Of birds and mice and men: comments on the use of animal models and molecular hybridization in the search for human tumor viruses

Human RNA "rumor" viruses: the search for novel human retroviruses in chronic disease

Retroviruses are an important group of pathogens that cause a variety of diseases in humans and animals. Four human retroviruses are currently known, including human immunodeficiency virus type 1, which causes AIDS, and human T-lymphotropic virus type 1, which causes cancer and inflammatory disease. For many years, there have been sporadic reports of additional human retroviral infections, particularly in cancer and other chronic diseases. Unfortunately, many of these putative viruses remain unproven and controversial, and some retrovirologists have dismissed them as merely "human rumor viruses." Work in this field was last reviewed in depth in 1984, and since then, the molecular techniques available for identifying and characterizing retroviruses have improved enormously in sensitivity. The advent of PCR in particular has dramatically enhanced our ability to detect novel viral sequences in human tissues. However, DNA amplification techniques have also increased the potential for false-positive detection due to contamination. In addition, the presence of many families of human endogenous retroviruses (HERVs) within our DNA can obstruct attempts to identify and validate novel human retroviruses. Here, we aim to bring together the data on "novel" retroviral infections in humans by critically examining the evidence for those putative viruses that have been linked with disease and the likelihood that they represent genuine human infections. We provide a background to the field and a discussion of potential confounding factors along with some technical guidelines. In addition, some of the difficulties associated with obtaining formal proof of causation for common or ubiquitous agents such as HERVs are discussed.

Detection and cloning of human DNA sequences related to the mouse mammary tumor virus genome

Sequences related to the mouse mammary tumor virus (MMTV) genome have been detected in fragments of restricted human cellular DNA. These results were obtained by using recombinant DNA containing the MMTV proviral genome and lowering the stringency of blot-hybridization conditions. The MMTV genome also reacts with unique families of fragments in restricted cellular DNA from other mammalian species but not with salmon sperm DNA. A clone that reacted with labeled MMTV proviral DNA was selected from a human DNA library in Charon 4A. Under stringent conditions, a 3.7-kilobase MMTV-related EcoRI fragment of this clone hybridized with many of the same EcoRI restriction fragments of human cellular DNA detectable with MMTV proviral DNA under low-stringency conditions. Specific fragments of the human clone were shown to contain sequences related to the molecularly cloned gag, pol, and env regions of the MMTV genome.

Regulation of mouse mammary tumor virus gene expression by glucocorticoid hormones

Several laboratories have documented that glucocorticoid hormones markedly stimulate the expression of mouse mammary tumor virus genes in a variety of mouse mammary tumor cells and in infected heterologous cells. The effect of the hormone appears to be a rapid and specific augmentation of the synthesis of viral RNA, mediated by interaction with glucocorticoid receptor proteins. The availability of virus-specific reagents and recent developments in the molecular biology of RNA tumor viruses now permit a highly refined analysis of hormonal regulation in this experimental system.