Enzyme-linked immunosorbent assay in the study of histone antigens and nucleosome structure

Mass Spectrometry-Based Proteomics: Analyses Related to Drug-Resistance and Disease Biomarkers

Mass spectrometry-based proteomics is a key player in research efforts to characterize aberrant epigenetic alterations, including histone post-translational modifications and DNA methylation. Data generated by this approach complements and enrich datasets generated by genomic, epigenetic and transcriptomics approaches. These combined datasets can provide much-needed information on various mechanisms responsible for drug resistance, the discovery and validation of potential biomarkers for different diseases, the identification of signaling pathways, and genes and enzymes to be targeted by future therapies. The increasing use of high-resolution, high-accuracy mass spectrometers combined with more refined protein labeling and enrichment procedures enhanced the role of this approach in the investigation of these epigenetic modifications. In this review, we discuss recent MS-based studies, which are contributing to current research efforts to understand certain mechanisms behind drug resistance to therapy. We also discuss how these MS-based analyses are contributing to biomarkers discovery and validation.

A new mass spectrometry-based method for the quantification of histones in plasma from septic shock patients

The aim of this study was to develop a novel method to detect circulating histones H3 and H2B in plasma based on multiple reaction monitoring targeted mass spectrometry and a multiple reaction monitoring approach (MRM-MS) for its clinical application in critical bacteriaemic septic shock patients. Plasma samples from 17 septic shock patients with confirmed bacteraemia and 10 healthy controls were analysed by an MRM-MS method, which specifically detects presence of histones H3 and H2B. By an internal standard, it was possible to quantify the concentration of circulating histones in plasma, which were significantly higher in patients, and thus confirmed their potential as biomarkers for diagnosing septic shock. After comparing surviving patients and non-survivors, a correlation was found between higher levels of circulating histones and unfavourable outcome. Indeed, histone H3 proved a more efficient and sensitive biomarker for septic shock prognosis. In conclusion, these findings suggest the accuracy of the MRM-MS technique and stable isotope labelled peptides to detect and quantify circulating plasma histones H2B and H3. This method may be used for early septic shock diagnoses and for the prognosis of fatal outcomes.

Immunogenicity of free histones and of histones complexed with RNA

Histone antibodies have been obtained by immunizing rabbits with histones H1, H2A, H2B, H3, H4 and triacetylated H4, uncomplexed to RNA. The reactivity of these antibodies was investigated by ELISA using as antigen isolated histones and chromatin as well as thirty-five different synthetic peptides covering the entire sequence of the four core histones, two peptides of H1 and two acetylated peptides of H4. The binding of these antibodies to histones was also measured in immunoblotting and in microcomplement fixation (MCF) tests. In parallel experiments using the same assays the various antigens were tested with antisera raised against histones complexed with RNA. Antibodies induced in the absence of RNA did not react with histones in MCF tests nor with chromatin in ELISA but reacted with the histones in ELISA, although the antibody titers were somewhat lower than in the case of antisera to histone-RNA complexes. Antibodies to RNA-histone complexes reacted with histones in both ELISA and MCF tests. When they were tested with peptide-coated microtiter plates in a direct binding ELISA format, antibodies induced with uncomplexed histones recognized very few fragments which were mainly located in the N- and C-terminal ends of the histones.

Solid-phase enzyme immunoassay for the detection of HMG nonhistone proteins in their native structure

The use of purified specific antibodies against HMG1 and HMG17 (high mobility group nonhistone chromosomal proteins), together with a very sensitive solid phase enzyme immunoassay, allows the detection of nanograms of these proteins free in solution or in chromatin native structure, and the measurement of their accessibility. The sensitivity of the assay is comparable to that of the radioimmunoassay, with the advantage of avoiding the handling of radioactive materials.

Immunological relationships among vertebrate lysine-rich histones

1. The relationship between sequence homology and immunological cross-reaction was investigated by enzyme-linked immunosorbent assay and immunoblotting using polyclonal antisera against lysine-rich histones (LRH) of known sequence, chicken H1 and H5, trout Hl and Xenopus H1s. 2. The order of immunological relatedness was consistent with known homologies among these LRH and goose H5, but quantitative correlations reflected varied locations of antigenic determinants. 3. When immunoblotting was extended to LRH from eight more vertebrates, it was evident that avian H5, mammalian H1o and anuran H1s form a sub-class, to which turtle H1s may belong, that urodele erythrocytes contain no H5-like histone and that fish "H5" is more like H1 than the H5/H1s/H1o subclass.

Antibodies to histones and disease activity in systemic lupus erythematosus: a comparative study with an enzyme-linked immunosorbent assay and immunoblotting

Enzyme-linked immunosorbent assay (ELISA) and immunoblotting were used to define comparatively the frequency of antibodies to total histones and different histone fractions - H1, H2A, H2B, H3, and H4 - in 16 patients with idiopathic systemic lupus erythematosus. H1 and H2B showed the most prominent antigenic properties; H3's were weaker, while antibodies to H2A and H4 were rarely detected and only with the more sensitive ELISA on microtiter plates. Detailed specification was carried out of the antigenic determinant on fragments obtained by specific cleavage of purified H1 at phenylalanine-106. Antibodies were detected against both the NH2 and COOH terminal halves of the molecule. The presence of antihistone antibodies was not associated with any particular clinical symptoms, but an obvious link with disease activity has been proved.

