The p53 response to DNA damage in vivo is independent of DNA-dependent protein kinase

Ionizing radiation (IR) exposure causes mammalian cells to undergo p53-dependent cell cycle arrest and/or apoptosis. The in vivo role of DNA-dependent protein kinase (DNA-PK) in the transduction of the DNA damage signal to p53 remains unresolved. To determine the relationship between DNA-PK and p53, we studied the cell cycle and apoptotic responses to IR in mice deficient in DNA-PK. Using the slip mouse, which harbors an inactivating mutation of the DNA-PK catalytic subunit (DNA-PKcs), we demonstrated not only that these DNA-PKcs null mutants were highly radiosensitive but also that upon IR treatment, p53 accumulated in their cultured cells and tissue. Induced p53 was transcriptionally active and mediated the induction of p21 and Bax in slip cells. Examination of the thymic cell cycle response to IR treatment indicated that the slip G(1)/S-phase cell cycle checkpoint function was intact. We further show that slip mice exhibited a higher level of spontaneous thymic apoptosis as well as a more robust apoptotic response to IR than wild-type mice. Together, these data demonstrate that the p53-mediated response to DNA damage is intact in cells devoid of DNA-PK activity and suggest that other kinases, such as the product of the gene (ATM) mutated in ataxia telangiectasia, are better candidates for regulating IR-induced phosphorylation and accumulation of p53.

Analysis of variable (diversity) joining recombination in DNAdependent protein kinase (DNA-PK)-deficient mice reveals DNA-PK-independent pathways for both signal and coding joint formation

Previous studies have suggested that ionizing radiation causes irreparable DNA double-strand breaks in mice and cell lines harboring mutations in any of the three subunits of DNA-dependent protein kinase (DNA-PK) (the catalytic subunit, DNA-PKcs, or one of the DNA-binding subunits, Ku70 or Ku86). In actuality, these mutants vary in their ability to resolve double-strand breaks generated during variable (diversity) joining [V(D)J] recombination. Mutant cell lines and mice with targeted deletions in Ku70 or Ku86 are severely compromised in their ability to form coding and signal joints, the products of V(D)J recombination. It is noteworthy, however, that severe combined immunodeficient (SCID) mice, which bear a nonnull mutation in DNA-PKcs, are substantially less impaired in forming signal joints than coding joints. The current view holds that the defective protein encoded by the murine SCID allele retains enough residual function to support signal joint formation. An alternative hypothesis proposes that DNA-PKcs and Ku perform different roles in V(D)J recombination, with DNA-PKcs required only for coding joint formation. To resolve this issue, we examined V(D)J recombination in DNA-PKcs-deficient (SLIP) mice. We found that the effects of this mutation on coding and signal joint formation are identical to the effects of the SCID mutation. Signal joints are formed at levels 10-fold lower than in wild type, and one-half of these joints are aberrant. These data are incompatible with the notion that signal joint formation in SCID mice results from residual DNA-PKcs function, and suggest a third possibility: that DNA-PKcs normally plays an important but nonessential role in signal joint formation.

DNA-PKcs: a T-cell tumour suppressor encoded at the mouse scid locus

Severe combined immunodeficiency (SCID) mice are defective in their ability to rearrange their variable (V), diversity (D) and joining (J) genetic elements to generate functional immunoglobulin (Ig) and T-cell receptor (TCR) molecules; as a result, they lack mature B and T cells. These mice are highly sensitive to ionizing radiation, suggesting that the product of the scid gene plays a critical role in both V(D)J recombination and DNA double-strand break repair. Recent studies suggest that the SCID defect lies in the gene encoding the catalytic subunit of DNA-dependent protein kinase (DNA-PK; refs 6-8), a nuclear protein made up of the Ku 70 and Ku 86 subunits as well as the large catalytic subunit, DNA-PKcs. Other reports have implied that the SCID phenotype correlates with nonsense mutations at the extreme 3' end of Prkdc, the DNA-PKcs gene. The identity of the gene remains in doubt, however, because the consequences of genetic inactivation of Prkdc have not been determined. This study shows that complete inactivation of Prkdc in a novel insertional mouse mutant recapitulates the SCID phenotype and that Prkdc and scid are alleic. Significantly, DNA-PKcs null mice demonstrate complete penetrance of thymic lymphoblastic lymphomas, strongly suggesting that Prkdc functions in mice as a T-cell tumour suppressor and, by virtue of its association with DNA repair and recombination, belongs to the 'caretaker' class of tumour-suppressor genes that includes ATM, BRCA1 and BRCA2 (ref. 15).

