ABACUS: A flexible UMI counter that leverages intronic reads for single-nucleus RNAseq analysis

Single-nucleus RNA sequencing (sNuc-RNAseq) is an emerging powerful genomics technology that combines droplet microfluidics with next-generation sequencing to interrogate transcriptome changes at single nucleus resolution. Here we developed Abacus, a flexible UMI counter software for sNuc-RNAseq analysis. Abacus draws extra information from sequencing reads mapped to introns of pre-mRNAs (~60% of total data) that are ignored by many single-cell RNAseq analysis pipelines. When applied to our pilot human brain sNuc-RNAseq data, ABACUS nearly doubled the number of nuclei identified by the CellRanger workflow, recovering a large number of nuclei from non-neuronal cells. By incorporating intronic reads into gene expression quantification, we showed that they encoded additional and valid transcription features of individual cells and could be used to improve cluster resolution of different cell types. By separately counting UMIs derived from forward and reverse intronic reads and from exonic reads, Abacus gives users flexibility in representing genes expressed at different abundance levels. In summary, Abacus represents a flexible, improved workflow for sNuc-RNAseq data processing and analysis.