Single-nucleus chromatin accessibility and gene expression co-profiling by ISSAAC-seq

Multimodal profiling of different molecular layers from the same single cell enables more comprehensive characterization of cellular heterogeneity compared with conventional single-modality approaches. A key example is co-detection of chromatin accessibility and gene expression that offers the opportunity to investigate cell type-resolved gene regulatory mechanisms. Here we describe a sensitive and robust protocol for in situ sequencing hetero RNA–DNA-hybrid after assay for transposase-accessible chromatin using sequencing (ISSAAC-seq) for the concurrent measurement of chromatin accessibility and gene expression from the same single nucleus. The method begins with dual Tn5 tagging of open chromatin regions and the RNA–cDNA hybrid produced by reverse transcription that take place in bulk nuclei. Then, various single-nucleus isolation strategies, including plate and droplet barcoding-based approaches, can be used based on the experimental purpose of the user. The protocol is highly modular with a flexible throughput ranging from several hundreds to tens of thousands of nuclei. The generated data are of high quality in both modalities. The entire workflow can be finished within 1 or 2 days, and the procedures work on multiple different single-nucleus isolation and barcoding platforms. This protocol describes a flexible workflow for joint profiling of chromatin accessibility and transcriptomes in single nuclei, compatible with either plate or droplet single-cell isolation platforms.