Engineering the poly(A) tail for therapeutic mRNA: from expression control to manufacturing robustness

Messenger RNA (mRNA) therapeutics have advanced rapidly, yet sequence design must now satisfy not only biological potency but also scalable manufacturing, product consistency, and immune compatibility. The 3'poly(A) tail is increasingly recognized as a pivotal factor linking these competing demands. While long homopolymeric tails can enhance translation and prolong RNA persistence, their repetitive nature can also compromise DNA template stability, increase in vitro transcriptional slippage and 3'-end heterogeneity, and promote the formation of immunostimulatory byproducts. We define this design trade-off as the Production-Expression paradox, in which sequence features that favor expression may simultaneously undermine manufacturability and immune safety. In this review, we summarize the molecular functions of the poly(A) tail in regulating translation and mRNA decay, and discuss emerging engineering strategies that move beyond conventional linear homopolymers, including segmented designs, chemical modifications, and experimental topological engineering through structured 3'modules, along with their potential impacts on protein yield (often termed a "translational tax"). We further connect upstream production-associated defects to downstream innate immune activation and outline a proposed Quality-by-Design framework for poly(A) optimization, linking critical quality attributes with fit-for-purpose analytical methods ranging from routine release assays to nucleotide-resolution profiling. Reframing poly(A) architecture as an engineerable design parameter, rather than a fixed default element, may improve product comparability, reduce immunogenic burden, and support the more robust translation of mRNA medicines into clinical applications.