cis regulation of the keratin 18 gene in transgenic mice

Differential sensitivity of mouse epithelial tissues to the polyomavirus middle T oncogene

To determine how different epithelial cell types respond to the same oncogenic stimulation, we have used a modified human keratin 18 gene to conditionally express the polyomavirus middle T antigen (PyMT) oncogene in simple epithelial tissues of transgenic mice. Activation of PyMT expression by transgenic Cre recombinase in mammary epithelial cells resulted in carcinomas in all bitransgenic females. PyMT expression induced by K18-driven Cre in internal epithelial organs resulted in pancreatic acinar metaplasia and ductal dysplasia with remarkable desmoplastic stromal responses in all 25 bitransgenic mice. Hepatoma formation with altered lipid metabolism and gastric adenocarcinoma occurred in 96 and 54% of these mice, respectively. Elevated PyMT RNA expression also correlated with intraepithelial neoplasia in the prostate. Activated Erk2 was found in mammary tumors, pancreatic tissues, and affected livers. Hes1 RNA, a target of Notch signaling that has been implicated downstream of Ras pathway activation, was elevated in pancreatic and liver lesions. The variety of responses of different epithelia to PyMT demonstrates the importance of the differentiated state in interpreting oncogenic signals.

Exploiting the keratin 17 gene promoter to visualize live cells in epithelial appendages of mice

Keratin genes afford, given their large number (>50) and differential regulation, a unique opportunity to study the mechanisms underlying specification and differentiation in epithelia of higher metazoans. Moreover, the small size and regulation in cis of many keratin genes enable the use of their regulatory sequence to achieve targeted gene expression in mice. Here we show that 2 kilobases of 5' upstream region from the mouse keratin 17 gene (mK17) confers expression of green fluorescent protein (GFP) in major epithelial appendages of transgenic mice. Like that of mK17, onset of [mK17 5']-GFP reporter expression coincides with the appearance of ectoderm-derived epithelial appendages during embryonic development. In adult mice, [mK17 5']-GFP is appropriately regulated within hair, nail, glands, and oral papilla. Tracking of GFP fluorescence allows for the visualization of growth cycle-related changes in hair follicles, and the defects engendered by the hairless mutation, in live skin tissue. Deletion of an internal 48-bp interval, which encompasses a Gli-responsive element, from this promoter results in loss of GFP fluorescence in most appendages in vivo, suggesting that sonic hedgehog participates in K17 regulation. The compact mK17 gene promoter provides a novel tool for appendage-preferred gene expression and manipulation in transgenic mice.

Transcriptional deregulation of the keratin 18 gene in human colon carcinoma cells results from an altered acetylation mechanism

We are investigating the mechanism responsible for the overexpression of the keratin 18 (K18) gene in tumorigenic clones from the SW613-S human colon carcinoma cell line, as compared with non-tumorigenic clones. We have previously shown that this mechanism affects the minimal K18 promoter (TATA box and initiation site). We report here that treatment of the cells with histone deacetylase inhibitors stimulates the activity of the promoter in non-tumorigenic cells but has no effect in tumorigenic cells, resulting in a comparable activity of the promoter in both cell types. The adenovirus E1A protein inhibits the activity of the K18 promoter specifically in tumorigenic cells. This inhibition can be reversed by an excess of CBP protein. The conserved region 1 (CR1) of E1A, which is involved in the interaction with the CBP/p300 co-activators, is necessary to the inhibitory capacity of E1A. A 79 amino acid long N-terminal fragment of E1A, encompassing the two domains of E1A necessary and sufficient for binding to CBP (N-terminus and CR1), has the same differential inhibitory capacity on the K18 promoter as wild-type E1A. Forced recruitment of GAL4-CBP fusion proteins to the K18 promoter results in a greater stimulation of its activity in non-tumorigenic than in tumorigenic cells. The histone acetyltransferase activity of CBP is essential for this differential stimulation and the presence of the CBP2 domain greatly augments the activation capacity of the fusion protein. Chromatin immunoprecipitation experiments carried out with anti-acetylated histone antibodies showed no difference in the level of histone acetylation in the region of the K18 promoter between the two cell types. The structure of chromatin in the promoter region is similar in tumorigenic and non-tumorigenic cells, as determined by mapping of DNase I hypersensitive sites and probing the accessibility of the DNA to restriction endonucleases. From all these results we conclude that alteration of an acetylation mechanism involving the CBP (or p300) protein and acting on a non-histone substrate is responsible for the higher activity of the K18 promoter in tumorigenic cells of the SW613-S cell line.

