Functional redundancy between UTY and UTX in regulating the localization of transcription factors involved in pluripotency

This study demonstrates that the Y-linked gene UTY functions redundantly with its homolog UTX in a demethylase-independent manner to maintain human pluripotency by ensuring proper chromatin accessibility and the recruitment of core transcription factors like OCT4 and SOX2.

The Gist

The Big Picture: A Tale of Two Brothers

Imagine the human body is a massive, bustling city. Inside every cell of this city, there is a "Master Control Room" (the nucleus) containing the blueprints for how to build and run everything. These blueprints are stored in a library called DNA.

To keep the city running smoothly, especially when it's just starting out as a tiny, flexible construction site (a stem cell), you need a team of Construction Managers (transcription factors like OCT4 and SOX2). These managers decide which blueprints to open, which buildings to construct, and when to keep the site flexible so it can become anything later.

For a long time, scientists knew about one very famous Construction Manager named UTX. UTX is like a senior foreman who is incredibly good at his job. He has a special tool (an enzyme) that can erase "Do Not Build" signs (repressive marks) on the blueprints, allowing new construction to happen.

But there is a second, lesser-known manager named UTY. UTY is the "Y-chromosome" brother. He looks very similar to UTX, but he has a broken tool. His "eraser" doesn't work very well, and he is much quieter and less visible in the office. Scientists used to think, "Well, since his tool is broken and he's so quiet, he probably doesn't do much."

This paper asks a simple question: If UTX is the boss, does UTY just sit around doing nothing, or is he actually helping out in a different way?

The Discovery: The "Shadow" Manager

The researchers decided to test this by creating human stem cells (the construction sites) with different combinations of these managers:

1. Normal cells: Both UTX and UTY are present.
2. Missing UTX: Only UTY is there.
3. Missing UTY: Only UTX is there.
4. Missing Both: Neither UTX nor UTY is there.

Here is what they found:

• The "Broken Tool" Myth: Even though UTY's tool (the eraser) is broken, he is still incredibly important. When the researchers removed both brothers, the construction site didn't just slow down; it completely collapsed. The stem cells lost their ability to stay flexible and started turning into the wrong things (like nerve cells) too early.
• The Redundancy: It turns out UTX and UTY are like a backup system. If you take away UTX, UTY steps up and does the job well enough to keep the cell alive. If you take away UTY, UTX handles everything. But if you take away both, the system fails.
• The Real Job: The study revealed that UTY and UTX aren't just there to erase "Do Not Build" signs. Their main job is actually holding the door open.

The Analogy: The Doorway and the Bouncers

Think of the DNA as a giant building with many rooms (genes).

• The Transcription Factors (OCT4/SOX2) are the VIP guests who need to get inside the rooms to give orders.
• UTX and UTY are the bouncers and door-holders standing at the entrance of these VIP rooms (called enhancers).

Even though UTY has a broken tool, he is still a strong bouncer. He stands next to UTX and helps hold the door open wide.

• When both are there, the VIP guests (OCT4/SOX2) can easily walk in and do their job.
• When both are gone, the doors slam shut or get jammed. The VIP guests can't get in, or they get lost and wander into the wrong rooms (like the "Neuron" room instead of the "Stem Cell" room).

Crucially, the paper shows that UTY does this without using his broken tool. He acts like a structural pillar. He physically helps recruit other machinery (like chromatin remodelers) that keeps the DNA "loose" and accessible, like a librarian keeping a book open on a table so you can read it.

Why Does This Matter?

1. The Y-Chromosome is Useful: For a long time, people thought the Y chromosome was just a "junk drawer" left over from evolution, mostly used for making sperm. This paper proves that the Y chromosome carries a critical gene (UTY) that is essential for the very beginning of human life (keeping stem cells healthy). Without it, the "backup system" fails.
2. It's Not About the Tool: It teaches us that proteins don't always need to be "enzymes" (chemical workers) to be important. Sometimes, just being there to hold the structure together is enough to save the day.
3. Sex Differences: Since males have one UTX and one UTY, and females have two UTXs (and no UTY), this study helps explain why some genetic diseases affect boys and girls differently. In early development, the male's "backup" (UTY) is enough to keep things running, but later on, the lack of a second full-strength UTX might cause issues.

The Bottom Line

Imagine a construction site where the main foreman (UTX) is busy. He has a quiet assistant (UTY) who doesn't have the right tools but is great at holding the ladder steady and keeping the door open. As long as the assistant is there, the site runs fine. But if you fire the assistant and the foreman, the whole building falls apart.

This paper shows that UTY is that essential assistant. He works in tandem with UTX to keep human stem cells flexible and ready to become any part of the human body, proving that the Y chromosome plays a vital, non-reproductive role in our very first days of life.

Technical Summary

1. Problem Statement

The Y chromosome contains very few protein-coding genes, and their specific roles in early human development remain poorly understood. UTY (Ubiquitously Transcribed Tetratricopeptide Repeat Gene on the Y chromosome) is a Y-linked homolog of the X-linked histone demethylase UTX (KDM6A). While UTX is a well-characterized H3K27 demethylase essential for lineage specification, UTY possesses significantly reduced enzymatic activity and has no detectable demethylase activity in cultured cells.

Despite this, UTY is evolutionarily conserved and broadly expressed. Previous mouse models suggest UTY can compensate for UTX loss during early embryogenesis, but the mechanism remains unclear. The central question is: How do UTY and UTX functionally interact to maintain human pluripotency, and is this role dependent on their histone demethylase activity?

