Although recent studies have identified genes important in hematopoietic stem cells (HSCs), human embryonic stem cells (hESCs), and induced pluripotent stem cells (iPSCs), the molecular underpinnings of normal stem cell function are unclear. A better understanding of key stem cell pathways will be essential for the safe use of stem cells in regenerative medicine and should also uncover novel therapeutic targets in aggressive hematologic malignancies and other stem-like cancer cells. To elucidate the molecular underpinnings of “stemness”, we investigated transcriptional networks in pluripotent stem cells. Our focus is the high mobility group A1 (HMGA1) gene, which encodes the HMGA1a and HMGA1b chromatin remodeling proteins. These proteins bind to AT-rich regions of DNA and orchestrate the assembly of transcription factor complexes to alter chromatin structure and modulate gene expression. HMGA1 is highly expressed during embryogenesis with low or undetectable levels in adult, differentiated tissues. HMGA1 is also enriched in HSCs, hESCs, iPSCs, refractory leukemia, and poorly differentiated solid tumors. Our group discovered that HMGA1 functions as a potent oncogene in cultured cells and causes aggressive leukemia in transgenic mice. We also found that high levels of HMGA1 expression correlate with relapse in acute lymphoblastic leukemia. Together, these findings suggest that HMGA1 drives a stem cell phenotype during normal development, hematopoiesis, and malignant transformation.
To further investigate the role of HMGA1 in a stem cell state, we compared its expression in iPSCs, hESCs, HSCs, and cancer cells. HMGA1 is highly expressed in fully reprogrammed iPSCs and hESCs, with intermediate levels in HSCs and cancer cells, and low levels in fibroblasts. When hESCs are induced to differentiate, HMGA1 decreases and parallels that of other pluripotency factors. Conversely, forced expression of HMGA1 blocks differentiation in hESCs. We also discovered that HMGA1 enhances cellular reprogramming of somatic cells (mesenchymal stem cells, HSCs, and fetal lung fibroblasts) to an iPSC together with the Yamanaka factors (OCT4, SOX2, KLF4, cMYC or OSKM). HMGA1 results in an increase in the number and size of iPSC colonies compared to OSKM controls. Surprisingly, there was normal differentiation in vitro and benign, teratoma formation in vivo of the HMGA1-derived iPSCs. During the reprogramming process, HMGA1 induces the expression of pluripotency genes, including SOX2, LIN28, and cMYC, while knockdown of HMGA1 in hESCs results in the repression of these genes. Chromatin immunoprecipitation shows that HMGA1 binds to the promoters of these pluripotency genes in vivo.
In summary, our findings uncover a key role for HMGA1 as a regulator of the stem cell state through transcriptional networks that induce pluripotency and an undifferentiated state. Further studies are needed to determine if HMGA1 pathways could be targeted in hematologic and other malignancies or exploited in regenerative medicine.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.