Defective trophoblast function in mice with a targeted mutation of Ets2

Members of the Ets family of transcription factors mediate transcriptional responses of multiple signaling pathways in diverse cell types and organisms. Targeted deletion of the conserved DNA binding domain of the Ets2 transcription factor results in the retardation and death of homozygous mouse embryos before 8.5 days of embryonic development. Defects in extraembryonic tissue gene expression and function include deficient expression of matrix metalloproteinase-9 (MMP-9, gelatinase B), persistent extracellular matrix, and failure of ectoplacental cone proliferation. Mutant embryos were rescued by aggregation with tetraploid mouse embryos, which complement the developmental defects by providing functional extraembryonic tissues. Rescued Ets2-deficient mice are viable and fertile but have wavy hair, curly whiskers, and abnormal hair follicle shape and arrangement, resembling mice with mutations of the EGF receptor or its ligands. However, these mice are not deficient in the production of TGFalpha or the EGF receptor. Homozygous mutant cell lines respond mitogenically to TGFalpha, EGF, FGF1, and FGF2. However, FGF fails to induce MMP-13 (collagenase-3) and MMP-3 (stromelysin-1) in the Ets2-deficient fibroblasts. Ectopic expression of Ets2 in the deficient fibroblasts restores expression of both matrix metalloproteinases. Therefore, Ets2 is essential for placental function, mediating growth factor signaling to key target genes including MMP-3, MMP-9, and MMP-13 in different cell types, and for regulating hair development.

Methylation of an ETS site in the intron enhancer of the keratin 18 gene participates in tissue-specific repression

The activities of ETS transcription factors are modulated by posttranscriptional modifications and cooperation with other proteins. Another factor which could alter the regulation of genes by ETS transcription factors is DNA methylation of their cognate binding sites. The optimal activity of the keratin 18 (K18) gene is dependent upon an ETS binding site within an enhancer region located in the first intron. The methylation of the ETS binding site was correlated with the repression of the K18 gene in normal human tissues and in K18 transgenic mouse tissues. Neither recombinant ETS2 nor endogenous spleen ETS binding activities bound the methylated site effectively. Increased expression of the K18 gene in spleens of transgenic mice by use of an alternative, cryptic promoter 700 bp upstream of the enhancer resulted in modestly decreased methylation of the K18 ETS site and increased RNA expression. Expression in transgenic mice of a mutant K18 gene, which was still capable of activation by ETS factors but was no longer a substrate for DNA methylation of the ETS site, was fivefold higher in spleen and heart. However, expression in other organs such as liver and intestine was similar to that of the wild-type gene. This result suggests that DNA methylation of the K18 ETS site may be functionally important in the tissue-specific repression of the K18 gene. Epigenetic modification of the binding sites for some ETS transcription factors may result in a refractory transcriptional response even in the presence of necessary trans-acting activities.

