Endogenous mammary tumour virus DNA varies among wild mice and segregates during inbreeding.

Proviruses of the mouse mammary tumour virus (MMTV) endogenous to normal mice can be identified by molecular hybridisation and distinguished using restriction endonucleases. Feral mice display marked heterogeneity with respect to the number of copies and the sites of insertion of endogenous MMTV-specific DNA, with occasional mice apparently free of MMTV DNA. Several different MMTV proviruses present in laboratory mice have segregated like stable, independent genetic elements during the inbreeding which followed a cross between Bagg albino and DBA mice 60 years ago. The results favour the hypothesis that endogenous proviruses have been established by multiple, independent infections of germ cells rather than by somatic mutation of ancestral proviruses or of cellular genes.     

Expression of endogenous murine leukaemia viruses in AKR/J streaker mice

MICE of the AKR/J strain show high rates of spontaneous thymic lymphoma and are chronically infected with murine leukaemia viruses. Total thymectomy in young mice of this strain results in a marked decrease in the incidence of leukaemia. The finding of athymic mutant mice on the AKR/J background (referred to as streaker mice) is of importance in the study of genetic factors which influence virus expression and leukaemogenesis. Here we report that mice of both normal and mutant genotypes vary in the expression of a particular class of murine leukaemia virus. This lower virus expression in conjunction with the absence of a thymus leads to a lower tumour incidence in streaker mice.     

Cell-surface antigens induced by RNA tumour viruses.

Host animals show an immune response to the surface of cells infected with RNA tumour viruses. An element of this response is due to expression of viral structural antigens, but the major part is due to virus-induced cell-surface antigens (CSAs). This article compares the properties of CSAs of the avian, murine and feline retrovirus systems.     

gag Gene of mammalian type-C RNA tumour viruses.

The translation product of the gag gene of mammalian type-c- RNA viruses is a 65,000-68,000 molecular weight precursor polypeptide (Pr65) whose cleavage leads to the formation of four virion proteins, p30, p15, p12 and p10. An immunological approach has been used to establish the arrangement of the sequences coding for these proteins within the viral genome as (5') p15-p12-p30-p10 (3').     

Clonal deletion of V beta 14-bearing T cells in mice transgenic for mammary tumour virus.

Autoreactive T lymphocytes are clonally deleted during maturation in the thymus. Deletion of T cells expressing particular receptor V beta elements is controlled by poorly defined autosomal dominant genes. A gene has now been identified by expression of transgenes in mice which causes deletion of V beta 14+ T cells. The gene lies in the open reading frame of the long terminal repeat of the mouse mammary tumour virus.     

Linkage of Marfan syndrome and a phenotypically related disorder to two different fibrillin genes.

Marfan syndrome (MFS), one of the most common genetic disorders of connective tissue, is characterized by skeletal, cardiovascular and ocular abnormalities. The incidence of the disease is about 1 in 20,000, with life expectancy severely reduced because of cardiovascular complications. As the underlying defect is unknown, MFS diagnosis is based solely on clinical criteria. Certain phenotypic features of MFS are also shared by other conditions, which may be genetically distinct entities although part of a clinical continuum. Immunohistochemical studies have implicated fibrillin, a major component of elastin-associated microfibrils, in MFS aetiology. Genetic linkage analysis with random probes has independently localized the MFS locus to chromosome 15. Here we report that these two experimental approaches converge with the cloning and mapping of the fibrillin gene to chromosome 15q15-21, and with the establishment of linkage to MFS. We also isolated a second fibrillin gene and mapped it to chromosome 5q23-31. We linked this novel gene to a condition, congenital contractural arachnodactyly, that shares some of the features of MFS. Thus, the cosegregation of two related genes with two related syndromes implies that fibrillin mutations are likely to be responsible for different MFS phenotypes.     

A maternally inherited superantigen encoded by a mammary tumour virus.

A collection of superantigens, molecules which in combination with class II major histocompatibility complex (MHC) engage T cells bearing particular V beta chains as part of their alpha beta receptors, have recently been described. The mouse self superantigen, Mls-1a, for example, in conjunction with many MHC class II proteins, engages mouse T cells bearing V beta 6, 7, 8.1 and 9, almost regardless of the sequences of the other variable components of the receptors on the T cells. Two types of superantigen have been identified so far: first, superantigens encoded in the mouse genome, such as Mls-1a; second, superantigens produced by bacteria, such as the staphylococcal enterotoxins. Although the latter type of superantigens are in many cases known to be proteins of about 220 amino acids, nothing is known about the structures of any of the superantigens encoded in mouse. Here we describe the properties of a new mouse superantigen. The antigen is maternally transmitted in milk and is probably encoded by a mammary tumour virus (MTV). Given the known genetic linkage between at least one of the mouse genomic superantigens and endogenous MTV integration sites, it is tempting to speculate that the superantigen described here and some of the endogenous mouse superantigens are encoded by MTVs.     

Steroid-free glucocorticoid receptor binds specifically to mouse mammary tumour virus DNA

Steroid hormones are thought to modulate gene expression through their interaction with receptor proteins. The intracellular localization of unoccupied receptor proteins has been a subject of controversy: free glucocorticoid receptor appears to reside in the cytoplasm and moves to the cell nucleus only after binding the steroid. The purified hormone-bound glucocorticoid receptor has been shown to bind selectively to hormone regulatory elements (HRE) in the vicinity of hormonally-inducible promoters and, in particular, in the long terminal repeat (LTR) region of mouse mammary tumour virus (MMTV). We have tackled the question of whether the hormone itself is required for the interaction of the receptor protein with the HRE. Using monoclonal antibodies to the receptor we find that upon heat-activation the steroid-free glucocorticoid receptor present in rat liver cytosol binds specifically in vitro to the HRE of MMTV. No qualitative differences in the DNaseI-footprints were detected when hormone-free receptor was compared to the hormone-receptor complex or even receptor complexed with the hormone antagonist RU486. We conclude that the steroid ligand is not an absolute requirement for generating the conformation of the glucocorticoid receptor that allows its interaction with the HRE in vitro. An alternative function of the hormone in vivo could be to modulate nuclear partitioning of the receptor.