Homologous recombination between transferred and chromosomal immunoglobulin kappa genes

Channel catfish immunoglobulins: repertoire and expression

The channel catfish, Ictalurus punctatus, is widely recognized as an important model for studying immune responses in ectothermic vertebrates. It is one of the few fish species for which defined viable in vitro culture systems have been established and is currently the only fish species from which a variety of functionally distinct clonal leukocyte lines are available. Moreover, there is a large basis of biochemical and molecular information on the structure and function of catfish immunoglobulins (Igs). Catfish, as other teleosts, have a tetrameric homolog of IgM as their predominant serum Ig plus a homolog of IgD. They also have genetic elements basically similar to those of mammals, which encode and regulate their expression. The catfish Ig heavy (H) chain locus is a translocon-type locus with three Igdelta genes linked to an Igmu gene or pseudogene. The catfish IgH locus is estimated to contain approximately 200 variable (V) region genes representing 13 families as well as at least three diversity (D) and 11 joining (JH) genes. The catfish has two light (L) chain isotypes, F and G, both encoded by loci organized in multiple cassettes of VL-JL-CL with the VL in the opposite transcriptional orientation. Hence, all requisite components for encoding antibodies are present in the catfish, albeit with certain variations. In the future, whether or not additional unique features of Ig function and expression will be found remains to be determined.

Mechanisms involved in targeted gene replacement in mammalian cells

The "ends-out" or omega (Omega)-form gene replacement vector is used routinely to perform targeted genome modification in a variety of species and has the potential to be an effective vehicle for gene therapy. However, in mammalian cells, the frequency of this reaction is low and the mechanism unknown. Understanding molecular features associated with gene replacement is important and may lead to an increase in the efficiency of the process. In this study, we investigated gene replacement in mammalian cells using a powerful assay system that permits efficient recovery of the product(s) of individual recombination events at the haploid, chromosomal mu-delta locus in a murine hybridoma cell line. The results showed that (i) heteroduplex DNA (hDNA) is formed during mammalian gene replacement; (ii) mismatches in hDNA are usually efficiently repaired before DNA replication and cell division; (iii) the gene replacement reaction occurs with fidelity; (iv) the presence of multiple markers in one homologous flanking arm in the replacement vector did not affect the efficiency of gene replacement; and (v) in comparison to a genomic fragment bearing contiguous homology to the chromosomal target, gene targeting was only slightly inhibited by internal heterology (pSV2neo sequences) in the replacement vector.

Formation and repair of heteroduplex DNA on both sides of the double-strand break during mammalian gene targeting

In this study, we examined homologous recombination in mammalian cells using a gene targeting assay in which the introduction of a double-strand-break (DSB) in the vector-borne region of homology to the chromosome resulted in targeted vector integration. The vector-borne DSB was flanked with small palindromic insertions that, when encompassed within heteroduplex DNA (hDNA) formed during targeted vector integration, were capable of avoiding the activity of the mismatch repair (MMR) system. When used in conjunction with an isolation procedure in which the product(s) of each targeted vector integration event were retained for molecular analysis, information about recombination mechanisms was obtained. The examination of marker segregation patterns in independent recombinants revealed the following, (i) hDNA tracts could form simultaneously on each side of the DSB and in both participating homologous regions. Clonal analysis of sectored recombinants revealed that, in the homologous repeats generated by the recombination event, vector-borne palindrome and chromosomal markers were linked in the expected way in each strand of the hDNA intermediate, (ii) hDNA tracts were subject to MMR processing that occurred on opposite sides of the DSB, and (iii) in the majority of recombinants, the vector-borne marker was replaced with the corresponding marker from the chromosome. Bidirectional hDNA formation and MMR processing of both sides of the DSB are consistent with the double-strand-break repair (DSBR) model of recombination.

The molecular basis of multiple vector insertion by gene targeting in mammalian cells

Gene targeting using sequence insertion vectors generally results in integration of one copy of the targeting vector generating a tandem duplication of the cognate chromosomal region of homology. However, occasionally the target locus is found to contain >1 copy of the integrated vector. The mechanism by which the latter recombinants arise is not known. In the present study, we investigated the molecular basis by which multiple vectors become integrated at the chromosomal immunoglobulin mu locus in a murine hybridoma. To accomplish this, specially designed insertion vectors were constructed that included six diagnostic restriction enzyme markers in the Cmu region of homology to the target chromosomal mu locus. This enabled contributions by the vector-borne and chromosomal Cmu sequences at the recombinant locus to be ascertained. Targeted recombinants were isolated and analyzed to determine the number of vector copies integrated at the chromosomal immunoglobulin mu locus. Targeted recombinants identified as bearing >1 copy of the integrated vector resulted from a Cmu triplication formed by two vector copies in tandem. Examination of the fate of the Cmu region markers suggested that this class of recombinant was generated predominantly, if not exclusively, by two targeted vector integration events, each involving insertion of a single copy of the vector. Both vector insertion events into the chromosomal mu locus were consistent with the double-strand-break repair mechanism of homologous recombination. We interpret our results, taken together, to mean that a proportion of recipient cells is in a predetermined state that is amenable to targeted but not random vector integration.