Oncogenic regulation and function of keratins 8 and 18

Keratin 8 (K8) and keratin 18 (K18) are the most common and characteristic members of the large intermediate filament gene family expressed in 'simple' or single layer epithelial tissues of the body. Their persistent expression in tumor cells derived from these epithelia has led to the wide spread use of keratin monoclonal antibodies as aids in the detection and identification of carcinomas. Oncogenes which activate ras signal transduction pathways stimulate expression of the K18 gene through transcription factors including members of the AP-1 (jun and fos) and ETS families. The persistent expression of K8 and K18 may reflect the integrated transcriptional activation of such transcription factors and, in the cases of ectopic expression, an escape from the suppressive epigenetic mechanisms of DNA methylation and chromatin condensation. Comparison of the mechanisms of transcriptional control of K18 expression with expression patterns documented in both normal and pathological conditions leads to the proposal that persistent K8 and K18 expression is a reflection of the action of multiple different oncogenes converging on the nucleus through a limited number of transcription factors to then influence the expression of a large number of genes including these keratins. Furthermore, correlation of various tumor cell characteristics including invasive behavior and drug sensitivity with K8 and K18 expression has stimulated consideration of the possible functions of these proteins in both normal development and in tumorigenesis. Recent developments in the analysis of the functions of these intermediate filament proteins provide new insights into diverse functions influenced by K8 and K18.

Contrasting effects of the SATB1 core nuclear matrix attachment region and flanking sequences of the keratin 18 gene in transgenic mice

The 2.3 kb and 3.5 kb of DNA that flank the human keratin 18 (K18) gene and synthetic nuclear matrix attachment regions (MAR) composed of the binding sites for the SATB1 nuclear protein were fused to a reporter gene that utilizes the mouse metallothionein promoter and the human growth hormone gene (MThGH). Transgenic mice were generated from both constructions and the control MThGH gene to test K18 and SATB1 MAR sequences for the ability to insulate the reporter gene from integration site-specific position effects. The MThGH control gene was variably expressed in brain, heart, intestine, kidney, liver, and testes, confirming previous studies. In contrast, the MThGH gene insulated by the K18 flanking sequences was expressed in the same tissues of four independent transgenic animals at levels correlated with the copy number except for intestine. The average level of expression on a per gene basis of the K18 insulated gene was from 9- to 49-fold higher than the control. The MThGH gene linked to the SATB1 MAR sequences was completely repressed in the brains and kidneys of all six transgenic mice. However, expression was nearly as efficient in testes as the K18-insulated gene. Both the SATB1 MAR and the K18 flanking sequences confer position-independent transcriptional status on the reporter gene in some or many tissues. However, the effects are stimulatory for the K18 elements and generally suppressive for the SATB1 MAR elements.

The consensus sequence of a major Alu subfamily contains a functional retinoic acid response element

Alu repeats are interspersed repetitive DNA elements specific to primates that are present in 500,000 to 1 million copies. We show here that an Alu sequence encodes functional binding sites for retinoic acid receptors, which are members of the nuclear receptor family of transcription factors. The consensus sequences for the evolutionarily recent Alu subclasses contain three hexamer half sites, related to the consensus AGGTCA, arranged as direct repeats with a spacing of 2 bp, which is consistent with the binding specificities of retinoic acid receptors. An analysis was made of the DNA binding and transactivation potential of these sites from an Alu sequence that has been previously implicated in the regulation of the keratin K18 gene. These Alu double half sites are shown to bind bacterially synthesized retinoic acid receptors as assayed by electrophoretic mobility shift assays. These sites are further shown to function as a retinoic acid response element in transiently transfected CV-1 cells, increasing transcription of a reporter gene by a factor of approximately 35-fold. This transactivation requires cotransfection with vectors expressing retinoic acid receptors, as well as the presence of all-trans-retinoic acid, which is consistent with the known function of retinoic acid receptors as ligand-inducible transcription factors. The random insertion of potentially thousands of Alu repeats containing retinoic acid response elements throughout the primate genome is likely to have altered the expression of numerous genes, thereby contributing to evolutionary potential.

