Base editing of haematopoietic stem cells rescues sickle cell disease in mice.

Sickle cell disease (SCD) is caused by a mutation in the ß-globin gene HBB1. We used a custom adenine base editor (ABE8e-NRCH)2,3 to convert the SCD allele (HBBS) into Makassar ß-globin (HBBG), a non-pathogenic variant4,5. Ex vivo delivery of mRNA encoding the base editor with a targeting guide RNA into haematopoietic stem and progenitor cells (HSPCs) from patients with SCD resulted in 80% conversion of HBBS to HBBG. Sixteen weeks after transplantation of edited human HSPCs into immunodeficient mice, the frequency of HBBG was 68% and hypoxia-induced sickling of bone marrow reticulocytes had decreased fivefold, indicating durable gene editing. To assess the physiological effects of HBBS base editing, we delivered ABE8e-NRCH and guide RNA into HSPCs from a humanized SCD mouse6 and then transplanted these cells into irradiated mice. After sixteen weeks, Makassar ß-globin represented 79% of ß-globin protein in blood, and hypoxia-induced sickling was reduced threefold. Mice that received base-edited HSPCs showed near-normal haematological parameters and reduced splenic pathology compared to mice that received unedited cells. Secondary transplantation of edited bone marrow confirmed that the gene editing was durable in long-term haematopoietic stem cells and showed that HBBS-to-HBBG editing of 20% or more is sufficient for phenotypic rescue. Base editing of human HSPCs avoided the p53 activation and larger deletions that have been observed following Cas9 nuclease treatment. These findings point towards a one-time autologous treatment for SCD that eliminates pathogenic HBBS, generates benign HBBG, and minimizes the undesired consequences of double-strand DNA breaks.

Efficient Production of On-Target Reads for Small RNA Sequencing of Single Cells Using Modified Adapters.

Although single-cell mRNA sequencing has been a powerful tool to explore cellular heterogeneity, the sequencing of small RNA at the single-cell level (sc-sRNA-seq) remains a challenge, as these have no consensus sequence, are relatively low abundant, and are difficult to amplify in a bias-free fashion. We present two methods of single-cell-lysis that enable sc-sRNA-seq. The first method is a chemical-based technique with overnight freezing while the second method leverages on-chip electrical lysis of plasma membrane and physical extraction and separation of cytoplasmic RNA via isotachophoresis. We coupled these two methods with off-chip small RNA library preparation using CleanTag modified adapters to prevent the formation of adapter dimers. We then demonstrated sc-sRNA-seq with single K562 human leukemic cells. Our approaches offer a relatively short hands-on time of 6 h and efficient generation of on-target reads. The sc-sRNA-seq with our approaches showed detection of miRNA with various abundances ranging from 16 000 copies/cell to about 10 copies/cell. We anticipate this approach will create a new opportunity to explore cellular heterogeneity through small RNA expression.

In vitro RNA SELEX for the generation of chemically-optimized therapeutic RNA drugs.

Aptamers are single-stranded DNA or RNA oligonucleotides that can bind with exquisitely high affinity and specificity to target molecules and are thus often referred to as 'nucleic acid' antibodies. Oligonucleotide aptamers are derived through a process of directed chemical evolution called SELEX (Systematic Evolution of Ligands by Exponential enrichment). This chemical equivalent of Darwinian evolution was first described in 1990 by Tuerk & Gold and Ellington & Szostak and has since yielded aptamers for a wide-range of applications, including biosensor technologies, in vitro diagnostics, biomarker discovery, and therapeutics. Since the inception of the original SELEX method, numerous modifications to the protocol have been described to fit the choice of target, specific conditions or applications. Technologies such as high-throughput sequencing methods and microfluidics have also been adapted for SELEX. In this chapter, we outline key steps in the SELEX process for enabling the rapid identification of RNA aptamers for in vivo applications. Specifically, we provide a detailed protocol for the selection of chemically-optimized RNA aptamers using the original in vitro SELEX methodology. In addition, methods for performing next-generation sequencing of the RNAs from each round of selection, based on Illumina sequencing technology, are discussed.

