AIDS-causing HI viruses preferentially integrate into transcriptionally active genes of the host cell genome. These gene regions are located in the immediate vicinity of the nuclear pores. A team of researchers from Heidelberg University Hospital and colleagues from Italy have now shown that the integration of the viruses is controlled by components of the nuclear pores. These findings are an important step towards gaining a detailed understanding of how HI viruses can hide from therapeutic drugs inside the nucleus.
Although great and decisive breakthroughs in the fight against AIDS have repeatedly been announced, infection with the AIDS-causing human immunodeficiency virus (HIV) cannot be completely cured. Although antiretroviral drugs are able to control the disease and eliminate the viruses from the blood, the genetic information of the virus nevertheless remains in the patient's DNA and can become active again after discontinuation of therapy. This leads to the production and rapid proliferation of new HI viruses that invade the body and cause a renewed outbreak of the disease.
The inactive state of the viruses when they remain dormant inside the host cells and cannot be reached by therapeutic drugs or eliminated by cellular defense mechanisms is referred to as latency. "It is only by understanding the mechanisms of HIV latency that we will be able to develop therapeutic countermeasures," explains Dr. Marina Lusic, senior research scientist in the Department of Infectious Diseases at Heidelberg University Hospital.
Marina Lusic, an internationally reputed virologist, moved from the International Centre for Genetic Engineering and Biotechnology in Trieste (Italy) to Heidelberg in 2014, where she has continued her previous work on HIV function in the Thematic Translation Unit "HIV" at the German Centre for Infection Research in close cooperation with researchers from all over Germany. Lusic and her team have recently published a paper in the prestigious journal Nature, in which they provide new insights into the integration of the HI viruses in the host genome.
It has long been known that HIV-1 (human immunodeficiency virus type 1), which is responsible for the global AIDS pandemic, does not integrate at random into the host genome following the infection of the CD4(+) T cells of the host's immune system. The process of integration starts with the reverse transcription of the HIV RNA into DNA by the viral reverse transcriptase enzyme in the cytoplasm. The DNA is then transported through the pores of the nuclear membrane into the cell nucleus where it is integrated into transcriptionally active genes that are involved in different cell functions. The reason why the virus selects only certain of the transcriptionally active genes remains largely unknown. Lusic and her team have now shown that HIV-1 integration occurs in the outer shell of the nucleus in close correspondence with the nuclear pore. The researchers did not find viral DNA in other transcriptionally active DNA regions located closer to the centre of the nucleus.
Marina Lusic has developed a seemingly trivial explanation for this: the genetic material of the HI viruses is unable to get further than the entrance area of the cell nucleus because it is integrated into the first transcriptionally active genes it encounters after passing through a nuclear pore. "Imagine that you are invited to a concert or play and you are late. The obvious thing to do is to take the first decent seat you see. And this is just what the viral genetic material does," says Lusic. However, the molecular mechanisms of this behaviour are much more complicated.
The integration of viral DNA into peripheral host DNA involves a viral integrase enzyme and two cellular cofactors of the enzyme. It therefore appears that certain viral genes are transcribed and translated into viral proteins prior to integration of the viral DNA into the host genome. One of the cofactors, Nup153, belongs to the nucleoporin class of proteins that make up the nuclear pore complex. Lusic and her team have been able to show that the HIV-1 DNA comes into contact with different nucleoporins as it passes through the nuclear pore. This association is necessary for integration into the host genome. The architecture of the cell nucleus therefore determines the host genes into which viral DNA is integrated.
HIV-1 persists in a latent state in resting CD4(+) T cells where the virus is protected against antiviral drugs and the host immune system that can only destroy free viruses in the blood. If a person is infected again, for example by another pathogen, the T cells will be activated and start to produce large amounts of virus copies, which will eventually destroy the host cell and infect new cells. However, some of the viruses will remain in an inactive state in the cell nucleus of a T-cell subpopulation known as CD4(+) memory T cells. This large number of latent HI viruses is the reason why the viruses can never be fully eradicated from the body and why the disease cannot be cured.
The newly published results are an important step towards a better understanding of how HI viruses hide their DNA in host cells. The elucidation of the mechanisms that lead to viral latency is a prerequisite for finding ways to reverse latency and make the hidden virus reservoir accessible for therapy. Finding a solution to this is the objective of Marina Lusic and her team as well as of many other groups of researchers who are part of the Thematic Translational Unit "HIV" (TTU HIV) of the German Centre for Infection Research. This Germany-wide collaboration is coordinated by Professor Hans-Georg Kräusslich, one of the world's leading HIV researchers and spokesperson of the Department of Infectious Diseases at Heidelberg University Hospital.
Bruna Marini, Attila Kertesz-Farkas, Hashim Ali, Bojana Lucic, Kamil Lisek, Lara Manganaro, Sandor Pongor, Roberto Luzzati, Alessandra Recchia, Fulvio Mavilio, Mauro Giacca & Marina Lusic. Nuclear architecture dictates HIV-1 integration site selection. Nature (2015) doi: 10.1038/nature14226