An Sp1 binding site and the minimal promoter contribute to overexpression of the cytokeratin 18 gene in tumorigenic clones relative to that in nontumorigenic clones of a human carcinoma cell line

Clones of cells tumorigenic or nontumorigenic in nude mice have been previously isolated from the SW613-S human colon carcinoma cell line. We have already reported that tumorigenic cells overexpress the cytokeratin 18 (K18) gene in comparison with nontumorigenic cells and that this difference is mainly due to a transcriptional regulation. We now report that a 2,532-bp cloned human K18 gene promoter drives the differential expression of a reporter gene in a transient assay. A 62-bp minimal K18 promoter (TATA box and initiation site) has a low but differential activity. Analysis of deletion and substitution mutants as well as hybrid SV40-K18 promoters and reconstructed K18 promoters indicated that an important element for the activity of the K18 promoter is a high-affinity binding site for transcription factor Sp1 located just upstream of the TATA box. This Sp1 binding element, as well as the intron 1 enhancer element, stimulates the basal activity of the minimal promoter through mechanisms that maintain the differential activity. Gel shift assays and the use of an anti-Sp1 antibody have shown that both tumorigenic and nontumorigenic SW613-S cells contain three factors able to bind to the Sp1 binding element site and that one of them is Sp1. A hybrid GAL4-Sp1 protein transactivated to comparable extents in tumorigenic and nontumorigenic cells a reconstructed K18 promoter containing GAL4 binding sites and therefore without altering its differential behavior. These results indicate that the Sp1 transcription factor is involved in the overexpression of the K18 gene in tumorigenic SW613-S cells through its interaction with a component of the basal transcription machinery.

AP-1, ETS, and transcriptional silencers regulate retinoic acid-dependent induction of keratin 18 in embryonic cells

The differentiation of both embryonal carcinoma (EC) and embryonic stem (ES) cells can be triggered in culture by exposure to retinoic acid and results in the transcriptional induction of both the endogenous mouse keratin 18 (mK18) intermediate filament gene and an experimentally introduced human keratin 18 (K18) gene as well as a variety of other markers characteristic of extraembryonic endoderm. The induction of K18 in EC cells is limited, in part, by low levels of ETS and AP-1 transcription factor activities which bind to sites within a complex enhancer element located within the first intron of K18. RNA levels of ETS-2, c-Jun, and JunB increase upon the differentiation of ES cells and correlate with increased expression of K18. Occupancy of the ETS site, detected by in vivo footprinting methods, correlates with K18 induction in ES cells. In somatic cells, the ETS and AP-1 elements mediate induction by a variety of oncogenes associated with the ras signal transduction pathway. In EC cells, in addition to the induction by these limiting transcription factors, relief from negative regulation is mediated by three silencer elements located within the first intron of the K18 gene. These silencer elements function in F9 EC cells but not their differentiated derivatives, and their activity is correlated with proteins in F9 EC nuclei which bind to the silencers and are reduced in the nuclei of differentiated F9 cells. The induction of K18, associated with the differentiation of EC cells to extraembryonic endoderm, is due to a combination of relief from negative regulation and activation by members of the ETS and AP-1 transcription factor families.

AP-1, ETS, and transcriptional silencers regulate retinoic acid-dependent induction of keratin 18 in embryonic cells

The differentiation of both embryonal carcinoma (EC) and embryonic stem (ES) cells can be triggered in culture by exposure to retinoic acid and results in the transcriptional induction of both the endogenous mouse keratin 18 (mK18) intermediate filament gene and an experimentally introduced human keratin 18 (K18) gene as well as a variety of other markers characteristic of extraembryonic endoderm. The induction of K18 in EC cells is limited, in part, by low levels of ETS and AP-1 transcription factor activities which bind to sites within a complex enhancer element located within the first intron of K18. RNA levels of ETS-2, c-Jun, and JunB increase upon the differentiation of ES cells and correlate with increased expression of K18. Occupancy of the ETS site, detected by in vivo footprinting methods, correlates with K18 induction in ES cells. In somatic cells, the ETS and AP-1 elements mediate induction by a variety of oncogenes associated with the ras signal transduction pathway. In EC cells, in addition to the induction by these limiting transcription factors, relief from negative regulation is mediated by three silencer elements located within the first intron of the K18 gene. These silencer elements function in F9 EC cells but not their differentiated derivatives, and their activity is correlated with proteins in F9 EC nuclei which bind to the silencers and are reduced in the nuclei of differentiated F9 cells. The induction of K18, associated with the differentiation of EC cells to extraembryonic endoderm, is due to a combination of relief from negative regulation and activation by members of the ETS and AP-1 transcription factor families.