Expression of a truncated Int3 gene in developing secretory mammary epithelium specifically retards lobular differentiation resulting in tumorigenesis

Insertional mutation of the Int3 gene, a member of the Notch gene family, is frequently associated with primary mouse mammary tumors induced by the mouse mammary tumor virus (MMTV). A major consequence of these mutations is the production of a shortened 2.4-kb tumor specific Int3 RNA transcript that encodes the entire intracellular domain of the Int3 protein. Previous studies have demonstrated that mammary gland development and function was severely impaired in transgenic mice expressing the truncated Int3 gene product from the MMTV viral promoter. Both mammary ductal growth and secretory lobule development were curtailed in these mice. These results were attributed to a gain of function modification of the Int3 gene, which led to a restriction of cell fate selection in the affected mammary epithelial cells. To confirm and extend these findings, truncated Int3 was expressed from the whey acidic protein (WAP) promoter, the activity of which, unlike that of the MMTV long terminal repeat, is restricted to the secretory mammary epithelial population. In transgenic mice carrying the WAP/Int3 construct, mammary ductal growth was unaffected in virgin females, but growth and differentiation of secretory lobules during gestation was profoundly inhibited. Coincidental with the block in lobular secretory differentiation, mammary dysplasia and tumorigenesis occurred in all breeding females by 25 weeks of age. In nonbreeding WAP/Int3 females, mammary tumor incidence also reached 100%, but only after 70 weeks. The WAP/Int3 mammary tumors were highly malignant, and most tumor-bearing females, irrespective of breeding history, developed metastatic lung lesions. These results suggest that WAP promotor-targeted Int3 function is associated with mammary secretory cell differentiation and maintenance in this transgenic model. Consistent with the conclusion that WAP-driven truncated Int3 expression influenced only lobular differentiation and not ductal growth and extension during mammary gland development, transplants of WAP/Int3 gland into nontransgenic mammary fat pads produced complete mammary ductal outgrowths in virgin FVB/N mice but failed to develop secretory lobules when the females were impregnated.

Transforming growth factor-alpha promotes mammary tumorigenesis through selective survival and growth of secretory epithelial cells

Transforming growth factor (TGF)-alpha stimulates the growth and development of mammary epithelial cells and is implicated in the pathogenesis of human breast cancer. In this report we evaluate the consequences of overexpressing TGF-alpha in the mammary gland of transgenic mice and examine associated cellular mechanisms. When operating on a FVB/N genetic background (line MT100), TGF-alpha induced the stochastic development of mammary adenomas and adenocarcinomas f secretory epithelial origin in 64% of multiparous females. In contrast, tumors were exceedingly rare in virgin MT100 females, MT100 males, and multiparous FVB/N females. In MT100 females multiple foci of hyperplastic secretory lesions preceded the development of frank tumors; these initial lesions appeared during the involution period after the first lactation. Serial transplantation of these hyperplasias indicated an absence of proliferative immortality. Nevertheless, they gave rise to tumors at a low frequency and after a prolonged latency in virgin hosts; in multiparous hosts, tumors developed earlier and at a high incidence. The TGF-alpha transgene was highly expressed in hyperplasias and tumors but not in virgin and nonlesion-bearing tissue, suggesting that TGF-alpha overexpression provides a selective growth advantage. TGF-alpha also induced at lactation a 6.4-fold increase in DNA synthesis in MT100 epithelial cells, many of which were binucleated. MT100 mammary tissue experienced an obvious delay in involution, resulting in the postlactational survival of a significant population of unregressed secretory epithelial cells. In contrast, another line of transgenic mice on a CD-1 genetic background (MT42), in which TGF-alpha overexpression induced liver but not mammary tumors, failed to demonstrate postlactational epithelial cell survival. These data show that TGF-alpha promotes mammary tumorigenesis in multiparous MT100 mice by stimulating secretory epithelial cell proliferation during lactation and prolonging survival during involution. These points support the notion that TGF-alpha can act as a mitogen and also as a differentiation factor in mammary epithelium.

