A miRaculous new therapy in myeloma?

In this issue of Blood, Morelli et al describe a novel method for targeting cancers with dysregulated c-MYC (MYC), such as multiple myeloma, by inhibiting the micro-RNA 17-92 (miR-17-92) cluster through degradation of its precursor RNA.¹ 

Early work by Evan and colleagues initially established that dysregulation of MYC could sensitize cells to apoptosis, providing evidence that overcoming apoptosis is required for cells to survive the inappropriate proliferation induced during oncogenesis.²  We now know that MYC dysregulation results in the induction of apoptosis in part through the upregulation of the pro-apoptotic BCL2 family member, BIM (BCL2L11).³  Cancer cells survive by neutralizing BIM through binding by antiapoptotic members of the BCL2 family including BCL2, BCL-XL (BCL2L1), and MCL1, the latter of which can adapt to changes in BIM levels through stabilization.⁴  Additionally, although MYC can induce BIM transcription, expression is also regulated posttranscriptionally by miRs of the miR-17-92 cluster.⁵ 

Expression of the miR-17-92 cluster is upregulated in a variety of cancers and has been previously associated with poor prognosis in myeloma,⁶  a finding verified in this report. The cluster consists of 6 related mIRs encoded from a polycistronic long, noncoding RNA (MIR17HG transcript variant 1, also known as pri-miR-17-92).⁵  Given its apparent role in oncogenesis and ability to regulate the expression of proapoptotic molecules such as BIM, the ability to modulate the miR-17-92 cluster to restore apoptotic function makes it an attractive therapeutic target. However, this has been challenging because the 6 miRs in the cluster have overlapping target specificity, decreasing the utility of targeting single miRs.⁵  To overcome this issue, the authors took advantage of a novel approach to target nuclear RNAs. Specifically, they designed antisense oligonucleotides against MIR17HG to target the precursor RNA for RNaseH-dependent degradation. The oligonucleotides were modified to block degradation as well as increase binding stability to RNA through the inclusion of locked nucleic acids at the 5′ and 3′ ends. Interestingly, they were also able to demonstrate that these modified oligonucleotides (called LNA gapmeRs) were able to enter and accumulate in cells in a transfection-free process referred to as gymnosis.⁷ 

The authors went on to test a panel of LNA gapmeRs and focused on 1 (called MIR17PTi for MIR17 precursor transcript inhibitor) that was able to deplete MIR17HG as well as subsequent processed miRs. Importantly, the authors demonstrated that this occurred in an RNaseH-dependent fashion. They next tested the effects of MIR17PTi on a panel of 48 cancer cell lines and 5 nontransformed lines and although sensitivity was observed in most tumor types tested, hematological malignancies, especially myeloma lines, were most sensitive. This result is consistent with previous studies demonstrating that MYC is the most commonly dysregulated gene in myeloma patients and that MYC translocations are found in the vast majority of myeloma cell lines.⁸  MIR17PTi was more effective than targeting individual mIRs from the cluster. In contrast, targeting all miRs simultaneously had similar activity as MIR17PTi, demonstrating the need to block the expression of all miRs in the cluster. Importantly, activity was also demonstrated in freshly isolated patient samples and could overcome the protective effects of coculture with bone marrow stromal cells. Additional mechanistic studies confirmed that MYC upregulation sensitized cells to MIR17PTi and that induction of BIM was important for activity. However, loss of BIM had only a partial effect on MIR17PTi activity, suggesting that additional factors or pathways targeted by the cluster are involved in MIR17PTi-induced cell death. In this regard, it is worth noting that the miR-17-92 cluster has also been shown to regulate PTEN/phosphatidylinositol 3-kinase, NF-κB, and p21, which might also play a role in the observed phenotypes.⁵  Moreover, derepression of BIM would not be expected to induce cell-cycle arrest or senescence, yet these outcomes were also observed in cells treated with MIR17PTi.

The final studies point to the potential for translation of MIR17PTi. First, the authors demonstrated that MIR17PTi can work in combination with commonly used myeloma drugs such as melphalan, dexamethasone, and bortezomib. Importantly, MIR17PTi was active in blocking tumor growth in vivo in multiple xenograft models. Finally, a potential concern regarding the ability to deliver an oligonucleotide-based therapy was addressed by pharmacokinetic data in Cynomolgus monkeys provided and, although no pathology data are shown, the authors reported that toxicity was not observed.

Additional questions remain regarding the use of MIR17PTi. Would this combine with other drugs used in myeloma therapy? The use of IMiDs such as lenalidomide with MIR17PTi was not tested; however, it would be interesting to determine if this combination is active. The direct cellular activity of IMiDs is associated with downregulation of MYC, which could potentially antagonize the effects of MIR17PTi. Similarly, BET domain inhibitors also regulate MYC expression in myeloma⁹ ; however, previous studies have suggested that the BRD4 inhibitor, JQ1, kills in part by upregulating BIM through inhibition of miR-17-92 expression in hematopoietic tumors.¹⁰  Therefore, additional tests are warranted. Use of MIR17PTi in combinations in vivo will also be of interest. Of course, challenges remain because oligonucleotide approaches for targeting BCL2 showed preclinical promise but were not successful in the clinic; thus, additional pharmacodynamic and toxicity studies beyond the initial pharmacokinetic experiments presented in the current paper are required.

These studies point to a potentially new therapeutic approach in myeloma that exploits a synthetic lethality created by the most common genetic alteration in this disease. It will be interesting to follow the progress of this approach in the clinic because it represents a new twist on targeting undruggable targets. LNA gapmeRs could be used to degrade the RNAs of not only primary transcripts of miR clusters, but also of other noncoding RNAs where a protein target is not available. Additionally, they could be used to target the messenger RNA of proteins where small molecule targeting has been elusive. This could be more efficient than proteolysis targeting chimera approaches because developing oligonucleotides may be easier than the targeting molecules for protein degradation.