Synergistic Reactivation Of Cancer/Testis Antigens By Combination Treatment With Decitabine and Histone Deacetylase Inhibitors (Valproate, Panobinostat) In Myeloid Leukemia Cells

Epigenetic therapies with azanucleoside DNA hypomethylating agents, alone or in combination with histone deacetylase inhibitors (HDACi), show clinical activity in myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML), particularly when given at non-cytotoxic doses. They are able to reactivate epigenetically silenced genes including, among others, a number of highly immunogenic proteins dubbed Cancer/testis antigens (CTAs), predominantly the CTAs located on the X chromosome. We have previously shown that decitabine can induce expression of several CTAs, including MAGEB2 and NY-ESO-1, in myeloid cells in vitro and thereby trigger an immune response (Almstedt et al., Leuk. Res. 2010). Induction of a CTA-specific cytotoxic T cell response in vivo was reported also in AML patients treated with azacitidine and sodium valproate (VPA) and correlated with clinical response (Goodyear et al., Blood 2010). To the best of our knowledge, no data have yet been reported on the effect of combination treatment with decitabine and panobinostat or sodium valproate (VPA) on CTA reactivation in myeloid leukemia.

We hypothesized that by combining decitabine with HDACi we could further enhance expression of CTAs in myeloid leukemia cells and thereby boost recognition of the malignant cells by the cytotoxic T lymphocytes.

The myeloid cell lines U937 and Kasumi-1 were treated with decitabine alone or in combination with the HDACi VPA or panobinostat applied at non-toxic concentrations (>80% cell viability). Expression of CTAs was analyzed by RT-qPCR and Western blot after 48 hours of HDACi treatment. DNA methylation of NY-ESO-1 and MAGEB2 promoter regions was quantified by pyrosequencing. Bone marrow mononuclear cells from 19 AML patients (treated with or without VPA as add-on to decitabine in the ongoing randomized phase II DECIDER clinical trial, NCT00867672) were collected before and on day 15 of treatment, in some patients also after 2 treatment cycles. CTA mRNA expression and promoter DNA methylation were quantified as described above.

VPA or panobinostat alone did not induce MAGEB2 or NY-ESO-1 expression in vitro. However the pretreatment of cells with decitabine prior to addition of either HDACi resulted in a synergistic dose-dependent reactivation of MAGEB2 and NY-ESO-1 on the mRNA level (confirmed for the latter on the protein level). Pyrosequencing analysis of the heavily methylated NY-ESO-1 and MAGEB2 promoters revealed, as expected, no methylation changes upon HDACi treatment, but a dose-dependent hypomethylation upon decitabine. In recently initiated in vivo studies (DECIDER trial), until now cells from 19 AML patients receiving epigenetic treatment were sequentially analyzed. Induction of MAGEB2 mRNA was observed in 9 patients (from absent to a median of 0.002 relative to GAPDH, range 0.0004-0.043), with concomitant DNA hypomethylation of the MAGEB2 promoter from median 83% pretreatment methylation (range 63%-90%) to 63% posttreatment (range 44%-74%). In 5 patients modest hypomethylation without changes in MAGEB2 expression was observed (from median pretreatment values of 89% [72%-92%] to 82% [58%-87%] posttreatment). Another 5 patients disclosed neither hypomethylation nor reexpression of MAGEB2 (results as yet blinded to treatment arm and clinical response).

Combined epigenetic treatment with the hypomethylating agent decitabine and the HDACi VPA or panobinostat synergistically induced a dose-dependent reactivation of the CTAs MAGEB2 and NY-ESO-1 in vitro, accompanied by promoter hypomethylation. First translational results of the DECIDER AML trial also indicate in vivo effects of the epigenetic treatment on CTA induction. The unmasking of CTAs to the immune system by epigenetically active drugs can increase anti-tumor immune responses, and thus has clear implications for future clinical trials combining epigenetic therapy and specific immunotherapy in myeloid neoplasia.

No relevant conflicts of interest to declare.