Rational Design of RNA Editing Guide Strands: Cytidine Analogs at the Orphan Position

Adenosine Deaminases Acting on RNA (ADARs) convert adenosine to inosine in double stranded RNA. Human ADARs can be directed to predetermined target sites in the transcriptome by complementary guide strands, allowing for the correction of disease-causing mutations at the RNA level. Here we use structural information available for ADAR2-RNA complexes to guide the design of nucleoside analogs for the position in the guide strand that contacts a conserved glutamic acid residue in ADARs (E488 in human ADAR2), which flips the adenosine into the ADAR active site for deamination. Mutating this residue to glutamine (E488Q) results in higher activity because of the hydrogen bond donating ability of Q488 to N3 of the orphan cytidine on the guide strand. We describe the evaluation of cytidine analogs for this position that stabilize an activated conformation of the enzyme-RNA complex and increase catalytic rate for deamination by the wild-type enzyme. A new crystal structure of ADAR2 bound to duplex RNA bearing a cytidine analog revealed a close contact between E488, stabilized by an additional hydrogen bond and altered charge distribution when compared to cytidine. In human cells and mouse primary liver fibroblasts, this single nucleotide modification increased directed editing yields when compared to an otherwise identical guide oligonucleotide. Our results show that modification of the guide RNA can mimic the effect of hyperactive mutants and advance the approach of recruiting endogenous ADARs for site-directed RNA editing.

Antisense oligonucleotide-based treatment of retinitis pigmentosa caused by USH2A exon 13 mutations

Mutations in USH2A are among the most common causes of syndromic and non-syndromic retinitis pigmentosa (RP). The two most recurrent mutations in USH2A, c.2299delG and c.2276G > T, both reside in exon 13. Skipping exon 13 from the USH2A transcript presents a potential treatment modality in which the resulting transcript is predicted to encode a slightly shortened usherin protein. Morpholino-induced skipping of ush2a exon 13 in zebrafish ush2a^rmc1 mutants resulted in the production of usherinΔexon 13 protein and a completely restored retinal function. Antisense oligonucleotides were investigated for their potential to selectively induce human USH2A exon 13 skipping. Lead candidate QR-421a induced a concentration-dependent exon 13 skipping in induced pluripotent stem cell (iPSC)-derived photoreceptor precursors from an Usher syndrome patient homozygous for the c.2299delG mutation. Mouse surrogate mQR-421a reached the retinal outer nuclear layer after a single intravitreal injection and induced a detectable level of exon skipping until at least 6 months post-injection. In conclusion, QR-421a-induced exon skipping proves to be a highly promising treatment option for RP caused by mutations in USH2A exon 13.

Probabilistic assessment of the impact of bottom sediment on doses to humans from a groundwater-mediated radionuclide release in a farm-lake scenario

Radionuclide transport with groundwater flow and subsequent doses to people are an aspect to be studied when assessing the long-term safety of geological nuclear waste repositories. A scenario where the radionuclide release migrates through a three-layer sediment structure of a lake in a farming environment is presented in this paper. The sediment column consists of deep (till), intermediate (glacio-aquatic sediment) and top layers (clay). The radionuclide release is assumed to enter the deep sediment layer from a bedrock fracture system at a rate of 1 Bq yr^-1. The main objectives of the paper are to investigate the most contributing parameters, especially linked to the sediment layers, to the overall dose estimates for humans. The sensitivity analysis was conducted in two phases where the Morris method was used for screening and the Extended Fourier Amplitude Sensitivity Testing and Sobol's methods were used for estimating total-order indices. The studied radionuclides, ³⁶Cl, ¹³⁵Cs, ¹²⁹I, ⁹⁴Nb, ²³⁷Np, ⁹⁰Sr, ⁹⁹Tc and ²³⁸U, exhibit differences in how the sediment layers affect the concentration in the lake water used for drinking, irrigation and watering cattle and subsequently the dose conversion factors for humans through ingestion, inhalation and external radiation.

B Cell Receptor Activation Predominantly Regulates AKT-mTORC1/2 Substrates Functionally Related to RNA Processing

Protein kinase B (AKT) phosphorylates numerous substrates on the consensus motif RXRXXpS/T, a docking site for 14-3-3 interactions. To identify novel AKT-induced phosphorylation events following B cell receptor (BCR) activation, we performed proteomics, biochemical and bioinformatics analyses. Phosphorylated consensus motif-specific antibody enrichment, followed by tandem mass spectrometry, identified 446 proteins, containing 186 novel phosphorylation events. Moreover, we found 85 proteins with up regulated phosphorylation, while in 277 it was down regulated following stimulation. Up regulation was mainly in proteins involved in ribosomal and translational regulation, DNA binding and transcription regulation. Conversely, down regulation was preferentially in RNA binding, mRNA splicing and mRNP export proteins. Immunoblotting of two identified RNA regulatory proteins, RBM25 and MEF-2D, confirmed the proteomics data. Consistent with these findings, the AKT-inhibitor (MK-2206) dramatically reduced, while the mTORC-inhibitor PP242 totally blocked phosphorylation on the RXRXXpS/T motif. This demonstrates that this motif, previously suggested as an AKT target sequence, also is a substrate for mTORC1/2. Proteins with PDZ, PH and/or SH3 domains contained the consensus motif, whereas in those with an HMG-box, H15 domains and/or NF-X1-zinc-fingers, the motif was absent. Proteins carrying the consensus motif were found in all eukaryotic clades indicating that they regulate a phylogenetically conserved set of proteins.