Specific combinations of human serum albumin introns direct high level expression of albumin in transfected COS cells and in the milk of transgenic mice

A new series of expression vectors, each comprised of the beta-lactoglobulin (BLG) promoter driving one of a variety of human serum albumin (HSA) minigenes or the entire gene, were evaluated for their ability to direct expression of HSA in vitro in COS tissue culture cells and into the milk of transgenic mice. Vectors directed a hierarchy of expression levels in vitro, dependent upon the specific complement of HSA introns included. HSA introns acted in a synergistic manner. In addition, minigenes comprised of specific subsets of introns were more efficacious than the entire HSA gene with all of its introns. Transgenic mice expressed as much as 10 mg ml-1 of HSA in their milk. Vectors comprised of specific intron subsets directed levels at 1 mg ml-1 or greater in the milk of 20% of generated transgenics. A statistical correlation between the expression level trend in vitro with the trend of expression in vivo (% which express) at detectable levels (p = 0.0015) and at the level of greater than 0.1 mg ml-1 (p = 0.0156) was demonstrated. A weak correlation existed (p = 0.0526) at in vivo levels of 1 mg ml-1 or greater. These new vectors are expected to direct the production of high levels of HSA in the milk of a large percentage of generated transgenic dairy animals.

Evolutionary selection against change in many Alu repeat sequences interspersed through primate genomes

Mutations have been examined in the 1500 interspersed Alu repeats of human DNA that have been sequenced and are nearly full length. There is a set of particular changes at certain positions that rarely occur (termed suppressed changes) compared to the average of identical changes of identical nucleotides in the rest of the sequence. The suppressed changes occur in positions that are clustered together in what appear to be sites for protein binding. There is a good correlation of the suppression in different positions, and therefore the joint probability of absence of mutation at many pairs of such positions is significantly higher than that expected at random. The suppression of mutation appears to result from selection that is not due to requirements for Alu sequence replication. The implication is that hundreds of thousands of Alu sequences have sequence-dependent functions in the genome that are selectively important for primates. In a few known cases Alu inserts have been adapted to function in the regulation of gene transcription.

Eukaryotic transcription termination factor La mediates transcript release and facilitates reinitiation by RNA polymerase III

Ample evidence indicates that Alu family interspersed elements retrotranspose via primary transcripts synthesized by RNA polymerase III (pol III) and that this transposition sometimes results in genetic disorders in humans. However, Alu primary transcripts can be processed posttranscriptionally, diverting them away from the transposition pathway. The pol III termination signal of a well-characterized murine B1 (Alu-equivalent) element inhibits RNA 3' processing, thereby stabilizing the putative transposition intermediary. We used an immobilized template-based assay to examine transcription termination by VA1, 7SL, and Alu class III templates and the role of transcript release in the pol III terminator-dependent inhibition of processing of B1-Alu transcripts. We found that the RNA-binding protein La confers this terminator-dependent 3' processing inhibition on transcripts released from the B1-Alu template. Using pure recombinant La protein and affinity-purified transcription complexes, we also demonstrate that La facilitates multiple rounds of transcription reinitiation by pol III. These results illustrate an important role for La in RNA production by demonstrating its ability to clear the termination sites of class III templates, thereby promoting efficient use of transcription complexes by pol III. The role of La as a potential regulatory factor in transcript maturation and how this might apply to Alu interspersed elements is discussed.

