Ultra-deep bisulfite sequencing to detect specific DNA methylation patterns of minor cell types in heterogeneous cell populations: An example of the pituitary tissue

Abstract

DNA methylation is an epigenetic modification important for cell fate determination and cell type-specific gene expression. Transcriptional regulatory regions of the mammalian genome contain a large number of tissue/cell type-dependent differentially methylated regions (T-DMRs) with DNA methylation patterns crucial for transcription of the corresponding genes. In general, tissues consist of multiple cell types in various proportions, making it difficult to detect T-DMRs of minor cell types in tissues. The present study attempts to detect T-DMRs of minor cell types in tissues by ultra-deep bisulfite sequencing of cell typerestricted genes and to assume proportions of minor cell types based on DNA methylation patterns of sequenced reads. For this purpose, we focused on transcriptionally active hypomethylated alleles (Hypo-alleles), which can be recognized by the high ratio of unmethylated CpGs in each sequenced read (allele). The pituitary gland contains multiple cell types including five hormone-expressing cell types and stem/progenitor cells, each of which is a minor cell type in the pituitary tissue. By ultra-deep sequencing of more than 100 reads for detection of Hypo-alleles in pituitary cell type-specific genes, we identified T-DMRs specific to hormone-expressing cells and stem/progenitor cells and used them to estimate the proportions of each cell type based on the Hypo-allele ratio in pituitary tissue. Therefore, introduction of the novel Hypo-allele concept enabled us to detect T-DMRs of minor cell types with estimation of their proportions in the tissue by ultra-deep bisulfite sequencing.