Engineered zinc finger nickases induce homology-directed repair with reduced mutagenic effects.

Engineered zinc finger nucleases (ZFNs) induce DNA double-strand breaks at specific recognition sequences and can promote efficient introduction of desired insertions, deletions or substitutions at or near the cut site via homology-directed repair (HDR) with a double- and/or single-stranded donor DNA template. However, mutagenic events caused by error-prone non-homologous end-joining (NHEJ)-mediated repair are introduced with equal or higher frequency at the nuclease cleavage site. Furthermore, unintended mutations can also result from NHEJ-mediated repair of off-target nuclease cleavage sites. Here, we describe a simple and general method for converting engineered ZFNs into zinc finger nickases (ZFNickases) by inactivating the catalytic activity of one monomer in a ZFN dimer. ZFNickases show robust strand-specific nicking activity in vitro. In addition, we demonstrate that ZFNickases can stimulate HDR at their nicking site in human cells, albeit at a lower frequency than by the ZFNs from which they were derived. Finally, we find that ZFNickases appear to induce greatly reduced levels of mutagenic NHEJ at their target nicking site. ZFNickases thus provide a promising means for inducing HDR-mediated gene modifications while reducing unwanted mutagenesis caused by error-prone NHEJ.

Identifying functional microRNAs in macrophages with polarized phenotypes.

Macrophages respond to external stimuli with rapid changes in expression of many genes. Different combinations of external stimuli lead to distinct polarized activation patterns, resulting in a spectrum of possible macrophage activation phenotypes. MicroRNAs (miRNAs) are small, noncoding RNAs that can repress the expression of many target genes. We hypothesized that miRNAs play a role in macrophage polarization. miRNA expression profiles were determined in monocyte-derived macrophages (MDMs) incubated in conditions causing activation toward M1, M2a, M2b, or M2c phenotypes. One miRNA guide strand and seven miRNA passenger strands were significantly altered. Changes were confirmed in MDMs from six separate donors. The amplitude of miRNA expression changes in MDMs was smaller than described studies of monocytes responding to inflammatory stimuli. Further investigation revealed this correlated with higher basal miRNA expression in MDMs compared with monocytes. The regulation of M1- and M2b-responsive miRNAs (miR-27a, miR-29b, miR-125a, miR-146a, miR-155, and miR-222) was similar in differentiated THP-1 cells and primary MDMs. Studies in this model revealed cross-talk between IFNγ- and LPS-associated pathways regulating miRNA expression. Furthermore, expression of M1-associated transcripts was increased in THP-1 cells transfected with mimics of miR-29b, miR-125a-5p, or miR-155. The apparent inflammatory property of miR-29b and miR-125a-5p can be at least partially explained by repression of TNFAIP3, a negative regulator of NF-κB signaling. Overall, these data suggest miRNAs can contribute to changes in macrophage gene expression that occur in different exogenous activating conditions.

Hepatitis C virus infection and hepatic stellate cell activation downregulate miR-29: miR-29 overexpression reduces hepatitis C viral abundance in culture.

