Altered Expression of Hematopoiesis Regulatory Genes in the Bone Marrow Mesenchymal Stem Cells of Patients of Aplastic Anemia

Mesenchymal stem cells (MSC) are an important component of the hematopoietic niche in the bone marrow (BM) and regulate hematopoiesis by producing a variety of cytokines and growth factors. In aplastic anemia (AA), most of the studies have attributed the reduced hematopoiesis to a defect in hematopoietic stem cells (HSC) and limited data is available on the role of BM-MSC in AA. Therefore, the objective of the present study was to evaluate the expression of hematopoiesis regulatory genes, viz. granulocyte colony stimulating factor (G-CSF), stromal cell derived factor (SDF-1), stem cell factor (SCF), tumor necrosis factor-alpha (TNF-α) macrophage inflammatory protein-1 alpha (MIP-1α) and transforming growth factor-beta (TGF-β) in BM-MSC of patients with AA and compare it with BM-MSC of control group.

Twenty patients of idiopathic acquired AA with a median age of 25.5 years (range: 12-64 years) were included in the study. The control group consisted of 10 healthy volunteers and 10 patients with iron deficiency anemia or immune thrombocytopenic purpura. The median age of the control group was 20 years (range: 11-62 years). The BM-MSC were isolated and cultured as per protocol standardized and previously published by us. Third passage cells were used in the study. The MSC were characterized both by their phenotypic markers and by their ability to differentiate into adipogenic and osteogenic lineages. The expression of hematopoiesis regulatory genes was studied by real-time quantitative polymerase chain reaction (qRT-PCR). The GAPDH was used as the housekeeping gene to normalize the transcript levels and the fold change in the gene expression was calculated by 2^-ΔΔCtmethod.

The BM-MSC of AA patients and controls had similar morphology and expression of mesenchymal markers CD73, CD105, CD90 and CD166, absence of expression of hematopoietic markers CD13, CD34 and CD45 and of HLA-DR. However, the BM-MSC of AA patients exhibited a higher adipogenic and a lower osteogenic differentiation in comparison to those of controls. Further, the BM-MSC of AA patients in comparison to those of control group, had a higher expression of G-CSF (fold increase: 1.99; p<0.0001), SDF-1 (fold increase: 1.37; p<0.01) and TNF-α (fold increase: 10.68; p<0.0001) and a very low expression of MIP-1α (fold decease: 50.0; p<0.0001) transcripts. The expression of SCF and TGF-β transcripts were comparable in the BM-MSC of both the groups (p>0.05).

Though AA patients have been shown to have elevated levels of G-CSF in the peripheral blood and BM but there is only one previous report on G-CSF gene expression in BM-MSC of AA, in which a higher expression was observed and thus corroborates with our data. There is no data available on SDF-1 levels in the peripheral blood and bone marrow of AA patients. We have observed higher gene expression of SDF-1 in BM-MSC of AA patients. The higher expression of G-CSF and SDF-1, pro-hematopoietic factors, in AA may be due to a compensatory response of the BM stroma to boost the hematopoiesis. Our observation of higher TNF-α gene expression in BM-MSC corroborates with previous reports on higher levels of this anti-hematopoietic cytokine in the BM plasma of patients with AA and indicates that MSC could contribute to the increase in the TNF-α level in the BM of AA patients. A conspicuous observation of our study was a markedly decreased expression of MIP-1α gene in BM-MSC of AA and to the best of our knowledge this is the first report on MIP-1α in AA. MIP-1α is a chemokine which has been shown to inhibit proliferation of HSC in vitro and thus may help to maintain HSC in an undifferentiated state. Furthermore, MIP-1α has also been reported to mediate interaction of HSC with stromal cells in BM and may have a role in supporting hematopoiesis. Its precise role in AA needs to be studied further. We are currently studying the levels of these cytokines/growth factors in the BM plasma of the same cohort of AA patients and controls and the data will be presented.

Our study thus shows that BM-MSC of AA patients have altered expression of hematopoiesis regulatory genes which may contribute to the pathobiology of the disease.