The Introduction of m7G Cap Structure Significantly Improves the Translation Efficiency of Circular mRNA

As an RNA therapy tool, circular mRNA has higher stability and durability than linear mRNA. However, its translation efficiency is low and it mainly relies on the internal ribosome entry site (IRES) to initiate translation. But the IRES-mediated translation efficiency is generally low, which severely limits the application of circular mRNA in therapy.

In February 2025, a research team from Nagoya University published a research paper titled "Internal cap-initiated translation for efficient protein production from circular mRNA" in the journal Nature Biotechnology. The study enhances the translation efficiency of circular mRNA by introducing an internal cap structure, thereby promoting its application in RNA therapy.

In this latest study, the research team proposed two molecular designs to enhance the translation efficiency of circular mRNA:

1. Covalent introduction of m7G cap structure: By covalently linking the N7-methylguanosine (m7G) cap structure (cap-circ mRNA) on the branch chain of circular mRNA, the translation efficiency of circular mRNA was significantly improved. Compared with circular mRNA containing IRES, cap-circ mRNA can recruit the translation machinery more efficiently and show lower immunostimulatory effects in mice.

2. Non-covalent introduction of m7G cap structure: By hybridizing the m7G cap structure with circular mRNA, the cap structure is non-covalently introduced, which increases the translation efficiency by more than 50 times. This design allows circular mRNA to hybridize with capped mRNA or long non-coding RNA and perform rolling circle translation.

Figure 1. Promotion of translation initiation on circular mRNA. (Fukuchi K, et al., 2025)

The experimental results showed that the translation efficiency of circular mRNA with m7G cap structure in cells was significantly improved, especially when the cap structure was located close to the start codon. Compared with linear mRNA, circular mRNA showed higher stability and the translation product lasted longer in vivo. In addition, by introducing N1-methyl pseudouridine (m1Ψ) modification, the immunostimulatory effect of cap-circ mRNA was further reduced while maintaining a high translation efficiency.

These designs not only improve the translation efficiency of circular mRNA, but also enable circular mRNA to initiate translation in specific cell types by designing specific hybrid sequences, thereby achieving cell type-selective protein production. In addition, this technology can also be used to detect the presence of long non-coding RNA, providing a new tool for disease diagnosis.

In general, the internal cap-initiated translation mechanism proposed in this study provides an efficient and flexible strategy for the translation of circular mRNA, overcoming the problem of low translation efficiency mediated by traditional IRES. By covalently or non-covalently introducing the m7G cap structure, the translation efficiency of circular mRNA is significantly improved, and it is compatible with mRNA chemical modification, reducing the immunostimulatory effect. These advances have opened up new avenues for the application of circular mRNA in disease treatment, vaccine development, and disease diagnosis.