The activation of cellular oncogenes by proviral insertion in murine mammary cancer

Mls: A Link Between Immunology and Retrovirology

The nature of the mysterious minor lymphocyte stimulating (Mls) antigens has recently been clarified. These molecules which were key elements for our current understanding of immune tolerance, have a strong influence on the mouse immune system and are encoded by the open reading frame (orf) of endogenous and exogenous mouse mammary tumor viruses (MMTV's). The knowledge that these antigens are encoded by cancerogenic retroviruses opens an interdisciplinary approach for understanding the mechanisms of immune responses and immune tolerance, retroviral carcinogenesis, and retroviral strategies for infection.

Mouse mammary tumor virus-like env sequences in human breast cancer

Mouse mammary tumor virus (MMTV) has long been known as a causal agent of breast cancer in mice. To date, varied MMTV-like envelope gene (env) sequences have been identified in up to 74% of human breast cancers. However, the role and origin of these MMTV-like sequences in human breast cancer remain uncertain. Our study was initiated to study the integration of MMTV-like env sequences in human breast cancer. PCR screening has identified 28 (56%) Australian breast cancer specimens and 7 (87.5%) human breast cancer cell lines to be positive for MMTV-like env sequence. In the MCF-7 genome, a fragment containing an MMTV-like env sequence of approximately 1.9 kb plus a downstream rodent-like sequence of approximately 200 bp was found to be integrated into a bacterial-like beta-lactamase sequence by insertional mutagenesis. The identified MMTV-rodent fragment is present in some MCF-7 sublines but absent in the screened specimens and other cell lines. Sporadic mutations found in this fragment indicate it has multiple copies in the MCF-7 genome. Sequence analysis has identified a novel ORF of approximately 1.6 kb which is 94-99% identical to MMTV env genes. RT-PCR was performed on the MCF-7 cDNA but no MMTV-like env transcript was detected. This is the first report to reveal the locus of MMTV-like env sequence in human cells. The MMTV-like env sequence was shown to be distinct from the human endogenous retroviral sequences and is closely related to rodents.

The Wnt family of cell signalling molecules in postimplantation development of the mouse

The mammalian Wnt gene family consists of at least ten members, all of which share a common structure. The N-terminus encodes a putative signal peptide sequence, suggesting that Wnt proteins are secreted. A number of absolutely conserved cysteine residues imply that inter- or intramolecular disulphide bonding is important to Wnt protein function. Wnt RNAs are localized to discrete regions of the postimplantation embryo and fetus, particularly within the developing central nervous system. Studies on Wnt gene expression strongly suggest that Wnt-mediated signalling is likely to be an important aspect of mouse development. One member of the family, Wnt-1, has been studied in some detail. By generating mutant alleles, we have demonstrated that Wnt-1 regulates regional development of the central nervous system at early somite stages. There is circumstantial evidence that some aspects of the pathway through which Wnt-1 action is mediated may be evolutionarily conserved. We propose that the Wnt family plays a major role in cell-cell interactions in the mouse.

Notch signaling in development and cancer

Notch is an evolutionarily conserved local cell signaling mechanism that participates in a variety of cellular processes: cell fate specification, differentiation, proliferation, apoptosis, adhesion, epithelial-mesenchymal transition, migration, and angiogenesis. These processes can be subverted in Notch-mediated pathological situations. In the first part of this review, we will discuss the role of Notch in vertebrate central nervous system development, somitogenesis, cardiovascular and endocrine development, with attention to the mechanisms by which Notch regulates cell fate specification and patterning in these tissues. In the second part, we will review the molecular aspects of Notch-mediated neoplasias, where Notch can act as an oncogene or as a tumor suppressor. From all these studies, it becomes evident that the outcome of Notch signaling is strictly context-dependent and differences in the strength, timing, cell type, and context of the signal may affect the final outcome. It is essential to understand how Notch integrates inputs from other signaling pathways and how specificity is achieved, because this knowledge may be relevant for future therapeutic applications.

