SMYD2
Most of the data concerning the relationship between SMYD2 and the immune system refer to its methyltransferase activity on histone 3 lysine 36 (H3K36) (Brown et al. 2006). Nonetheless, SMYD2 enzymatic activity is not restricted to H3K36, and this enzyme can also act on histone 4 lysine 20 (H4K20), among other targets (Boehm et al. 2017; Yi, Jiang, and Fang 2019).
With reference to haematopoiesis, Velinder et al. showed that SMYD2 methylates growth factor independent 1 transcription repressor (GFI1), which is a master regulator of normal and malignant haematopoiesis. The proposed mechanism was that this specific methylation in GFI1 protein provokes recruitment of lysine demethylase 1A (LSD1) and repression of the transcription of GFI1-targeted genes, leading to normal and/or malignant haematopoiesis (Velinder et al. 2016).
In addition, SMYD2 plays a role in macrophages. Xu et al.identified SMYD2 as a negative regulator of macrophage activation and M1 polarization. Smyd2 upregulation abrogated macrophage production of IL-6 and TNFα, among other proinflammatory cytokines, by catalysing H3K36 dimethylation on Tnf and Il6 promoters and suppression of nuclear factor kappa B subunit 1 (NFκB) and extracellular signal-regulated kinase (ERK) signalling (Xu et al. 2015). Li et al. studied macrophages in pathological conditions, such as exposure to the plastics industry widely-used chemicals bisphenol A and phthalate. These chemicals affect peritoneal macrophages by hampering their capacity to produce proinflammatory cytokines, express scavenger receptors and phagocytise. SMYD2 seemed to be involved in this process since SMYD2 knockdown or inhibition of its methyltransferase activity led to an expected decrease in H3K36 dimethylation and activation of IL-6 and TNFα expression, although the phagocytic capacity of the macrophages was unexpectedly reduced (Li et al. 2018).
Immune cells are directly involved in inflammatory disorders such as rheumatoid arthritis. The TNFα inhibitor etanercept (commonly used for the treatment of rheumatoid arthritis) diminished the protein levels of SMYD2 and also H3K36 trimethylation in the C-C motif chemokine ligand 2 (CCL2) promoter region in a lipopolysaccharide (LPS)-stimulated human monocytic cell line. These and other data presented by the authors indicate that the mechanism of action of this drug involves suppression of LPS-induced CCL2 expression by inhibiting SMYD2 and other methyltransferases (Y. C. Lin et al. 2017).
With respect to immune system-related malignancies, high expression levels of SMYD2 were described in many types of leukaemia by Brownet al . In B-cell acute lymphoblastic leukaemia there was a positive correlation between high SMYD2 and low overall patient survival. Haematopoietic stem cell-specific SMYD2 KO mice displayed aberrant haematopoiesis caused by apoptosis induction and changes in gene expression. Indeed, SMYD2 controls gene expression programmes related to wingless-related integration site (WNT) regulation of haematopoietic stem cells proliferation, in part by its association with Frizzled Receptor 2 (FZD2) (Brown et al. 2020). Another study confirmed the previous results (Brown et al. 2020); SMYD2 was overexpressed in acute lymphoblastic leukaemia compared to non-malignant bone marrow and high SMYD2 expression negatively correlated with overall survival in acute lymphoblastic leukaemia. A decrease in SMYD2expression was observed 29 days after chemotherapy in most of the patients who responded to the treatment (leukaemia remission was determined by the presence of less than 5% of leukaemic cells in bone marrow) (Sakamoto et al. 2014). Similar results were obtained by Zhanget al. who observed overexpression of SMYD2 in the bone marrow of children with B lineage acute lymphoblastic leukaemia, which was associated with bad prognosis (including increased white blood cell count and less overall survival and tumour-free survival) and a reduction of SMYD2 expression levels after remission (Zhang et al. 2020). Animal studies demonstrated that, although germ-line SMYD2 KO mice are viable, healthy and display normal haematopoiesis, SMYD2 deficiency delayed and reduced penetrance of death due to MLL-AF9/NRasG12D-induced acute myeloid leukaemia. After disease establishment, both wild-type (WT) and SMYD2 KO cells gave rise to indistinguishable acute myeloid leukaemia phenotypes, but SMYD2 deletion caused competitive disadvantages over their WT counterpartsin vitro and in vivo . The authors also described direct binding of Myc to Smyd2 promoter as a possible molecular mechanism for this phenotype (Bagislar et al. 2016). Moreover, Oliveira-Santos et al. found that both SMYD2 andSMYD3 were upregulated in malignant B cells from the same patients with chronic lymphocytic leukaemia, indicating that these two SMYD proteins may be controlled by the same molecular mechanism in this biological context. Interestingly, patients with very low expression ofSMYD2 and SMYD3 displayed elevated white blood cell counts and a complex karyotype (Oliveira-Santos et al. 2016). Furthermore, Hayet al. noticed that premalignant thymus from RUNX family transcription factor 2 (RUNX2) / myelocytomatosis oncogene (MYC) transgenic mice overexpress Smyd2 and, by extrapolating data from murine primary fibroblasts, they speculated that SMYD2 (but not SMYD5) may inhibit the senescence-like growth arrest caused by RUNX upregulation in RUNX2/MYC lymphoma, thereby acting as an oncogene (Hay et al. 2019). Indeed, altogether, these data indicate that SMYD2 has an oncogenic role in leukaemia.
On the contrary, Zipin-Roitman et al. observed that low expression of SMYD2 in acute myeloid leukaemia patients negatively correlated with treatment response and the probability of tumour-free survival. In addition, human acute myeloid leukaemia cells devoid of SMYD2 were more resistant to DNA damage caused by irradiation compared to control cells, and the mechanism leading to this resistance seemed to be the induction of transient quiescence. The decrease in SMYD2 levels led to a compensatory increase of SET domain-containing protein (SET)7/9 methyltransferase levels, suggesting an interplay between the two enzymes that induces quiescence of leukaemia cells, which then become more resistant to chemotherapy (DNA damage) (Zipin-Roitman et al. 2017).
In addition to macrophages and blood malignancies, SMYD2 is also implicated in host-pathogen interactions. Infection withLeishmania donovani , the intracellular parasite responsible for leishmaniasis, activated the expression of Smyd2 via c-Mycin murine cell lines and primary macrophages. This parasitic infection also caused H3K36 dimethylation at the TNFα promoter, probably by the enzymatic action of SMYD2. Pharmacological inhibition of SMYD2 using AZ505 enhanced the protective inflammatory response in infected macrophage cell lines and decreased parasite multiplication in infected mice. Thus, SMYD2, along with other methyltransferases, aidsLeishmania donovani in the process of infecting the host (Parmar, Chandrakar, and Kar 2020). Moreover, using human CD4+ cells lines and T cells from human immunodeficiency virus type 1 (HIV-1)-infected donors, Boehm et al. demonstrated that SMYD2 mediates H4K20 monomethylation in the HIV-1 promoter located in the 5’ long terminal repeat. H4K20 monomethylation promotes association of the reader protein L3MBTL histone methyl-lysine binding protein 1 (L3MBTL1) with the 5’ long terminal repeat. The presence of SMYD2 is required for this association and necessary for HIV latency, since SYMD2 knockdown or inhibition reactivates the virus. Thus, SMYD2 positively regulates HIV-1 latency (Boehm et al. 2017).