Monitoring cell physiology by expression profiles and discovering cell type-specific genes by compiled expression profiles.

A gene expression profile is the list showing the expressed gene species and the abundance of their transcripts in a given cell or tissue. This list is made by constructing 3'-directed cDNA libraries consisting of only the 3'-termini of mRNA and sequencing randomly selected clones from such libraries: genes are identified by the sequences, and the composition of mRNA, which reflects gene activities, is measured from the frequency of appearance of the gene transcripts. For practical reasons, the number of sequenced clones has been limited to approximately 1000 per library at present, but the resulting profile covers almost all highly or moderately expressed genes, along with many less active genes. We constructed expression profiles from the HL60 human promyelocytic cell line and two of its derivatives, granulocytoids induced by DMSO and monocytoids induced by TPA. In HL60, a significant fraction of the abundantly expressed genes was for protein synthesis. Upon induction, these genes were partially or totally silenced; transcripts for proteins that characterize the granulocytes and monocyte-macrophages became abundant. By compiling and comparing different expression profiles, genes can be categorized into those expressed in diverse cell types and those active only in limited cell types. Although at present, the number of expression profiles that can be compiled is limited and this categorization is applicable only to abundantly expressed genes, 13 novel genes that may represent granulocyte- or monocyte-specific functions have been discovered.