DNA rearrangement causes a high rate of spontaneous mutation at the immunoglobulin heavy-chain locus of a mouse myeloma cell line

Cis-acting regulatory sequences promote high-frequency gene conversion between repeated sequences in mammalian cells

In mammalian cells, little is known about the nature of recombination-prone regions of the genome. Previously, we reported that the immunoglobulin heavy chain (IgH) mu locus behaved as a hotspot for mitotic, intrachromosomal gene conversion (GC) between repeated mu constant (Cmu) regions in mouse hybridoma cells. To investigate whether elements within the mu gene regulatory region were required for hotspot activity, gene targeting was used to delete a 9.1 kb segment encompassing the mu gene promoter (Pmu), enhancer (Emu) and switch region (Smu) from the locus. In these cell lines, GC between the Cmu repeats was significantly reduced, indicating that this 'recombination-enhancing sequence' (RES) is necessary for GC hotspot activity at the IgH locus. Importantly, the RES fragment stimulated GC when appended to the same Cmu repeats integrated at ectopic genomic sites. We also show that deletion of Emu and flanking matrix attachment regions (MARs) from the RES abolishes GC hotspot activity at the IgH locus. However, no stimulation of ectopic GC was observed with the Emu/MARs fragment alone. Finally, we provide evidence that no correlation exists between the level of transcription and GC promoted by the RES. We suggest a model whereby Emu/MARS enhances mitotic GC at the endogenous IgH mu locus by effecting chromatin modifications in adjacent DNA.

Ectopic recombination within homologous immunoglobulin mu gene constant regions in a mouse hybridoma cell line

We have transferred a pSV2neo vector containing the wild-type constant region of the immunoglobulin mu gene (C mu) into the mutant hybridoma igm482, which bears a 2-bp deletion in the third constant-region exon of its haploid chromosomal mu gene (C mu 3). Independent igm482 transformants contain the wild-type immunoglobulin C mu region stably integrated in ectopic chromosomal positions. We report here that the wild-type immunoglobulin C mu region can function as the donor sequence in a gene conversion event which corrects the 2-bp deletion in the mutant igm482 chromosomal C mu 3 exon. The homologous recombination event restores normal immunoglobulin M production in the mutant cell.

Ectopic recombination within homologous immunoglobulin mu gene constant regions in a mouse hybridoma cell line

We have transferred a pSV2neo vector containing the wild-type constant region of the immunoglobulin mu gene (C mu) into the mutant hybridoma igm482, which bears a 2-bp deletion in the third constant-region exon of its haploid chromosomal mu gene (C mu 3). Independent igm482 transformants contain the wild-type immunoglobulin C mu region stably integrated in ectopic chromosomal positions. We report here that the wild-type immunoglobulin C mu region can function as the donor sequence in a gene conversion event which corrects the 2-bp deletion in the mutant igm482 chromosomal C mu 3 exon. The homologous recombination event restores normal immunoglobulin M production in the mutant cell.

Spontaneous deletions in Ig heavy chain genes: flanking sequences influence splice site selection

The cell line G403.4.7, isolated as a spontaneous variant of the MPC-11 derived myeloma G403.4, produces a truncated gamma 2b HC protein, but no light chain (LC), and a single gamma 2b specific transcript of 2.4kb. This gamma 2b transcript consists of the VDJ and CH1 exons, the CH1 to Hinge (Hi) intervening sequence (IVS) and HI exon, part of the IVS between the two membrane exons M1 and M2, and most of the membrane 3' untranslated (UT) region. Even though the mature mRNA contains intronic sequences, it is abundant in the cytoplasm. Analysis of the gamma 2b genomic organization reveals that this unusual transcript results in part from two genomic deletions of 2.5kb and 588bp and in part from an altered splicing pattern. This altered splicing pattern is probably a consequence of the sequence alterations resulting from the genomic deletions. Analysis of these events provides some interesting insights into the mechanism of splice site selection and the evolution of introns and exons.

Genes activated in the presence of an immunoglobulin enhancer or promoter are negatively regulated by a T-lymphoma cell line

The tissue-specific expression of immunoglobulin genes can be partially explained by a requirement for activating factors found only in B lymphocytes and their derivatives. However, loss of immunoglobulin expression upon fusion of an immunoglobulin-producing myeloma cell with a T lymphoma cell (BW5147) or fibroblast (L cell) suggests that negatively acting factors also play a role in the tissue specificity of immunoglobulin genes. Expression of a cloned immunoglobulin heavy-chain gene introduced into myeloma cells was suppressed after fusion of the myeloma transformants with BW5147. The presence of either the immunoglobulin heavy-chain enhancer or promoter conferred suppression, under similar conditions, upon a heterologous gene that is normally expressed in both B and T lymphocytes. These immunoglobulin heavy-chain gene control regions, or gene modifications induced by them, are subject to negative control by T-lymphocyte-derived factors.

Genes activated in the presence of an immunoglobulin enhancer or promoter are negatively regulated by a T-lymphoma cell line

The tissue-specific expression of immunoglobulin genes can be partially explained by a requirement for activating factors found only in B lymphocytes and their derivatives. However, loss of immunoglobulin expression upon fusion of an immunoglobulin-producing myeloma cell with a T lymphoma cell (BW5147) or fibroblast (L cell) suggests that negatively acting factors also play a role in the tissue specificity of immunoglobulin genes. Expression of a cloned immunoglobulin heavy-chain gene introduced into myeloma cells was suppressed after fusion of the myeloma transformants with BW5147. The presence of either the immunoglobulin heavy-chain enhancer or promoter conferred suppression, under similar conditions, upon a heterologous gene that is normally expressed in both B and T lymphocytes. These immunoglobulin heavy-chain gene control regions, or gene modifications induced by them, are subject to negative control by T-lymphocyte-derived factors.

Site of unequal sister chromatid exchange contains a potential Z-DNA-forming tract

Previous studies from this laboratory have provided evidence that the duplication of the IgG2a heavy chain constant region gene (C gamma 2a) in the murine myeloma cell line MPC-11 occurred via unequal sister chromatid exchange. We now report the determination of the DNA sequences of the germ-line regions in which this exchange has occurred. The two donor sequences have long regions of virtual identity. Furthermore, both chromatids contain stretches of T-C and T-G dinucleotides; the latter has the potential to form Z-DNA. Localization of the actual breakpoints via genomic Southern blot analysis suggests a novel mechanism for the recombination and strongly implicates the simple sequence in the process.