Spacer Fidelity Assessments of Guide RNA by Top-Down Mass Spectrometry.

The advancement of CRISPR-based gene editing tools into biotherapeutics offers the potential for cures to genetic disorders and for new treatment paradigms for even common diseases. Arguably, the most important component of a CRISPR-based medicine is the guide RNA, which is generally large (>100-mer) synthetic RNA composed of a "tracr" and "spacer" region, the latter of which dictates the on-target editing site as well as potential undesired off-target edits. Aiming to advance contemporary capabilities for gRNA characterization to ensure the spacer region is of high fidelity, top-down mass spectrometry was herein implemented to provide direct and quantitative assessments of highly modified gRNA. In addition to sequencing the spacer region and pinpointing modifications, top-down mass spectra were utilized to quantify single-base spacer substitution impurities down to <1% and to decipher highly dissimilar spacers. To accomplish these results in an automated fashion, we devised custom software capable of sequencing and quantifying impurities in gRNA spacers. Notably, we developed automated tools that enabled the quantification of single-base substitutions, including advanced isotopic pattern matching for C > U and U > C substitutions, and created a de novo sequencing strategy to facilitate the identification and quantification of gRNA impurities with highly dissimilar spacer regions.

In vivo CRISPR base editing of PCSK9 durably lowers cholesterol in primates.

Gene-editing technologies, which include the CRISPR-Cas nucleases1-3 and CRISPR base editors4,5, have the potential to permanently modify disease-causing genes in patients6. The demonstration of durable editing in target organs of nonhuman primates is a key step before in vivo administration of gene editors to patients in clinical trials. Here we demonstrate that CRISPR base editors that are delivered in vivo using lipid nanoparticles can efficiently and precisely modify disease-related genes in living cynomolgus monkeys (Macaca fascicularis). We observed a near-complete knockdown of PCSK9 in the liver after a single infusion of lipid nanoparticles, with concomitant reductions in blood levels of PCSK9 and low-density lipoprotein cholesterol of approximately 90% and about 60%, respectively; all of these changes remained stable for at least 8 months after a single-dose treatment. In addition to supporting a 'once-and-done' approach to the reduction of low-density lipoprotein cholesterol and the treatment of atherosclerotic cardiovascular disease (the leading cause of death worldwide7), our results provide a proof-of-concept for how CRISPR base editors can be productively applied to make precise single-nucleotide changes in therapeutic target genes in the liver, and potentially in other organs.

Data-independent oxonium ion profiling of multi-glycosylated biotherapeutics.

The characterization of glycosylation is required for many protein therapeutics. The emergence of antibody and antibody-like molecules with multiple glycan attachment sites has rendered glycan analysis increasingly more complicated. Reliance on site-specific glycopeptide analysis is therefore necessary to fully analyze multi-glycosylated biotherapeutics. Established glycopeptide methodologies have generally utilized a priori knowledge of the glycosylation states of the investigated protein(s), database searching of results generated from data-dependent liquid chromatography-tandem mass spectrometry workflows, and extracted ion quantitation of the individual identified species. However, the inherent complexity of glycosylation makes predicting all glycoforms on all glycosylation sites extremely challenging, if not impossible. That is, only the "knowns" are assessed. Here, we describe an agnostic methodology to qualitatively and quantitatively assess both "known" and "unknown" site-specific glycosylation for biotherapeutics that contain multiple glycosylation sites. The workflow uses data-independent, all ion fragmentation to generate glycan oxonium ions, which are then extracted across the entirety of the chromatographic timeline to produce a glycan-specific "fingerprint" of the glycoprotein sample. We utilized both HexNAc and sialic acid oxonium ion profiles to quickly assess the presence of Fab glycosylation in a therapeutic monoclonal antibody, as well as for high-throughput comparisons of multi-glycosylated protein drugs derived from different clones to a reference product. An automated method was created to rapidly assess oxonium profiles between samples, and to provide a quantitative assessment of similarity.

