A candidate cytokine gene selectively expressed in human CD34+ cells

Residing in bone marrow (BM) and neonatal cord blood (CB), a rare population of human cells bearing the CD34 cell surface marker (CD34+) functions as hematopoietic stem/progenitor cells. In contrast, cells lacking CD34 expression (CD34−) are largely mature hematopoietic cells of various lineages. To elucidate molecular mechanisms governing functional differences between CD34+ and CD34− cells, we set up a molecular screen to identify genes that are preferentially expressed in CD34+ cells. After 3 rounds of PCR-based subtraction (CB CD34+ vs. CD34−) using the RDA method, genes expressed preferentially in CD34+ cells were highly enriched (10% and 20% of cloned fragments were from the CD34 and c-kit gene, respectively). Among the 73 initially sequenced RDA fragments, 27 (37%) were novel, or homologous only to entries in EST databases. One of these Novel/EST clones (C17) was found 4 times (5.5%). By RT-PCR and Northern blot analyses, C17 gene expression was detected in CD34+ cells from CB, BM and G-CSF mobilized peripheral blood, but not in bulk CD34− cells, peripheral blood leukocytes (PBL) or marrow stromal cells. The C17 cDNA is approximately 1.0 kb, and encodes a polypeptide of 136 amino acids. It has a putative signal peptide at its N-terminus. Transient expression in transfected human 293 cells demonstrated that the C17 peptide is indeed a secreted molecule. To date we have not found any existing molecule that shows significant homology to the amino acid sequence of the C17 gene. A secondary structure analysis predicts that the C17 peptide contains 4 alpha-helices, a characteristic of hematopoietic cytokines and interleukins. We are now investigating whether the C17 protein functions as a cytokine.