BACKGROUND: Chronic hepatitis C virus (HCV)-induced liver fibrosis involves upregulation of transforming growth factor (TGF)-ß and subsequent hepatic stellate cell (HSC) activation. MicroRNAs (miRNAs) regulate HCV infection and HSC activation. METHODS: TaqMan miRNA profiling identified 12 miRNA families differentially expressed between chronically HCV-infected human livers and uninfected controls. To identify pathways affected by miRNAs, we developed a new algorithm (pathway analysis of conserved targets), based on the probability of conserved targeting. RESULTS: This analysis suggested a role for miR-29 during HCV infection. Of interest, miR-29 was downregulated in most HCV-infected patients. miR-29 regulates expression of extracellular matrix proteins. In culture, HCV infection downregulated miR-29, and miR-29 overexpression reduced HCV RNA abundance. miR-29 also appears to play a role in HSCs. Hepatocytes and HSCs contribute similar amounts of miR-29 to whole liver. Both activation of primary HSCs and TGF-ß treatment of immortalized HSCs downregulated miR-29. miR-29 overexpression in LX-2 cells decreased collagen expression and modestly decreased proliferation. miR-29 downregulation by HCV may derepress extracellular matrix synthesis during HSC activation. CONCLUSIONS: HCV infection downregulates miR-29 in hepatocytes and may potentiate collagen synthesis by reducing miR-29 levels in activated HSCs. Treatment with miR-29 mimics in vivo might inhibit HCV while reducing fibrosis.

ZFN-site searches genomes for zinc finger nuclease target sites and off-target sites.

BACKGROUND: Zinc Finger Nucleases (ZFNs) are man-made restriction enzymes useful for manipulating genomes by cleaving target DNA sequences. ZFNs allow therapeutic gene correction or creation of genetically modified model organisms. ZFN specificity is not absolute; therefore, it is essential to select ZFN target sites without similar genomic off-target sites. It is important to assay for off-target cleavage events at sites similar to the target sequence. RESULTS: ZFN-Site is a web interface that searches multiple genomes for ZFN off-target sites. Queries can be based on the target sequence or can be expanded using degenerate specificity to account for known ZFN binding preferences. ZFN off-target sites are outputted with links to genome browsers, facilitating off-target cleavage site screening. We verified ZFN-Site using previously published ZFN half-sites and located their target sites and their previously described off-target sites. While we have tailored this tool to ZFNs, ZFN-Site can also be used to find potential off-target sites for other nucleases, such as TALE nucleases. CONCLUSIONS: ZFN-Site facilitates genome searches for possible ZFN cleavage sites based on user-defined stringency limits. ZFN-Site is an improvement over other methods because the FetchGWI search engine uses an indexed search of genome sequences for all ZFN target sites and possible off-target sites matching the half-sites and stringency limits. Therefore, ZFN-Site does not miss potential off-target sites.

Biliverdin inhibits hepatitis C virus nonstructural 3/4A protease activity: mechanism for the antiviral effects of heme oxygenase?

UNLABELLED: Induction of heme oxygenase-1 (HO-1) inhibits hepatitis C virus (HCV) replication. Of the products of the reaction catalyzed by HO-1, iron has been shown to inhibit HCV ribonucleic acid (RNA) polymerase, but little is known about the antiviral activity of biliverdin (BV). Herein, we report that BV inhibits viral replication and viral protein expression in a dose-dependent manner in replicons and cells harboring the infectious J6/JFH construct. Using the SensoLyte 620 HCV Protease Assay with a wide wavelength excitation/emission (591 nm/622 nm) fluorescence energy transfer peptide, we found that both recombinant and endogenous nonstructural 3/4A (NS3/4A) protease from replicon microsomes are potently inhibited by BV. Of the tetrapyrroles tested, BV was the strongest inhibitor of NS3/4A activity, with a median inhibitory concentration (IC(50)) of 9 µM, similar to that of the commercial inhibitor, AnaSpec (Fremont, CA) #25346 (IC(50) 5 µM). Lineweaver-Burk plots indicated mixed competitive and noncompetitive inhibition of the protease by BV. In contrast, the effects of bilirubin (BR) on HCV replication and NS3/4A were much less potent. Because BV is rapidly converted to BR by biliverdin reductase (BVR) intracellularly, the effect of BVR knockdown on BV antiviral activity was assessed. After greater than 80% silencing of BVR, inhibition of viral replication by BV was enhanced. BV also increased the antiviral activity of α-interferon in replicons. CONCLUSION: BV is a potent inhibitor of HCV NS3/4A protease, which likely contributes to the antiviral activity of HO-1. These findings suggest that BV or its derivatives may be useful in future drug therapies targeting the NS3/4A protease.

