Spherical nucleic acids for precision medicine

With the advent of functional cancer genomics, precision medicine has begun to enter clinical practice. Therapeutic regimens are informed by the tumor’s genotype to specifically target critical genetic aberrations that are the driving forces of disease progression. While several targeted small molecule inhibitors and biotherapeutic antibodies have been FDA-approved, in particular those targeting a cancer cell’s kinome, it has become apparent that the majority of cancer genes represent unprecedented, non-enzymatic targets with unknown modi operandi that cooperate to drive cancer progression and therapy resistance. How can multiple, undruggable, and uncharacterized genes be therapeutically targeted? RNA interference (RNAi) comes to mind, but due to difficulties in delivering small interfering (si) or small hairpin (sh)RNAs robustly and safely to tumor sites, the delivery of oligonucleotide payloads represents a significant challenge and an unmet clinical need. Nanotechnology continues to provide fundamentally different approaches to the treatment of genetic disease. In particular, spherical nucleic acids (SNAs), gold-based nanoconjugates functionalized with densely packed, highly oriented antisense DNA or siRNA oligonucleotides represent one of the most prominent and promising nanoscale gene regulation platforms [1]. Specifically, these nanoconjugates typically are comprised of a 13 nm gold nanoparticle core that is decorated with a corona of thiolated double-stranded RNAs (Figure 1). SNAs show robust cellular uptake via scavengerreceptor-dependent endocytosis without the use of toxic, auxiliary transfection agents or viral delivery platforms [1] and exhibit extraordinary stability in physiological environments and robust resistance to nuclease degradation [1, 2]. To evaluate SNAs as a platform for biotherapeutic gene silencing in cancer, we synthesized SNAs targeted to the oncogene Bcl2-Like12 (Bcl2L12), which is overexpressed in several cancer types, most prominently in Glioblastoma multiforme (GBM), the most aggressive and prevalent manifestation of malignant brain tumors. Bcl2L12 is a an atypical member of the Bcl-2 protein family, characterized by proline-rich and a C-terminal 14 amino acid sequence with homology to the BH (Bcl-2 Homology) 2 domain found in several members of the Bcl-2 family [3, 4]. Bcl2L12 is overepxressed in >90% of human primary GBM specimens and exhibits low or undetectable levels in cells of glial origin in the normal brain surrounding tumor tissue or in low-grade astrocytoma [4]. Analysis of 343 glioma patients in the Repository of Molecular Brain Neoplasia Data (REMBRANDT) identified Bcl2L12 as a potential prognostic factor, as GBM patients with high-level overexpression of Bcl2L12 mRNA have shorter progression-free survival compared to patients with low expression or underexpression of Bcl2L12 (p<0.001). On the cellular level, enforced expression of Bcl2L12 confers marked apoptosis resistance, induces malignant transformation, and promotes high-grade glioma progression in vivo [4]. Mechanistically, Bcl2L12 inhibits apoptosis by neutralizing effector caspases [4]. Bcl2L12 physically interacts with caspase-7, blocks proteolytical processing by upstream caspases, and induces transcriptional upregulation of the small heat shock protein αB-crystallin, which directly binds to and inhibits caspase-3. In the cell nucleus, Bcl2L12 interacts with the p53 tumor suppressor. Consequently, Bcl2L12 expression antagonizes replicative senescence without concomitant loss of p53 or p19Arf, blocks p53-dependent apoptosis, impedes the capacity of p53 to bind to target gene promoters and to transcriptionally induce target mRNA expression, e.g. p21 [5]. Correspondingly, copy number and mRNA profiles obtained from The Cancer Editorial Material