Chromatin function regulates retroviral gene expression

Synopsis: experimental work described in this Thesis has been mainly focused on the study of the regulation of viral gene expression, in two different members of the Retroviridae family, namely the lentivirus HIV-1 and the gammaretrovirus MoMLV. The results are divided into two parts, Part A and Part B. The work reported in Part A gives critical clues for the comprehension of the still obscure mechanisms that regulate function of the HIV-1 transactivator Tat in the initiation and elongation of HIV-1 transcription. In particular, we exploited a proteomic screening aimed at characterizing novel Tat partners. This approach allowed us to identify the cellular histone chaperone NAP-1 (Nucleosome Assembly Protein-1) as a Tat-binding protein. We found that this interaction exerts a positive role on Tat-driven LTR transactivation and on HIV-1 infection. To our knowledge, this is the first demonstration of an interaction between Tat and a cellular histone chaperone. We propose a mechanism by which Tat benefits from this class of proteins to relieve the repression imposed by chromatin conformation on proviral transcription. The findings described in Part B are part of an ongoing project, that we derived from our previous observation that HIV-1 transcription is also regulated by long-range chromatin interactions. In particular, we originally found that a gene loop structure is imposed on the provirus upon transcriptional activation. We now show that MoMLV also adopts a transcription-dependent LTR-LTR gene loop conformation. This observation leads us to hypothesize that gene looping might be a general hallmark of retroviral transcription. Moreover, we also demonstrate that an aberrant loop might form between the retroviral MoMLV LTRs and regulatory regions of the host cell genome. We suggest that this event might be at the basis of the phenomenon of insertional mutagenesis observed in some of the gene therapy clinical trials that so far have exploited members of the gammaretroviridae family of viruses.