Enhancers are critical cis-regulatory elements that regulate gene expression in a context-dependent manner by integrating transcription factor binding, chromatin state, and the three-dimensional organization of the genome. Recent advances in functional genomics and synthetic biology have increased interest in harnessing enhancer activity to regulate the expression of therapeutic genes. Unlike traditional approaches that rely on promoter-driven gene regulation, enhancer-based approaches can bias transgene expression toward specific cellular states or disease contexts; however, this control remains probabilistic and highly dependent on the chromatin environment. This review summarizes current knowledge of enhancer biology, discusses new strategies for utilizing enhancer function directly, and examines the potential benefits and drawbacks of using enhancer-based strategies for gene therapy applications. Often delivered using adeno-associated virus (AAV) vectors with tailored capsids, enhancers in gene therapy can be included into expression cassettes. Astrocyte- or microglia-specific enhancers in the brain enable enriched or preferential distribution of neuroprotective or immunomodulatory genes, hence lowering unintentional expression in non-target cell types. It is important to control gene expression for specific cell types for the treatment of neurodegenerative conditions such as Alzheimer’s or Parkinson’s disease, were unintentional gene expression results in negative consequences. However, the uses of enhancer-guided gene therapy go beyond the central nervous system. In cancer, therapeutic constructs are designed to inhibit oncogenic expression or induce tumor suppression pathways, using enhancers in either malignant or immune cells as a target. Similarly, through the use of tissue-specific enhancers in cardiovascular and regenerative medicine, lineage-enriched genes can be used to promote repair of damaged tissues and enhance functional recovery. Enhancer-based systems that modulate the levels of gene expression (enhancer systems that adjust gene expression to levels that are physiologically appropriate for a given cell type) may also be useful in diseases caused by imbalances of gene dosage (e.g., haploinsufficiency and copy number variations). However, despite the potential promise of enhancer-driven gene therapy, many technical and translational hurdles remain. Mapping and validating the function of cell-type-specific enhancers is hampered by the dynamic, context-dependent regulation of chromatin. The identification of enhancers across a variety of developmental stages and clinical states is being accelerated through the combination of recent advances in single-cell epigenomic techniques (e.g., ATAC-seq, ChIP-seq, and multi-omic integration). Recent advances in non-viral delivery methods and AAV capsid engineering are improving the safety, efficacy, and scalability of enhancer-driven gene therapies. However, there must be careful regulatory oversight to avoid unintentional activation of enhancers and ensure continuing efficacy of enhancer-guided therapies. This paper provides an overview of the conceptual basis of enhancer-driven gene therapies, the currently available applications, and barriers to their clinical application. We show how the combination of delivery technology, synthetic biology, and genomics is enabling new possibilities for tailored gene therapy particular to cell- and disease-specific. Enhancer-driven gene therapy could become an important component of next-generation precision medicine by addressing current challenges and using creative technology.
Hany E. Marei (Sat,) studied this question.