Molecular Restoration Through Replacement Therapies: Current Advances and Future Directions in Treating Diseases

Replacement therapy is an advanced therapeutic approach for diseases caused by molecular deficits. It aims to restore normal physiology by replacing deficient molecules such as enzymes, proteins, genes, or other molecules. Here, we discuss different modalities, including protein replacement, gene therapy, messenger RNA (mRNA) replacement, noncoding RNA (ncRNA) therapies, cell replacement, and gene editing, aimed at addressing and treating the fundamental genetic defects underlying a range of diseases. These therapies could have potentially curative and disease-modifying effects when used to directly replace deficient or dysfunctional components, addressing the inherent limitations of conventional therapies, such as off-target effects and control of disease-related symptoms. Some replacement therapies, such as protein therapy, gene therapy, and cell therapy, are already approved for clinical use, while emerging approaches-including mRNA therapy, ncRNA therapy, and gene editing-remain primarily in the preclinical or clinical trial stages. To achieve broad clinical translation of these emerging approaches, key challenges, including delivery, safety, specificity, and ethical concerns, must be addressed. This review provides an overview of the existing modalities of replacement therapies, their mechanisms of action, and future directions for improving clinical translation, efficacy, and accessibility.