Zinc-finger nucleases as a novel therapeutic strategy for targeting hepatitis B virus DNAs.

Hepatitis B virus (HBV) chronically infects 350-400 million people worldwide and causes >1 million deaths yearly. Current therapies prevent new viral genome formation, but do not target pre-existing viral genomic DNA, thus curing only approximately 1/2 of patients. We targeted HBV DNA for cleavage using zinc-finger nucleases (ZFNs), which cleave as dimers. Co-transfection of our ZFN pair with a target plasmid containing the HBV genome resulted in specific cleavage. After 3 days in culture, 26% of the target remained linear, whereas approximately10% was cleaved and misjoined tail-to-tail. Notably, ZFN treatment decreased levels of the hepatitis C virus pregenomic RNA by 29%. A portion of cleaved plasmids are repaired in cells, often with deletions and insertions. To track misrepair, we introduced an XbaI restriction site in the spacer between the ZFN sites. Targeted cleavage and misrepair destroys the XbaI site. After 3 days in culture, approximately 6% of plasmids were XbaI-resistant. Thirteen of 16 clones sequenced contained frameshift mutations that would lead to truncations of the viral core protein. These results demonstrate, for the first time, the possibility of targeting episomal viral DNA genomes using ZFNs.

Correlation between microRNA expression levels and clinical parameters associated with chronic hepatitis C viral infection in humans.

MicroRNAs (miRNAs) are small RNAs that regulate gene expression pathways. Previous studies have shown interactions between hepatitis C virus (HCV) and host miRNAs. We measured miR-122 and miR-21 levels in HCV-infected human liver biopsies relative to uninfected human livers and correlated these with clinical patient data. miR-122 is required for HCV replication in vitro, and miR-21 is involved in cellular proliferation and tumorigenesis. We found that miR-21 expression correlated with viral load, fibrosis and serum liver transaminase levels. miR-122 expression inversely correlated with fibrosis, liver transaminase levels and patient age. miR-21 was induced ∼twofold, and miR-122 was downregulated on infection of cultured cells with the HCV J6/JFH infectious clone, thus establishing a link to HCV. To further examine the relationship between fibrosis and the levels of miR-21 and miR-122, we measured their expression levels in a mouse carbon tetrachloride fibrosis model. As in the HCV-infected patient samples, fibrotic stage positively correlated with miR-21 and negatively correlated with miR-122 levels. Transforming growth factor ß (TGF-ß) is a critical mediator of fibrogenesis. We identified SMAD7 as a novel miR-21 target. SMAD7 is a negative regulator of TGF-ß signaling, and its expression is induced by TGF-ß. To confirm the relationship between miR-21 and the TGF-ß signaling pathway, we measured the effect of miR-21 on a TGF-ß-responsive reporter. We found that miR-21 enhanced TGF-ß signaling, further supporting a relationship between miR-21 and fibrosis. We suggest a model in which miR-21 targeting of SMAD7 could increase TGF-ß signaling, leading to increased fibrogenesis.

MicroRNA-21 is upregulated during the proliferative phase of liver regeneration, targets Pellino-1, and inhibits NF-kappaB signaling.