Covalent Labeling Denaturation Mass Spectrometry for Sensitive Localized Higher Order Structure Comparisons.

Protein higher order structure (HOS) describes the three-dimensional folding arrangement of a given protein and plays critical roles in structure/function relationships. As such, it is a key product quality attribute that is monitored during biopharmaceutical development. Covalent labeling of surface residues, combined with mass spectrometry analysis, has increasingly played an important role in characterizing localized protein HOS. Since the label can potentially induce conformation changes, protocols generally use a small amount of label to ensure that the integrity of the protein HOS is not disturbed. The present study, however, describes a method that purposely uses high amounts of isobaric label (levels that induce denaturation) to enhance the sensitivity and resolution for detecting localized structural differences between two or more biological products. The method proved to be highly discriminative, detecting differences in HOS affecting as little as 2.5-5% of the molecular population, levels at which circular dichroism and nuclear magnetic resonance spectroscopy fingerprinting, both gold standard HOS techniques, were unable to adequately differentiate. The methodology was shown to have comparable sensitivity to differential scanning calorimetry for detecting HOS differences. In addition, the workflow presented herein can also quantify other product attributes such as post-translational modifications and site-specific glycosylation, using a single liquid chromatography-tandem mass spectrometry (LC-MS/MS) run with automated data analysis. We applied this technique to characterize a large (>90 kDa), multiply glycosylated therapeutic protein under different heat stress conditions and aggregation states.

Toward the complete characterization of host cell proteins in biotherapeutics via affinity depletions, LC-MS/MS, and multivariate analysis.

Host cell protein (HCP) impurities are generated by the host organism during the production of therapeutic recombinant proteins, and are difficult to remove completely. Though commonly present in small quantities, if levels are not controlled, HCPs can potentially reduce drug efficacy and cause adverse patient reactions. A high resolution approach for thorough HCP characterization of therapeutic monoclonal antibodies is presented herein. In this method, antibody samples are first depleted via affinity enrichment (e.g., Protein A, Protein L) using milligram quantities of material. The HCP-containing flow-through is then enzymatically digested, analyzed using nano-UPLC-MS/MS, and proteins are identified through database searching. Nearly 700 HCPs were identified from samples with very low total HCP levels (< 1 ppm to ∼ 10 ppm) using this method. Quantitation of individual HCPs was performed using normalized spectral counting as the number of peptide spectrum matches (PSMs) per protein is proportional to protein abundance. Multivariate analysis tools were utilized to assess similarities between HCP profiles by: 1) quantifying overlaps between HCP identities; and 2) comparing correlations between individual protein abundances as calculated by spectral counts. Clustering analysis using these measures of dissimilarity between HCP profiles enabled high resolution differentiation of commercial grade monoclonal antibody samples generated from different cell lines, cell culture, and purification processes.

Concurrent automated sequencing of the glycan and peptide portions of O-linked glycopeptide anions by ultraviolet photodissociation mass spectrometry.

O-Glycopeptides are often acidic owing to the frequent occurrence of acidic saccharides in the glycan, rendering traditional proteomic workflows that rely on positive mode tandem mass spectrometry (MS/MS) less effective. In this report, we demonstrate the utility of negative mode ultraviolet photodissociation (UVPD) MS for the characterization of acidic O-linked glycopeptide anions. This method was evaluated for a series of singly and multiply deprotonated glycopeptides from the model glycoprotein kappa casein, resulting in production of both peptide and glycan product ions that afforded 100% sequence coverage of the peptide and glycan moieties from a single MS/MS event. The most abundant and frequent peptide sequence ions were a/x-type products which, importantly, were found to retain the labile glycan modifications. The glycan-specific ions mainly arose from glycosidic bond cleavages (B, Y, C, and Z ions) in addition to some less common cross-ring cleavages. On the basis of the UVPD fragmentation patterns, an automated database searching strategy (based on the MassMatrix algorithm) was designed that is specific for the analysis of glycopeptide anions by UVPD. This algorithm was used to identify glycopeptides from mixtures of glycosylated and nonglycosylated peptides, sequence both glycan and peptide moieties simultaneously, and pinpoint the correct site(s) of glycosylation. This methodology was applied to uncover novel site-specificity of the O-linked glycosylated OmpA/MotB from the "superbug" A. baumannii to help aid in the elucidation of the functional role that protein glycosylation plays in pathogenesis.

