A Unified Statistical Framework for Single Cell and Bulk RNA Sequencing Data

TL;DR

This paper introduces a hierarchical statistical model that integrates single cell and bulk RNA sequencing data, effectively addressing dropout noise and improving gene expression estimation across cell types.

Contribution

The paper presents URSM, a unified framework that models both data types and dropout events, enhancing accuracy in gene expression analysis and cell type deconvolution.

Findings

URSM outperforms existing methods in dropout correction.

URSM accurately estimates cell type proportions in bulk data.

Application to fetal brain data reveals meaningful cell-specific expression patterns.

Abstract

Recent advances in technology have enabled the measurement of RNA levels for individual cells. Compared to traditional tissue-level bulk RNA-seq data, single cell sequencing yields valuable insights about gene expression profiles for different cell types, which is potentially critical for understanding many complex human diseases. However, developing quantitative tools for such data remains challenging because of high levels of technical noise, especially the "dropout" events. A "dropout" happens when the RNA for a gene fails to be amplified prior to sequencing, producing a "false" zero in the observed data. In this paper, we propose a Unified RNA-Sequencing Model (URSM) for both single cell and bulk RNA-seq data, formulated as a hierarchical model. URSM borrows the strength from both data sources and carefully models the dropouts in single cell data, leading to a more accurate estimation of cell type specific gene expression profile. In addition, URSM naturally provides inference on the dropout entries in single cell data that need to be imputed for downstream analyses, as well as the mixing proportions of different cell types in bulk samples. We adopt an empirical Bayes approach, where parameters are estimated using the EM algorithm and approximate inference is obtained by Gibbs sampling. Simulation results illustrate that URSM outperforms existing approaches both in correcting for dropouts in single cell data, as well as in deconvolving bulk samples. We also demonstrate an application to gene expression data on fetal brains, where our model successfully imputes the dropout genes and reveals cell type specific expression patterns.