During liver regeneration, normally quiescent liver cells reenter the cell cycle, nonparenchymal and parenchymal cells divide, and proper liver architecture is restored. The gene expression programs regulating these transitions are not completely understood. MicroRNAs are a newly discovered class of small regulatory RNAs that silence messenger RNAs by binding to their 3'-untranslated regions (UTRs). A number of microRNAs, including miR-21, have been shown to be involved in regulation of cell proliferation. We performed partial hepatectomies on mice and allowed the liver to regenerate for 1, 6, 12, 24, and 48 h and 4 and 7 days. We compared the expression of miR-21 in the posthepatectomy liver to the prehepatectomy liver by Northern blot and found that miR-21 was upregulated during the early stages of liver regeneration. NF-kappaB signaling is also activated very early during liver regeneration. It has been previously reported that NF-kappaB upregulates the miR-21 precursor transcript. The predicted miR-21 target, Pellino (Peli1), is a ubiquitin ligase involved in activating NF-kappaB signaling. We observed an inverse correlation between miR-21 and Peli1 mRNA levels during liver regeneration. miR-21 overexpression in cultured cells inhibited a Peli1 3'-UTR luciferase reporter. Using NF-kappaB reporter assays, we determined that miR-21 overexpression inhibits NF-kappaB signaling. In conclusion, miR-21 expression was upregulated during early stages of liver regeneration. Targeting of Peli1 by miR-21 could potentially provide the basis for a negative feedback cycle regulating NF-kappaB signaling.

Rational design leads to more potent RNA interference against hepatitis B virus: factors effecting silencing efficiency.

RNA interference (RNAi) can be an effective antiviral agent; however, overexpression of RNAi can be toxic through competition with the endogenous microRNA (miRNA) machinery. We used rational design to identify highly potent RNAi that is effective at nontoxic doses. A statistical analysis was conducted to pinpoint thermodynamic characteristics correlated with activity. Sequences were selected that conformed to a consensus internal stability profile (ISP) associated with active RNAi, and RNAi triggers were expressed in the context of an endogenous miRNA. These approaches yielded highly active hepatitis B virus (HBV) RNAi. A statistical analysis found a correlation between activity and nucleation by binding within the seed sequence to accessible regions in the target RNA. Guide strands were selected for favorable strand biasing, but increased strand biasing did not correlate with potency, suggesting a threshold effect. Exogenous short hairpin RNAs (shRNAs), but not miRNAs were previously reported to compete with miRNAs for the miRNA/RNAi machinery. In contrast, we show that exogenous Polymerase III- but not Polymerase II-driven miRNAs compete with exogenous miRNAs, at multiple steps in the miRNA pathway. Exogenous miRNAs also compete with endogenous miR-21. Thus, competition with endogenous miRNAs should be monitored even when using miRNA-based therapeutics. However, potent silencing was achieved at doses where competition was not observed.

Chemical modifications of adenine base editor mRNA and guide RNA expand its application scope.

CRISPR-Cas9-associated base editing is a promising tool to correct pathogenic single nucleotide mutations in research or therapeutic settings. Efficient base editing requires cellular exposure to levels of base editors that can be difficult to attain in hard-to-transfect cells or in vivo. Here we engineer a chemically modified mRNA-encoded adenine base editor that mediates robust editing at various cellular genomic sites together with moderately modified guide RNA, and show its therapeutic potential in correcting pathogenic single nucleotide mutations in cell and animal models of diseases. The optimized chemical modifications of adenine base editor mRNA and guide RNA expand the applicability of CRISPR-associated gene editing tools in vitro and in vivo.

The host response to adenovirus, helper-dependent adenovirus, and adeno-associated virus in mouse liver.

Understanding host responses to viral gene therapy vectors is necessary for the development of safe and efficacious in vivo gene transfer agents. We describe the use of high-density spotted complementary DNA microarrays in monitoring the in vivo host transcriptional responses in mouse liver upon administration of either a "first-generation"adenoviral (Ad) vector, a helper-dependent "gutless" adenoviral (HD) vector, or an adeno-associated viral (AAV) vector containing human factor IX (hFIX) expression cassettes. Since HD and AAV do not contain any viral genes, they allow us to assess the host response to the viral capsid and packaged nonviral DNA in whole animals. Comparison of the host response to Ad and HD helps assess the importance of leaky adenoviral gene expression. While all three vectors induced characteristic temporally sequenced programs of gene expression, the gene expression programs induced by the Ad and HD adenovirus vectors were remarkably similar, including the induction of a prominent type I interferon (IFN)-dependent cluster within 6 hours of administration. In contrast, the AAV-based vector caused far fewer alterations of host-gene expression. Our results indicate that recognition of the Ad capsid or double-stranded DNA (of nonviral origin) in the vector elicits a robust type I IFN response that is, however, not elicited by AAV-derived vector transduction.