High-throughput database search and large-scale negative polarity liquid chromatography-tandem mass spectrometry with ultraviolet photodissociation for complex proteomic samples.

The use of ultraviolet photodissociation (UVPD) for the activation and dissociation of peptide anions is evaluated for broader coverage of the proteome. To facilitate interpretation and assignment of the resulting UVPD mass spectra of peptide anions, the MassMatrix database search algorithm was modified to allow automated analysis of negative polarity MS/MS spectra. The new UVPD algorithms were developed based on the MassMatrix database search engine by adding specific fragmentation pathways for UVPD. The new UVPD fragmentation pathways in MassMatrix were rigorously and statistically optimized using two large data sets with high mass accuracy and high mass resolution for both MS(1) and MS(2) data acquired on an Orbitrap mass spectrometer for complex Halobacterium and HeLa proteome samples. Negative mode UVPD led to the identification of 3663 and 2350 peptides for the Halo and HeLa tryptic digests, respectively, corresponding to 655 and 645 peptides that were unique when compared with electron transfer dissociation (ETD), higher energy collision-induced dissociation, and collision-induced dissociation results for the same digests analyzed in the positive mode. In sum, 805 and 619 proteins were identified via UVPD for the Halobacterium and HeLa samples, respectively, with 49 and 50 unique proteins identified in contrast to the more conventional MS/MS methods. The algorithm also features automated charge determination for low mass accuracy data, precursor filtering (including intact charge-reduced peaks), and the ability to combine both positive and negative MS/MS spectra into a single search, and it is freely open to the public. The accuracy and specificity of the MassMatrix UVPD search algorithm was also assessed for low resolution, low mass accuracy data on a linear ion trap. Analysis of a known mixture of three mitogen-activated kinases yielded similar sequence coverage percentages for UVPD of peptide anions versus conventional collision-induced dissociation of peptide cations, and when these methods were combined into a single search, an increase of up to 13% sequence coverage was observed for the kinases. The ability to sequence peptide anions and cations in alternating scans in the same chromatographic run was also demonstrated. Because ETD has a significant bias toward identifying highly basic peptides, negative UVPD was used to improve the identification of the more acidic peptides in conjunction with positive ETD for the more basic species. In this case, tryptic peptides from the cytosolic section of HeLa cells were analyzed by polarity switching nanoLC-MS/MS utilizing ETD for cation sequencing and UVPD for anion sequencing. Relative to searching using ETD alone, positive/negative polarity switching significantly improved sequence coverages across identified proteins, resulting in a 33% increase in unique peptide identifications and more than twice the number of peptide spectral matches.

The outer membrane of Gram-negative bacteria: lipid A isolation and characterization.

The isolation and characterization of the lipid A domain of lipopolysaccharide (LPS) are important methodologies utilized to gain understanding of the Gram-negative cell envelope. Here, we describe protocols often employed by our laboratory for small- and large-scale isolation of lipid A from bacterial cells. Additionally, we describe various methodologies including isolation of radiolabeled lipid A, thin layer chromatography, and various mass spectrometry methods. Tandem mass spectrometry is an integral tool for the structural characterization of lipid A molecules, and both coventional collision induced dissociation (CID) and new ultraviolet photodissociation (UVPD) methods are described.

Tyrosine sulfation in a Gram-negative bacterium.

Tyrosine sulfation, a well-characterized post-translation modification in eukaryotes, has not previously been reported in prokaryotes. Here, we demonstrate that the RaxST protein from the Gram-negative bacterium, Xanthomonas oryzae pv. oryzae, is a tyrosine sulfotransferase. We used a newly developed sulfotransferase assay and ultraviolet photodissociation mass spectrometry to demonstrate that RaxST catalyses sulfation of tyrosine 22 of the Xoo Ax21 (activator of XA21-mediated immunity) protein. These results demonstrate a previously undescribed post-translational modification in a prokaryotic species with implications for studies of host immune responses and bacterial cell-cell communication systems.