High efficiency site-specific modification of the chromosomal immunoglobulin locus by gene targeting

Site-specific modification of the chromosomal immunoglobulin locus by gene targeting is a powerful tool in studying the molecular requirements for immunoglobulin gene structure and function and in the production of engineered antibodies. Here, we describe a two step- integration then excision-gene targeting procedure for introducing planned genetic alterations into the chromosomal immunoglobulin locus. The efficiency of gene targeting with an enhancer-trap vector in which an enhancerless neo and HSV-tk gene were inserted into the vector backbone was compared to that of the corresponding enhancer-positive vector. Both insertion vectors also contained homology to the chromosomal immunoglobulin target locus along with the desired genetic alteration. The first step involved insertion of the transferred vector into the target locus by homologous recombination. An approximately 15-fold enrichment in the frequency of vector insertion was obtained with the enhancer-trap compared to the enhancer-positive vector. The majority of targeted cells (75%) contained a single copy of the vector integrated into the chromosomal immunoglobulin locus. The second step involved excision of the integrated vector by intrachromosomal homologous recombination between the duplicated region of homology that removed the integrated vector, neo and tk genes along with one copy of homologous DNA. Vector excision was very efficient generating G418S, FIAU(R) secondary recombinants at the high rate of approximately 10(-3)/cell generation. In the secondary recombinants, the overall structure of the chromosomal immunoglobulin locus was restored with the desired genetic alteration being present in an expected proportion of the cells.

High-frequency loss of specific immunoglobulin production in hybridoma cell lines bearing a chromosomal immunoglobulin kappa gene modified by homologous recombination

We have examined the stability of trinitrophenyl (TNP) -specific IgM production in hybridoma cell lines in which homologous recombination was used to change the variable region of an endogenous chromosomal immunoglobulin kappa gene to one specific for TNP. Mutant hybridomas that have lost TNP-specific IgM production are detected with a frequency of approximately 1%. Characterization of the mutant cells reveals a variety of gross rearrangements in the recombinant kappa TNP gene as well as in the endogenous kappa and muTNP genes.

Replacement-like recombination induced by an integration vector with a murine homology flank at the immunoglobulin heavy-chain locus in mouse and rat hybridoma cells

Vectors for homologous recombination are commonly designed as replacement or integration constructs. We have evaluated integration vectors for the substitution of the immunoglobulin heavy-chain constant region by various human isotypes in mouse and rat hybridomas. It is known that under certain circumstances replacement vectors exhibit a lower target efficiency and can be incorporated by integration events. Conversely, we show here that an integration vector can undergo a replacement event despite having free homologous adjacent DNA ends, which would be expected to initiate integration according to the double-strand break repair model. Moreover, in cases of replacement recombination the 5' crossover is not necessarily located within the homology region, thereby giving rise to a truncated gene product. Whether or not the replacement leads to such deletions is clearly dependent on the isotypes involved in the targeting reaction. The fact that the vector is correctly targeted to the heavy-chain locus, but that the homology region is not always the site of recombination, points to a novel recombination mechanism that may be specific for the immunoglobulin loci and that seems to be predominant even in the presence of the free homologous adjacent ends of an integration vector. Furthermore we demonstrate that homologous recombination at the heavy-chain locus is also possible between sequences from different species. The implications of our findings for the production of chimeric antibodies are discussed.

Restoration of a normal level of immunoglobulin production in a hybridoma cell line following modification of the chromosomal immunoglobulin mu gene by gene replacement

In the present investigation, we have measured the levels of IgM mRNA and protein in recombinant cell lines in which the chromosomal immunoglobulin mu gene has been modified by gene replacement (gene conversion or double reciprocal recombination) or vector integration (single reciprocal recombination) events. Our studies reveal that chromosomal immunoglobulin mu genes modified by gene replacement are expressed at wild-type levels whereas those modified by vector integration have lower levels of immunoglobulin mu gene expression. These results suggest that gene replacement may a preferred method for the construction of hybridoma and myeloma cell lines producing optimized immunoglobulins and for studies of immunoglobulin gene function.

Analysis of mutations introduced into the chromosomal immunoglobulin mu gene

We have introduced a pSV2neo-derived vector that contains a 2-base-pair (bp) deletion in its immunoglobulin mu gene constant region into hybridoma cells bearing a single copy of the wild-type chromosomal immunoglobulin mu gene. Homologous recombination between the transferred mutant C mu region and the wild-type chromosomal C mu region is expected to introduce the 2-bp deletion into the chromosomal mu gene, generating recombinant cells synthesizing noncytolytic IgM. Analysis of the DNA in independent noncytolytic transformants indicates that in one case the mu gene has the structure expected for correct homologous recombination. Unexpectedly, the remaining transformants, bear chromosomal mu gene deletions.

Targeted homologous recombination in mammalian cells

This article presents a review of recent progress in the field of targeted homologous recombination in mammalian cells. Beginning with an introduction of basic terminology and why 'gene targeting' is potentially such a powerful genetic tool, the article explores some of the obstacles that must be overcome in order for targeting to be generally useful. In particular, the different ways in which investigators have been able to work around the great inefficiency of gene targeting is covered in some detail. When possible, insights into the mechanisms(s) of gene targeting are extracted from the published literature. The use of targeted gene 'knockout' in mouse embryonic stems cells to create animal disease models is discussed. The need for systematic studies into the mechanisms(s) of targeting to make gene targeting useful for human gene therapy is recognized, and some suggestions are made.

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.