2. Methodology

The study utilized a combination of genome editing, multi-omics sequencing, and functional assays in human embryonic stem cells (hESCs):

• Cell Line Generation:
  • Knock-in (KI): CRISPR-Cas9 was used to insert a 3×Flag-HA epitope tag at the C-terminus of endogenous UTX and UTY loci in male hESCs (SEES3 line) to enable precise protein detection.
  • Knockout (KO): Single (UTX-KO, UTY-KO) and Double (DKO) knockout cell lines were generated using CRISPR-Cas9 to delete specific exons, creating frameshift mutations.
• Expression Analysis:
  • RNA-seq: Quantified transcript levels of UTX and UTY across multiple pluripotent cell lines.
  • Protein Quantification: Immunoblotting and immunostaining compared protein levels of tagged UTX and UTY.
• Genomic Binding Profiling:
  • ChIP-seq: Utilized a dual crosslinking method (DSG followed by formaldehyde) to capture protein-DNA interactions, as UTX/UTY do not bind DNA directly. This was performed on KI cell lines to map genome-wide occupancy.
  • ATAC-seq: Assessed chromatin accessibility.
  • Motif Analysis: Identified transcription factor binding motifs (e.g., OCT4, SOX2) within UTX/UTY bound regions.
• Functional Assays:
  • Differentiation: Induced differentiation into endoderm, mesoderm, and ectoderm lineages.
  • Teratoma Formation: Injected cells into immunodeficient mice to assess in vivo pluripotency and differentiation potential.
• Data Integration: Integrated ChIP-seq, RNA-seq, and ATAC-seq data to correlate protein binding, chromatin state, and gene expression changes.

3. Key Contributions & Results

A. Expression and Localization

• Expression Levels: UTY is expressed in hESCs but at significantly lower levels (2–12 TPM) compared to UTX (7–48 TPM). Protein levels of UTY are ~37% of UTX levels. This disparity is not due to promoter activity (RNA Pol II, H3K27ac, and accessibility are similar) but likely due to post-transcriptional or post-translational instability.
• Co-occupancy: Despite lower abundance, UTY co-localizes with UTX at ~80% of UTY binding sites. Both proteins bind to promoters and distal enhancers. Regions bound by both ("Both-enriched") show higher transcriptional activity than UTX-only regions.

B. Functional Redundancy in Pluripotency

• Transcriptional Impact: Single knockouts (UTX-KO or UTY-KO) showed minimal global transcriptomic changes. However, the Double Knockout (DKO) resulted in widespread gene expression changes, specifically:
  • Downregulation: Key pluripotency genes (e.g., NODAL, LEFTY) and epithelial markers.
  • Upregulation: Neural differentiation genes (e.g., MAP2, WLS).
• Sex-Dependent Compensation: In female hESCs (XX), UTX loss mimics the DKO phenotype seen in males, confirming that UTY in males functionally compensates for the loss of one UTX allele.

C. Mechanism: Demethylase-Independent Regulation

• H3K27me3 Levels: Crucially, the loss of UTX and UTY did not result in detectable changes in global H3K27me3 levels. This indicates their role in maintaining pluripotency is independent of histone demethylase activity.
• Transcription Factor Relocalization:
  • DKO cells exhibited a redistribution of core pluripotency factors OCT4 and SOX2.
  • OCT4/SOX2 binding was lost at enhancers associated with downregulated pluripotency genes.
  • Conversely, OCT4/SOX2 binding was gained at aberrant sites associated with upregulated neural genes.
• Chromatin Remodeling: The loss of UTX/UTY led to reduced recruitment of ATP-dependent chromatin remodelers (e.g., BRG1, CHD7) and the histone methyltransferase MLL4 at enhancers. This resulted in reduced chromatin accessibility (ATAC-seq) and decreased H3K27ac at pluripotency enhancers.

D. Phenotypic Consequences

• In Vitro: DKO cells showed subtle morphological changes, reduced alkaline phosphatase staining, and aberrant expression of the neuronal marker NESTIN, indicating a compromised pluripotent state.
• In Vivo (Teratoma): While single KO cells formed teratomas with tissues from all three germ layers, DKO cells failed to form recognizable teratomas, producing only small, undifferentiated cell masses. This confirms that the combined loss of UTX and UTY abolishes the ability to maintain a stable pluripotent state in vivo.

4. Significance

• Non-Enzymatic Function: The study establishes that UTY and UTX function as scaffolds or linchpins for the recruitment of chromatin remodeling complexes (SWI/SNF, COMPASS) to enhancers. Their primary role in early development is to stabilize the binding of core transcription factors (OCT4/SOX2) and maintain an open chromatin state, rather than acting as enzymatic demethylases.
• Evolutionary Conservation: The findings explain why UTY, despite its weak catalytic activity and low expression, has been evolutionarily conserved on the Y chromosome. It provides a critical, redundant backup for UTX to ensure the robustness of the pluripotency network during early human development.
• Disease Relevance: The study highlights that the combined loss of UTX and UTY (observed in certain cancers) disrupts enhancer function and transcriptional plasticity, potentially driving tumorigenesis or developmental defects.
• Sex Chromosome Biology: It reveals a specific mechanism where Y-linked genes regulate fundamental developmental processes in a manner that parallels X-linked genes, challenging the view that Y-chromosome genes are solely involved in male reproduction.

Conclusion

UTY and UTX function redundantly to safeguard pluripotency in human embryonic stem cells. They achieve this not through histone demethylation, but by facilitating the recruitment of chromatin remodelers to enhancers, thereby ensuring the proper localization of OCT4 and SOX2. The loss of both proteins leads to a collapse of the pluripotent state due to the mislocalization of transcription factors and the failure to maintain active enhancer landscapes.