The phenomenon of adeno-associated virus (AAV) capsid-promoter interaction recently seen in the rat central nervous system has now been shown to occur in the non-human primate brain. This interaction can directly determine cell-specific transgene expression, as described in the article, “Adeno-Associated Virus Capsid-Promoter Interactions in the Brain Translate from Rat to the Nonhuman Primate,” published in Human Gene Therapy.

An AAV contains a single-stranded DNA genome encapsulated in a capsid comprised of three structural proteins.

“Recently, we established an AAV9 capsid-promoter interaction that directly determined cell-specific gene expression across two synthetic promoters, Cbh and CBA, in the rat striatum. These studies not only expand this capsid-promoter interaction to include another promoter in the rat striatum but also establish AAV capsid-promoter interactions in the nonhuman primate brain,” the investigators wrote.

“When AAV serotype 9 (AAV9) vectors were injected into the rat striatum, the minimal synthetic promoter JetI drove green fluorescent protein (GFP) gene expression predominantly in oligodendrocytes. However, similar to our previous findings, the insertion of six alanines into VP1/VP2 of the AAV9 capsid (AAV9AU) significantly shifted JetI-driven GFP gene expression to neurons.

“In addition, previous retrograde tracing studies in the nonhuman primate brain also revealed the existence of a capsid-promoter interaction. When rAAV2-Retro vectors were infused into the frontal eye field (FEF) of rhesus macaques, local gene expression was prominent using either the hybrid chicken beta actin (CAG) or human synapsin (hSyn) promoters. However, only the CAG promoter, not the hSyn promoter, led to gene expression in the ipsilateral claustrum and contralateral FEF. Conversely, infusion of rAAV2-retro-hSyn vectors, but not rAAV2-retro-CAG, into the macaque superior colliculus led to differential and selective retrograde gene expression in cerebellotectal afferent cells.

“Clearly, this differential promoter/capsid expression profile could not be attributed to promoter inactivation from retrograde transport of the rAAV2-Retro vector. In summary, we document the potential for AAV capsid/promoter interactions to impact cell-specific gene expression across species, experimental manipulations, and engineered capsids, independent of capsid permissivity.”

“We document a unique attribute of AAV vectors in both rodent and primate models that until recently remained undescribed: namely capsid/promoter interactions, that dictated cell type transduction profiles regardless of virus permissivity,” said R. Jude Samulski, PhD, professor at the University of North Carolina School of Medicine.

“Up until now, we have thought of the AAV capsid as a ‘delivery truck,’ and once it dropped off its payload at the right cellular address, the vector promoter would do the rest,” according to Terence R. Flotte, MD, editor-in-chief of Human Gene Therapy, the Celia and Isaac Haidak professor of medical education, and dean, provost, and executive deputy chancellor, University of Massachusetts Medical School.

“This work refutes that concept, showing that the capsid continues to have an effect on gene expression within specific cells of the brain and spinal cord. This has profound implications for vector design in the future.”

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