Can MMTV exploit TLR4?

The recognition of microbial pathogens based on their molecular patterns is essential for host defense. Recently, Toll-like receptors have been shown not only to recognize viruses as well as bacteria and fungi, but also to trigger an efficient immune response. A recent publication proposed that the retrovirus mouse mammary tumor virus exploits the pattern-recognition receptor Toll-like receptor 4 to achieve more efficient infection.

Transgenic mouse models of breast cancer

Although valuable initial information can be gathered about transformation from in vitro studies, human cancer occurs in the context of a complex interaction with its environment and must ultimately be studied in living animals. Transgenic animal models have been used to study breast transformation for a number of years and have yielded valuable information on the subject. In this paper, we will summarize results from our laboratories, and others, regarding the use of transgenic mice to study breast tumorigenesis. We will also suggest future directions for the use of transgenic models to understand, and hopefully, one day to cure the disease.

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.

Wnt-3, a gene activated by proviral insertion in mouse mammary tumors, is homologous to int-1/Wnt-1 and is normally expressed in mouse embryos and adult brain

We have isolated a common insertion site, Wnt-3, for proviruses of the mouse mammary tumor virus (MMTV). Of mammary tumors induced by the GR variant of MMTV, 5% contains a provirus at Wnt-3, which is located on mouse chromosome 11. The gene is transcribed into a 3.8-kilobase (kb) mRNA in tumors with nearby proviral insertions but not in tumors with proviruses at other loci or in most adult tissues. Normal expression of Wnt-3 is detected in mouse embryos (with a peak around day 12 of gestation) and at low levels in adult brain. The transcriptional unit of the Wnt-3 gene spans approximately 55 kb, with a first intron of 36 kb. The deduced amino acid sequence of the Wnt-3 protein is 47% identical to the int-1/Wnt-1 gene product.

Insertion mutation of the int-1 and int-2 loci by mouse mammary tumor virus in premalignant and malignant neoplasms from the GR mouse strain

Mouse mammary tumor virus (MMTV)-induced mammary adenocarcinomas can develop from several different premalignant precursors common in GR mice. Insertion mutagenesis of the mammary protooncogenes int-1 and int-2 was studied in this multistep system by analyzing samples from various stages of neoplastic development for novel int-1 and int-2 restriction fragments generated by MMTV provirus integration. int-1 and int-2 insertion mutations were observed in both premalignant lesions and malignant tumors. Some of the tumors with insertion mutations were experimentally derived from insertion mutation-free premalignant precursors. Each class of neoplasm examined had a characteristic frequency of int-1 and int-2 insertion mutations; however, no correspondence was observed between neoplasm morphology and mutation of either gene. These results indicate that insertion mutation of the int-1 and int-2 loci by MMTV provirus can be involved in the earliest identifiable stages of neoplastic development as well as during progression of premalignant lesions to tumors. Insertion mutation of int-1 and int-2 is therefore not stage specific in this system.

Isolation of a pathogenic clone of mouse mammary tumor virus

Exogenous mouse mammary tumor virus (MMTV) was cloned from a GR mammary tumor. Clone lambda GRT39 contained a full-length integrated MMTV(GR) provirus and both 5' and 3' host flanking DNA. The lambda GRT39 provirus had no apparent structural changes associated with cloning and retained the exogenous MMTV gag gene poison sequence. When introduced into rat mammary adenocarcinoma LA7 cells, the lambda GRT39 provirus was fully expressed. lambda GRT39-transfected LA7 cells made MMTV RNA, had gp52 SU protein on the cell surface, and produced B-type retrovirus particles characteristic of MMTV. Mammary tumors developed in hormone-stimulated BALB/c females injected with MMTV from lambda GRT39-transfected LA7 cells [MMTV (lambda GRT39)]. The tumors had new, clonally integrated copies of the MMTV(lambda GRT39) provirus and were expressing MMTV antigen. These data indicate that the lambda GRT39 provirus is biologically active and pathogenic.