Targeted delivery of small noncoding RNA for glioblastoma

Highlights

• MicroRNAs (miRNAs) are found to play critical roles in human cancers.

• •Three-way-junction (3WJ) motif-based RNA nanoparticle is developed for the targeted delivery of small nonconding RNAs for cancer therapy.

• •Folate-conjugation of 3WJ-RNA nanoparticle is capable of specifically recognizing glioblastoma.

Abstract

Aberrant expression of certain genes and microRNAs (miRNAs) has been shown to drive cancer development and progression, thus the modification of aberrant gene and miRNA expression presents an opportunity for therapeutic targeting. Ectopic modulation of a single dysregulated miRNA has the potential to revert therapeutically unfavorable gene expression in cancer cells by targeting multiple genes simultaneously. Although the use of noncoding RNA-based cancer therapy is a promising approach, the lack of a feasible delivery platform for small noncoding RNAs has hindered the development of this therapeutic modality. Recently, however, there has been an evolution in RNA nanotechnology, in which small noncoding RNA is loaded onto nanoparticles derived from the pRNA-3WJ viral RNA motif of the bacteriophage phi29. Preclinical studies have shown the capacity of this technology to specifically target tumor cells by conjugating these nanoparticles with ligands specific for cancer cells and resulting in the endocytic delivery of siRNA and miRNA inhibitors directly into the cell. Here we provide a systematic review of the various strategies, which have been utilized for miRNA delivery with a specific focus on the preclinical evaluation of promising RNA nanoparticles for glioblastoma (GBM) targeted therapy.

Introduction

Although great strides have been made in cancer therapy, cancer is still a leading cause of death in modern world. Over the past century, a major focus of cancer research has been limited to understanding genetic and epigenetic alterations of protein coding genes. However, decades of research on noncoding RNA (ncRNA) has clearly shown that the sole analysis of molecular changes in protein coding genes is not sufficient to understand the whole disease. Indeed, cumulative data clearly indicates that ncRNAs play a critical role in cancer, not merely as diagnostic or prognostic markers but also as therapeutically druggable targets. NcRNAs are endogenously transcribed functional RNA moieties which do not undergo protein translation, as they lack an open reading frame (ORF) [1]. In general, ncRNAs are categorized into two groups depending on size: long noncoding RNAs (lncRNAs) with over 200 nucleotides and short noncoding RNAs (sncRNAs) which are shorter than 200 nucleotides.

MicroRNAs (miRNAs), the smallest sncRNAs, are approximately 22 nucleotides long and have been characterized as some of the key regulators in the translation of messenger RNAs (mRNAs) into proteins [2]. MiRNAs were first discovered in Caenorhabditis elegans (C. elegans) [3]. Victor Ambros and his colleagues found that a 22 nucleotide-long piece of RNA derived from abnormal cell LINeage gene (Lin-4) inhibited the translation of the protein coding gene Lin-14 through its direct interaction with the 3′ untranslated region (3′ UTR) of the Lin-14 mRNA [3,4]. A similar small RNA regulator, Let-7, was also found in C. elegans [5], prompting further research towards the identification of miRNAs in other organisms, including humans. It was discovered that miRNAs are regulators of gene expression in both normal cells as well as diseased. In human cancers, the first pathological evidence that miRNAs are directly involved in cancer development and progression was reported in 2002 by Carlo Croce's laboratory [6]. In a landmark study, authors identified a frequently deleted miR-15/16 cluster of genes in chronic lymphocytic leukemia (CLL) patient tissues. It was discovered that miR-15/16 is a tumor suppressor which is deleted in CLL, and normally functions to directly inhibit the expression of the oncogene B-cell lymphoma gene 2 (BCL2) [6]. Since this seminal finding, numerous additional studies have revealed that miRNAs take on various roles in many aspects of cancer initiation, progression, and metastasis [7], such as cell proliferation and apoptosis [8], differentiation [9], cancer cell metabolism [10] and cell cycle [11]. The aberrant dysregulation of miRNAs in cancer cells results in a unique expression profile and led to the identification of “signature miRNAs”, which have been proposed to serve as diagnostic or prognostic markers. Many of these signature miRNAs have also been explored as potential tools in the development of cancer therapies. However, a lack of proper strategies to ensure efficient targeting and delivery of miRNAs into cancer cells has hampered the therapeutic potential of miRNAs in the clinical setting. Recent advances in RNA nanotechnology have enabled new strategies for targeted delivery of therapeutically useful sncRNAs. This review will describe the current progress of RNA nanotechnology-based targeted delivery of sncRNAs for glioblastoma (GBM), highlighting an important translational implication for future clinical trials.