bonds for formation of IgM polymers and monomers by analyzing the effects of eliminating one or more of these disulfide bondings. Ablation of all inter-chain bonds by either chemical reduction and alkylation or by mutagenesis resulted in the exclusive production of halfmers (muL) molecules. IgM composed of mu-chains bearing each of the other six possible combinations of cysteine to serine replacements was produced as different mixtures of polymers, monomers, and halfmers. Cysteine 575 was both necessary and sufficient for efficient assembly of IgM polymers and sufficient but not necessary for efficient assembly of monomers. Cysteine 337 was sufficient but not necessary for efficient assembly of monomers, and was neither sufficient nor necessary for formation of polymers. Cysteine 414 was neither necessary nor sufficient for efficient formation of either monomers or polymers. Data also suggest that noncovalent interactions between C mu 2 domains are stronger than the interactions between C mu 4/tail domains.

C1q binding properties of monomer and polymer forms of mouse IgM mu-chain variants. Pro544Gly and Pro434Ala

The effect of replacing proline with alanine at position 434 in the C mu 3 domain (P434A) and with glycine at position 544 in the C mu 4 domain (P544G) of the mu-chain of mouse IgM has been studied. The P434A substitution results in the loss of measurable complement-mediated cytolytic activity (CML) and a decrease in the association rate constant at low ionic strength (mu = 0.06), that results in a diminished Ka for C1q binding to P434A IgM bound to haptenated cells (0.4 x 10(9) M-1). Binding of C1(qr2s2) could not be detected. In contrast, replacement of proline at 544 had no measurable effect on the cytolytic or C1q/C1 binding properties of the polymeric molecule, supporting the view that the C mu 3 domain is important in C1q binding and CML. The secreted monomeric subunit of P544G was not able to mediate CML. Also, whereas hapten-bound P544G polymer bound C1q with a functional affinity of 1.5 x 10(9) M-1 at low ionic strength (mu = 0.06), similar to that observed with wild-type polymer (1.7 x 10(9) M-1) and wild-type IgG monomer (4.7 x 10(9) M-1), no C1q binding was detected with the P544G IgM monomer. This could not be attributed to differences in glycosylation. Inasmuch as the P544G mutation per se had no effect on the C1q binding properties of the polymer, we conclude that unlike IgG, aggregation does not sufficiently enhance the avidity of IgM monomer to enable it to activate complement. Augmentation of the site must occur during polymerization or when the IgM binds to Ag.

C1q binding properties of monomer and polymer forms of mouse IgM mu-chain variants. Pro544Gly and Pro434Ala

The effect of replacing proline with alanine at position 434 in the C mu 3 domain (P434A) and with glycine at position 544 in the C mu 4 domain (P544G) of the mu-chain of mouse IgM has been studied. The P434A substitution results in the loss of measurable complement-mediated cytolytic activity (CML) and a decrease in the association rate constant at low ionic strength (mu = 0.06), that results in a diminished Ka for C1q binding to P434A IgM bound to haptenated cells (0.4 x 10(9) M-1). Binding of C1(qr2s2) could not be detected. In contrast, replacement of proline at 544 had no measurable effect on the cytolytic or C1q/C1 binding properties of the polymeric molecule, supporting the view that the C mu 3 domain is important in C1q binding and CML. The secreted monomeric subunit of P544G was not able to mediate CML. Also, whereas hapten-bound P544G polymer bound C1q with a functional affinity of 1.5 x 10(9) M-1 at low ionic strength (mu = 0.06), similar to that observed with wild-type polymer (1.7 x 10(9) M-1) and wild-type IgG monomer (4.7 x 10(9) M-1), no C1q binding was detected with the P544G IgM monomer. This could not be attributed to differences in glycosylation. Inasmuch as the P544G mutation per se had no effect on the C1q binding properties of the polymer, we conclude that unlike IgG, aggregation does not sufficiently enhance the avidity of IgM monomer to enable it to activate complement. Augmentation of the site must occur during polymerization or when the IgM binds to Ag.

On the linkage between RNA processing and RNA translatability

The immunoglobulin mu heavy chain gene of mouse hybridoma cells is expressed in two forms, microseconds and microns, differing in their use of 3' exons. As for many other mammalian genes, mutations in the mu gene which prematurely terminate translation often have the effect of reducing the amount of these mu RNAs. To test the generality of this relationship, we selected mutant hybridoma cell lines defective in IgM production and searched both for translation termination mutations which do not reduce the amount of mu RNA as well as for mutants which show the more commonly observed reduction in mu RNA. As observed previously, the amount of microseconds RNA is normal in mutants terminating in the C mu 4 exon; by contrast the amount of microns RNA is reduced in these mutants, indicating that the effect of the mutation is influenced by some feature near the 3' end of the RNA. Mutations terminating translation in other C region exons have a graded effect on RNA content, ranging from 10% the normal level for termination in the C mu 3 exon down to 1% for termination in the C mu 2 exon. By contrast, a mutant cell line terminating in the leader exon contained 25% the normal amount of mu RNA, suggesting that translation past some point might be required to fully engage the RNA degradation process.

