1- Introduction

2- Structural elements of mRNA

3- Improving the stability and translation of mRNA

4- Avoiding immunogenicity of mRNA

5- 5 mRNA delivery

6- Preclinical and clinical advances of mRNA therapeutics

7- Concerns over the mRNA medicinal industry

8- Summary and perspective

References

Abstract

Messenger RNA (mRNA)-based therapeutics hold the potential to cause a major revolution in the pharmaceutical industry because they can be used for precise and individualized therapy, and enable patients to produce therapeutic proteins in their own bodies without struggling with the comprehensive manufacturing issues associated with recombinant proteins. Compared with the current therapeutics, the production of mRNA is much cost-effective, faster and more flexible because it can be easily produced by in vitro transcription, and the process is independent of mRNA sequence. Moreover, mRNA vaccines allow people to develop personalized medications based on sequencing results and/or personalized conditions rapidly. Along with the great potential from bench to bedside, technical obstacles facing mRNA pharmaceuticals are also obvious. The stability, immunogenicity, translation efficiency, and delivery are all pivotal issues need to be addressed. In the recently published research results, these issues are gradually being overcome by state-of-the-art development technologies. In this review, we describe the structural properties and modification technologies of mRNA, summarize the latest advances in developing mRNA delivery systems, review the preclinical and clinical applications, and put forward our views on the prospect and challenges of developing mRNA into a new class of drug.

INTRODUCTION

Messenger RNA (mRNA) has become an attractive subject of basic and applied research since it was first discovered in 1960s (Brenner et al., 1961). Accordingly, the understanding of mRNA has shifted from a simple link between DNA and protein to a versatile molecule that regulates the functions of genes in all living organisms. Based on this change, numerous types of mRNA-based therapeutics have emerged. In 1990, Wolff et al. firstly reported that intramuscular injection of mRNA into the skeletal muscle of mice led to the expression of encoding proteins (Wolff et al., 1990). Since then, mRNA- based therapeutics have been exploited in a variety of applications, including cancer immunotherapy, infectious disease vaccines, protein substitution and cellular genetic engineering. In 2001, ex vivo mRNA transfected dendritic cells entered clinical trial for the first time (Heiser et al., 2002), and hundreds of mRNA-based clinical trials have been launched over the past two decades. However, mRNA was not considered a new class of drug in the first decades after its discovery. Obstacles such as instability and immunogenicity have hampered its development, making it less pursued than DNA in gene therapy (Burnett and Rossi, 2012; Crooke et al., 2018; Geall et al., 2013; Hajj and Whitehead, 2017; Kallen and Thess, 2014; Kreiter et al., 2011; Reautschnig et al., 2017; Sahin et al., 2014; Van et al., 2015). In recent years, these key problems have been mainly solved by introducing modified nucleosides into mRNA sequences and developing various RNA packaging and delivery systems. A lot of evidences not only proved that mRNA can mediate superior transfection efficiency and longer protein expression time, but also revealed the main advantages of mRNA over DNA. The advantages of mRNA include: (1) mRNA does not need to enter the nucleus to be functional.