Protein HMG1 is different from a DNA helix unwinding protein in calf thymus

A number of criteria were used--chromatography on columns with single-stranded and double-stranded DNA, electrophoresis, peptide analysis, immunological tests and thermal denaturation of DNA--to show that protein (high mobility group) HMG1 and an unwinding protein from calf thymus are two distinct, unrelated proteins. While both proteins are thought to be related to DNA replication this might involve different mechanisms of action.

Characterization of a monoclonal antibody reacting with histone H3

A hybridoma cell line, 1GB3, has been obtained from a fusion between SP/O-Ag 14 myeloma cells and lymphocytes from BALB/c mice immunized with rat liver nuclear proteins. This hybridoma secreted a monoclonal antibody of the IgG2b class which reacted specifically with histone H3 in enzyme-linked immunosorbent assay (ELISA) as well as in immunoblotting and immunodot assays. Stringent test conditions were necessary to eliminate the presence of nonspecific or contaminating reactions with other histones than H3. The monoclonal antibody appears to recognize an epitope situated in the N-terminal residues 20-50 of histone H3; it recognizes this epitope in the octamer aggregate of core histones but not in the core particle.

Patterns of antihistone antibody specificity in systemic rheumatic disease. I Systemic lupus erythematosus, mixed connective tissue disease, primary sicca syndrome, and rheumatoid arthritis with vasculitis

A new fluorimetric assay was used to measure the relative amounts of antibodies to individual nuclear histones in sera from 102 patients with systemic lupus erythematosus (SLE), mixed connective tissue disease, primary sicca syndrome, and rheumatoid arthritis with vasculitis. In SLE sera, the predominant responses were to histones H-1, H-2B, and H-3, with marked elevations of binding to H-1 and H-2B in one-third of the patients, and to H-3 in one-fourth; antibodies of both the IgG and IgM classes were also detected. In a few SLE sera, the pattern of histone response differed or was restricted to 1 immunoglobulin class. In mixed connective tissue disease, only 2 of 9 sera showed elevated histone binding activity, the response being predominantly to H-3 in 1 patient and to H-1 and H-2B in the other. Binding to H-2B was also prominent in 2 of 3 patients with primary sicca syndrome. The highest antihistone reactivity and the most heterogeneous response patterns were observed in patients who had rheumatoid arthritis with vasculitis; 6 of 8 of those sera had elevated histone reactivity. In SLE, the highest histone binding results were found among patients with a history of photosensitivity. Histones are closely associated with DNA in the nucleosome, and we speculate that antihistone antibodies could arise as a result of damage to DNA, induced by drugs or irradiation.

Electron-microscopical evidence that ribosomal chromatin of human circulating lymphocytes is devoid of histones

We have studied the distribution of histones in the nucleolus of human circulating lymphocytes in situ, using thin sections, either treated with antibodies against the core histones revealed by colloidal gold, or stained with the acrolein-silver methenamine technique for basic proteins. Gold particles were not found in the fibrillar centre, nor were silver-stained structures visible in this nucleolar component. Since the fibrillar centre contains the bulk of the ribosomal chromatin which is in a completely extended, non-nucleosomal configuration, our results indicate that this chromatin is devoid of histones.

Antigenic structure of histone H2B

Antigenic determinants of histone H2B were localized using a series of 23 overlapping fragments of H2B obtained either by chemical and enzymatic cleavage of the histone or by solid-phase peptide synthesis. The ability of peptides to bind H2B antibodies was measured in an enzyme-linked immunosorbent assay, using antisera directed against calf thymus and chicken erythrocyte H2B as well as four anti H2B monoclonal antibodies obtained from autoimmune mice. Seven antigenic determinants were localized in the H2B molecule in the vicinity of residues 1-11, 6-18, 15-25, 26-35, 50-65, 94-113 and 114-125. Two of these determinants (residues 6-18 and 26-35) were revealed only through the binding properties of antibodies isolated from autoimmune mice. The usual correlation between hydrophilicity and antigenicity was found to hold for four of the epitopes, and the N- and C-termini of H2B were both antigenically active.

Involvement of protein HMG1 in DNA replication

Antibodies against HMG1 inhibit the incorporation of [3H]thymidine in Ehrlich ascites cell nuclei. By the use of specific inhibitors it is shown that HMG1 is needed for the action of the replicative DNA polymerase and not for the reparative one. This is supported by the fact that the addition of exogenous HMG1 to the nuclei enhances the replication process.