An improved system of somatic cell molecular genetics for analyzing the requirements of Ig synthesis and function

A general method of relating molecular function and structure is to examine the biological and chemical effects of defined mutations. In many cases, particularly those concerned with the rate or efficiency of gene expression, it is important to assess mutations in the normal chromosomal context. There are two methods of obtaining such mutants: (i) site-directed mutagenesis of the chromosomal locus, using homologous recombination to target defined mutations to the gene of interest, and (ii) phenotypic selection of mutant organisms. For most mammalian genes the rarity of targeted recombinants and phenotypically evident mutants impede the use of either of these approaches. However, various genetic and biochemical features render the Ig heavy chain locus in B cell lines amenable to both gene targeting and phenotypic selection of mutants. We describe here a replacement-type vector in which the selectable marker is an enhancerless gpt gene which is particularly suitable for targeting the IgH locus. Deletion of the enhancer greatly decreased the frequency of gpt+ random transformants while still allowing properly targeted transformants to be gpt+, such that transformants with the predicted recombinant structure comprised 25% of the gpt+ population. Thus, the labor involved in mutagenizing the chromosomal locus using this method is comparable to the usual method of isolating randomly inserted transformants, but offers the important advantages that the copy number and integration site are the same in independent transformants. In the hybridoma cell lines which we have tested, the consistent copy number and integration site are sufficient to yield a uniform level of recombinant gene expression.(ABSTRACT TRUNCATED AT 250 WORDS)

Mapping of amino acid residues in the C mu 3 domain of mouse IgM important in macromolecular assembly and complement-dependent cytolysis

By analyzing the effects of single site mutations of a TNP-binding mouse IgM we have identified amino acid residues clustered in two regions in the C mu 3 domain that are important in the complement-dependent cytolytic activity of polymeric IgM. Some of the mutations also impaired IgM polymerization. For one of these clusters, D432G, P434A, and P436S, which lies on the fy2 and fy3 strands and their connecting loop, polymerization was little affected and the effect on the cytolytic activity of the polymer fraction was taken to imply direct involvement of the residue in C1 binding. The other cluster, involving residues D356A K361A and D417G, is situated at the other end of the C mu 3 domain closer to the center of the Fc mu disc. The D356A K361A and D417G mutations significantly impaired polymer formation, suggesting that these residues are necessary for proper folding or packing of the C mu 3 domains and may affect cytolysis only indirectly. Some other mutations had little or no effect on polymerization or cytolytic activity (E423A, E527G), whereas some mutations impaired only IgM polymerization without affecting cytolytic activity (D344A, K361A, K443A P544G). In others the defect in polymerization was so profound that only the monomer formed (H430A/N/Q and K438G). Our results also suggest that the C1 binding site of IgM is not strictly homologous to the C1 binding site of IgG. Although mutation of E318 of IgG has been shown to reduce its cytolytic activity, mutation of the homologous residue in IgM, E423, was without effect as were mutations of other flanking-charged residues. Proline at 436 in IgM and 331 in IgG may, however, be a common element.

Mapping of amino acid residues in the C mu 3 domain of mouse IgM important in macromolecular assembly and complement-dependent cytolysis

By analyzing the effects of single site mutations of a TNP-binding mouse IgM we have identified amino acid residues clustered in two regions in the C mu 3 domain that are important in the complement-dependent cytolytic activity of polymeric IgM. Some of the mutations also impaired IgM polymerization. For one of these clusters, D432G, P434A, and P436S, which lies on the fy2 and fy3 strands and their connecting loop, polymerization was little affected and the effect on the cytolytic activity of the polymer fraction was taken to imply direct involvement of the residue in C1 binding. The other cluster, involving residues D356A K361A and D417G, is situated at the other end of the C mu 3 domain closer to the center of the Fc mu disc. The D356A K361A and D417G mutations significantly impaired polymer formation, suggesting that these residues are necessary for proper folding or packing of the C mu 3 domains and may affect cytolysis only indirectly. Some other mutations had little or no effect on polymerization or cytolytic activity (E423A, E527G), whereas some mutations impaired only IgM polymerization without affecting cytolytic activity (D344A, K361A, K443A P544G). In others the defect in polymerization was so profound that only the monomer formed (H430A/N/Q and K438G). Our results also suggest that the C1 binding site of IgM is not strictly homologous to the C1 binding site of IgG. Although mutation of E318 of IgG has been shown to reduce its cytolytic activity, mutation of the homologous residue in IgM, E423, was without effect as were mutations of other flanking-charged residues. Proline at 436 in IgM and 331 in IgG may, however, be a common element.

