Expression of the int-1 and int-2 loci in endogenous mouse mammary tumor virus-induced mammary tumorigenesis in the C3Hf mouse

Endogenous retroviruses in systemic response to stress signals

Infection of germline cells with retroviruses initiates permanent proviral colonization of the germline genome. The germline-integrated proviruses, called endogenous retroviruses (ERVs), are inherited to offspring in a Mendelian order and belong to the transposable element family. Endogenous retroviruses and other long terminal repeat retroelements constitute ~8% and ~10% of the human and mouse genomes, respectively. It is likely that each individual has a distinct genomic ERV profile. Recent studies have revealed that a substantial fraction of ERVs retains the coding potentials necessary for virion assembly and replication. There are several layers of potential mechanisms controlling ERV expression: intracellular transcription environment (e.g., transcription factor pool, splicing machinery, hormones), epigenetic status of the genome (e.g., proviral methylation, histone acetylation), profile of transcription regulatory elements on each ERV's promoter, and a range of stress signals (e.g., injury, infection, environment). Endogenous retroviruses may exert pathophysiologic effects by infection followed by random reintegration into the genome, by their gene products (e.g., envelope, superantigen), and by altering the expression of neighboring genes. Several studies have provided evidence that ERVs are associated with a range of pathogenic processes involving multiple sclerosis, systemic lupus erythematosus, breast cancer, and the response to burn injury. For instance, the proinflammatory properties of the human ERV-W envelope protein play a central role in demyelination of oligodendrocytes. As reviewed in this article, recent advances in ERV biology and mammalian genomics suggest that ERVs may have a profound influence on various pathogenic processes including the response to injury and infection. Understanding the roles of ERVs in the pathogenesis of injury and infection will broaden insights into the underlying mechanisms of systemic immune disorder and organ failure in these patients.

Biological significance of human endogenous retroviral sequences

Endogenous retroviruses (ERVs) have been known for many years to exist in numerous natural and laboratory animal species. In humans it has been demonstrated that at least 1% of the genome consists of retrovirus-related sequences. Involvement of ERVs in the development of neoplastic and autoimmune diseases in the mouse model implicated a potentially pathogenic role of ERVs for humans, too. The research in this field led to a number of results strongly suggesting that human endogenous retroviral sequences (HERVs) are biologically active, on the RNA and even on the protein level. Particle formation, regulation or dysregulation of cellular gene expression, and synthesis of potentially pathogenic viral proteins indicate the broad spectrum of mechanisms by which HERVs may obtain biological significance.

Amplification of the Int-2 gene in head and neck squamous cell carcinoma

Cellular oncogenes have been implicated in head and neck cancer development since 1986. More recently interest has focused on chromosome 11q13; oncogenes therein undergoing ongoing investigation include Bcl-1/Prad-1, Hst-1 and Int-2. Our laboratory has studied the Int-2 oncogene for several years, primarily in the breast. This paper presents our investigations of Int-2 in the head and neck. Thirty-four paraffin-embedded primary squamous cell carcinomas were studied for Int-2 gene amplification using a carefully controlled method of sequence quantification by DNA dot blots. Amplification, mostly low level, was identified in 62 per cent of samples studied. No clinical correlation to amplification could be found. Further studies are underway looking for evidence of expression of Int-2 in fresh tissues and for amplification and expression of other oncogenes on this amplicon.