Constitutive expression of a truncated INT3 gene in mouse mammary epithelium impairs differentiation and functional development

INT3 is interrupted by retroviral DNA insertion in approximately 18% of primary Czech mouse mammary tumors induced by mouse mammary tumor virus. One consequence of these insertions is the production of a 2.4-kilobase, tumor-specific RNA transcript encoding the entire intracellular domain of the Int3 protein which is initiated from the 3' long terminal repeat promoter of the inserted viral genome. Female mice (FVB-3) transgenic for a genomic fragment comprised of this truncated region of INT3 express the 2.4-kilobase truncated INT3 transcript and exhibit focal mammary tumors at 100% penetrance. INT3 is a member of a family of genes, highly conserved through evolution and characterized by Drosophila melanogaster Notch and Caenorhabditis elegans lin-12, the function of which relates to cell fate determination. Upon transfection into the appropriate hosts, expression vectors of truncated Notch and lin-12, representing their respective cytoplasmic domains, have been demonstrated to effect their complete gene function with respect to cell fate determination. This suggests that the extracellular portion of these proteins function only to regulate activity. Reciprocal transplantation of transgenic FVB-3 and normal mammary tissue to the epithelium-divested fat pads of the respective donor females demonstrates that FVB-3 mammary epithelium is unable to grow and/or to functionally differentiate. However, normal epithelium grows and fully differentiates in transgenic FVB-3 fat pads, indicating that the dysfunction of FVB-3 mammary glands is due to a deficiency inherent in their epithelium. Electron microscopy reveals that transgenic INT3 epithelial cells do not form intercellular junctional complexes in the developing subadult mammary gland. The hormonal stimulation of pregnancy overcomes the deficiency for ductal growth so apparent in the virgin gland such that pregnant FVB-3 glands produce complete ductal systems. Nevertheless, during pregnancy, FVB-3 mammary cells fail to form secretory lobules and to produce milk. Examination of INT3 expression by immunocytochemistry and reverse transcriptase-PCR show that INT3 is expressed constitutively in mammary stroma and epithelia at all stages of postpubertal mammary evolution. These results indicate that deregulated expression of a truncated Int3 in mammary epithelial cells limits their capacity to perform the cell fate decisions required for morphogenesis and functional differentiation.

Ectopic TGF beta 1 expression in the secretory mammary epithelium induces early senescence of the epithelial stem cell population

An important feature of the mammary gland is the regenerative capacity of its epithelium which is demonstrated upon successive cycles of lactation and involution. Pregnant mice expressing a whey-acidic protein (WAP) promoter-driven transforming growth factor-beta 1 (TGF beta 1) cDNA are unable either to generate a secretory mammary epithelium or to lactate. Here we investigate whether ectopic TGF beta 1 induces this phenotype by affecting the transgenic epithelium directly or in trans. Reciprocal transplantation of mammary tissue between normal and transgenic hosts resulted in the development of the respective phenotypes of the transplants within the same mammary fat pad. When isolated mammary epithelial cells from both were mixed before implantation so that transgenic and normal epithelium would develop together more proximately, both phenotypes were simultaneously observed in the resultant chimeric mammary outgrowths. Since no trans effect was detectable, we hypothesize that early expression of the transgene results in compromised lobular progenitor cells through an intracrine mechanism. Consistent with this posit, WAP promoter-driven protein expression was detected in individual cells of the subtending ducts of immature females at estrus. Transplantation of WAP-TGF beta 1 mammary gland into nonpregnant hosts revealed that transgenic implants, even those from young postpubertal virgin females, had a diminished ability to repopulate epithelium-free mammary fat pads. Accordingly, the ectopic expression of WAP-TGF beta 1 not only impairs lobular progenitors, but also promotes an early senescence of the regenerative capacity of the mammary ductal epithelium. This leads us to propose that mammary epithelial stem cells give rise to two functionally distinct progenitor cells in the mammary gland epithelium: one capable of producing daughters committed to ductal formation, the other capable only of producing daughters committed to lobular function.

