Steroid hormone responsiveness of a family of closely related mouse proviral elements

Mammalian Endogenous Retroviruses

Over 40% of mammalian genomes comprise the products of reverse transcription. Among such retrotransposed sequences are those characterized by the presence of long terminal repeats (LTRs), including the endogenous retroviruses (ERVs), which are inherited genetic elements closely resembling the proviruses formed following exogenous retrovirus infection. Sequences derived from ERVs make up at least 8 to 10% of the human and mouse genomes and range from ancient sequences that predate mammalian divergence to elements that are currently still active. In this chapter we describe the discovery, classification and origins of ERVs in mammals and consider cellular mechanisms that have evolved to control their expression. We also discuss the negative effects of ERVs as agents of genetic disease and cancer and review examples of ERV protein domestication to serve host functions, as in placental development. Finally, we address growing evidence that the gene regulatory potential of ERV LTRs has been exploited multiple times during evolution to regulate genes and gene networks. Thus, although recently endogenized retroviral elements are often pathogenic, those that survive the forces of negative selection become neutral components of the host genome or can be harnessed to serve beneficial roles.

Impact of transposable elements on the evolution of mammalian gene regulation

Transposable elements (TEs) are present in all organisms and nearly half of the human and mouse genome is derived from ancient transpositions. This fact alone suggests that TEs have played a major role in genome organization and evolution. Studies undertaken over the last two decades or so clearly show that TEs of various kinds have played an important role in organism evolution. Here we review the impact TEs have on the evolution of gene regulation and gene function with an emphasis on humans. Understanding the mechanisms resulting in genomic change is central to our understanding of gene regulation, genetic disease and genome evolution. Full comprehension of these biological processes is not possible without an in depth knowledge of how TEs impact upon the genome.

Effect of 17beta-estradiol and progesterone on the expression of FeLV in chronically infected cells

In a previous study, it was found that even though more male cats were infected by feline leukaemia virus (FeLV), females seemed to progress easier to overt disease. To study the effect of female hormones, 17beta-estradiol and progesterone were added in different concentrations (10(-3) M to 10(-12) M) to a culture of persistently FeLV-infected cells. The effect of both hormones was very similar. After 24 h the cell viability was very low at 10(-3) M and 10(-4) M but similar to controls at the remaining concentrations. Liberation of viral particles was estimated by the reverse transcriptase activity (RT), which was the lowest also at 10(-3) M and 10(-4) M. However, low viability could not account for this low RT, as when cells were lysed with lysis buffer RT was high. Thus, cells were dying without freeing viral particles, suggestive of apoptosis. This possibility was confirmed by staining hormone-treated cells with annexin V and propidium iodide. The FeLV antigen p27 measured in the cultures had a maximum at 10(-3) M and 10(-4) M, higher than controls and lysed cells, so the presence of p27 in the supernatant was not only due to cell lysis but a consequence of hormone effect. In conclusion, 17beta-estradiol and progesterone induce death of FeLV-infected cells at high concentrations, probably through a process of apoptosis, which might limit the spread of the infection, as infective viral particles would be hampered from budding.

Dancing with complement C4 and the RP-C4-CYP21-TNX (RCCX) modules of the major histocompatibility complex

The number of the complement component C4 genes varies from 2 to 8 in a diploid genome among different human individuals. Three quarters of the C4 genes in Caucasian populations have the endogenous retrovirus, HERV-K(C4), in the ninth intron. The remainder does not. The C4 serum proteins are highly polymorphic and their concentrations vary from 100 to approximately 1000 microg/ml. There are two distinct classes of C4 protein, C4A and C4B, which have diversified to fulfill (a) the opsonization/immunoclearance purposes and (b) the well-known complement function in the killing of microbes by lysis and neutralization, respectively. Many infectious and autoimmune diseases are associated with complete or partial deficiency of C4A and/or C4B. The adverse effects of high C4 gene dosages, however, are just emerging, as the concepts of human C4 genetics are revised and accurate techniques are applied to distinguish partial deficiencies from differential expression caused by unequal C4A and C4B gene dosages and gene sizes. This review attempts to dissect the sophisticated genetics of complement C4A and C4B. The emphases are on the qualitative and quantitative diversities of C4 genotypes and phenotypes. The many allotypic variants and the processed products of human and mouse C4 proteins are described. The modular variation of C4 genes together with the serine/threonine nuclear kinase gene RP, the steroid 21-hydroxylase CYP21, and extracellular matrix protein TNX (RCCX modules) are investigated for the effects on homogenization of C4 protein polymorphisms, and on the unequal genetic crossovers that knocked out the functions of CYP21 and/or TNX. Furthermore, the influence of the endogenous retrovirus HERV-K(C4) on C4 gene expression and the dispersal of HERV-K(C4) family members in the human genome are discussed.

The porcine endogenous retrovirus long terminal repeat contains a single nucleotide polymorphism that confers distinct differences in estrogen receptor binding affinity between PERV A and PERV B/C subtypes

Porcine endogenous retroviruses (PERV) have been shown to have zoonotic potential, both in vitro and in vivo. Once integrated into the host cell genome activation of the proviral genes is ultimately dependent upon transactivation of the long terminal repeat (LTR). Currently there is no direct evidence of host cell transcription factors interacting with PERV LTRs. Using comparative genomics we discovered a potentially functional single nucleotide polymorphism (SNP) within the U5 region downstream of the TATA box in the PERV LTR that distinguishes PERV A from PERV B and PERV C subtypes. We demonstrated that the SNP occurs within a potential hormone-responsive region where it has a profound effect, not only upon estrogen receptor binding but also upon the binding of other transcription factors at this site. These results suggest that differences in transcriptional regulation between PERV subtypes are subtle and, as for other retroviruses, transcription can be mediated by steroid hormone receptors.

Organizations and gene duplications of the human and mouse MHC complement gene clusters

The MHC complement gene cluster (MCGC) in most people contains thirteen structural genes, pseudogenes and gene segments. Novel genes RD, SKI2W, DOM3Z and RP1 are organized as two head-to-head gene pairs between complement gene Bf and the first locus of C4. Southern blot analysis shows that single-copy genes for DOM3Z are detectable in primates and other mammals. Sequence analyses revealed that the exon- intron structures of human and mouse DOM3Z genes are identical. Both human and mouse DOM3Z transcripts exhibit splice variants at the 5' regions, although the open reading frames remain identical. Cloning and characterization of the mouse RP1 cDNA revealed a reading frame for 254 amino acids with a bipartite nuclear localization signal close to the amino terminus. The mouse RP1 gene consists of 7 exons and spans 12.9 kb. Located in intron 4 of mouse RP1 is an endogenous retrovirus that probably confers the androgen-responsive expression of the Slp protein in certain male mice. The availability of the complete human and mouse MCGC genomic and cDNA sequences allows further deliberate analyses of gene duplications and evolution. The intergenic region between mouse SLP and C4 genes is more than six times larger than the corresponding region in humans. It contains the functional gene steroid CYP21A, long stretches of repetitive DNA elements, and three partially duplicated gene segments TNXA, SKI2W2 and RP2. The modular duplications of human and mouse RP-C4-CYP21-TNX (RCCX) are sharply different as SKI2W2 is absent in the human MCGC, and TNXA and RP2 are smaller in size but higher in sequence conservation in humans.