Dietary effects on gene expression in mammary tumorigenesis

Studies were undertaken to determine the effect a high fat diet has on the hormonally controlled transcription of the Mtv-1 locus in C3Hf mice. The expression of this locus in the initiating event in mammary tumor development in the C3Hf mouse. Mice were weaned at 21 days to either a high fat diet containing 23.5 percent corn oil or to a low fat diet containing 5 percent corn oil. Mice were sacrificed at first, second, and third parity, or when they had developed mammary tumors, and their mammary glands and mammary tumors were isolated. RNA was isolated from all mammary glands and breast tumors and analyzed. The high fat diet accelerated the transcription of the Mtv-1 locus. The transcripts of this locus, which are never seen in C3Hf mouse mammary glands until second parity, were present in first parity mammary glands of 6 out of 10 high fat diet C3Hf mice which were studied. The mammary glands of 15 first parity C3Hf mice which were on the low fat diet were analyzed and none contained the Mtv-1 specific transcripts. In addition, mammary tumor development was detected earlier (11 vs 17.8 months) and after fewer litters (2.1 vs. 4.2) on the average in high fat diet C3Hf mice. One C3Hf mouse on the high fat diet developed a breast tumor at six months without going through pregnancy. These results indicate that a high fat diet of 23.5 percent corn oil can accelerate hormonally controlled gene expression specifically linked to mammary tumorigenesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Dietary fat: gene expression and mammary tumorigenesis

Although discovered as an exogenous agent of mammary carcinoma, the mouse mammary tumor virus (MMTV) is also transmitted vertically as endogenous proviral DNA present in the germ line of all inbred mice. In the C3Hf mouse, which receives no exogenous virus, the initial event of mammary tumorigenesis is the transcription of the endogenous MMTV proviral DNA present at the Mtv-1 locus. Transcription occurs as a result of the hormonal effects of pregnancy, and Mtv-1 specific transcripts are seen in second-parity lactating mammary glands of these mice. As a means of studying the effects of diet on mammary carcinoma at the molecular/genetic level, we have studied the transcription of the Mtv-1 locus in C3Hf mice on a high-fat diet containing 46% fat in calories or a low-fat diet containing 10% fat in calories. We have detected an accelerated transcription of the Mtv-1 locus (first- vs. second-parity lactating mammary glands) in > 50% of the C3Hf mice on the high-fat diet. In addition, mice on the high-fat diet developed mammary tumors earlier (11 vs. 17.8 mos) and after fewer litters (2.1 vs. 4.2). Our results indicate that fat in the diet can affect gene expression related to mammary carcinoma.

Homozygous deletion, rearrangement and hypermethylation implicate chromosome region 3p14.3-3p21.3 in sporadic breast-cancer development

DNAs from 19 malignant human breast tumors and 2 benign fibroadenomas were analyzed for heterozygosity at 5 polymorphic loci on the short arm of chromosome 3. One homozygous deletion and one rearrangement were identified using probe D3S2 which maps to 3p14.3-3p21.1. This probe also detected novel hybridizing fragments of 2.0 kb and/or 3.4 kb in 6/18 (33%) of the malignant tumor samples that hybridized with the D3S2 probe following digestion with the 5'-methylcytosine-insensitive enzyme MspI. Comparisons of HpaII and MspI digestion showed that all but one of the tumor DNAs analyzed were hypermethylated. The two fibroadenoma DNAs were not as highly methylated and had hybridizing fragments of 3.4 kb after HpaII digestion. These malignant breast-tumor DNAs exhibit 3 mechanisms by which a tumor-suppressor gene hypothesized to reside at 3p14-3p21 could be inactivated: homozygous deletion, rearrangement and hypermethylation, and strongly implicate this 3p chromosome region in breast-tumor development.

Mouse mammary-tumor virus activates Fgf-3/Int-2 less frequently in tumors from virgin than from parous mice

Tumorigenesis by mouse mammary-tumor virus (MMTV) involves proviral disruption and transcriptional activation of a number of cellular oncogenes, generically termed Int. The frequencies with which different Int genes are activated in different mouse strains can be quite variable, and previous surveys have suggested that insertions at Int-2/Fgf-3 occur primarily in strains that develop pregnancy-dependent mammary tumors. To address this issue, we have determined the relative contributions of 5 known Int genes (Wnt-1, Wnt-3, Fgf-3, Fgf-4 and Int-3) in mammary tumors from virgin BR6 and multiparous BR6, BALB/cfBR6 and RIII mice. Whereas Fgf-3 was implicated in 66%, 80% and 92% of the tumors from the respective parous animals, only 20% of the tumors from virgin mice expressed Fgf-3. This reduced involvement of Fgf-3 was compensated by proviral insertions in Fgf-4, Int-3 and Wnt-3, but the frequency of Wnt-1 activation was relatively constant. These data strengthen the link between Fgf-3 and a pregnancy-dependent phenotype and suggest that, in the strains analyzed, the frequency of Int-gene activation was influenced more by the hormonal status than by the genetic background.