Advances in microRNAs: implications for gene therapists.

MicroRNAs (miRNAs) are a class of small regulatory RNAs that are thought to regulate the expression of as many as one-third of all human messenger RNAs (mRNAs). miRNAs are thought to be involved in diverse biological processes, including tumorigenesis. Analysis of miRNA levels may have diagnostic implications. Evidence shows that numerous viruses interact with the miRNA machinery, and that a number of viruses encode their own miRNAs. It seems likely that miRNAs will be implicated in many human diseases. Manipulation of miRNA levels by gene therapy provides an attractive new approach for therapeutic development. This review focuses on approaches to manipulate miRNA levels in cells and in vivo, and the implications for gene therapy. Furthermore, we discuss the use of endogenous miRNAs as scaffolds for the expression of RNA interference (RNAi) as well as competition between exogenous RNAi triggers and endogenous miRNAs. Because short interfering RNAs can also act as miRNAs, seed matches with the 3' untranslated regions of genes should be avoided to prevent off-target effects. Last, we discuss the use of miRNAs to avoid immune responses to viral vectors.

CleanTag Adapters Improve Small RNA Next-Generation Sequencing Library Preparation by Reducing Adapter Dimers.

Next-generation small RNA sequencing is a valuable tool which is increasing our knowledge regarding small noncoding RNAs and their function in regulating genetic information. Library preparation protocols for small RNA have thus far been restricted due to higher RNA input requirements (>10 ng), long workflows, and tedious manual gel purifications. Small RNA library preparation methods focus largely on the prevention or depletion of a side product known as adapter dimer that tends to dominate the reaction. Adapter dimer is the ligation of two adapters to one another without an intervening library RNA insert or any useful sequencing information. The amplification of this side reaction is favored over the amplification of tagged library since it is shorter. The small size discrepancy between these two species makes separation and purification of the tagged library very difficult. Adapter dimer hinders the use of low input samples and the ability to automate the workflow so we introduce an improved library preparation protocol which uses chemically modified adapters (CleanTag) to significantly reduce the adapter dimer. CleanTag small RNA library preparation workflow decreases adapter dimer to allow for ultra-low input samples (down to approx. 10 pg total RNA), elimination of the gel purification step, and automation. We demonstrate how to carry out this streamlined protocol to improve NGS data quality and allow for the use of sample types with limited RNA material.

Uridine Depletion and Chemical Modification Increase Cas9 mRNA Activity and Reduce Immunogenicity without HPLC Purification.

The Cas9/guide RNA (Cas9/gRNA) system is commonly used for genome editing. mRNA expressing Cas9 can induce innate immune responses, reducing Cas9 expression. First-generation Cas9 mRNAs were modified with pseudouridine and 5-methylcytosine to reduce innate immune responses. We combined four approaches to produce more active, less immunogenic second-generation Cas9 mRNAs. First, we developed a novel co-transcriptional capping method yielding natural Cap 1. Second, we screened modified nucleotides in Cas9 mRNA to identify novel modifications that increase Cas9 activity. Third, we depleted the mRNA of uridines to improve mRNA activity. Lastly, we tested high-performance liquid chromatography (HPLC) purification to remove double-stranded RNAs. The activity of these mRNAs was tested in cell lines and primary human CD34+ cells. Cytokines were measured in whole blood and mice. These approaches yielded more active and less immunogenic mRNA. Uridine depletion (UD) most impacted insertion or deletion (indel) activity. Specifically, 5-methoxyuridine UD induced indel frequencies as high as 88% (average ± SD = 79% ± 11%) and elicited minimal immune responses without needing HPLC purification. Our work suggests that uridine-depleted Cas9 mRNA modified with 5-methoxyuridine (without HPLC purification) or pseudouridine may be optimal for the broad use of Cas9 both in vitro and in vivo.