Splice-correction strategies for treatment of X-linked agammaglobulinemia

X-linked agammaglobulinemia (XLA) is a primary immunodeficiency disease caused by mutations in the gene coding for Bruton’s tyrosine kinase (BTK). Deficiency of BTK leads to a developmental block in B cell differentiation; hence, the patients essentially lack antibody-producing plasma cells and are susceptible to various infections. A substantial portion of the mutations in BTK results in splicing defects, consequently preventing the formation of protein-coding mRNA. Antisense oligonucleotides (ASOs) are therapeutic compounds that have the ability to modulate pre-mRNA splicing and alter gene expression. The potential of ASOs has been exploited for a few severe diseases, both in pre-clinical and clinical studies. Recently, advances have also been made in using ASOs as a personalized therapy for XLA. Splice-correction of BTK has been shown to be feasible for different mutations in vitro, and a recent proof-of-concept study demonstrated the feasibility of correcting splicing and restoring BTK both ex vivo and in vivo in a humanized bacterial artificial chromosome (BAC)-transgenic mouse model. This review summarizes the advances in splice correction, as a personalized medicine for XLA, and outlines the promises and challenges of using this technology as a curative long-term treatment option.

HnRNPH1/H2, U1 snRNP, and U11 snRNP cooperate to regulate the stability of the U11-48K pre-mRNA

Alternative splicing (AS) is a major contributor to proteome diversity, but it also regulates gene expression by introducing premature termination codons (PTCs) that destabilize transcripts, typically via the nonsense-mediated decay (NMD) pathway. Such AS events often take place within long, conserved sequence elements, particularly in genes encoding various RNA binding proteins. AS-NMD is often activated by the protein encoded by the same gene, leading to a self-regulating feedback loop that maintains constant protein levels. However, cross-regulation between different RNA binding proteins is also common, giving rise to finely tuned regulatory networks. Recently, we described a feedback mechanism regulating two protein components of the U12-dependent spliceosome (U11-48K and U11/U12-65K) through a highly conserved sequence element. These elements contain a U11 snRNP-binding splicing enhancer (USSE), which, through the U11 snRNP, activates an upstream U2-type 3'ss, resulting in the degradation of the U11-48K mRNA by AS-NMD. Through phylogenetic analysis, we now identify a G-rich sequence element that is conserved in fishes as well as mammals. We show that this element binds hnRNPF/H proteins in vitro. Knockdown of hnRNPH1/H2 or mutations in the G-run both lead to enhanced activation of the 3'ss in vivo, suggesting that hnRNPH1/H2 proteins counteract the 3'ss activation. Furthermore, we provide evidence that U1 binding immediately downstream from the G-run similarly counteracts the U11-mediated activation of the alternative 3'ss. Thus, our results elucidate the mechanism in which snRNPs from both spliceosomes together with hnRNPH1/H2 proteins regulate the recognition and activation of the highly conserved alternative splice sites within the U11-48K pre-mRNA.

The significant other: splicing by the minor spliceosome

The removal of non-coding sequences, introns, from the mRNA precursors is an essential step in eukaryotic gene expression. U12-type introns are a minor subgroup of introns, distinct from the major or U2-type introns. U12-type introns are present in most eukaryotes but only account for less than 0.5% of all introns in any given genome. They are processed by a specific U12-dependent spliceosome, which is similar to, but distinct from, the major spliceosome. U12-type introns are spliced somewhat less efficiently than the major introns, and it is believed that this limits the expression of the genes containing such introns. Recent findings on the role of U12-dependent splicing in development and human disease have shown that it can also affect multiple cellular processes not directly related to the functions of the host genes of U12-type introns. At the same time, advances in understanding the regulation and phylogenetic distribution of the minor spliceosome are starting to shed light on how the U12-type introns and the minor spliceosome may have evolved. © 2012 John Wiley & Sons, Ltd.

An ancient mechanism for splicing control: U11 snRNP as an activator of alternative splicing

Alternative pre-mRNA splicing is typically regulated by specific protein factors that recognize unique sequence elements in pre-mRNA and affect, directly or indirectly, nearby splice site usage. We show that 5' splice site sequences (5'ss) of U12-type introns, when repeated in tandem, form a U11 snRNP-binding splicing enhancer, USSE. Binding of U11 to the USSE regulates alternative splicing of U2-type introns by activating an upstream 3'ss. The U12-type 5'ss-like sequences within the USSE have a regulatory role and do not function as splicing donors. USSEs, present both in animal and plant genes encoding the U11/U12 di-snRNP-specific 48K and 65K proteins, create sensitive switches that respond to intracellular levels of functional U11 snRNP and alter the stability of 48K and 65K mRNAs. We conclude that U11 functions not only in 5'ss recognition in constitutive splicing, but also as an activator of U2-dependent alternative splicing and as a regulator of the U12-dependent spliceosome.

The U11-48K protein contacts the 5' splice site of U12-type introns and the U11-59K protein

Little is currently known about proteins that make contact with the pre-mRNA in the U12-dependent spliceosome and thereby contribute to intron recognition. Using site-specific cross-linking, we detected an interaction between the U11-48K protein and U12-type 5' splice sites (5'ss). This interaction did not require branch point recognition and was sensitive to 5'ss mutations, suggesting that 48K interacts with the 5'ss during the first steps of prespliceosome assembly in a sequence-dependent manner. RNA interference-induced knockdown of 48K in HeLa cells led to reduced cell growth and the inhibition of U12-type splicing, as well as the activation of cryptic, U2-type splice sites, suggesting that 48K plays a critical role in U12-type intron recognition. 48K knockdown also led to reduced levels of U11/U12 di-snRNP, indicating that 48K contributes to the stability and/or formation of this complex. In addition to making contact with the 5'ss, 48K interacts with the U11-59K protein, a protein at the interface of the U11/U12 di-snRNP. These studies provide important insights into the protein-mediated recognition of the U12-type 5'ss, as well as functionally important interactions within the U11/U12 di-snRNP.