H3K4me3 interactions with TAF3 regulate preinitiation complex assembly and selective gene activation.

Histone modifications regulate chromatin-dependent processes, yet the mechanisms by which they contribute to specific outcomes remain unclear. H3K4me3 is a prominent histone mark that is associated with active genes and promotes transcription through interactions with effector proteins that include initiation factor TFIID. We demonstrate that H3K4me3-TAF3 interactions direct global TFIID recruitment to active genes, some of which are p53 targets. Further analyses show that (1) H3K4me3 enhances p53-dependent transcription by stimulating preinitiation complex (PIC) formation; (2) H3K4me3, through TAF3 interactions, can act either independently or cooperatively with the TATA box to direct PIC formation and transcription; and (3) H3K4me3-TAF3/TFIID interactions regulate gene-selective functions of p53 in response to genotoxic stress. Our findings indicate a mechanism by which H3K4me3 directs PIC assembly for the rapid induction of specific p53 target genes.

Reprogramming human T cell function and specificity with non-viral genome targeting.

Decades of work have aimed to genetically reprogram T cells for therapeutic purposes1,2 using recombinant viral vectors, which do not target transgenes to specific genomic sites3,4. The need for viral vectors has slowed down research and clinical use as their manufacturing and testing is lengthy and expensive. Genome editing brought the promise of specific and efficient insertion of large transgenes into target cells using homology-directed repair5,6. Here we developed a CRISPR-Cas9 genome-targeting system that does not require viral vectors, allowing rapid and efficient insertion of large DNA sequences (greater than one kilobase) at specific sites in the genomes of primary human T cells, while preserving cell viability and function. This permits individual or multiplexed modification of endogenous genes. First, we applied this strategy to correct a pathogenic IL2RA mutation in cells from patients with monogenic autoimmune disease, and demonstrate improved signalling function. Second, we replaced the endogenous T cell receptor (TCR) locus with a new TCR that redirected T cells to a cancer antigen. The resulting TCR-engineered T cells specifically recognized tumour antigens and mounted productive anti-tumour cell responses in vitro and in vivo. Together, these studies provide preclinical evidence that non-viral genome targeting can enable rapid and flexible experimental manipulation and therapeutic engineering of primary human immune cells.

LEDGF/p75 interacts with mRNA splicing factors and targets HIV-1 integration to highly spliced genes.

The host chromatin-binding factor LEDGF/p75 interacts with HIV-1 integrase and directs integration to active transcription units. To understand how LEDGF/p75 recognizes transcription units, we sequenced 1 million HIV-1 integration sites isolated from cultured HEK293T cells. Analysis of integration sites showed that cancer genes were preferentially targeted, raising concerns about using lentivirus vectors for gene therapy. Additional analysis led to the discovery that introns and alternative splicing contributed significantly to integration site selection. These correlations were independent of transcription levels, size of transcription units, and length of the introns. Multivariate analysis with five parameters previously found to predict integration sites showed that intron density is the strongest predictor of integration density in transcription units. Analysis of previously published HIV-1 integration site data showed that integration density in transcription units in mouse embryonic fibroblasts also correlated strongly with intron number, and this correlation was absent in cells lacking LEDGF. Affinity purification showed that LEDGF/p75 is associated with a number of splicing factors, and RNA sequencing (RNA-seq) analysis of HEK293T cells lacking LEDGF/p75 or the LEDGF/p75 integrase-binding domain (IBD) showed that LEDGF/p75 contributes to splicing patterns in half of the transcription units that have alternative isoforms. Thus, LEDGF/p75 interacts with splicing factors, contributes to exon choice, and directs HIV-1 integration to transcription units that are highly spliced.

A Combination of Amino Acid Mutations Leads to Resistance to Multiple Nucleoside Analogs in Reverse Transcriptases from HIV-1 Subtypes B and C.

Resistance to anti-HIV drugs has been a problem from the beginning of antiviral drug treatments. The recent expansion of combination antiretroviral therapy worldwide has led to an increase in resistance to antiretrovirals; understanding the mechanisms of resistance is increasingly important. In this study, we analyzed reverse transcriptase (RT) variants based on sequences derived from an individual who had low-level rebound viremia while undergoing therapy with abacavir, azidothymidine (AZT) (zidovudine), and (-)-l-2',3'-dideoxy-3'-thiacytidine (3TC) (lamivudine). The RT had mutations at positions 64, 67, 70, 184, and 219 and a threonine insertion after amino acid 69 in RT. The virus remained partially susceptible to the nucleoside RT inhibitor (NRTI) regimen. We show how these mutations affect the ability of NRTIs to inhibit DNA synthesis by RT. The presence of the inserted threonine reduced the susceptibility of the RT mutant to inhibition by tenofovir.

