Thelma Escobar, an assistant professor of biochemistry at the University of Washington, presented at the Hopkins Department of Biology’s Seminar Series on Thursday, Sept. 25, 2025. She discussed the progress her lab has recently made regarding chromatin modifications in hematopoietic stem cells (HSCs) and the adaptive immune system.
HSCs exist in our bone marrow and proliferate to both maintain their HSC identity and differentiate to give rise to our blood and immune cells. This maintenance and transition is highly regulated, and epigenetic factors like histone modifications have an important role in this process.
Inside a cell’s nucleus, there contains DNA, negatively charged, wrapped around positively charged histones that regulate their expression. Looser associations in euchromatin, determined by the presence of certain euchromatic histone modifications, allow for increased gene expression, while tightly-bound heterochromatin regions are sequestered from the transcription machinery due to their distinct histone modifications. An intermediate “poised” chromatin state also exists, containing both repressive and activating histone modifications.
As a cell divides and replicates its negatively charged DNA, the positively charged histones must double to maintain balance.
“[This process involves] a very complex orchestration of destruction [of] parental histones and recycling of histones, as well as de novo deposition, all occurring during DNA replication,” Escobar explained.
Histones need help finding their place on the DNA, and are guided by histone chaperones, the proteins that bring histones to their genomic locations. Escobar’s lab studies these histone chaperone proteins – particularly one she worked on during her postdoctoral studies named Nucleophosmin 1, which is encoded by the NPM1 gene. This protein has been implicated in acute myeloid leukemia (AML), a deadly blood cancer which begins in the bone marrow.
“NPM1 mutations… [are] shown… to be present in 30 to 35% of all AMLs,” Escobar began, and explained further that “there are no targeted therapies for NPM1-mutated AML.” However, given the association between NPM1 mutations and AML, Escobar decided to study the gene to determine its potential role in causing the leukemia. A current hypothesis is that mutated NPM1 leads to increased HOX gene activity, which promotes proliferation of cells and decreases cellular differentiation. Therefore, if NPM1 is mutated in the bone marrow, hematopoietic stem cells could overproliferate and fail to fulfill their role as blood-cell precursors.
To study HSCs in the lab, the Escobar lab differentiated induced pluripotent stem cells (iPSCs) into HSCs, which allowed them to attain large enough quantities of HSCs for experiments.
One series of HSC experiments done in the lab is comparing how NPM1 and the mutated NMP1’s protein interactions differ. A recent discovery from this work is that NPM1 interacts with another protein whose gene is often found mutated in a precursor of AML. Escobar’s group is currently exploring this further.
Another surprising result was the discovery of a NPM1 protein fragment in the nucleus of HSCs both in vitro and in vivo. This had usually not been observed in other cells, but the lab recently determined the 5 amino acid sequence where NPM1 gets cleaved to create this fragment they see in the HSC nuclei.
To find the cut site, they mutated different NPM1 protein sites and observed if one or two fragments were produced. A mutation in the ‘incorrect’ site would produce two fragments as observed in the wild type. But, mutating around these five possible amino acids produced only one segment, indicating it contains the cleavage site. Escobar is planning further work to investigate and characterize the cellular and physiological significance of this protein fragment.
Lastly, Escobar briefly discussed another project looking at chromatin changes in T cells in the adaptive immune system. Much work in the field previously focused on the cells involved in the adaptive immune response – that B and T cells proliferate quickly to increase in number upon reinvasion by a pathogen – but chromatin states and epigenetics also show an important role that the lab wants to better understand.
“Naive [T] cells have more repressive chromatin domains. The goal is to identify whether the chromatin composition is similar, or are there important players for this ‘poised’ chromatin in new cells,” Escobar explained. Poised chromatin, as mentioned before, is an intermediate chromatin state that contains both euchromatic and heterochromatic histone modifications. Escobar reasoned that an immune cell with poised chromatin will activate faster upon re-infection, as it contains activating histone modifications that make particular genes more poised to activate the adaptive immune response.
Altogether, Escobar discussed her work studying a histone chaperone protein called NMP1 in HSCs that, when mutated, causes acute myeloid leukemia. She studied protein interaction differences between NMP1 and mutant NMP1 to find a new protein/NMP1 interaction. The lab found a site where NMP1 is likely cleaved to explain how a fragment was found in HSC nuclei. Lastly, they are exploring chromatin inheritance’s role in the adaptive immune response. Escobar hinted her excitement at her lab’s future projects, asking the audience to keep an eye out for updates.
“Stay tuned, because we're mixing biochemistry and also hematopoietic stem cells differentiation. Hopefully we can have this story published in the near future,” she said.