Transgenic mice provide genetic evidence that transforming growth factor alpha promotes skin tumorigenesis via H-ras-dependent and H-ras-independent pathways

The epidermal growth factor receptor (EGFR), which mediates the mitogenic activity of transforming growth factor alpha (TGF-alpha), has been shown to activate Ras in cultured cells through well-defined intermediary proteins. To examine the in vivo relationship between EGFR and Ras, chemical carcinogenesis of TGF-alpha transgenic mouse skin was chosen as an experimental model. Transgenic mice overexpressing TGF-alpha in a wide variety of epithelial tissues by virtue of a metallothionein promoter demonstrate a multitude of premalignant and neoplastic lesions but not spontaneous skin tumors. Transgenic skin was initiated with a single dose of 7,12-dimethylbenz[a]anthracene (DMBA), shown previously to induce, in concert with a tumor promoter, murine papillomas that consistently contain specific H-ras mutations. Virtually all DMBA-treated TGF-alpha transgenic mice, but not treated control animals, developed hyperplasias, papillomas, sebaceous adenomas, and more infrequently, sebaceous and squamous cell carcinomas. Therefore, TGF-alpha functions as an autonomous tumor promoter in DMBA-initiated transgenic skin. Skin tumors could be separated into two mutually exclusive genetic classes. In tumors harboring mutant H-ras, TGF-alpha transgene expression was relatively low and essentially unchanged relative to untreated skin; however, only 42% of skin tumors contained mutations in H-ras. Conversely, in most tumors with wild-type H-ras, transgenic TGF-alpha transcripts were enhanced 10- to 20-fold. These results suggest that strong constitutive EGFR stimulation, through TGF-alpha transgene overexpression, can substitute functionally for mutational activation of H-ras in skin tumorigenesis. Moreover, because H-ras mutational activation could not induce skin tumors without TGF-alpha transgene activity, simultaneous stimulation of an EGFR-mediated H-Ras-independent pathway appears to be required for tumor development as well.

An epidermal growth factor receptor promoter construct selectively expresses in the thymus and spleen of transgenic mice

To determine the range of tissues in which the human epidermal growth factor (EGF) receptor promoter is active, we created 12 independently derived lines of mice expressing a transgene consisting of human EGF receptor promoter and enhancer sequences fused to the bacterial chloramphenicol acetyltransferase (CAT) gene. Analysis for the presence of CAT activity in these transgenic mice revealed that the human EGF receptor promoter construct was consistently active in the thymus and spleen. Thymocytes separated from thymic stromal cells of EGF receptor-CAT mice demonstrated no CAT activity suggesting that expression in the thymus is confined to the thymic stroma, which consists mainly of epithelial cells. Thus, it should be possible to use the EGF receptor promoter construct described in this study to direct expression of a variety of foreign genes to the nonlymphocytic cells of the thymus and spleen so that the effect of these genes on the maturation and proliferation of thymocytes may be addressed.

