Unveiling the Potential of YY1 in Blood Cell Development and Therapy

B cells, a type of white blood cell responsible for producing antibodies, develop through several stages: common lymphoid progenitors, pre-pro-B cells, pro-B cells, pre-B cells, immature B cells, and finally, more mature specialized B cells. By the time they reach the pro-B stage, these cells are committed to becoming B cells rather than any other type of cell.

However, research from the School of Veterinary Medicine and Perelman School of Medicine found that deleting the transcription factor YY1 in pro-B cells disrupts this commitment, allowing the cells to develop into other types of blood cells. YY1, a transcription factor with both activation and repression functions, plays a key role in DNA repair, cell division, proliferation, and embryonic development.

In both in vitro studies and a mouse model, they discovered that pro-B cells lacking YY1 could differentiate into T cells, which help B cells produce antibodies. These findings were published in the journal Genes & Development.

The data has come out better than my wildest fantasy. Since it is expressed everywhere and it is involved with so many lineages, the potential for regenerative medicine is quite high.”

Michael Atchison, Professor and Study Senior Author, Biomedical Sciences, School of Veterinary Medicine, University of Pennsylvania

He imagines being able to drive cells in a different direction, temporarily knock down YY1, and then remove the block to create a new lineage. For instance, a patient suffering from a muscular condition might have their muscle cells reimplanted.

The author said, “Although B cell lineage plasticity has been observed following knockout of several lineage-specific transcription factors, YY1 is unique in being a ubiquitous transcription factor expressed in all cell types suggesting a potentially universal mechanism of lineage commitment.”

Atchison claims that one of his graduate students cloned YY1 shortly after he arrived at Penn Vet as an Assistant Professor in 1989. He says that at the time, it was startling that a transcription factor could suppress the expression of some genes while activating others. More than 30 years ago, other labs published this at the same time, and another researcher called the transcription factor Yin Yang 1 because of its dual role.

Atchison notes that the foundation for this paper was laid almost 10 years ago by a former postdoctoral researcher who, while analyzing RNA sequencing data, proposed that YY1 knockout pro-B cells could potentially differentiate into T cells. Sarmistha Banerjee, the paper's first author, later continued this research as a senior research investigator at Penn.

The researchers removed YY1 using Mb1-driven CRE. Mb1 is a promoter that stimulates the expression of CRE in pro-B cells, and CRE is a protein that can destroy DNA.

We saw this transition happening from B to T and thought it would be a gradual transition, but there were a few weird-looking genes that were expressed in the mature T cells that developed.”

Michael Atchison, Professor and Study Senior Author, Biomedical Sciences, School of Veterinary Medicine, University of Pennsylvania

Co-author Joshua Rhoades, a bioinformatician, suggested doing single-cell RNA sequencing, which Atchison says “turned out to be really transformative. Then we saw all these other cell types coming up.”

As YY1 deletion pro-B cells grew in culture, the sequencing data revealed that 85% of them were classified as monocytic or dendritic cells, which help B and T cells deliver antigens to each other so they may build an immune response, while only 3% were classified as T cells. However, the majority of these alternative lineage genes were downregulated after three weeks of incubation, while T lineage genes were more highly expressed.

A very useful, insightful, and unexpected finding was that while we were pushing these B cells to T cells, in the appropriate T cell environment, we observed that during this transition (midway) they were making a lot of other cell types because we showed that Notch, a receptor in a highly conserved cell signaling pathway, was involved in the process, this could mean that it is the gradient of Notch signaling that is required to make these other cell types during the early phase while also inhibiting their differentiation during later stages, pushing them to T cells.”

Sarah Naiyer, Research Associate and Study Co-Author, Biomedical Sciences, School of Veterinary Medicine, University of Pennsylvania

Atchison expressed satisfaction in witnessing these results being successfully replicated in vivo.

“It is coming out pretty close to what we fantasized about how the genome has changed when you knock out YY1 and why that allows the cells to be less committed to one lineage and able to be pushed into another direction,” Atchison said.

Looking forward, the mountain of data the team amassed “can be mined for a long time to look at what is happening when you knock out YY1,” Atchison said.

Atchison adds, “We have looked at a small handful of genes, but there are 20,000 genes in the genome, and there is a lot we can do just with the data we have.”

Two more study directions are examining how well YY1 deletion functions in different cell types and tissues and whether T cells can still migrate to a different lineage.