Enzyme Expression Abnormalities: mRNA and protein expression of DNMT3A and DNMT3B, responsible for establishing DNA methylation imprints, are significantly lower in peripheral blood mononuclear cells (PBMCs) of ITP patients compared to healthy controls [5] . Although no difference exists between chronic ITP (>12 months) and newly diagnosed ITP patients, the persistently low expression of DNA methyltransferases (DNMTs) suggests their involvement in disease maintenance rather than serving as activity markers [5] .

Methyl Donor Metabolic Dysregulation: Plasma S-adenosylhomocysteine (SAH) concentration is significantly elevated in ITP patients. SAH, a potent methyltransferase inhibitor, competitively binds to the enzyme active site, inhibiting DNMT activity and leading to global DNA hypomethylation [6] [7] . Increased SAH levels positively correlate with lymphocyte DNA hypomethylation and represent a key metabolic driver of epigenetic dysregulation in ITP [6] .

T Cell Polarization Imbalance: DNA hypomethylation promotes a pro-inflammatory environment:

Th1/Th2 Imbalance: Abnormal hypomethylation at gene loci for IFN-γ (Th1) and IL-4 (Th2) leads to a Th1 polarization bias, exacerbating inflammation [8] .

Treg/Th17 Imbalance: The gene locus for forkhead box P3 (FOXP3), the key Treg transcription factor, exhibits physiological hypomethylation in Treg cells to maintain their function. However, in ITP, Treg numbers are reduced and their function is suppressed, potentially related to abnormal methylation at other regulatory sites (e.g., CTLA4) [9] . Concurrently, pro-inflammatory Th17 cells may be activated due to de-repression at gene loci like IL-17A [10] .

B Cell Abnormal Activation and Autoantibody Production: DNA hypomethylation can lift the inhibition of autoreactive B cells, promoting their differentiation into plasma cells and the production of anti-platelet autoantibodies (e.g., anti-GPIIb/IIIa, GPIb/IX) [11] . These antibodies facilitate platelet phagocytosis by macrophages via Fcγ receptors and may inhibit megakaryopoiesis.

Direct Cytotoxic Effect of Cytotoxic T Cells (CTLs): CD8⁺ CTLs can directly lyse platelets via Fas/FasL and perforin/granzyme pathways and kill megakaryocytes in the bone marrow [12] . DNA hypomethylation may participate in regulating the expression of CTL-related effector molecules (e.g., perforin, granzyme B).

Expression of key genes for megakaryocyte differentiation and maturation (e.g., TRAIL, NF-E2, GATA1, FLI1) is finely regulated by DNA methylation [13] .

Bone marrow megakaryocytes in ITP patients often exhibit maturation arrest (increased small megakaryocytes) and increased apoptosis. Studies found that plasma from ITP patients can induce downregulation of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) expression in megakaryocytes, promoting their apoptosis. Hypermethylation of the TRAIL gene promoter region may be one cause of its silencing [14] .

Serum levels of 1,25(OH)₂D₃ and expression of the vitamin D receptor (VDR) are significantly reduced in active ITP patients and positively correlate with platelet count [15] . Vitamin D deficiency is considered a risk factor for ITP development and progression.

1,25(OH)₂D₃ exerts potent immunomodulatory functions by binding to VDR:

Inhibits PBMC proliferation and reduces inflammatory cell infiltration.

Reverses abnormal T cell polarization: Suppresses Th1 (IFN-γ) and Th17 (IL-17) differentiation, promotes Th2 (IL-4, IL-10) responses [16] .

Induces Treg differentiation and function: Promotes FOXP3 expression and enhances Treg suppressive capacity [14] .

Modulates cytokine networks: Downregulates pro-inflammatory cytokines (TNF-α, IL-6, IL-17), upregulates anti-inflammatory cytokines (IL-10, TGF-β) [16] .

Vitamin D can profoundly influence the DNA methylation landscape through the VDR signaling pathway:

Regulation of Methyltransferase Expression: Reduced expression of DNMT1 and DNMT3B was observed in CD4⁺ T cells of mice supplemented with vitamin D (cholecalciferol) [18] . Downregulation of DNMT1 and DNMT3B was also noted in prostate cancer cells treated with calcitriol [19] .

Molecular Mechanisms Regulating DNMTs: Transcriptional Regulation: VDR can directly bind to or interfere with the binding of other transcription factors (e.g., SP1) to the DNMT3B promoter region, inhibiting its transcription [19] . miRNA-Mediated: Vitamin D upregulates miR-98-5p, which targets the 3’UTR region of DNMT3B mRNA, promoting its degradation [19] . Influence on Methyl Donor Metabolism: Vitamin D can upregulate the expression of Bhmt1 (betaine-homocysteine methyltransferase) in CD4⁺ T cells [18] . Bhmt1 catalyzes the conversion of homocysteine to methionine, which is subsequently converted to S-adenosylmethionine (SAM)—the primary methyl donor for DNA methylation. This pathway may impact the methylation cycle. Indirect Regulation Dependent on Histone Modifications: Targeting of specific genomic regions by DNMT3A/3B depends on histone marks (e.g., H3K36me3) [20] . VDR can alter local chromatin states by regulating the expression or recruiting chromatin remodeling complexes of histone-modifying enzymes (histone methyltransferases/demethylases, acetyltransferases/deacetylases), indirectly influencing the recruitment and activity of DNA methylation enzymes [21] .

