Summary Table — Liver / Hepatocyte-Specific Promoters for AAV

Summary Table: Liver / Hepatocyte-Specific Promoters

Promoter	Target Cell	Size (bp)	Origin	Key Characteristics
TBG	Hepatocytes	~450–600 bp	Human	Thyroxine-Binding Globulin. The “gold standard” for AAV liver targeting. Compact, extremely specific, and potent.
Albumin (Alb)	Hepatocytes	~300 bp – 2 kb	Human/Mouse	Highly specific, but the native version is relatively weak. Often requires enhancers (like Ealb) to be effective.
A1AT (hAAT)	Hepatocytes	~400 bp	Human	Alpha-1 Antitrypsin. A compact, naturally occurring promoter with robust activity in the liver.
ApoE-hAAT	Hepatocytes	~800–1000 bp	Human	A hybrid promoter using the ApoE enhancer with the hAAT promoter. Very high expression; used in clinical trials.
HCR	Hepatocytes	~300–500 bp	Human	Hepatic Control Region. Usually used as an enhancer combined with other promoters (like hAAT) to boost strength.

Insights into Liver-Directed Gene Therapy

1. The Potency of Hybrids (HLP and LP1) Because native liver promoters can be modest in strength, researchers often engineer synthetic hybrids. The HLP (Hybrid Liver-specific Promoter) combines the HCR enhancer, the hAAT promoter, and an ApoE fragment. At roughly 800 bp, it provides enough strength to produce therapeutic levels of proteins from a single AAV dose.

2. TBG: Small but Mighty TBG is favored for its “cleanliness.” While many promoters have low-level leakiness in the heart or kidneys, TBG remains strictly silent outside the liver. Its small size is also ideal for large transgenes like Factor VIII, which is nearly 4.4 kb and barely fits in AAV.

3. Enhancer Stacking The HCR (Hepatic Control Region) is often “stacked” upstream of the Albumin or hAAT promoter. This configuration acts like a volume knob, increasing the rate of transcription without losing the liver-specific restriction.