Future of epigenetic immunotherapy in kidney cancer

In clinical practice, immune checkpoint inhibition based on the use of antibodies against PD-1 (programmed death 1), PD-L1 (programmed death-ligand 1) and CTLA-4 (cytotoxic T-lymphocyte-associated antigen 4) is actively used for treatment of kidney cancer. However, objective response to monotherapy with these drugs is observed only in 9–24 % of patients, and combinations with other anticancer drugs in most cases cause severe adverse reactions. At the same time, there is an increased risk of toxic liver damage, immune-dependent pneumonitis, and rash. Therefore, it is necessary to search for new methods of immunotherapy, the most promising of which is the method of viral mimicry based on epigenetic stimulation of retroelement expression. Double-stranded retroelement transcripts activate antiviral interferon response that induces apoptosis of tumor cells. To achieve this, inhibitors of DNA methyltransferase, deacetylase and histone methyltransferase are used which have been successfully applied to treat various malignant neoplasms. In the experiment, DNA methyltransferase inhibitor 5-aza-2-deoxytidine (decitabine) effectively inhibited clear cell renal cell carcinoma cells proliferation which indicates their potential in treatment of kidney cancer. However, similarly to other neoplasms, activation of retroelements in renal cell carcinoma serves as initiator of the tumor process as it leads to increased expression of oncogenes, inactivation of tumor suppressors, and genomic instability. Therefore, the method of viral mimicry requires a differentiated approach with inhibition of retroelements involved in carcinogenesis and simultaneous stimulation of expression of retrotransposons that are not involved in the mechanisms of tumor development and have immunogenic properties. For this, microRNAs derived from transposons can be used as guides for DNA methyltransferases. An analysis of scientific literature revealed 41 such microRNAs of which decreased expression in kidney cancer was established for miR-95, -887, -652, -585, -511, -502, -495, -493, -487b, -335; increased for miR-1249, -1266, -151a, -211, -2114, -2355, -28, -3144, -340, -342, -374a, -374b, -3934, -421, -545, -576, -582, -584, -616, -769; and specific expression in different tumor subtypes for miR-708, -577, -450b, -326, -3200, -31, -224, -192, -1271. Since activation of retroelements can lead to insertions into new genome loci with formation of new mutations involved in carcinogenesis, a promising direction in integrated immunotherapy of kidney cancer is the use of reverse transcriptase inhibitors.