Clonal expansion of SIV-infected cells in macaques on antiretroviral therapy is similar to that of HIV-infected cells in humans.

Clonal expansion of HIV infected cells plays an important role in the formation and persistence of the reservoir that allows the virus to persist, in DNA form, despite effective antiretroviral therapy. We used integration site analysis to ask if there is a similar clonal expansion of SIV infected cells in macaques. We show that the distribution of HIV and SIV integration sites in vitro is similar and that both viruses preferentially integrate in many of the same genes. We obtained approximately 8000 integration sites from blood samples taken from SIV-infected macaques prior to the initiation of ART, and from blood, spleen, and lymph node samples taken at necropsy. Seven clones were identified in the pre-ART samples; one persisted for a year on ART. An additional 100 clones were found only in on-ART samples; a number of these clones were found in more than one tissue. The timing and extent of clonal expansion of SIV-infected cells in macaques and HIV-infected cells in humans is quite similar. This suggests that SIV-infected macaques represent a useful model of the clonal expansion of HIV infected cells in humans that can be used to evaluate strategies intended to control or eradicate the viral reservoir.

Mutations in HIV-1 reverse transcriptase affect the errors made in a single cycle of viral replication.

UNLABELLED: The genetic variation in HIV-1 in patients is due to the high rate of viral replication, the high viral load, and the errors made during viral replication. Some of the mutations in reverse transcriptase (RT) that alter the deoxynucleoside triphosphate (dNTP)-binding pocket, including those that confer resistance to nucleoside/nucleotide analogs, affect dNTP selection during replication. The effects of mutations in RT on the spectrum (nature, position, and frequency) of errors made in vivo are poorly understood. We previously determined the mutation rate and the frequency of different types of mutations and identified hot spots for mutations in a lacZα (the α complementing region of lacZ) reporter gene carried by an HIV-1 vector that replicates using wild-type RT. We show here that four mutations (Y115F, M184V, M184I, and Q151M) in the dNTP-binding pocket of RT that had relatively small effects on the overall HIV-1 mutation rate (less than 3-fold compared to the wild type) significantly increased mutations at some specific positions in the lacZα reporter gene. We also show that changes in a sequence that flanks the reporter gene can affect the mutations that arise in the reporter. These data show that changes either in HIV-1 RT or in the sequence of the nucleic acid template can affect the spectrum of mutations made during viral replication. This could, by implication, affect the generation of drug-resistant mutants and immunological-escape mutants in patients. IMPORTANCE: RT is the viral enzyme that converts the RNA genome of HIV into DNA. Errors made during replication allow the virus to escape from the host's immune system and to develop resistance to the available anti-HIV drugs. We show that four different mutations in RT which are known to be associated with resistance to anti-RT drugs modestly increased the overall frequency of errors made during viral replication. However, the increased errors were not uniformly distributed; the additional errors occurred at a small number of positions (hot spots). Moreover, some of the RT mutations preferentially affected the nature of the errors that were made (some RT mutations caused an increase in insertion and deletion errors; others caused an increase in substitution errors). We also show that sequence changes in a region adjacent to a target gene can affect the errors made within the target gene.

Enhancers are major targets for murine leukemia virus vector integration.

UNLABELLED: Retroviral vectors have been used in successful gene therapies. However, in some patients, insertional mutagenesis led to leukemia or myelodysplasia. Both the strong promoter/enhancer elements in the long terminal repeats (LTRs) of murine leukemia virus (MLV)-based vectors and the vector-specific integration site preferences played an important role in these adverse clinical events. MLV integration is known to prefer regions in or near transcription start sites (TSS). Recently, BET family proteins were shown to be the major cellular proteins responsible for targeting MLV integration. Although MLV integration sites are significantly enriched at TSS, only a small fraction of the MLV integration sites (<15%) occur in this region. To resolve this apparent discrepancy, we created a high-resolution genome-wide integration map of more than one million integration sites from CD34(+) hematopoietic stem cells transduced with a clinically relevant MLV-based vector. The integration sites form ∼60,000 tight clusters. These clusters comprise ∼1.9% of the genome. The vast majority (87%) of the integration sites are located within histone H3K4me1 islands, a hallmark of enhancers. The majority of these clusters also have H3K27ac histone modifications, which mark active enhancers. The enhancers of some oncogenes, including LMO2, are highly preferred targets for integration without in vivo selection. IMPORTANCE: We show that active enhancer regions are the major targets for MLV integration; this means that MLV preferentially integrates in regions that are favorable for viral gene expression in a variety of cell types. The results provide insights for MLV integration target site selection and also explain the high risk of insertional mutagenesis that is associated with gene therapy trials using MLV vectors.