Systematic comparison of ultraviolet photodissociation and electron transfer dissociation for peptide anion characterization.

Ultraviolet photodissociation at 193 nm (UVPD) and negative electron transfer dissociation (NETD) were compared to establish their utility for characterizing acidic proteomes with respect to sequence coverage distributions (a measure of product ion signals across the peptide backbone), sequence coverage percentages, backbone cleavage preferences, and fragmentation differences relative to precursor charge state. UVPD yielded significantly more diagnostic information compared with NETD for lower charge states (n ≤ 2), but both methods were comparable for higher charged species. While UVPD often generated a more heterogeneous array of sequence-specific products (b-, y-, c-, z-, Y-, d-, and w-type ions in addition to a- and x- type ions), NETD usually created simpler sets of a/x-type ions. LC-MS/UVPD and LC-MS/NETD analysis of protein digests utilizing high pH mobile phases coupled with automated database searching via modified versions of the MassMatrix algorithm was undertaken. UVPD generally outperformed NETD in stand-alone searches due to its ability to efficiently sequence both lower and higher charge states with rapid activation times. However, when combined with traditional positive mode CID, both methods yielded complementary information with significantly increased sequence coverage percentages and unique peptide identifications over that of just CID alone.

Amino acid addition to Vibrio cholerae LPS establishes a link between surface remodeling in gram-positive and gram-negative bacteria.

Historically, the O1 El Tor and classical biotypes of Vibrio cholerae have been differentiated by their resistance to the antimicrobial peptide polymyxin B. However, the molecular mechanisms associated with this phenotypic distinction have remained a mystery for 50 y. Both gram-negative and gram-positive bacteria modify their cell wall components with amine-containing substituents to reduce the net negative charge of the bacterial surface, thereby promoting cationic antimicrobial peptide resistance. In the present study, we demonstrate that V. cholerae modify the lipid A anchor of LPS with glycine and diglycine residues. This previously uncharacterized lipid A modification confers polymyxin resistance in V. cholerae El Tor, requiring three V. cholerae proteins: Vc1577 (AlmG), Vc1578 (AlmF), and Vc1579 (AlmE). Interestingly, the protein machinery required for glycine addition is reminiscent of the gram-positive system responsible for D-alanylation of teichoic acids. Such machinery was not thought to be used by gram-negative organisms. V. cholerae O1 El Tor mutants lacking genes involved in transferring glycine to LPS showed a 100-fold increase in sensitivity to polymyxin B. This work reveals a unique lipid A modification and demonstrates a charge-based remodeling strategy shared between gram-positive and gram-negative organisms.

Charge-site-dependent dissociation of hydrogen-rich radical peptide cations upon vacuum UV photoexcitation.

Here, 193 nm vacuum ultraviolet photodissociation (VUVPD) was used to investigate the fragmentation of hydrogen-rich radical peptide cations generated by electron transfer reactions. VUVPD offers new insight into the factors that drive radical- and photon-directed processes. The location of a basic Arg site influences photon-activated C(α)-C(O) bond cleavages of singly charged peptide radical cations, an outcome attributed to the initial conformation of the peptide as supported by molecular dynamics simulated annealing and the population of excited states upon UV excitation. This hybrid ETD/VUVPD method was employed to identify phosphorylation sites of the kinase domain of human TRPM7/ChaK1.

193 nm ultraviolet photodissociation of imidazolinylated Lys-N peptides for de novo sequencing.

