Effect of DNMT SNPs on DNA Methylation in Primary ITP

Immune thrombocytopenia (ITP), is an immune-mediated acquired disease of adults and children characterized by transient or persistent decrease of the platelet count and, depending upon the degree of thrombocytopenia, increased risk of bleeding (Rodeghiero et al., 2009). International guidelines define thrombocytopenia as a peripheral blood platelet count less than 100 000/uL (Raj, 2017). Platelet count values between 100 000/uL and 150 000/uL are frequently found in apparently healthy people (Adibi et al., 2007) and this threshold reduces the concern over the mild "physiologic" thrombocytopenia associated with pregnancy (Neunert et al., 2011).

Immune thrombocytopenia may be primary or secondary. Primary ITP is a diagnosis of exclusion. Secondary ITP occur due to an underlying diseases/conditions. Causes of secondary ITP include infection (CMV, Helicobacter pylori, HCV, HIV, Varicella zoster), Systemic lupus erythematosus, Antiphospholipid syndrome, Drug-induced, Lymphoproliferative disorders, Post bone marrow transplantation, and Post vaccination (Neunert et al., 2011).

Primary ITP is an acquired autoimmune disease causing both increased platelet destruction and insufficient platelet production. More than one mechanism could contribute to this including autoreactive B lymphocytes, Th1/Tc1 polarization, T-cell-mediated platelet lysis and abnormal circulating Treg cells. Also, DNA methylation may participate in the pathophysiology of ITP (Semple et al., 2010, Wang et al., 2011).

DNA methylation is a heritable, stable, and also reversible way of DNA modification; it can regulate gene expression without changing the nucleotide sequences (Huiyuan et al., 2013). DNA methylation is mediated by DNA methyltransferases (DNMTs). There are five members in DNMT group including DNMT3A and DNMT3B (Okano et al., 1999). DNMT3A and DNMT3B catalyze de novo methylation and it is important in the establishment of DNA methylation patterns within the embryo and during fetal development (Sawalha, 2008).

DNA methylation status takes part in the regulation of immune response, the loss of methylation pattern in immune cells will result in autoimmune disease by inducing aberrant gene expression. ITP is an autoimmune disease with many immune deficiencies with abnormal DNA methylation involved in the disease etiology (Huiyuan et al., 2013). Chen et al., 2011, concluded that aberrant DNA methylation may take part in the pathogenesis of ITP by quantifying the methylcytosine concentration of genomic DNA. They found hypomethylation pattern in CD4 T cells of ITP patients.

DNMT3A, and DNMT3B DNA methyltransferases are encoded by different genes on distinct chromosomes (Sawalha, 2008). There are many single nucleotide polymorphisms (SNPs) in DNMT3A gene which may affect catalytic activity of the DNMT3A enzyme, including -448G/A SNP (Zhao et al., 2012). Twenty-one polymorphisms have been identified in the DNMT3B gene including -149 C/T SNP and -579 G/T SNP (Zhang et al., 2015).

In a previous study in Assiut University Hospital (AbdelKader et al., 2018), the DNMT3A -448 G/A SNP variant A allele was significantly associated with decreased risk of primary ITP, while, DNMT3B -149C/T SNP variant T-allele was significantly associated with nearly double-fold increase in risk of primary ITP. However, the underlying mechanism behind this association was not investigated. Authors recommended to study mRNA expression of DNMT genes, enzymatic activity of DNA methyltransferases, quantification of global methylated DNA, and/or methylation status of methylation-sensitive genes involved in primary ITP pathogenesis to understand this association.