The population of elderly people is increasing both in the United States and worldwide, and this increase presents challenges in addressing the medical risk factors associated with aging, including an increased risk of developing acute myeloid leukemia (AML). New research out of the laboratory of Maria "Ken" Figueroa, Associate Professor and Co-Leader of the Cancer Epigenetics Research Program at University of Miami Sylvester Comprehensive Cancer Center, indicates that taking a closer look at the hematopoietic stem cells (HSCs) in aged individuals may provide a clue as to why those middle aged or older are at greater risk.
Background on Previous Research
A common feature of aging is a change in the behavior of HSCs, which are self-renewing cells that have the ability to differentiate into all types of mature blood cells. Aging HSCs have an increase in myeloid differentiation potential and a decrease in lymphoid differentiation potential. Increase in myeloid potential correlates with an increase in myeloid cancers, such as AML. Decreased immune function, which may partly result from decreased B and T cell numbers due to these myeloid-biased HSCs, is also observed in aged individuals and may also contribute to the increased cancer risk.
HSC behavior likely changes for a number of reasons, which may include both environmental and cell-intrinsic factors. Numerous studies in mice have shown that aging changes the HSCs in a manner that is relatively stable over generations of HSC progeny. These observations led to the identification of epigenetic changes correlating with age-related HSC dysfunction. Epigenetic changes are reversible modifications to DNA and/or histones (proteins with structural and regulatory functions in DNA). These changes alter the activity of the modified region and may increase or decrease gene expression. Therefore, epigenetic modifications likely have a major effect on HSC function in the mouse. However, it is unclear to what extent human HSCs undergo age-related epigenetic changes and what effect this has on the HSCs.
New Developments from the Figueroa Lab
Dr. Figueroa is leading efforts to better understand the role of epigenetic changes, including age-related, in AML and normal blood cell development. A new study from Dr. Figueroa’s lab, led by postdoctoral fellow Emmalee Adelman, was published online recently in Cancer Discovery.
Dr. Figueroa’s team compared samples of HSCs from young donors (generally <35 years old) to samples from aged donors (generally >65 years old). They focused on specific histone modifications that are associated with either enhanced or repressed transcription: H3K4me1, H3K27me3, H3K4me3, and H3K27ac. All of these histone marks show changes in aged HSCs that are absent in young HSCs.
How might these histone changes affect the blood?
Epigenetic changes can occur in enhancer regions, which are DNA regulatory regions that function to modify the transcriptional behavior of genes. Dr. Figueroa’s team demonstrated deregulation of enhancers that are associated with genes that determine blood cell function and immune signaling. This suggests that the histone epigenetic changes are not random changes but rather specific changes with functional consequences to the HSCs. These results are consistent with dysfunction of blood cell maturation and immunity seen with aging.
Are there any observed changes with possible connections to cancer development?
In addition to observing enhancer deregulation of immune-related genes, Dr. Figueroa’s group found that enhancers that regulate genes involved in AML are also altered with age. These enhancers, called bivalent promoters, are gene regulatory regions that have both a repressive mark (H3K27me3), which normally serves to keep the gene from being transcribed, and an expressive mark (H3K4me3), which normally serves to allow the gene to be transcribed. These marks act in opposition to each other to tightly regulate genes that are important in various developmental systems, including HSCs. When both marks are present, the gene is repressed yet poised for very rapid active transcription given the appropriate signals. This state is tightly regulated and prevents genes from being turned on until the appropriate developmental cue is given. Many genes regulated by bivalent promoters are thought to be important as either positive or negative regulators in AML and other blood cancers.
Dr. Figueroa’s team identified a number of bivalent promoters that change with aging. The most common of these changes is a loss of expressive H3K4me3 with little to no change in repressive H3K27me3, which changes the bivalent state to a repressed state. Other researchers have shown that this change is comparable to that seen in bivalent genes during the development of cancer, suggesting that these age-related changes in HSCs shown by Dr. Figueroa’s team may be one of the keys to understanding why some blood cancers are seen more often in elderly people.
