Hematopoietic Stem Cells “Stemness” Genes Contain Many Genes Mutated or Abnormally Expressed in Hematopoietic Malignancies.

Umbilical cord blood (CB) and mobilized peripheral blood (PB) are relatively new sources of hematopoietic stem cells (HSC) that have been increasingly used in clinical transplantations. The genetic basis of the underlying mechanism of HSC self- renewal and differentiation has not been elucidated yet.

Several groups have recently used microarray technology to study the common characteristics of stem cells from different tissues (“stemness”) and the typical features of each source of stem cell (“specificity”). Most groups focused their study on mice, used relatively small cDNA microarrays, and used CD34 as the cell surface marker for stem cells isolation. We studied human stem cells characterized by expression of the more primitive CD133 antigen, and used the Affymetrix Human Hu133A oligonucleotide arrays containing 22,215 probe sets to study the expression profile of these cells. An unsupervised hierarchical clustering of 14,025 “valid” probe sets showed a clear distinction between the CD133 + cells representing the stem cell population, and CD133 − cells that represent various stages of cell differentiation. CD133+ cells isolated from CB were compared to CD133− cells identifying 304 genes that were up regulated by at least two folds and the comparison between PB CD133+ cells and CD133− cells identified 218 genes which were up regulated by at least two-folds. These genes were considered as source specific and maybe relevant to the unique properties of CB and PB derived HSC.

We were concentrated in the 244 genes that were found to be up regulated by at least two folds in the CD133 positive cells as compared to the CD133 negative cells and were common to both CB and PB. Comparison of these “stemness” genes, to the lists of “stemness” genes that were identified by 2 recent studies that analyzed mainly murine HSC identified 39 (Ramalho et al.) and 83 (Ivanova et al.) common genes.

Twenty-four genes were common to another study that analyzed human HSC (Georgantas et al.).

Among these common “stemness” genes we identified 4 groups of genes that have an important role in hematopoiesis: Growth factor receptors (Receptor thyrosine kinases (RTK) and c-mpl), a group of transcription factors which includes several homeobox genes and TGF-β targeted genes, genes that have an important role in the process of development and genes involved in cell growth.

Among these 4 groups we identified 16 “stemness” genes (MPL, FLT3, HOXA9, MEIS 1, MLLT3, KIT, TIE, GATA-2, HOXA5, BAALC, HLF, MYCN, EVI1, MYB, FHL1, and HMGA2) that are known to be mutated or abnormally regulated in acute leukemias. It can be suggested that perturbation of expression of key hematopoietic stemness machinery genes may lead to abnormal proliferation and leukemia.]