Eukaryotic transcription termination factor La mediates transcript release and facilitates reinitiation by RNA polymerase III

Ample evidence indicates that Alu family interspersed elements retrotranspose via primary transcripts synthesized by RNA polymerase III (pol III) and that this transposition sometimes results in genetic disorders in humans. However, Alu primary transcripts can be processed posttranscriptionally, diverting them away from the transposition pathway. The pol III termination signal of a well-characterized murine B1 (Alu-equivalent) element inhibits RNA 3' processing, thereby stabilizing the putative transposition intermediary. We used an immobilized template-based assay to examine transcription termination by VA1, 7SL, and Alu class III templates and the role of transcript release in the pol III terminator-dependent inhibition of processing of B1-Alu transcripts. We found that the RNA-binding protein La confers this terminator-dependent 3' processing inhibition on transcripts released from the B1-Alu template. Using pure recombinant La protein and affinity-purified transcription complexes, we also demonstrate that La facilitates multiple rounds of transcription reinitiation by pol III. These results illustrate an important role for La in RNA production by demonstrating its ability to clear the termination sites of class III templates, thereby promoting efficient use of transcription complexes by pol III. The role of La as a potential regulatory factor in transcript maturation and how this might apply to Alu interspersed elements is discussed.

Oncogene activation of human keratin 18 transcription via the Ras signal transduction pathway

Keratin 8 (K8) and keratin 18 (K18) are intermediate filament proteins normally expressed in simple epithelial tissues and persistently expressed in a wide variety of carcinomas. Ectopic expression of K8 and K18 occurs in some epidermal and murine skin carcinomas induced by chemical carcinogenesis or oncogenic ras expression. We show here that K18 is a direct target of the Ras signal transduction pathway, by demonstrating that activated Ha-Ras, as well as activated Src, Lck, or Raf, stimulates the transcription of K18. This activation is mediated by an enhancer element containing essential and closely spaced Ets and AP-1 transcription factor binding sites. Oncogene activation of K18 transcription provides a molecular explanation for the persistent and sometimes unexpected expression of K18 in such a wide variety of tumors.

Alu sequence involvement in transcriptional insulation of the keratin 18 gene in transgenic mice

The human keratin 18 (K18) gene is expressed in a variety of adult simple epithelial tissues, including liver, intestine, lung, and kidney, but is not normally found in skin, muscle, heart, spleen, or most of the brain. Transgenic animals derived from the cloned K18 gene express the transgene in appropriate tissues at levels directly proportional to the copy number and independently of the sites of integration. We have investigated in transgenic mice the dependence of K18 gene expression on the distal 5' and 3' flanking sequences and upon the RNA polymerase III promoter of an Alu repetitive DNA transcription unit immediately upstream of the K18 promoter. Integration site-independent expression of tandemly duplicated K18 transgenes requires the presence of either an 825-bp fragment of the 5' flanking sequence or the 3.5-kb 3' flanking sequence. Mutation of the RNA polymerase III promoter of the Alu element within the 825-bp fragment abolishes copy number-dependent expression in kidney but does not abolish integration site-independent expression when assayed in the absence of the 3' flanking sequence of the K18 gene. The characteristics of integration site-independent expression and copy number-dependent expression are separable. In addition, the formation of the chromatin state of the K18 gene, which likely restricts the tissue-specific expression of this gene, is not dependent upon the distal flanking sequences of the 10-kb K18 gene but rather may depend on internal regulatory regions of the gene.

Alu sequence involvement in transcriptional insulation of the keratin 18 gene in transgenic mice

The human keratin 18 (K18) gene is expressed in a variety of adult simple epithelial tissues, including liver, intestine, lung, and kidney, but is not normally found in skin, muscle, heart, spleen, or most of the brain. Transgenic animals derived from the cloned K18 gene express the transgene in appropriate tissues at levels directly proportional to the copy number and independently of the sites of integration. We have investigated in transgenic mice the dependence of K18 gene expression on the distal 5' and 3' flanking sequences and upon the RNA polymerase III promoter of an Alu repetitive DNA transcription unit immediately upstream of the K18 promoter. Integration site-independent expression of tandemly duplicated K18 transgenes requires the presence of either an 825-bp fragment of the 5' flanking sequence or the 3.5-kb 3' flanking sequence. Mutation of the RNA polymerase III promoter of the Alu element within the 825-bp fragment abolishes copy number-dependent expression in kidney but does not abolish integration site-independent expression when assayed in the absence of the 3' flanking sequence of the K18 gene. The characteristics of integration site-independent expression and copy number-dependent expression are separable. In addition, the formation of the chromatin state of the K18 gene, which likely restricts the tissue-specific expression of this gene, is not dependent upon the distal flanking sequences of the 10-kb K18 gene but rather may depend on internal regulatory regions of the gene.