Targeting TET Enzymes: VDR binding sites are also found near the ten-eleven translocation (TET)2 and TET3 genes (involved in active DNA demethylation) [22] , suggesting vitamin D may influence hydroxymethylation levels (5 hmC)—a key step in active demethylation—by regulating TET enzymes.

Mechanism of Action: DAC is a DNMT inhibitor. At low doses, it primarily exerts demethylating effects, promoting gene re-expression.

Efficacy Evidence: Promoting Megakaryocyte Maturation and Platelet Release: Low-dose DAC reverses ITP plasma-induced megakaryocyte maturation arrest and apoptosis. This mechanism is associated with demethylation of key gene promoters (e.g., TRAIL), restoring their expression, and promoting megakaryocyte maturation and platelet production [14] [23] . In animal models, DAC significantly increased platelet release from the spleen and bone marrow [23] . Immunomodulation: In treating refractory ITP patients, low-dose DAC significantly reduced CD16⁺ inflammatory monocytes and activated CD4⁺ T cell numbers, while enhancing the suppressive function of Treg cells [24] .

Clinical Outcomes: For adult ITP patients refractory or relapsed after first-line (corticosteroids, IVIG) and second-line (TPO-RAs, rituximab) therapies, low-dose DAC demonstrates good response rates (ORR approximately 60% - 80%) and sustained responses, significantly increasing platelet counts and improving bleeding symptoms [24] [25] .

Corrects the vitamin D deficiency state in patients.

Increases Platelet Count: Multiple observational studies and small randomized controlled trials (RCTs) indicate that vitamin D supplementation correlates with elevated platelet counts [26] .

Improves Immune Parameters: Upregulates Treg proportion and function, suppresses Th17 responses, and shifts the cytokine profile towards an anti-inflammatory direction [16] [17] .

Potential as Adjunctive Therapy: Vitamin D may enhance the efficacy of other ITP treatments (e.g., corticosteroids, TPO-RAs) or reduce their required dosages [26] .

Theoretical Basis for Synergy:

1) Co-targeting Immune Dysregulation: DAC directly corrects DNA hypomethylation, restoring immune tolerance (e.g., enhancing Tregs); Vitamin D suppresses excessive inflammation (e.g., inhibiting Th1/Th17) via VDR signaling. Together, they synergistically reshape immune homeostasis.

2) Co-promoting Megakaryopoiesis: DAC promotes megakaryocyte maturation by demethylating key genes (TRAIL, etc.); Vitamin D may support megakaryocyte differentiation by modulating the bone marrow microenvironment or directly acting on megakaryocyte VDR.

3) Vitamin D Enhances DAC’s Epigenetic Effect: Vitamin D, by downregulating DNMTs (e.g., DNMT1, DNMT3B) [18] [19] and potentially influencing methyl donor metabolism (upregulating Bhmt1) [18] , may reduce the enzyme activity and substrate levels required to maintain abnormal methylation states. Theoretically, this could enhance the demethylation efficiency of low-dose DAC or prolong its duration of action.

4) Vitamin D May Mitigate Potential DAC Side Effects: The immunomodulatory and anti-inflammatory effects of vitamin D may help alleviate treatment-related inflammatory responses.

Core Hypothesis Needing Investigation: The central hypothesis of this review is that baseline patient 1,25(OH)₂D₃ levels may influence the efficacy of demethylating therapy (e.g., DAC).

Hypothesis: Patients with severe vitamin D deficiency may have more severe DNA methylation dysregulation (e.g., lower DNMT expression, more pronounced hypomethylation of pro-inflammatory genes) and more impaired immune cell (especially Treg) function. In such cases, DAC monotherapy may be insufficient to fully restore immune homeostasis and megakaryocyte function. Conversely, patients with sufficient vitamin D levels may have an epigenetic background and immune baseline state more conducive to DAC efficacy.

Validation Directions: Prospective clinical studies are needed to compare treatment outcomes (platelet response rate, response depth, duration) in ITP patients with high vs. low baseline 1,25(OH)₂D₃ levels receiving DAC therapy. Dynamic monitoring before and after treatment should include: T/B cell subset changes (e.g., Treg frequency/function, Th17, activated T/B cells); Key cytokine profiles (IFN-γ, IL-4, IL-10, IL-17, TGF-β, etc.); DNA methylation status (genome-wide/specific gene loci e.g., FOXP3 TSDR, TRAIL promoter methylation levels); Megakaryocyte-related parameters (bone marrow megakaryocyte count, maturity, platelet production rate).