Construction of a gene expression profile of a human fetal liver by single-pass cDNA sequencing

We have obtained an overall gene expression profile of a human fetal liver by sequencing the 5' ends of random cDNA clones from an unbiased cDNA library. As a result, many novel genes that might be related to liver growth and hemopoiesis have been identified. Poly (A)+ RNA was purified from the liver of a human fetus obtained at the 22nd week of gestation, and a directional library was constructed with oligo d(T)-primed cDNAs synthesized without any normalizing procedures. The 5' end of each randomly chosen clone was sequenced by the dideoxy-chain termination methods, and each sequence was used for homology search in the public databases such as GenBank, SWISS-PROT, and PIR. Of 1231 random cDNA clones analyzed, 697 clones representing 204 different transcripts (57%), were identical to previously known human genes. The spectrum of the genes in this category reflected well the physiological characteristics of the fetal liver, a combination of hepatic and hemopoietic functions. About 4% of the clones represented novel gene transcripts with significant homologies to known genes of human or other organisms. These included several genes that are known to be involved in cellular differentiation and/or proliferation. About 25% of the clones had no statistically significant match to any known genes. In summary, we have identified 546 different gene transcripts consisting of 204 known human genes, 42 homologous genes, and 300 unknown genes. Thus, this approach appears to be a highly efficient way to identify novel genes of biological interest.