High Mobility Group A1 Chromatin Remodeling Protein Regulates Self-Renewal, Niche Formation, and Regenerative Function in Adult Stem Cells through Wnt/β-Catenin Signaling

Introduction: Nuclear chromatin structure is a key determinant of stem cell function and cell fate, although factors that regulate this are only beginning to emerge. While High Mobility Group A1(HMGA1) chromatin remodeling proteins are among the most abundant, nonhistone chromatin binding proteins in adult stem cells (ASCs), their role in this setting has been unknown. HMGA1/2 proteins modulate gene expression by binding to DNA, bending chromatin, and recruiting transcription factor complexes to enhancers throughout the genome. The HMGA1 gene is highly expressed during embryogenesis with low or undetectable levels in mature, differentiated tissues. In cancer, HMGA1 re-expression occurs through oncogenic transcription factors, other epigenetic alterations, or in rare cases, chromosomal translocation events. Importantly, HMGA1 levels correlate with adverse clinical outcomes in diverse malignancies. We previously reported that Hmga1 transgenic mice develop leukemic transformation by inducing transcriptional networks involved in stem cell function and cell cycle progression.

Methods: To elucidate the role of Hmga1 in normal development and ASCs in vivo, we generated mouse models with transgenic overexpression or deletion of Hmga1. To define the function of Hmga1 in adult stem cells (ASCs), we used gain-of-function (overexpression) and loss-of-function (silencing or genetic deletion) approaches in human and murine intestinal stem cells (ISCs) and hematopoietic stem and progenitor cells.

Results:Transgenic mice overexpressing Hmga1 in ISCs develop hyperproliferation, aberrant crypt formation, and polyposis in the intestinal epithelium by expanding the ISC and niche compartments. Hmga1 enhances self-renewal in ISCs by amplifying Wnt/β-catenin signaling, inducing genes that encode both Wnt agonist receptors and downstream Wnt effectors. Surprisingly, Hmga1 also "builds" an epithelial niche by directly up-regulating Sox9 to induce Paneth cell differentiation. Paneth cells constitute the epithelial ISC niche by secreting Wnt agonists. This is the first example of Hmga1 fostering terminal differentiation to establish a stem cell niche. In human intestine, HMGA1 and SOX9 are highly correlated, and both become up-regulated in colorectal cancer.

Human CD34+ cells engineered to overexpress Hmga1 expand more efficiently, while those with Hmga1 deficiency have defective proliferation and colony forming capability. Both colony number and size were decreased, and differentiation was skewed towards myeloid lineages. In mice, Hmga1 deletion causes partial embryonic lethality; over 50% of expected offspring die before mid-gestation. Those that survive develop premature aging phenotypes with early kyphosis, decreased bone density, grip strength, gait velocity, and hearing deficits. Knock-out mice also have early thymic aplasia, decreased numbers of early T-cell precursors, as well as decreased B-cell differentiation. Long-term (LT)-hematopoietic stem cells were decreased and preliminary data suggests aberrant regenerative function in serial, competitive transplant experiments.Preliminary ChIP-seq and gene expression studies in CD34+ cells suggest that Hmga1 regulates transcriptional networks involved in Wnt, JAK-STAT, and PI3K signaling.

Conclusions:Our results in ASCs reveal a novel role for Hmga1 in tissue homeostasis by inducing pathways involved in Wnt and regenerative function. In ISCs, Hmga1 maintains both the stem cell pool and niche compartment whereas deregulated Hmga1 may perturb this equilibrium during carcinogenesis. Functional studies in HSCs suggest that Hmga1 also regulates self-renewal, regenerative potential, and the capacity for balanced differentiation. These findings indicate that HMGA1 is required for normal stem cell function, both during embryogenesis, and postnatally, in ASCs. Our prior work in tumor models demonstrates that a subset of HMGA1 stem cell pathways are hi-jacked by cancer cells to drive tumor progression. Together, these studies provide compelling rationale for further research to determine how to harness HMGA1 for regenerative medicine and to target it in cancer therapy.