Targeting expression of a transforming growth factor beta 1 transgene to the pregnant mammary gland inhibits alveolar development and lactation

Transforming growth factor-beta 1 (TGF-beta 1) possesses highly potent, diverse and often opposing cell-specific activities, and has been implicated in the regulation of a variety of physiologic and developmental processes. To determine the effects of in vivo overexpression of TGF-beta 1 on mammary gland function, transgenic mice were generated harboring a fusion gene consisting of the porcine TGF-beta 1 cDNA placed under the control of regulatory elements of the pregnancy-responsive mouse whey-acidic protein (WAP) gene. Females from two of four transgenic lines were unable to lactate due to inhibition of the formation of lobuloalveolar structures and suppression of production of endogenous milk protein. In contrast, ductal development of the mammary glands was not overtly impaired. There was a complete concordance in transgenic mice between manifestation of the lactation-deficient phenotype and expression of RNA from the WAP/TGF-beta 1 transgene, which was present at low levels in the virgin gland, but was greatly induced at mid-pregnancy. TGF-beta 1 was localized to numerous alveoli and to the periductal extracellular matrix in the mammary gland of transgenic females late in pregnancy by immunohistochemical analysis. Glands reconstituted from cultured transgenic mammary epithelial cells duplicated the inhibition of lobuloalveolar development observed in situ in the mammary glands of pregnant transgenic mice. Results from this transgenic model strongly support the hypothesis that TGF-beta 1 plays an important in vivo role in regulating the development and function of the mammary gland.

Hypertrophic gastropathy resembling Ménétrier's disease in transgenic mice overexpressing transforming growth factor alpha in the stomach

Transforming growth factor alpha (TGF alpha) is thought to participate in the normal and pathologic processes of numerous tissues, including the gastric mucosa. To explore its role in vivo, transgenic mice were generated overexpressing TGF alpha in the stomach. TGF alpha induced dramatic structural and functional lesions of the glandular stomach that were similar to Ménétrier's disease in humans. Transgenic mice developed severe adenomatous hyperplasia that resulted in a striking nodular thickening or hypertrophy of the gastric mucosa. Secretions obtained from affected stomachs contained no detectable gastric acid, suggesting that parietal cell function had been greatly impaired. These findings demonstrate that overproduction of TGF alpha can stimulate cellular proliferation, suppress acid secretion, and perturb organogenesis of the stomach of transgenic mice. Moreover, TGF alpha may contribute to the pathogenesis of related human hypertrophic gastropathies, such as Ménétrier's disease.

Expression of an activated Notch-related int-3 transgene interferes with cell differentiation and induces neoplastic transformation in mammary and salivary glands

Expression of the int-3 locus is activated in mouse mammary tumors as a consequence of insertional mutagenesis by the mouse mammary tumor virus (MMTV). Integration of the MMTV provirus into the int-3 locus promotes the transcription and translation of flanking cellular int-3 sequences sharing significant homology with the intracellular domain of the neurogenic Notch gene of Drosophila, and with the yeast cell cycle regulatory genes cdc10 and SWI6. To determine the in vivo consequences of activated int-3 expression, transgenic mice were generated harboring a genomic tumor DNA fragment consisting of the MMTV LTR and the flanking cellular int-3 sequences. All six int-3 founder transgenic mice and the progeny of one established line exhibited similar dramatic phenotypic abnormalities in tissues in which the transgene was expressed. Focal and often multiple poorly differentiated mammary and salivary adenocarcinomas appeared in the majority of transgenic mice between 2 and 7 months of age. Significantly, mammary glands were arrested in development and were lactation deficient in all female int-3 mice. The salivary glands, glands of the nasal mucosa and maxillary sinus, the extraorbital lacrimal glands, and the Harderian glands of juvenile and adult transgenic mice all contained proliferating immature ductule cells and were incompletely differentiated. In addition, all male int-3 transgenic mice were sterile, apparently the result of severe hyperplasia of the epididymis. These findings demonstrate in vivo that expression of the activated Notch-related int-3 gene causes deregulation of normal developmental controls and hyperproliferation of glandular epithelia.