Inhibition of hepatitis B virus in mice by RNA interference.

Hepatitis B virus (HBV) infection substantially increases the risk of chronic liver disease and hepatocellular carcinoma in humans. RNA interference (RNAi) of virus-specific genes has emerged as a potential antiviral mechanism. Here we show that RNAi can be applied to inhibit production of HBV replicative intermediates in cell culture and in immunocompetent and immunodeficient mice transfected with an HBV plasmid. Cotransfection with plasmids expressing short hairpin RNAs (shRNAs) homologous to HBV mRNAs induced an RNAi response. Northern and Southern analyses of mouse liver RNA and DNA showed substantially reduced levels of HBV RNAs and replicated HBV genomes upon RNAi treatment. Secreted HBV surface antigen (HBsAg) was reduced by 94.2% in cell culture and 84.5% in mouse serum, whereas immunohistochemical detection of HBV core antigen (HBcAg) revealed >99% reduction in stained hepatocytes upon RNAi treatment. Thus, RNAi effectively inhibited replication initiation in cultured cells and mammalian liver, showing that such an approach could be useful in the treatment of viral diseases.

Small RNA Library Preparation Method for Next-Generation Sequencing Using Chemical Modifications to Prevent Adapter Dimer Formation.

For most sample types, the automation of RNA and DNA sample preparation workflows enables high throughput next-generation sequencing (NGS) library preparation. Greater adoption of small RNA (sRNA) sequencing has been hindered by high sample input requirements and inherent ligation side products formed during library preparation. These side products, known as adapter dimer, are very similar in size to the tagged library. Most sRNA library preparation strategies thus employ a gel purification step to isolate tagged library from adapter dimer contaminants. At very low sample inputs, adapter dimer side products dominate the reaction and limit the sensitivity of this technique. Here we address the need for improved specificity of sRNA library preparation workflows with a novel library preparation approach that uses modified adapters to suppress adapter dimer formation. This workflow allows for lower sample inputs and elimination of the gel purification step, which in turn allows for an automatable sRNA library preparation protocol.

5' Unlocked Nucleic Acid Modification Improves siRNA Targeting.

Optimization of small interfering RNAs (siRNAs) is important in RNA interference (RNAi)-based therapeutic development. Some specific chemical modifications can control which siRNA strand is selected by the RNA-induced silencing complex (RISC) for gene silencing. Intended strand selection will increase potency and reduce off-target effects from the unintended strand. Sometimes, blocking RISC loading of the unintended strand leads to improved intended strand-silencing potency, but the generality of this phenomenon is unclear. Specifically, unlocked nucleic acid (UNA) modification of the 5' end of canonical (i.e., 19+2) siRNAs abrogates gene silencing of the modified strand, but the fate and potency of the unmodified strand has not been investigated. Here, we show that 5' UNA-modified siRNAs show improved silencing potency of the unmodified strand. We harness this advantageous property in a therapeutic context, where a limited target region in a conserved HIV 5' long terminal repeat U5 region would otherwise yield siRNAs with undesired strand selection properties and poor silencing. Applying 5' UNA modification to the unintended sense (S) strand of these otherwise poorly targeted siRNAs dramatically improves on-target silencing by the intended antisense (AS) strand in pNL4-3.luciferase studies. This study highlights the utility of 5' UNA siRNA modification in therapeutic contexts where siRNA sequence selection is constrained.Molecular Therapy-Nucleic Acids (2013) 2, e103; doi:10.1038/mtna.2013.36; published online 2 July 2013.