New exciting possibilities for the development of precision medicine therapies to restore the expression of the INK4/ARF locus

Loss of expression of the genes encoded in the INK4/ARF locus (p14^ARF, p15I^NK4B and p16^INK4A) is a common event in human cancers (1). Recent publications shed light on the mechanisms involved in the INK4/ARF locus silencing, which opens an exciting window for the design of new therapeutic strategies for cancer patients.

As recently reviewed by Minggang Fang in Annals of Research Hospitals, several mechanisms account for silencing of the INK4/ARF locus in the context of cancer cells. Besides somatic deletions of the genes encoded in this locus, the entire locus is frequently subdued by epigenetic repression. Loss of the expression of the INK4/ARF locus results from aberrant DNA hypermethylation. This has been demonstrated for some patients with KRAS-driven colorectal cancer, where upregulation of the transcriptional repressor ZNF304 recruits co-repressor proteins to the promoter of the INK4/ARF encoded genes, leading to promoter hypermethylation and transcriptional silencing (2). Importantly, Serra and co-authors demonstrated that in colorectal cancer KRAS increases ZNF304 protein levels, triggering promoter hypermethylation and thereby initiating oncogenesis.

Another KRAS-driven cancer that frequently presents with loss of INK4/ARF gene expression is Non-Small Cell Lung Cancer (NSCLC) (3). Since inactivation of the INK4/ARF locus in NSCLC patients is frequently due to promoter hypermethylation, it will be pertinent to explore if these patients present high levels of ZNF304, as demonstrated in colorectal cancer patients. In this context, we recently published an alternative mechanism for the silencing of the INK4/ARF locus in NSCLC patients, independent of promoter hypermethylation. This mechanism involves the E3 ligase and transcription co-factor E6AP (E6-Associated Protein, encoded by the UBE3A gene) (4). We demonstrated that in normal cells, E6AP restricts the abundance of CDC6, a key negative regulator of the INK4/ARF locus, resulting in high expression of the tumour suppressor genes encoded in the INK4/ARF locus. Loss of E6AP expression results in high levels of CDC6 and repression of the INK4/ARF locus, which is tumorigenic in lung. Importantly, we were able to identify a subset of KRAS-positive NSCLC patients with a E6AP-low/CDC6-high/p16^INK4a-low expression profile that showed low frequency of p16^INK4a promoter hypermethylation. The clinical relevance of this novel regulatory pathway is supported by the observation that this cohort of patients presented with the worst overall survival, indicating the potential prognostic value for the E6AP-low/CDC6-high/p16^INK4a-low expression profile.

These newly identified pathways not only have prognostic value but also provide attractive opportunities to explore new therapeutic options. Generic epigenetic-based therapies (i.e., DNA demethylating agents and histone deacetylase inhibitors) have shown promising results in pre-clinical models as well as in some cancers (5). However, the use of non-selective drugs is frequently associated with a broad spectrum of adverse effects. The inhibition of specific components of pathways, such as the ones described by Serra et al. and by us, is likely to provide more selective therapeutic options.

For those NSCLC patients with E6AP-low/CDC6-high/p16^INK4a-low expression profile, one exciting therapeutic possibility is to restore p16^INK4a expression and tumour suppression by reducing CDC6 abundance. Recently published data demonstrated that CDC6 induction and the consequent repression of the INK4/ARF locus, is stimulated by the histone acetyl-transferase, MOZ (6). This opens the opportunity to design novel therapies to restore the expression of the tumour suppressor genes encoded in the INK4/ARF locus by targeting MOZ activity. Small molecule inhibitors of MOZ have now been developed (7) and their therapeutic potential in the treatment of cancer is currently being assessed.

Acknowledgements

None.

Footnote

Conflicts of Interest: The authors have no conflicts of interest to declare.

References

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