BackgroundOne of the hallmarks of retroviral life cycle is the efficient and specific packaging of two copies of retroviral gRNA in the form of a non-covalent RNA dimer by the assembling virions. It is becoming increasingly clear that the process of dimerization is closely linked with gRNA packaging, and in some retroviruses, the latter depends on the former. Earlier mutational analysis of the 5’ end of the MMTV genome indicated that MMTV gRNA packaging determinants comprise sequences both within the 5’ untranslated region (5’ UTR) and the beginning of gag.ResultsThe RNA secondary structure of MMTV gRNA packaging sequences was elucidated employing selective 2’hydroxyl acylation analyzed by primer extension (SHAPE). SHAPE analyses revealed the presence of a U5/Gag long-range interaction (U5/Gag LRI), not predicted by minimum free-energy structure predictions that potentially stabilizes the global structure of this region. Structure conservation along with base-pair covariations between different strains of MMTV further supported the SHAPE-validated model. The 5’ region of the MMTV gRNA contains multiple palindromic (pal) sequences that could initiate intermolecular interaction during RNA dimerization. In vitro RNA dimerization, SHAPE analysis, and structure prediction approaches on a series of pal mutants revealed that MMTV RNA utilizes a palindromic point of contact to initiate intermolecular interactions between two gRNAs, leading to dimerization. This contact point resides within pal II (5’ CGGCCG 3’) at the 5’ UTR and contains a canonical “GC” dyad and therefore likely constitutes the MMTV RNA dimerization initiation site (DIS). Further analyses of these pal mutants employing in vivo genetic approaches indicate that pal II, as well as pal sequences located in the primer binding site (PBS) are both required for efficient MMTV gRNA packaging.ConclusionsEmploying structural prediction, biochemical, and genetic approaches, we show that pal II functions as a primary point of contact between two MMTV RNAs, leading to gRNA dimerization and its subsequent encapsidation into the assembling virus particles. The results presented here enhance our understanding of the MMTV gRNA dimerization and packaging processes and the role of structural motifs with respect to RNA-RNA and possibly RNA-protein interactions that might be taking place during MMTV life cycle.Electronic supplementary materialThe online version of this article (doi:10.1186/s12977-014-0096-6) contains supplementary material, which is available to authorized users.

Earlier genetic and structural prediction analyses revealed that the packaging determinants of Mason Pfizer monkey virus (MPMV) include two discontinuous core regions at the 5 ′ end of its genomic RNA. RNA secondary structure predictions suggested that these packaging determinants fold into several stem-loops (SLs). To experimentally validate this structural model, we employed selective 2 ′ hydroxyl acylation analyzed by primer extension (SHAPE), which examines the flexibility of the RNA backbone at each nucleotide position. Our SHAPE data validated several predicted structural motifs, including U5/Gag longrange interactions (LRIs), a stretch of single-stranded purine (ssPurine)-rich region, and a distinctive G-C-rich palindromic (pal) SL. Minimum free-energy structure predictions, phylogenetic, and in silico modeling analyses of different MPMV strains revealed that the U5 and gag sequences involved in the LRIs differ minimally within strains and maintain a very high degree of complementarity. Since the pal SL forms a helix loop containing a canonical "GC" dyad, it may act as a RNA dimerization initiation site (DIS), enabling the virus to package two copies of its genome. Analyses of wild-type and pal mutant RNAs revealed that disruption of pal sequence strongly affected RNA dimerization. However, when in vitro transcribed transcomplementary pal mutants were incubated together showed RNA dimerization was restored authenticating that the pal loop (5 ′ -CGGCCG-3 ′ ) functions as DIS.

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The feline immunodeficiency virus (FIV) is a lentivirus that is related to human immunodeficiency virus (HIV), causing a similar pathology in cats. It is a potential small animal model for AIDS and the FIV-based vectors are also being pursued for human gene therapy. Previous studies have mapped the FIV packaging signal (ψ) to two or more discontinuous regions within the 5′ 511 nt of the genomic RNA and structural analyses have determined its secondary structure. The 5′ and 3′ sequences within ψ region interact through extensive long-range interactions (LRIs), including a conserved heptanucleotide interaction between R/U5 and gag. Other secondary structural elements identified include a conserved 150 nt stem–loop (SL2) and a small palindromic stem–loop within gag open reading frame that might act as a viral dimerization initiation site. We have performed extensive mutational analysis of these sequences and structures and ascertained their importance in FIV packaging using a trans-complementation assay. Disrupting the conserved heptanucleotide LRI to prevent base pairing between R/U5 and gag reduced packaging by 2.8–5.5 fold. Restoration of pairing using an alternative, non-wild type (wt) LRI sequence restored RNA packaging and propagation to wt levels, suggesting that it is the structure of the LRI, rather than its sequence, that is important for FIV packaging. Disrupting the palindrome within gag reduced packaging by 1.5–3-fold, but substitution with a different palindromic sequence did not restore packaging completely, suggesting that the sequence of this region as well as its palindromic nature is important. Mutation of individual regions of SL2 did not have a pronounced effect on FIV packaging, suggesting that either it is the structure of SL2 as a whole that is necessary for optimal packaging, or that there is redundancy within this structure. The mutational analysis presented here has further validated the previously predicted RNA secondary structure of FIV ψ.

SummaryBendamustine (B) with rituximab (R) has become the preferred regimen for patients with indolent lymphoma in Ontario, Canada, compared to R with cyclophosphamide, vincristine, prednisone (CVP) or cyclophosphamide, doxorubicin, vincristine and prednisone (CHOP). We conducted a propensity‐matched retrospective cohort population‐based study of patients treated with R‐CVP/CHOP from 2005 to 2012 and patients treated with BR from 2013 to 2018. The primary outcome was 5‐year overall survival (OS), and secondary outcomes included toxicities and healthcare utilization. The 5‐year OS for patients treated with BR (n = 2023) and R‐CVP/CHOP (n = 2023) was 80% and 75% respectively. Treatment with BR was associated with improved OS (HR 0.79, 95% CI 0.69–0.91). During the first 9 months, patients treated with BR versus R‐CVP/CHOP had a higher number of admissions for infection (22% compared to 17%, p < 0.01) and a higher number of mean ED visits (mean 1.01 ± 1.68 visits vs. 0.85 ± 1.51 visits, p < 0.01). This trend persisted for 3 years. The adjusted 5‐year OS for patients 75 years and older did not differ based on treatment regimen (55.5% for BR vs. 55.4% for R‐CVP/CHOP). Our study supports the use of BR for patients with indolent lymphoma requiring treatment but suggests increased risk of certain toxicities warranting careful patient selection.