Production of mouse V/human C chimeric kappa genes by homologous recombination in hybridoma cells. Analysis of vector design and recombinant gene expression

Homologous recombination between transferred and chromosomal Ig genes in mouse hybridoma cells offers a general method of altering the chromosomal Ig genes in predetermined ways. Recombination is infrequent in hybridoma cells, and we have been interested in improving the methods for identifying and recovering the rare recombinants. We have used vectors that are designed to replace the mouse chromosomal C kappa segment with the human equivalent, so that recombinants produce mouse V/human C chimeric kappa-chains. We describe an enhancerless, replacement type vector that can be used with the herpes thymidine kinase counterselection to provide such enrichment that homologous recombinants constitute 15% of the selected G418-resistant, FIAU-resistant cells. We have also measured the level of chimeric kappa gene expression and found surprisingly that (1) it is very variable among transformants with the same recombinant gene structure, (2) there is no systematic difference in the level of production by recombinants that retain or have lost the J-C kappa intron enhancer, and (3) the amount of chimeric kappa mRNA in even the highest producing transformants is much less than the amount of the corresponding mouse kappa mRNA.

Production of mouse V/human C chimeric kappa genes by homologous recombination in hybridoma cells. Analysis of vector design and recombinant gene expression

Homologous recombination between transferred and chromosomal Ig genes in mouse hybridoma cells offers a general method of altering the chromosomal Ig genes in predetermined ways. Recombination is infrequent in hybridoma cells, and we have been interested in improving the methods for identifying and recovering the rare recombinants. We have used vectors that are designed to replace the mouse chromosomal C kappa segment with the human equivalent, so that recombinants produce mouse V/human C chimeric kappa-chains. We describe an enhancerless, replacement type vector that can be used with the herpes thymidine kinase counterselection to provide such enrichment that homologous recombinants constitute 15% of the selected G418-resistant, FIAU-resistant cells. We have also measured the level of chimeric kappa gene expression and found surprisingly that (1) it is very variable among transformants with the same recombinant gene structure, (2) there is no systematic difference in the level of production by recombinants that retain or have lost the J-C kappa intron enhancer, and (3) the amount of chimeric kappa mRNA in even the highest producing transformants is much less than the amount of the corresponding mouse kappa mRNA.

A hit-and-run system for introducing mutations into the Ig H chain locus of hybridoma cells by homologous recombination

The traditional method of site-specific mutagenesis is to introduce predetermined mutations into an expression vector, which is then transferred to cells so that the relevant gene product and the effects of the mutations can be measured. A problem with this approach is that the expression of the transferred genes varies from transformant to transformant, presumably because the number of vector copies and the site of chromosomal integration vary among transformants. While it should be possible to avoid this variability by mutagenizing the chromosomal gene itself, the labor involved in introducing predetermined mutations by homologous recombination with a mutagenized vector is usually so intense that this has not been the favored method. We describe here a system for introducing mutations into the IgH locus of hybridoma cells. This system greatly reduces the labor that would usually be required to identify and recover the rare recombinants. This is a two-step, so-called "hit-and-run," method, whereby mutations are first introduced into the chromosomal locus by targeted vector integration, after which the vector is excised so as to leave the mutation in the chromosomal target. The first step employs an enhancer trap vector bearing an enhancerless gpt gene; using this vector the frequency of randomly inserted transformants which grow in mycophenolic acid containing selective medium is so low that approximately 25% of the selected transformants have integrated the vector into the IgH locus by homologous recombination. Properly targeted transformants can then be used to derive secondary recombinants that have excised the vector and thus become gpt-. This second step which involves selection of gpt- cells by their resistance to 6-thioxanthine is also efficient, in that approximately 75% of the treated cells have excised the gpt gene by homologous recombination. Overall the labor involved in mutagenizing the chromosomal locus is not much more than is needed to produce the traditional transformants expressing a mutagenized transferred gene.

