Molecular modeling study of the multidrug resistance modifiers cis- and trans-flupentixol

Chromatin structural analyses of the mouse Igkappa gene locus reveal new hypersensitive sites specifying a transcriptional silencer and enhancer

To identify new regulatory elements within the mouse Igkappa locus, we have mapped DNase I hypersensitive sites (HSs) in the chromatin of B cell lines arrested at different stages of differentiation. We have focused on two regions encompassing 50 kilobases suspected to contain new regulatory elements based on our previous high level expression results with yeast artificial chromosome-based mouse Igkappa transgenes. This approach has revealed a cluster of HSs within the 18-kilobase intervening sequence, which we cloned and sequenced in its entirety, between the Vkappa gene closest to the Jkappa region. These HSs exhibit pro/pre-B cell-specific transcriptional silencing of a Vkappa gene promoter in transient transfection assays. We also identified a plasmacytoma cell-specific HS in the far downstream region of the locus, which in analogous transient transfection assays proved to be a powerful transcriptional enhancer. Deletional analyses reveal that for each element multiple DNA segments cooperate to achieve either silencing or enhancement. The enhancer sequence is conserved in the human Igkappa gene locus, including NF-kappaB and E-box sites that are important for the activity. In summary, our results pinpoint the locations of presumptive regulatory elements for future knockout studies to define their functional roles in the native locus.

High-level rearrangement and transcription of yeast artificial chromosome-based mouse Ig kappa transgenes containing distal regions of the contig

The mouse Ig kappa L chain gene locus has been extensively studied, but to date high-level expression of germline transgenes has not been achieved. Reasoning that each end of the locus may contain regulatory elements because these regions are not deleted upon V kappa-J kappa joining, we used yeast artificial chromosome-based techniques to fuse distal regions of the contig to create transgene miniloci. The largest minilocus (290 kb) possessed all members of the upstream V kappa 2 gene family including their entire 5' and 3' flanking sequences, along with one member of a downstream V kappa 21 gene family. In addition, again using yeast artificial chromosome-based technology, we created Ig kappa miniloci that contained differing lengths of sequences 5' of the most distal V kappa 2 gene family member. In transgenic mice, Ig kappa miniloci exhibited position-independent and copy number-dependent germline transcription. Ig kappa miniloci were rearranged in tissue and developmental stage-specific manners. The levels of rearrangement and transcription of the distal and proximal V kappa gene families were similar to their endogenous counterparts and appeared to be responsive to allelic exclusion, but were differentially sensitive to numerous position effects. The minilocus that contained the longest 5' region exhibited significantly greater recombination of the upstream V kappa 2 genes but not the downstream V kappa 21 gene, providing evidence for a local recombination stimulating element. These results provide evidence that our miniloci contain nearly all regulatory elements required for bona fide Ig kappa gene expression, making them useful substrates for functional analyses of cis-acting sequences in the future.

A targeted deletion of a region upstream from the Jkappa cluster impairs kappa chain rearrangement in cis in mice and in the 103/bcl2 cell line

We have shown previously that a mutation of the KI-KII site immediately 5' to J(kappa)1 on the mouse immunoglobulin light chain kappa locus reduces the rearrangement level in cis, although it does not affect transcription. Here we deleted by homologous recombination in mouse embryonic stem cells a 4-kb DNA fragment, located immediately upstream of the KI-KII element, which contains the promoter of the long germline transcript. Analysis of gene-targeted heterozygous mouse splenic B cells showed a strong decrease in rearrangement for the allele bearing the deletion. When both the KI-KII mutation and the 4-kb deletion were present on the same allele, the overall reduction in rearrangement was stronger than with the 4-kb deletion alone underlying the role of these two elements in the regulation of rearrangement. The same deletion was performed by homologous recombination on one allele of the rearrangement-inducible mouse 103/bcl2-hygro(R) pre-B cell line, and resulted in a similar reduction in the induction of rearrangement of the mutated allele. This result validates this cell line as an in vitro model for studying the incidence of gene-targeted modifications of the kappa locus on the regulation of rearrangement.

Negative regulation of Ig gene rearrangement by a 150-bp transcriptional silencer

We previously showed that the V-J intervening sequence of the chicken lambda immunoglobulin locus contains a strong silencer that acts both on transcription and rearrangement. We show here that the transcriptional silencer activity can be ascribed to a minimal 150-bp fragment. The rearrangement silencing activity was previously shown by the replacement of the V-J intervening sequence with a neutral DNA fragment that dramatically increased the rate of rearrangement of the transgene. Insertion of the minimal silencer in this neutral fragment is shown here to result in a marked decrease in rearrangement of the transgenic construct. Strikingly, deletion of 28 bp from the 150-bp fragment abolished most of the transcriptional silencing activity and had a similar effect on rearrangement. These results conclusively correlate the silencing activity on both rearrangement and transcription.

Reevaluation of 3'Ekappa function in stage- and lineage-specific rearrangement and somatic hypermutation

Transgenic studies have led to the conclusion that the 3'Ekappa enhancer functions to suppress kappa variable region gene assembly in T lineage cells and in progenitor B cells and have also implicated 3'Ekappa as a critical element in promoting somatic hypermutation of kappa variable region genes. To assess the role of the endogenous 3'Ekappa, we assayed these processes in mice homozygous for mutations in which the 3'Ekappa sequences were deleted by the loxP/Cre method (3'Ekappa delta/delta mice). In contrast to transgenic findings, we found that deletion of the endogenous 3'Ekappa did not deregulate kappa gene rearrangement in T lineage cells or in pro-B cells. Furthermore, immunization of the 3'Ekappa delta/delta mice led to the generation of specific antibodies with mutation patterns typical of affinity maturation, showing that there is no absolute requirement for the 3'Ekappa with respect to somatic mutation of endogenous kappa genes.