The goal of many MS/MS de novo sequencing strategies is to generate a single product ion series that can be used to determine the precursor ion sequence. Most methods fall short of achieving such simplified spectra, and the presence of additional ion series impede peptide identification. The present study aims to solve the problem of confounding ion series by enhancing the formation of "golden" sets of a, b, and c ions for sequencing. Taking advantage of the characteristic mass differences between the golden ions allows N-terminal fragments to be readily identified while other ion series are excluded. By combining the use of Lys-N, an alternate protease, to produce peptides with lysine residues at each N-terminus with subsequent imidazolinylation of the ε-amino group of each lysine, peptides with highly basic sites localized at each N-terminus are generated. Subsequent MS/MS analysis by using 193 nm ultraviolet photodissociation (UVPD) results in enhanced formation of the diagnostic golden pairs and golden triplets that are ideal for de novo sequencing.

Characterization of unique modification of flagellar rod protein FlgG by Campylobacter jejuni lipid A phosphoethanolamine transferase, linking bacterial locomotion and antimicrobial peptide resistance.

Gram-negative bacteria assemble complex surface structures that interface with the surrounding environment and are involved in pathogenesis. Recent work in Campylobacter jejuni identified a gene encoding a novel phosphoethanolamine (pEtN) transferase Cj0256, renamed EptC, that serves a dual role in modifying the flagellar rod protein, FlgG, and the lipid A domain of C. jejuni lipooligosaccharide with a pEtN residue. In this work, we characterize the unique post-translational pEtN modification of FlgG using collision-induced and electron transfer dissociation mass spectrometry, as well as a genetic approach using site-directed mutagenesis to determine the site of modification. Specifically, we show that FlgG is modified with pEtN at a single site (Thr(75)) by EptC and demonstrate enzyme specificity by showing that EptC is unable to modify other amino acids (e.g. serine and tyrosine). Using Campylobacter strains expressing site-directed FlgG mutants, we also show that defects in motility arise directly from the loss of pEtN modification of FlgG. Interestingly, alignments of FlgG from most epsilon proteobacteria reveal a conserved site of modification. Characterization of EptC and its enzymatic targets expands on the increasingly important field of prokaryotic post-translational modification of bacterial surface structures and the unidentified role they may play in pathogenesis.

Elucidation of a novel Vibrio cholerae lipid A secondary hydroxy-acyltransferase and its role in innate immune recognition.

Similar to most Gram-negative bacteria, the outer leaflet of the outer membrane of Vibrio cholerae is comprised of lipopolysaccharide. Previous reports have proposed that V. cholerae serogroups O1 and O139 synthesize structurally different lipid A domains, which anchor lipopolysaccharide within the outer membrane. In the current study, intact lipid A species of V. cholerae O1 and O139 were analysed by mass spectrometry. We demonstrate that V. cholerae serogroups associated with human disease synthesize a similar asymmetrical hexa-acylated lipid A species, bearing a myristate (C14:0) and 3-hydroxylaurate (3-OH C12:0) at the 2'- and 3'-positions respectively. A previous report from our laboratory characterized the V. cholerae LpxL homologue Vc0213, which transfers a C14:0 to the 2'-position of the glucosamine disaccharide. Our current findings identify V. cholerae Vc0212 as a novel lipid A secondary hydroxy-acyltransferase, termed LpxN, responsible for transferring the 3-hydroxylaurate (3-OH C12:0) to the V. cholerae lipid A domain. Importantly, the presence of a 3-hydroxyl group on the 3'-linked secondary acyl chain was found to promote antimicrobial peptide resistance in V. cholerae; however, this functional group was not required for activation of the innate immune response.

IR and UV photodissociation as analytical tools for characterizing lipid A structures.

