Do retroviruses preferentially integrate within highly plastic regions of the human genome?

Whether retroviral integration is a phenomenon specific to properties of the surrounding genomic region is a widely debated question. In this paper we attempt to enlight the involvement of genomic regions prone to DNA double strand breaks in such process, as well as the more general concept of genome plasticity concerning repair, recombination, transposition events. While performing a differential display analysis of the promonocytic cell line U937 and clone U42 HIV infected counterpart, we found, out of about 15 highly dysregulated genes, expected according to our previous proteomic analysis, two dysregulated cellular transcripts that are shown in the present study to colocalize on band 22q11. The LB14 transcript maps within the DiGeorge critical region. Whereas the AG46 transcript encodes the immunoglobulin-lambda like polypeptide 1 (IGLL1) 4.7Mb apart from LB14. The 22q11 band is remarkable for its high plasticity involving DNA double strand breaks, that may lead to translocations, large deletions, and immunoglobulin rearrangements, frequently observed in this region. We suggest that provirus integration preferentially occurs in such genomic regions and that the subsequent insertional mutagenesis leads to the present observations. Finally, we stress out the possibility that the small size of chromosome 22 is associated with this physical property of the genome.

Human immunodeficiency virus type 1 central DNA flap: dynamic terminal product of plus-strand displacement dna synthesis catalyzed by reverse transcriptase assisted by nucleocapsid protein.

To terminate the reverse transcription of the human immunodeficiency virus type 1 (HIV-1) genome, a final step occurs within the center of the proviral DNA generating a 99-nucleotide DNA flap (6). This step, catalyzed by reverse transcriptase (RT), is defined as a discrete strand displacement (SD) synthesis between the first nucleotide after the central priming (cPPT) site and the final position of the central termination sequence (CTS) site. Using recombinant HIV-1 RT and a circular single-stranded DNA template harboring the cPPT-CTS sequence, we have developed an SD synthesis-directed in vitro termination assay. Elongation, strand displacement, and complete central flap behavior were analyzed using electrophoresis and electron microscopy approaches. Optimal conditions to obtain complete central flap, which ended at the CTS site, have been defined in using nucleocapsid protein (NCp), the main accessory protein of the reverse transcription complex. A full-length HIV-1 central DNA flap was then carried out in vitro. Its synthesis appears faster in the presence of the HIV-1 NCp or the T4-encoded SSB protein (gp32). Finally, a high frequency of strand transfer was shown during the SD synthesis along the cPPT-CTS site with RT alone. This reveals a local and efficient 3'-5' branch migration which emphasizes some important structural fluctuations within the flap. These fluctuations may be stabilized by the NCp chaperone activity. The biological implications of the RT-directed NCp-assisted flap synthesis are discussed within the context of reverse transcription complexes, assembly of the preintegration complexes, and nuclear import of the HIV-1 proviral DNA to the nucleus toward their chromatin targets.

Cloning of a long HIV-1 readthrough transcript and detection of an increased level of early growth response protein-1 (Egr-1) mRNA in chronically infected U937 cells.

To identify the pathways involved in HIV-1 modification of cellular gene expression, chronically infected U937 cells were screened by mRNA differential display. A chimeric transcript consisting of the 3' end of the LTR of a HIV-1 provirus, followed by 3.7 kb of cellular RNA was identified suggesting that long readthrough transcription might be one of the mechanisms by which gene expression could be modified in individual infected cells. Such a phenomenon may also be the first step towards the potential transduction of cellular sequences. Furthermore, the mRNA encoding for the transcription factor Egr-1 was detected as an over-represented transcript in infected cells. Northern blot analysis confirmed the increase of Egr-1 mRNA content in both HIV-1 infected promonocytic U937 cells and T cell lines such as Jurkat and CEM. Interestingly a similar increase of Egr-1 mRNA has previously been reported to occur in HTLV-1 and HTLV-2 infected T cell lines. Despite the consistent increase in the level of Egr-1 mRNA, the amount of the encoded protein did not appear to be modified in HIV-1 infected cells, suggesting an increased turn over of the protein in chronically infected cells.