Where do the age-related changes to HSCs originate?
One unresolved question of HSC aging is whether the observed epigenetic changes are the result of an expansion of a preexisting clone already containing an epigenetic profile with characteristics observed in aged HSCs or of an epigenetic reprogramming of the HSCs. By analyzing the gene expression profiling of individual cells obtained from young and aged HSC populations and comparing their profiles to reference HSC profiles, Dr. Figueroa’s team concluded that the more likely scenario is that aging results in an epigenetic reprogramming of HSCs.
How does the epigenetic profile of HSCs in older people relate to this population’s increased risk of developing AML?
The epigenetic reprogramming of aged HSCs may explain their distinct characteristics, including a predisposition to AML. To more directly address this, Dr. Figueroa and colleagues looked at the histone marks H3K4me1, H3K4em3, H3K27me3, and H3K27ac in AML from a publicly available database. They found that a subset of enhancers and bivalent promoters are altered in AML in a similar manner to that which was observed in aged HSCs, suggesting that the epigenetic changes with aging may be part of the reason that the elderly are more likely to develop AML.
To correlate these aging epigenetic changes with gene transcription, Dr. Figueroa compared gene expression changes in young and aged HSCs. One of the most strongly downregulated genes is KLF6, which showed marked loss of histone epigenetic marks associated with active transcription. KLF6 is known to be involved in myeloid differentiation and inflammation, the latter of which is increased during aging and may be associated with increased cancer risk. Dr. Figueroa showed in laboratory models that decreasing KLF6 expression increases the potential of HSCs to form progenitor cells but decreases the potential of HSCs to form mature myeloid cells. Furthermore, experimentally decreased expression of KLF6 in HSCs results in a gene expression profile that strongly overlaps with the gene expression profiles seen in publicly available AML datasets.
Significance of these New Developments
Dr. Figueroa’s research is shedding light on aging in HSCs. Her work confirms that epigenetic reprogramming, which was previously demonstrated in aging HSCs in mice, is also a key factor affecting aging HSCs in humans. She also detailed the specific epigenetic changes that occur and correlated these with the transcription of key hematopoietic genes. Importantly, this correlation extends to epigenetic changes and gene transcription in aged HSCs, and these changes are also seen in AML.
Researchers continue to learn more about aging in HSCs and may discover a way to intervene to prevent or reverse certain epigenetic changes. This development would establish the possibility to prevent or reverse age-related pathology, such as reduced immune function, pre-cancerous conditions, and blood cancer. The development of drugs to target epigenetic modifiers is a robust field of investigation and has resulted in the FDA approval of some such drugs for certain cancers.
Dr. Figueroa’s data showing that reduction of KLF6 mirrors the changes in myeloid cell biology that correlate with AML suggest that KLF6 re-expression may have the opposite effect, potentially reversing some of the age-related problems seen in HSCs. Other genes may also be involved, which opens up the possibility that indirectly modulating their expression through epigenetic targeting drugs, or directly modulating their expression, may lead to a new era of the rejuvenation of aging HSCs.
Dr. Figueroa’s research was supported by The Leukemia & Lymphoma Society through a Quest for Cures grant, and she is a current Scholar of The Leukemia & Lymphoma Society.
Adelman ER, Huang HT, Roisman A, Olsson A, Colaprico A, Qin T, Lindsley RC, Bejar R, Salomonis N, Grimes HL, Figueroa ME. Aging Human Hematopoietic Stem Cells Manifest Profound Epigenetic Reprogramming of Enhancers That May Predispose to Leukemia. Cancer Discov. 2019 May 13. [Epub ahead of print]
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Elias HK, Bryder D, Park CY. Molecular mechanisms underlying lineage bias in aging hematopoiesis. Semin Hematol. 2017; 54(1):4-11.