Highlights d Human single-nucleus ATAC-seq dataset reveals neocortical enhancers d The human neocortical open chromatin landscape is compared to mouse d Enhancer-AAV vectors can drive expression in neocortical subclasses d PVALB-specific AAVs function in vivo in mice and primates

Viral genetic tools to target specific brain cell types in humans and non-genetic model organisms will transform basic neuroscience and targeted gene therapy. Here we used comparative epigenetics to identify thousands of human neuronal subclass-specific putative enhancers to regulate viral tools, and 34% of these were conserved in mouse. We established an AAV platform to evaluate cellular specificity of functional enhancers by multiplexed fluorescent in situ hybridization (FISH) and single cell RNA sequencing. Initial testing in mouse neocortex yields a functional enhancer discovery success rate of over 30%. We identify enhancers with specificity for excitatory and inhibitory classes and subclasses including PVALB, LAMP5, and VIP/LAMP5 cells, some of which maintain specificity in vivo or ex vivo in monkey and human neocortex. Finally, functional enhancers can be proximal or distal to cellular marker genes, conserved or divergent across species, and could yield brain-wide specificity greater than the most selective marker genes.

Myriad cell types comprise the human neocortex, but their roles in normal brain function and disease are largely unknown because few tools exist. To find enhancer elements useful for cell type-specific genetic tools, we examined chromatin accessibility in >2,800 highquality single human neocortical nuclei. Accessible elements frequently are conserved in mouse distributing tissue.