The utility of 193-nm ultraviolet photodissociation (UVPD) and 10.6-µm infrared multiphoton dissociation (IRMPD) for the characterization of lipid A structures was assessed in an ion trap mass spectrometer. The fragmentation behavior of lipid A species was also evaluated by activated-electron photodetachment (a-EPD), which uses 193-nm photons to create charge reduced radicals that are subsequently dissociated by collisional activation. In contrast to collision-induced dissociation (CID), IRMPD offered the ability to selectively differentiate product ions with varying degrees of phosphorylation because of the increased photoabsorption cross sections and thus dissociation of phosphate-containing species. Both 193-nm UVPD and a-EPD yielded higher abundances and a larger array of product ions arising from C-C cleavages, as well as cross-ring and inter-ring glucosamine cleavages, compared to CID and IRMPD, because of high energy, single-photon absorption, and/or radical-directed dissociation. UVPD at 193 nm also exhibited enhanced cleavage between the amine and carbonyl groups on the 2- and 2'-linked primary acyl chains. Lastly, UVPD of phosphorylethanolamine-modified lipid A species resulted in preferential cleavage of the C-O bond between ethanolamine and phosphate, enabling the selective identification of this modification.

193-nm photodissociation of singly and multiply charged peptide anions for acidic proteome characterization.

193-nm ultraviolet photodissociation (UVPD) was implemented to sequence singly and multiply charged peptide anions. Upon dissociation by this method, a-/x-type, followed by d and w side-chain loss ions, were the most prolific and abundant sequence ions, often yielding 100% sequence coverage. The dissociation behavior of singly and multiply charged anions was significantly different with higher charged precursors yielding more sequence ions; however, all charge states investigated (1- through 3-) produced rich diagnostic information. UVPD at 193 nm was also shown to successfully differentiate and pinpoint labile phosphorylation modifications. The sequence ions were produced with high abundances, requiring limited averaging for satisfactory spectral quality. The intact, charge-reduced radical products generated by UV photoexcitation were also subjected to collision-induced dissociation (termed, activated-electron photodetachment dissociation (a-EPD)), but UVPD alone yielded more predictable and higher abundance sequence ions. With the use of a basic (pH∼11.5), piperidine-modified mobile phase, LC-MS/UVPD was implemented and resulted in the successful analysis of mitogen-activated pathway kinases (MAPKs) using ultrafast activation times (5 ns).

Ultrafast ultraviolet photodissociation at 193 nm and its applicability to proteomic workflows.

Ultraviolet photodissociation (UVPD) at 193 nm was implemented on a linear ion trap mass spectrometer for high-throughput proteomic workflows. Upon irradiation by a single 5 ns laser pulse, efficient photodissociation of tryptic peptides was achieved with production of a, b, c, x, y, and z sequence ions, in addition to immonium ions and v and w side-chain loss ions. The factors that influence the UVPD mass spectra and subsequent in silico database searching via SEQUEST were evaluated. Peptide sequence aromaticity and the precursor charge state were found to influence photodissociation efficiency more so than the number of amide chromophores, and the ion trap q-value and number of laser pulses significantly affected the number and abundances of diagnostic product ions (e.g., sequence and immonium ions). Also, photoionization background subtraction was shown to dramatically improve SEQUEST results, especially when peptide signals were low. A liquid chromatography-mass spectrometry (LC-MS)/UVPD strategy was implemented and yielded comparable or better results relative to LC-MS/collision induced dissociation (CID) for analysis of proteolyzed bovine serum albumin and lysed human HT-1080 cytosolic fibrosarcoma cells.

Asymmetric charge partitioning upon dissociation of DNA duplexes.

Upon collisional activation, a series of DNA duplexes exhibited a significant degree of asymmetric dissociation with respect to charge partitioning among the single strands. That is, the charge states of the single strand product ions did not equal q/2 for even precursor charge states or (q + 1)/2 and (q-1)/2 for odd precursor charge states (where q is the charge of the precursor). The factors that affect this asymmetric charge partitioning were assessed. The smaller, lower charged duplexes resulted in more symmetric dissociation compared with larger duplexes in higher charge states, which displayed a high degree of asymmetry upon dissociation. The composition of the duplexes influenced charge partitioning, with those containing a greater number of A/T base pairs showing more symmetric dissociation relative to the more G/C rich duplexes. The use of higher collisional energies resulted in significantly more asymmetric dissociation. Comparisons were made with the dissociation behavior previously studied for protein noncovalent complexes and past studies of the gas-phase conformations and dissociation of DNA complexes.