Non-specific binding of heat-aggregated IgG to histone detected by ELISA

Anti-histone antibodies are currently detected by micro ELISA in systemic lupus erythematosus sera from humans, mice and dogs. Here we show that the control-heated sera may bind non-specifically to the whole histones and histone fractions. The heated immunoglobulins binding to histones are mainly IgG and to a lesser extent IgA, but never IgM. These false positive ELISA reactions occurred only with aggregated IgG which binds to histones via Fc; IgM rheumatoid factor prevented their fixation. Immune complexes do not seem to interfere significantly in the detection of anti-histone antibodies with the ELISA test.

A fluorimetric assay for human antibodies to all the histones

We describe a fluorescence immunoassay for anti-histone antibodies in human sera. Histones are bound to immobilised tyrosine-glutamic acid copolymer on a polystyrene cuvette. With mixed histones as antigen normal sera showed low levels of antibody binding. Much higher values were obtained with some sera from rheumatoid arthritis (RA) patients positive for antinuclear antibodies, and from patients with vasculitic RA, systemic lupus erythematosus (SLE) and drug induced LE. Antibodies to all 5 individual histones were elevated in SLE and vasculitic RA patients. Preliminary results suggest that differences in response patterns may be disease related.

Comparison of radioimmunoassay and ELISA methods for detection of antibodies to chromatin components

A solid phase radioimmunoassay has been compared with an enzyme-linked immunosorbent assay (ELISA) for efficacy in measuring anti-chromatin antibodies. The low backgrounds achieved with the radioimmunoassay method produced a high signal-to-noise ratio and enabled detection of the human test antiserum at a dilution of 1:102,400. By contrast, the ELISA could detect the same antiserum only at a dilution of 1:3200 and above. The radioimmunoassay was consistently more sensitive than the ELISA for detection of anti-chromatin antibodies in a number of human and mouse sera and ascites fluid containing a monoclonal antibody. Factors affecting sensitivity in both assays are discussed.

Localization of two antigenic determinants in histone H4

Four overlapping synthetic peptides corresponding to the carboxy-terminal region 80-102 of histone H4 were prepared by solid-phase peptide synthesis. Their antigenic activity was analysed by inhibition of the H4-anti-H4 reaction in complement fixation and enzyme-linked immunosorbent assay. One antigenic determinant was localized in residues 88-96 of the H4 molecule. No antigenic activity was found in peptides 80-89 and 97-102. Antibodies induced by peptide 85-102 were found to bind to free H4 in solution but not to chromatin subunits, suggesting a lack of accessibility of the C-terminal region of H4 in nucleosomes. A second epitope was found to be situated in the N-terminal region 1-53 of histone H4.

Visualization of anti-histone antibodies on SV40 minichromosomes by scanning transmission electron microscopy (STEM)

The unique capabilities of the scanning transmission electron microscope (STEM) have been used for a high resolution study of antibody binding to individual SV40 minichromosomes. A method of sample preparation has been developed which allows direct visualization of the antibody molecules in a clearly recognizable form. Using this technique, we have studied the binding of anti-H2B and anti-H3 immunoglobulins to SV40 minichromosomes. The results indicate that histones H2B and H3 are located only in the nucleosomes and are absent in the linker regions.

Autoantibodies to nonhistone nuclear antigens and their clinical significance

Identification of the immunologic specificity of antinuclear antibodies (ANAs) in the various systemic rheumatic diseases has become increasingly important. The standard immunofluorescence technique may enable detection of antibodies to nuclear antigens present in abundance in the nucleus, such as DNA, histones, Sm, nuclear ribonucleoprotein (nRNP), and SS-B/La. The nuclear antigens present in low concentrations, such as SS-A, proliferating cell nuclear antigen (PCNA), rheumatoid arthritis nuclear antigen (RANA), and Ku antigens, are unique to cell types, and their detection requires special substrates or reagent systems. Anti-Sm, anti-Scl-70, anticentromere, and anti-PM-1 are characteristic serologic markers for systemic lupus erythematosus, scleroderma, the CREST syndrome of scleroderma, and polymyositis, respectively. Distinct profiles of ANA characterize different rheumatic diseases. A number of ANAs are found in SLE, whereas other diseases are characterized by the presence or absence of a certain ANA or by differences in mean ANA titers. Specific ANAs have been used to isolate and characterize nuclear antigens at molecular and functional levels.

Detection of antibodies to histones in human systemic lupus erythematosus and in murine lupus-like syndromes using micro-ELISA

An enzyme-linked immunosorbent microassay (micro-ELISA) was improved for the detection of anti-histone antibodies. Detection of these antibodies in autoimmune diseases such as systemic lupus erythematosus (SLE), rheumatoid arthritis (RA) and systemic sclerodermitis sclerosis (SCL), showed a positive correlation between the presence of antinuclear antibodies detected by indirect immunofluorescence and the level of anti-histone antibodies (p less than or equal to 0.02) having anti-H1 (p less than or equal to 0.001) and anti-H2B (p less than or equal to 0.01) specificities. These antibodies were more frequently detected, and at a higher level, in human SLE than in RA and SCL. Anti-H1 and anti-H2B (and less frequently anti-H3) antibodies were also found in mice with lupus-like syndrome (MRL/1, PN and Swan). Their level rose with age and they were prevalent in the severe forms, especially in MRL/1 and PN.