Intermolecular recombination between DNAs introduced into mouse L cells is mediated by a nonconservative pathway that leads to crossover products.

We describe experiments designed to measure the efficiency of intermolecular recombination between mutant herpesvirus thymidine kinase (tk) genes introduced into mouse L cells. Recombinants were scored as stable transformants containing a functional tk gene. The two recombination substrates used were ptkB8, a pBR322-based plasmid containing a mutant tk gene, with a BamHI linker in an SphI restriction site that is centrally located within the gene, and mp10tk delta 3' delta 5', an mp10 vector with a tk gene deleted at both the 3' and 5' ends. The only homology shared by the two DNAs is 885 base pairs within the tk gene. To determine whether the double-strand break repair model that has been used to explain recombination in yeast cells (J. W. Szostak, T. L. Orr-Weaver, R. J. Rothstein, and F. W. Stahl, Cell 33:25-35, 1983) can account for recombination during the introduction of these DNAs into mammalian cells, we transformed cells with BamHI-linearized ptkB8 and supercoiled mp10tk delta 3' delta 5' replicative-form DNA. These two DNAs should recombine efficiently according to that model and should generate gene conversion products. In this reaction, the supercoiled DNA acts as the donor of information to repair the cleaved tk gene. Our results indicated that the efficiency of this reaction was very low (less than 10 transformants were obtained per 0.1 microgram of each DNA used in the reaction per 10(6) cells). In contrast, if BamHI-cleaved ptkB8 DNA was cotransformed into cells along with a circular DNA molecule containing a tk gene deleted only at its 3' end or only at its 5' end (mp10tk delta 3' or mp10tk delta 5'), then the efficiency of recombination could be more than 4 orders of magnitude higher than it was with circular mp10tk delta 3' delta 5' DNA. Recombination frequencies were highest when the tk delta 3' or tk delta 5' DNA used was cleaved at the tk deletion junction. Southern analyses of DNA from TK+ transformants generated with BamHI-cleaved ptkB8 and BamHI-cleaved mp10tk delta 3' DNAs indicated that recombination was almost always associated with the reassortment of markers flanking the reconstructed tk DNA. Together, these results are more consistent with the nonconservative single-strand annealing model for recombination that we proposed several years ago (F.-L. Lin, K. Sperle, and N. Sternberg, Mol. Cell. Biol. 4:1020-1034, 1984) than they are with the double-strand break repair model.

Repair of double-stranded DNA breaks by homologous DNA fragments during transfer of DNA into mouse L cells.

To test the validity of various models for recombination between extrachromosomal DNAs in mammalian cells, we measured recombination between a plasmid containing a herpesvirus thymidine kinase (tk) gene with an internal BamHI linker insertion mutation (ptkB8) and a tk gene deleted at both ends (tk delta 3' delta 5'). The two DNAs shared 885 base pairs of perfect tk homology except for the interruption at the linker insertion site. Recombination events that restored the mutated insertion site to wild type were monitored by the generation of hypoxanthine-aminopterine-thymidine-resistant colonies after cotransformation of Ltk- cells with the two DNAs. We found that cleavage of the ptkB8 DNA at the linker insertion site was essential for gene restoration. If the tk delta 3' delta 5' DNA was ligated into mp10 vector DNA, then recombination with the cleaved ptkB8 DNA was inefficient. In contrast, if it was excised from that vector by cleavage at flanking restriction sites, then recombination was stimulated about 150-fold. Using restriction site polymorphisms, we showed that most of the recombination events leading to restoration of the tk gene with the excised tk delta 3' delta 5' fragment involved three double-strand duplexes: two ptkB8 DNAs and one tk delta 3' delta 5' fragment. These results are much more readily explained by the single-strand annealing model of recombination than by the double-strand break repair model, and they suggest that the deficiency of the latter pathway for extrachromosomal mammalian recombination may be due, at least in part, to the obligate tripartite nature of the reaction. Finally, we measured the effect of DNA homology on the efficiency of the ptkB8-tk delta 3' delta 5' reaction. Our results showed a near-linear relationship between the efficiency of recombination and the amount of homology flanking either side of the linker insertion site. Moreover, we could detect thymidine kinase-positive transformants with as little as 10 base pairs of homology.

Model for homologous recombination during transfer of DNA into mouse L cells: role for DNA ends in the recombination process.

We have constructed phage lambda and plasmid DNA substrates (lambda tk2 and ptk2) that contain two defective herpesvirus thymidine kinase (tk) genes that can be used to detect homologous recombination during the transfer of DNA into mouse L cells deficient in thymidine kinase activity. The recombination event reconstructs a wild-type tk gene and is scored because it converts Tk- cells to Tk+. Using this system, we have shown that (i) both intramolecular and intermolecular homologous recombination can be detected after gene transfer; (ii) the degree of recombination decreases with decreasing tk gene homology; and (iii) the efficiency of recombination can be stimulated 10- to 100-fold by cutting the tk2 DNA with restriction enzymes at appropriate sites relative to the recombining sequences. Based on the substrate requirements for these recombination events, we propose a model to explain how recombination might occur in mammalian cells. The essential features of the model are that the cut restriction site ends are substrates for cellular exonucleases that degrade DNA strands. This process exposes complementary strands of the two defective tk genes, which then pair. Removal of unpaired DNA at the junction between the paired and unpaired regions permits a gap repair process to reconstruct an intact gene.