Transcriptional insulation of the human keratin 18 gene in transgenic mice

Expression of the 10-kb human keratin 18 (K18) gene in transgenic mice results in efficient and appropriate tissue-specific expression in a variety of internal epithelial organs, including liver, lung, intestine, kidney, and the ependymal epithelium of brain, but not in spleen, heart, or skeletal muscle. Expression at the RNA level is directly proportional to the number of integrated K18 transgenes. These results indicate that the K18 gene is able to insulate itself both from the commonly observed cis-acting effects of the sites of integration and from the potential complications of duplicated copies of the gene arranged in head-to-tail fashion. To begin to identify the K18 gene sequences responsible for this property of transcriptional insulation, additional transgenic mouse lines containing deletions of either the 5' or 3' distal end of the K18 gene have been characterized. Deletion of 1.5 kb of the distal 5' flanking sequence has no effect upon either the tissue specificity or the copy number-dependent behavior of the transgene. In contrast, deletion of the 3.5-kb 3' flanking sequence of the gene results in the loss of the copy number-dependent behavior of the gene in liver and intestine. However, expression in kidney, lung, and brain remains efficient and copy number dependent in these transgenic mice. Furthermore, herpes simplex virus thymidine kinase gene expression is copy number dependent in transgenic mice when the gene is located between the distal 5'- and 3'-flanking sequences of the K18 gene. Each adult transgenic male expressed the thymidine kinase gene in testes and brain and proportionally to the number of integrated transgenes. We conclude that the characteristic of copy number-dependent expression of the K18 gene is tissue specific because the sequence requirements for transcriptional insulation in adult liver and intestine are different from those for lung and kidney. In addition, the behavior of the transgenic thymidine kinase gene in testes and brain suggests that the property of transcriptional insulation of the K18 gene may be conferred by the distal flanking sequences of the K18 gene and, additionally, may function for other genes.

Transcriptional insulation of the human keratin 18 gene in transgenic mice

Expression of the 10-kb human keratin 18 (K18) gene in transgenic mice results in efficient and appropriate tissue-specific expression in a variety of internal epithelial organs, including liver, lung, intestine, kidney, and the ependymal epithelium of brain, but not in spleen, heart, or skeletal muscle. Expression at the RNA level is directly proportional to the number of integrated K18 transgenes. These results indicate that the K18 gene is able to insulate itself both from the commonly observed cis-acting effects of the sites of integration and from the potential complications of duplicated copies of the gene arranged in head-to-tail fashion. To begin to identify the K18 gene sequences responsible for this property of transcriptional insulation, additional transgenic mouse lines containing deletions of either the 5' or 3' distal end of the K18 gene have been characterized. Deletion of 1.5 kb of the distal 5' flanking sequence has no effect upon either the tissue specificity or the copy number-dependent behavior of the transgene. In contrast, deletion of the 3.5-kb 3' flanking sequence of the gene results in the loss of the copy number-dependent behavior of the gene in liver and intestine. However, expression in kidney, lung, and brain remains efficient and copy number dependent in these transgenic mice. Furthermore, herpes simplex virus thymidine kinase gene expression is copy number dependent in transgenic mice when the gene is located between the distal 5'- and 3'-flanking sequences of the K18 gene. Each adult transgenic male expressed the thymidine kinase gene in testes and brain and proportionally to the number of integrated transgenes. We conclude that the characteristic of copy number-dependent expression of the K18 gene is tissue specific because the sequence requirements for transcriptional insulation in adult liver and intestine are different from those for lung and kidney. In addition, the behavior of the transgenic thymidine kinase gene in testes and brain suggests that the property of transcriptional insulation of the K18 gene may be conferred by the distal flanking sequences of the K18 gene and, additionally, may function for other genes.