A hit-and-run system for introducing mutations into the Ig H chain locus of hybridoma cells by homologous recombination

The traditional method of site-specific mutagenesis is to introduce predetermined mutations into an expression vector, which is then transferred to cells so that the relevant gene product and the effects of the mutations can be measured. A problem with this approach is that the expression of the transferred genes varies from transformant to transformant, presumably because the number of vector copies and the site of chromosomal integration vary among transformants. While it should be possible to avoid this variability by mutagenizing the chromosomal gene itself, the labor involved in introducing predetermined mutations by homologous recombination with a mutagenized vector is usually so intense that this has not been the favored method. We describe here a system for introducing mutations into the IgH locus of hybridoma cells. This system greatly reduces the labor that would usually be required to identify and recover the rare recombinants. This is a two-step, so-called "hit-and-run," method, whereby mutations are first introduced into the chromosomal locus by targeted vector integration, after which the vector is excised so as to leave the mutation in the chromosomal target. The first step employs an enhancer trap vector bearing an enhancerless gpt gene; using this vector the frequency of randomly inserted transformants which grow in mycophenolic acid containing selective medium is so low that approximately 25% of the selected transformants have integrated the vector into the IgH locus by homologous recombination. Properly targeted transformants can then be used to derive secondary recombinants that have excised the vector and thus become gpt-. This second step which involves selection of gpt- cells by their resistance to 6-thioxanthine is also efficient, in that approximately 75% of the treated cells have excised the gpt gene by homologous recombination. Overall the labor involved in mutagenizing the chromosomal locus is not much more than is needed to produce the traditional transformants expressing a mutagenized transferred gene.

Isolation of new nonsense and frameshift mutants in the immunoglobulin mu heavy-chain gene of hybridoma cells

In order to expand the experimental material available for genetic and biochemical analyses of the natural immunoglobulin genes, we have isolated a variety of mutant mouse hybridoma cell lines. Some of these mutants have partial or complete deletions of the mu gene. Other mutants have nonsense or frameshift mutations in the exons encoding the variable and the second and third constant region domains of the mu heavy chain. When combined with earlier mutant data, this collection of genotypically and phenotypically tight mutants of known sequence spans most of the 10 kb of the mu gene, providing material for a variety of studies of genetic recombination and mRNA metabolism.

Effects of mutation position on frequency of marker rescue by homologous recombination

Homologous recombination between transferred and chromosomal DNA can be used for mapping mutations by marker rescue, i.e., by identifying which segment of wild-type DNA can recombine with the mutant chromosomal gene and restore normal function. In order to define how much the fragments should overlap each other for reliable mapping, we have measured how the frequency of marker rescue is affected by the position of the chromosomal mutation relative to the ends of the transferred DNA fragments. For this purpose, we used several DNA fragments to effect marker rescue in two mutant hybridomas which bear mutations 673 bp apart in the exons encoding the second and third constant region domains of the immunoglobulin mu heavy chain. The frequency of marker rescue decreased greatly when the mutation was located near one of the ends of the fragments, the results indicating that fragments should be designed to overlap by at least several hundred base pairs. Possible explanations for this "end effect" are considered.

Gene replacement with one-sided homologous recombination

Homologous recombination is now routinely used in mammalian cells to replace endogenous chromosomal sequences with transferred DNA. Vectors for this purpose are traditionally constructed so that the replacement segment is flanked on both sides by DNA sequences which are identical to sequences in the chromosomal target gene. To test the importance of bilateral regions of homology, we measured recombination between transferred and chromosomal immunoglobulin genes when the transferred segment was homologous to the chromosomal gene only on the 3' side. In each of the four recombinants analyzed, the 5' junction was unique, suggesting that it was formed by nonhomologous, i.e., random or illegitimate, recombination. In two of the recombinants, the 3' junction was apparently formed by homologous recombination, while in the other two recombinants, the 3' junction as well as the 5' junction might have involved a nonhomologous crossover. As reported previously, we found that the frequency of gene targeting increases monotonically with the length of the region of homology. Our results also indicate that targeting with fragments bearing one-sided homology can be as efficient as with fragments with bilateral homology, provided that the overall length of homology is comparable. The frequency of these events suggests that the immunoglobulin locus is particularly susceptible to nonhomologous recombination. Vectors designed for one-sided homologous recombination might be advantageous for some applications in genetic engineering.