The effect of parity, tumor latency and transplantation on the activation ofint loci in mmtv-induced, transplanted C3H mammary pre-neoplasias and their tumors

Mouse mammary tumor virus (MMTV) infection of mammary glands results in proviral insertion into host DNA and activation of cellular genes. Clonal expansion of cells bearing insertional mutations results in hyperplastic alveolar nodules (HAN) and tumors. HAN, transplanted into epithelium-cleared mammary fat pads, form hyperplastic alveolar outgrowths (HOGs). Previous work indicates the commonly MMTV-activated genes wnt-1 and int-2 are rarely affected in HOGs and HOG-derived tumors. To determine the basis of the dichotomy between the frequency of wnt/int gene activation in HOG-derived tumors and tumors from breeders of the identical inbred mouse strain, we compared the activation of wnt-1, int-2 and int-3 in tumors from virgin and breeding C3H mice, in consecutive primary tumors arising in individual C3H breeders and in C3H HOGs at early passages. Activation of wnt-1 or int-2 was rare in HOG-derived tumors (6% and 0%) compared with primary tumors in breeders (52% and 14%). int-3 was never found to be activated. wnt-1 was activated in the same percentage of primary tumors from virgins as from breeders. int-2 was activated only in tumors from breeders. wnt-1 activation also did not correlate with shorter tumor latency in multiple tumors from individual breeders. wnt-1 RNA was not detected in HOGs at early transplant generations, however, low levels of wnt-1 RNA were variably found in the epithelium of virgin mammary glands. We cannot explain why C3H HOGs and their derivative tumors develop without wnt-1 expression when the majority of C3H primary mammary tumors possess an MMTV-activated wnt-1 gene.

Host genetic background effect on the frequency of mouse mammary tumor virus-induced rearrangements of the int-1 and int-2 loci in mouse mammary tumors

The frequency with which int-1 and int-2 are rearranged in mouse mammary tumors by mouse mammary tumor virus (MMTV)-induced insertional mutagenesis is a consequence of the host genetic background. In 75% of C3H mammary tumors, int-1 is rearranged by MMTV insertion, whereas only 30% of BALB/cfC3H tumors contain a virus-induced rearrangement of int-1. This difference is significant (P less than 0.005) and could not be accounted for by the potentially additive effect of the genetically transmitted Mtv-1-encoded virus in C3H mice. Similarly, MMTV-induced rearrangement of the int-2 gene in mammary tumors of the R111 mouse strain (59%) occurred at a significantly (P less than 0.025) higher frequency than in BALB/cfR111 (25%) mammary tumors. Moreover, in BALB/cfR111 mammary tumors, there is evidence that rearrangement of int-1 and int-2 does not occur independently (P less than 0.025). These results suggest that the long history of inbreeding for high tumor incidence of C3H and R111 mouse strains has selected for the fixation of host mutations which either complement the action of the particular int gene or affect the sensitivity of specific subpopulations of mammary epithelium to infection by particular strains of MMTV.

Insertional mutagenesis: neoplasia arising from retroviral integration

The integration of retroviral proviruses near cellular genes can profoundly affect their expression. Painstaking analysis of insertion sites from a large number of tumors has revealed a number of previously unknown proto-oncogenes, and has elucidated new mechanisms whereby known proto-oncogenes can be activated. A number of these genes have been implicated in tumors of clinical relevance. At the time of writing a great deal remains to be learned of the normal function of these genes in the cell. While it has yet to be demonstrated that retroviral insertion mechanisms play some role in naturally occurring human neoplasms, they must be considered in the context of retroviral gene therapy protocols now being contemplated.