CMV early enhancer/chicken beta-actin/SV40 intron (CBA) promoter
Synthetic hybrid promoter combining the CMV immediate-early enhancer, the chicken β-actin promoter, and an SV40 intron; drives strong, ubiquitous constitutive expression in mammalian cells. Compact ~800 bp variant used in AAV and other gene-therapy vectors, distinct from the larger CAG promoter, which substitutes a rabbit β-globin intron for the SV40 intron.
Origin: CMV enhancer + chicken β-actin promoter + SV40 intron (synthetic)
Characteristics
CMV IE enhancer fused upstream of the chicken β-actin promoter, with an SV40 intron inserted downstream of the transcription start site (~800 bp total). In AAV vectors delivered by portal vein injection in mice, this CMV/β-actin cassette produces 9.5-fold higher expression than an EF1α promoter and 137-fold higher expression than the CMV promoter/enhancer alone (Xu et al. 2001). In rat hippocampal and spinal cord slice cultures transduced by AAV9, it drives mostly ubiquitous, high-level neural expression but yields far fewer GFP-positive spinal motor neurons (~2-4 per 40-µm section) than the CMV promoter or the hybrid-intron CBh variant (~30-40 per section) (Gray et al. 2011).
Applications: Drives therapeutic and reporter transgenes in AAV vectors for liver-directed gene therapy, including hepatic expression of human factor X in mice (Xu et al. 2001), and in AAV9 vectors for CNS/PNS gene delivery, where it produces widespread neuronal and glial expression but weak motor neuron transduction (Gray et al. 2011).
Limitations: Drives poor expression in spinal and brainstem motor neurons relative to the CMV promoter or the CBh hybrid-intron variant, a documented effect of the SV40 intron in this cassette (Gray et al. 2011). At ~800 bp, still adds meaningful size to AAV constructs with a ~4.7 kb packaging limit.