Extrachromosomal recombination in mammalian cells as studied with single- and double-stranded DNA substrates.

We have previously proposed a model to account for the high levels of homologous recombination that can occur during the introduction of DNA into mammalian cells (F.-L. Lin, K. Sperle, and N. Sternberg, Mol. Cell. Biol. 4:1020-1034, 1984). An essential feature of that model is that linear molecules with ends appropriately located between homologous DNA segments are efficient substrates for an exonuclease that acts in a 5'----3' direction. That process generates complementary single strands that pair in homologous regions to produce an intermediate that is processed efficiently to a recombinant molecule. An alternative model, in which strand degradation occurs in the 3'----5' direction, is also possible. In this report, we describe experiments that tested several of the essential features of the model. We first confirmed and extended our previous results with double-stranded DNA substrates containing truncated herpesvirus thymidine kinase (tk) genes (tk delta 5' and tk delta 3'). The results illustrate the importance of the location of double-strand breaks in the successful reconstruction of the tk gene by recombination. We next transformed cells with pairs of single-stranded DNAs containing truncated tk genes which should anneal in cells to generate the recombination intermediates predicted by the two alternative models. One of the intermediates would be the favored substrate in our original 5'----3' degradative model and the other would be the favored substrate in the alternative 3'----5' degradative model. Our results indicate that the intermediate favored by the 3'----5' model is 10 to 20 times more efficient in generating recombinant tk genes than is the other intermediate.

Molecular identification and analysis of a novel human corticotropin-releasing factor (CRF) receptor: the CRF2gamma receptor.

We report the discovery of a new CRF2 receptor splice isoform found in human brain, which we have termed the CRF2gamma receptor. The CRF2gamma cDNA encodes for a 397-amino acid receptor that has an amino terminus with no significant homology to the already reported alpha- and beta-termini. When expressed in 293-EBNA (Epstein-Barr nuclear antigen) cells, the CRF2gamma receptor responds in a dose-dependent manner to CRF and related peptides with a rank order of potency of urocortin > or = sauvagine>urotensin>r/h CRF, with EC50 values more similar to CRF2alpha than CRF2beta. Equilibrium saturation isotherm analysis with radiolabeled sauvagine reveals a two site/state model for binding to CRF2gamma with a 60 pM Kd high-affinity site and a 5 nM Kd low-affinity site. Analysis of CRF2gamma RNA expression in human brain demonstrates expression in septum and hippocampus, with weaker but detectable expression in amygdala, nucleus accumbens, midbrain, and frontal cortex.

Recombination in mouse L cells between DNA introduced into cells and homologous chromosomal sequences.

In this paper, we show that DNA added to mouse L cells by the calcium phosphate method can be inserted into the genome of those cells by homologous recombination. The insertion event is detected because it reconstructs a functional thymidine kinase (tk) gene from two defective genes that share 320 base pairs of homology. One of the genes is missing its 5' portion (tk delta 5') and is in the cell's chromosome, and the other is missing its 3' portion (tk delta 3') and is in the introduced DNA. Gene reconstruction by homologous insertion is relatively inefficient; approximately one Tk+ transformant is produced per 10(6) cells per 4 micrograms of added tk DNA, a frequency of about 10(-5) that of normal tk gene transformation. The Tk+ transformants produced by homologous recombination contain Sma I and Pvu II fragments that are diagnostic of the intact tk gene, contain a herpesvirus-specific thymidine kinase activity, and can transfer the Tk+ phenotype to Tk- cells by DNA-mediated gene transfer. Two surprising observations made in the course of these studies were that only 1 of 10 Tk- cell lines containing defective tk genes could be transformed to Tk+ by homologous insertion of the complementary defective tk gene and that relatively little illegitimate insertion of introduced tk DNA into cellular DNA was detected in those cells that were transformed to Tk+ by homologous recombination.

Rapid generation of stable cell lines expressing corticotropin-releasing hormone receptor for drug discovery.

Human HEK293 cells that stably express the Epstein Barr nuclear antigen 1 (EBNA1) support the episomal replication of plasmids containing the Epstein Barr virus origin of replication (EBV oriP). A 293EBNA (293E) cell line expressing the human corticotropin-releasing hormone receptor subtype I (CRHR1) from an episomal plasmid was generated (293CR1s), analyzed, adapted to spinner culture, and scaled-up for production in less than 6 weeks. Forty-seven stable CHO cell lines transfected with CRHR1 were also isolated. Expression of the receptor in the best of these lines (as judged by CRH-induced cAMP production), CHO-R22, was compared to that in 293CR1s cells. Results indicate that the CRHR1 episomal expression vector in 293E cells (1) rapidly generates stable cell lines suitable for scale-up; (2) is stably maintained during 3 months in culture; (3) expresses high levels of CRHR1 mRNA; and (4) expresses significantly more CRHR1 than the CHO-R22 line. Coexpression of additional G protein alpha subunit (G alpha s) with CRHR1 in 293E cells converts a higher percentage of receptor to the agonist high-affinity G-protein-coupled state. Our data support the idea that using the EBV oriP-driven episomal system for gene expression results in greater production of protein in a relatively short period of time.