Nucleosomes can be found circulating in blood of healthy as well as diseased individuals, mostly as a consequence of cell death. Nucleosomes, the basic units of chromatin, are composed of DNA wrapped around an octamer of histones (2 H2A, 2 H2B, 2 H3, and 2 H4 histones), and are often released as mono- or oligonucleosomes with circulating cell-free DNA (cfDNA). Increased levels of circulating nucleosomes or circulating histones are frequently found in the serum of patients with cancer and benign conditions, as well as patients suffering from stroke, sepsis, autoimmune diseases and inflammation (1).
 The finding that circulating nucleosomes are elevated in patients with several types of cancers, and other non-cancer conditions, has led to exploring these as possible diagnostic and prognostic biomarkers. Thus measuring nucleosomes in the serum or plasma of patient samples can prove valuable and have utility as a non-invasive biomarker strategy. In addition, several studies suggest that circulating nucleosome levels could be a useful predictor of response to therapy. In breast and lung cancer, patients with lower pre-therapeutic levels of circulating nucleosomes responded better to chemotherapy than patients with higher circulating nucleosome levels (1). Similar findings have been reported after chemo- or radiation therapy, in which decreased circulating nucleosome levels were associated with disease regression, while elevated levels were associated with progression of disease in several cancers (2).
Increased levels of circulating histones are also reported during severe trauma, and these can mediate inflammatory responses and damage to endothelial cells, particularly in lung, resulting in acute lung injury (3). In patients that have undergone heart surgery, circulating histones increase post-operatively and reach peak levels before other biomarkers (4). In addition, higher circulating histone levels were predictive of more post-operative adverse events. Thus it has been proposed that very high levels of circulating histones in blood may be harmful and correlate with toxicity and worse prognosis after trauma.
Histone post-translational modifications can also be detected in circulating histones in serum of healthy and diseased individuals. For instance, in cancer patients, circulating histone H3 lysine 9 tri-methylation (H3K9me3) levels were lower as compared to healthy individuals (5).
 To quantitatively detect circulating histone H3K4me3 from plasma and serum within 2.5 hours, use the EpiQuik Circulating Trimethyl Histone H3K4 ELISA Kit (Colorimetric).
In another study, a 5-gene epigenetic biomarker panel consisting of histone modifications (H3K4me2 and H2AK119Ub), specific histone variants (H2A.Z and H2A.A) and 5-methylcytosine proved useful in distinguishing pancreatic cancer from healthy controls, and was superior to the standard carbohydrate antigen 19-9 (CA 19-9) serum biomarker (6). Other histone modification marks, such as H3K9me1, H3K27me3, and H4K20me3 have been detected in circulating histones, and often levels differ between cancer and healthy individuals (7, 8).
In sepsis, citrullinated histone H3 (H3cit) has been suggested as a serum biomarker, as increased levels of H3cit correlate with fatality of disease (9, 10). In addition, inhibition of the circulating H3cit with an antibody improved survival in mice, suggesting H3cit as a possible therapeutic target. Altogether these results highlight the clinical relevance of circulating histones and their associated modifications, and developing methods for easy and accurate measurement that would allow monitoring of these epigenetic marks will be instrumental in advancing this research field.
 For ELISA-like measurement of total citrullinated histone H3 (at arginines R2, R8, R17) directly from plasma or serum, use the EpiQuik Circulating Histone H3 Citrullination ELISA Kit (Colorimetric).
To support investigators in their circulating histones research projects, EpiGentek has developed a series of ELISA kits for quantification of circulating histones in serum or plasma. These kits include the EpiQuik Circulating Total Histone H3 Quantification Kit for quantification of total histone H3, a site-specific series for quantification of citrullinated histone H3 (H3cit) and other modifications, as well as an EpiQuik Circulating Modified Histone H3 Multiplex Assay Kit, for simultaneous screening of 22 different histone H3 modifications from circulating histone. The 96-well format allows high-throughput measurement, and the kits require just a very small amount of serum or plasma (10-40 ul), saving precious sample for use in other applications.
References:
- McAnena P, et al., Circulating Nucleosomes and Nucleosome Modifications as Biomarkers in Cancer. Cancers (Basel). 2017 Jan 8;9(1).
- Holdenrieder S, et al., Nucleosomes in serum of patients with benign and malignant diseases. Int J Cancer. 2001 Mar 20;95(2):114-20.
- Abrams ST, et al., Circulating histones are mediators of trauma-associated lung injury. Am J Respir Crit Care Med. 2013 Jan 15;187(2):160-9.
- Gao H et al., Circulating histones for predicting prognosis after cardiac surgery: a prospective study. Interact Cardiovasc Thorac Surg. 2016 Nov;23(5):681-687.
- Rasmussen L, et al., Pre-analytical variables of circulating cell-free nucleosomes containing 5-methylcytosine DNA or histone modification H3K9Me3. Scand J Clin Lab Invest. 2016 Oct;76(6):448-53.
- Bauden M, et al., Circulating nucleosomes as epigenetic biomarkers in pancreatic cancer. Clin Epigenetics. 2015 Oct 7;7:106.
- Gezer U, et al., Histone Methylation Marks on Circulating Nucleosomes as Novel Blood-Based Biomarker in Colorectal Cancer. Int J Mol Sci. 2015 Dec 11;16(12):29654-62.
- Deligezer U, et al., Sequence-specific histone methylation is detectable on circulating nucleosomes in plasma. Clin Chem. 2008 Jul;54(7):1125-31.
- Li Y, et al., Citrullinated Histone H3 – A Novel Target for Treatment of Sepsis. Surgery. 2014 August ; 156(2): 229–234.
- Li Y, et al., Identification of Cit H3 as a Potential Serum Protein Biomarker in a Lethal Model of LPS-induced Shock. Surgery. 2011 September ; 150(3): 442–451.
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