Inactivation of a sperm motility gene by insertion of an epidermal growth factor receptor transgene whose product is overexpressed and compartmentalized during spermatogenesis

Transgenic mice were generated with a human epidermal growth factor (EGF) receptor cDNA driven by the chicken beta-actin gene promoter. One line (AE24) that exhibited a unique expression pattern in which dramatically elevated levels of EGF receptor RNA were found only in the testis was established, suggesting that the beta-actin promoter was being influenced by an adjacent testis-specific enhancer. EGF receptor RNA was detected in primary spermatocytes, whereas the synthesis of receptor protein was restricted to elongate spermatids, indicating that transgene expression was under translational control. At spermiation, the EGF receptor was sequestered in residual bodies and excluded from mature sperm by a compartmentalization mechanism. About half of AE24 homozygous males were sterile because of sperm paralysis, whereas heterozygous males and females of either genotype were completely fertile. Electron microscopic analysis of sperm flagella from sterile AE24 homozygotes revealed an aberrant axonemal structure in which outer doublet microtubules were missing from the middle piece, resembling changes observed in the sperm of some infertile humans. Flagellar axonemal disassembly was observed in the vas deferens and epididymis but not in the testis, suggesting that outer doublets were assembled in a grossly normal manner but possessed a latent instability. These results demonstrate that in the AE24 mouse line the EGF receptor transgene was integrated into and inactivated an endogenous autosomal gene, causing sperm flagellar axonemal disruption and male sterility.

TGF alpha overexpression in transgenic mice induces liver neoplasia and abnormal development of the mammary gland and pancreas

To define the role of TGF alpha in normal tissue function and in pathogenesis, transgenic mice have been generated bearing a fusion gene consisting of the mouse metallothionein 1 promoter and a human TGF alpha cDNA. In these mice, human TGF alpha RNA and protein are abundant in many tissues and TGF alpha is detectable in blood and urine. The effects of TGF alpha overproduction in transgenic mice are pleiotropic and tissue specific. The liver frequently contains multifocal, well-differentiated hepatocellular carcinomas that express enhanced levels of human TGF alpha RNA. The mammary gland exhibits impeded morphogenetic penetration of epithelial duct cells into the stromal fat pad. The pancreas shows progressive interstitial fibrosis and a florid acinoductular metaplasia, during which acinar cells appear to degranulate, dedifferentiate, and assume characteristics of intercalated or centroacinar duct cells. TGF alpha therefore plays an important role in cellular proliferation, organogenesis, and neoplastic transformation.

Transduction of host cellular sequences by a retroviral shuttle vector

We studied the frequencies and types of excision events which can occur with a retroviral shuttle vector containing the simian virus 40 origin of DNA replication. Analysis of the recloned vector plasmids by size and restriction enzyme mapping indicated that most contain one long terminal repeat. By hybridizing the plasmids to a mouse genomic repetitive DNA probe, we also determined that approximately 1 to 3% contain transduced cellular DNA sequences.

In vitro methylation of specific regions of the cloned Moloney sarcoma virus genome inhibits its transforming activity

The transforming activity of cloned Moloney sarcoma virus (MSV) proviral DNA was inhibited by in vitro methylation of the DNA at cytosine residues, using HpaII and HhaI methylases before transfection into NIH 3T3 cells. The inhibition of transforming activity due to HpaII methylation was reversed by treatment of the transfected cells with 5-azacytidine, a specific inhibitor of methylation. Analysis of the genomic DNA from the transformed cells which resulted from the transfection of methylated MSV DNA revealed that the integrated MSV proviral DNA was sensitive to HpaII digestion in all cell lines examined, suggesting that loss of methyl groups was necessary for transformation. When cells were infected with Moloney murine leukemia virus at various times after transfection with methylated MSV DNA, the amount of transforming virus produced indicated that the loss of methyl groups occurred within 24 h. Methylation of MSV DNA at HhaI sites was as inhibitory to transforming activity as methylation at HpaII sites. In addition, methylation at both HpaII and HhaI sites did not further reduce the transforming activity of the DNA. These results suggested that; whereas methylation of specific sites on the provirus may not be essential for inhibiting the transforming activity of MSV DNA, methylation of specific regions may be necessary. Thus, by cotransfection of plasmids containing only specific regions of the MSV provirus, it was determined that methylation of the v-mos gene was more inhibitory to transformation than methylation of the viral long terminal repeat.