Effects of vector cutting on its recombination with the chromosomal immunoglobulin gene in hybridoma cells

We have analyzed the effects of linearizing vector DNA on the frequency and pathway of its recombination with the homologous chromosomal gene. The pSV2neo vector bearing a 4.3-kb fragment encoding the mouse immunoglobulin mu heavy chain constant (C mu) region was cut either at sites within the C mu segment or outside C mu and then transferred to hybridoma cells bearing a mutant mu gene. The frequency of recombinant cells producing normal mu was then measured. For most cut sites, whether in regions of homology or of nonhomology, linearization of the transferred DNA enhanced the recombination frequency between the vector and chromosomal mu genes. When the vector was either uncut or cut at SacI in the region of homology, G418-resistant mu m+ recombinants were found to have integrated the vector by a single reciprocal homologous crossover; the enzyme site (SacI) used for cutting was present in the recombinants. By contrast, when the vector had been linearized at PvuI or SfiI in the region of nonhomology, vector integration involved nonhomologous crossovers, either between transferred DNA molecules or between transferred and chromosomal DNA, and the vector cut sites were absent in these recombinants. Some recombinants were found to have an unaltered as well as recombinant mu gene, suggesting that the nonhomologous recombination process might have involved sister chromatids.

Substitution of asparagine for serine-406 of the immunoglobulin mu heavy chain alters glycosylation at asparagine-402

Previous work suggested that the substitution of Asn for Ser at position 406 of the mu heavy chain of mouse IgM results in aberrant glycosylation at Asn402. In order to characterise the apparently abnormal glycosylation process more precisely, the mutant and wildtype mu chains were fragmented by cleavage with cyanogen bromide, and the resulting glycopeptides were analysed further. Measurements of lectin binding specificity as well as glycosidase sensitivity suggest that the oligosaccharide at Asn402 of wildtype mu is a hybrid type which does not contain terminal alpha(2-6) or alpha(2-3) linked sialic acid. By contrast, the corresponding oligosaccharide on Asn402 of mutant mu is complex and contains terminal sialic acid linked alpha(2-6) to galactose. The structural features for specifying the abnormal glycosylation are present in monomeric mutant IgM.

A V region mutation in a phosphocholine-binding monoclonal antibody results in loss of antigen binding

A V region mutant producing an antibody that had lost the ability to bind phosphocholine was isolated from a hybridoma producing a germline encoded T15 antibody. The mutation resulted in a single aspartic acid to asparagine substitution at residue 95 of the H chain V region. This confirms that the aspartic acid at residue 95 plays a major role in Ag binding. The results also suggest that somatic cell genetic techniques can be used to generate mAb with useful changes in Ag binding.

A V region mutation in a phosphocholine-binding monoclonal antibody results in loss of antigen binding

A V region mutant producing an antibody that had lost the ability to bind phosphocholine was isolated from a hybridoma producing a germline encoded T15 antibody. The mutation resulted in a single aspartic acid to asparagine substitution at residue 95 of the H chain V region. This confirms that the aspartic acid at residue 95 plays a major role in Ag binding. The results also suggest that somatic cell genetic techniques can be used to generate mAb with useful changes in Ag binding.

Homologous recombination in hybridoma cells: dependence on time and fragment length

Mutant hybridoma-myeloma cell lines that are defective in immunoglobulin production are expected to be useful for defining the molecular requirements of immunoglobulin gene expression. The analysis of such mutants would be greatly facilitated if they could be mapped by marker rescue, i.e., by identifying the segments of wild-type DNA that can restore the normal phenotype by homologous recombination with the mutant chromosomal immunoglobulin gene. To assess the feasibility of this type of mapping, we have measured the efficiency with which fragments of wild-type DNA recombine with a mutant hybridoma immunoglobulin gene and restore normal immunoglobulin production. We found that most if not all recombinants were detectable 2 days after DNA transfer and that the frequency of gene restoration increased with increasing length of the transferred mu gene fragments, between 1.2 and 9.5 kilobases. These results indicate that the available technology should be adequate to map mutations in the mu gene to within approximately 1 kilobase.

C1 binding by mouse IgM. The effect of abnormal glycosylation at position 402 resulting from a serine to asparagine exchange at residue 406 of the mu-chain

We have previously shown that IgM-Asn406, a mutant IgM which has asparagine in place of the serine which is normally found at position 406, also has an abnormally glycosylated mu-chain and is defective in complement-dependent cytolysis. Here we show by analyzing cyanogen bromide fragments from normal and mutant mu-chains that the site of abnormal glycosylation is at the neighboring position, Asn402. The cytolytic defect was shown to be due to impaired C1 binding. At physiological ionic strength, the C1 binding defect was estimated to be 12-fold, which correlates well with the measured defect in cytolytic activity; also, the severity of the defect in C1 binding by the mutant protein decreases with decreasing ionic strength. Kinetic studies showed that the difference in affinities is due to a proportional difference in the association rate for C1q. By comparing IgM made in the presence and absence of deoxymannojirimycin, we show further that the defect in cytolytic activity derives mostly from the abnormal oligosaccharide.