Differential effects of human immunodeficiency virus type 1 capsid and cellular factors nucleoporin 153 and LEDGF/p75 on the efficiency and specificity of viral DNA integration.

Retroviruses integrate into cellular DNA nonrandomly. Lentiviruses such as human immunodeficiency virus type 1 (HIV-1) favor the bodies of active genes and gene-enriched transcriptionally active regions of chromosomes. The interaction between lentiviral integrase and the cellular protein lens epithelium-derived growth factor (LEDGF)/p75 underlies the targeting of gene bodies, whereas recent research has highlighted roles for the HIV-1 capsid (CA) protein and cellular factors implicated in viral nuclear import, including transportin 3 (TNPO3) and nucleoporin 358 (NUP358), in the targeting of gene-dense regions of chromosomes. Here, we show that CA mutations, which include the substitution of Asp for Asn74 (N74D), significantly reduce the dependency of HIV-1 on LEDGF/p75 during infection and that this difference correlates with the efficiency of viral DNA integration. The distribution of integration sites mapped by Illumina sequencing confirms that the N74D mutation reduces integration into gene-rich regions of chromosomes and gene bodies and reveals previously unrecognized roles for NUP153 (another HIV-1 cofactor implicated in viral nuclear import) and LEDGF/p75 in the targeting of the viral preintegration complex to gene-dense regions of chromatin. A role for the CA protein in determining the dependency of HIV-1 on LEDGF/p75 during infection highlights a connection between the viral capsid and chromosomal DNA integration.

HRP2 determines the efficiency and specificity of HIV-1 integration in LEDGF/p75 knockout cells but does not contribute to the antiviral activity of a potent LEDGF/p75-binding site integrase inhibitor.

The binding of integrase (IN) to lens epithelium-derived growth factor (LEDGF)/p75 in large part determines the efficiency and specificity of HIV-1 integration. However, a significant residual preference for integration into active genes persists in Psip1 (the gene that encodes for LEDGF/p75) knockout (KO) cells. One other cellular protein, HRP2, harbors both the PWWP and IN-binding domains that are important for LEDGF/p75 co-factor function. To assess the role of HRP2 in HIV-1 integration, cells generated from Hdgfrp2 (the gene that encodes for HRP2) and Psip1/Hdgfrp2 KO mice were infected alongside matched control cells. HRP2 depleted cells supported normal infection, while disruption of Hdgfrp2 in Psip1 KO cells yielded additional defects in the efficiency and specificity of integration. These deficits were largely restored by ectopic expression of either LEDGF/p75 or HRP2. The double-KO cells nevertheless supported residual integration into genes, indicating that IN and/or other host factors contribute to integration specificity in the absence of LEDGF/p75 and HRP2. Psip1 KO significantly increased the potency of an allosteric inhibitor that binds the LEDGF/p75 binding site on IN, a result that was not significantly altered by Hdgfrp2 disruption. These findings help to rule out the host factor-IN interactions as the primary antiviral targets of LEDGF/p75-binding site IN inhibitors.

Lens epithelium-derived growth factor fusion proteins redirect HIV-1 DNA integration.

Lens epithelium-derived growth factor (LEDGF) fusion proteins can direct HIV-1 DNA integration to novel sites in the host genome. The C terminus of LEDGF contains an integrase binding domain (IBD), and the N terminus binds chromatin. LEDGF normally directs integrations to the bodies of expressed genes. Replacing the N terminus of LEDGF with chromatin binding domains (CBDs) from other proteins changes the specificity of HIV-1 DNA integration. We chose two well-characterized CBDs: the plant homeodomain (PHD) finger from ING2 and the chromodomain from heterochromatin binding protein 1alpha (HP1alpha). The ING2 PHD finger binds H3K4me3, a histone mark that is associated with the transcriptional start sites of expressed genes. The HP1alpha chromodomain binds H3K9me2,3, histone marks that are widely distributed throughout the genome. A fusion protein in which the ING2 PHD finger was linked to the LEDGF IBD directed integrations near the start sites of expressed genes. A similar fusion protein in which the HP1alpha chromodomain was linked to the LEDGF IBD directed integrations to sites that differed from both the PHD finger fusion-directed and LEDGF-directed integration sites. The ability to redirect HIV-1 DNA integration may help solve the problems associated with the activation of oncogenes when retroviruses are used in gene therapy.