p63 and p73 regulate convergent and factor-specific transcriptional programs in cutaneous squamous cell carcinoma

This study reveals that p63 and p73 form heteromeric complexes to co-occupy distal enhancers in cutaneous squamous cell carcinoma, where they cooperatively drive shared proliferative programs while exerting factor-specific roles—p63 reinforcing epithelial lineage and p73 supporting DNA replication and stress responses—ultimately sustaining tumorigenesis through a feed-forward EGFR signaling module dominated by Amphiregulin.

The Gist

The Big Picture: The "Boss Duo" of Skin Cancer

Imagine your skin is a bustling city. The cells in the bottom layer (the basal layer) are the construction workers who keep building new skin to replace the old stuff. To keep this city running, you need a strict set of rules and a foreman to give orders.

In healthy skin, there are two foremen, p63 and p73. They are like a management team. Usually, p63 is the loud, dominant boss who tells the workers to stay in their lane and keep building skin. p73 is the quieter partner who helps out but isn't as famous.

The Problem: In a type of skin cancer called Squamous Cell Carcinoma (SCC), things go wrong. Both p63 and p73 get "amped up" (overactive). Instead of just managing the city, they start acting like a criminal gang, forcing the cells to multiply uncontrollably and ignore safety warnings.

This paper asks a simple question: Do these two bosses work together as a team, or do they have different jobs?

The Discovery: A Perfect Partnership

The researchers found that p63 and p73 aren't just working in the same office; they are literally holding hands.

• The "Handshake" Analogy: Think of p63 and p73 as two people who fit together perfectly to form a super-stable "heterotetramer" (a fancy word for a four-person team). They bind together physically inside the cell nucleus.
• The "Control Room": Once they are holding hands, they go to the cell's "control room" (the DNA). They sit on the same switches (called enhancers) to turn genes on or off.
• The Result: Because they are sitting on the same switches, they create a shared "instruction manual" for the cancer cells. This manual tells the cells: "Keep dividing! Don't stop! Ignore the pain!"

The Twist: They Have Different Specialties

Even though they hold hands and sit on the same switches, they aren't identical twins. They have different strengths.

• p63 (The Identity Keeper): p63 is obsessed with making sure the cells remember they are "skin cells." It reinforces the "skin" identity and keeps the construction crew focused on building skin structures.
• p73 (The Repair Specialist): p73 is more focused on the machinery of the cell. It helps with DNA copying (replication) and fixing mistakes. It's like the mechanic who keeps the construction trucks running smoothly so they can work faster.

The Takeaway: If you remove p63, the cells forget they are skin cells. If you remove p73, the cells can't copy their DNA fast enough to keep up with the cancer's growth. But if you remove both, the cancer falls apart completely.

The Secret Weapon: The "Gas Pedal" (EGFR Ligands)

How do these two bosses actually make the cancer grow so fast? They found the answer in a specific signaling pathway called EGFR.

Think of EGFR as the gas pedal of the car.

• p63 and p73 work together to press the gas pedal down.
• They do this by ordering the production of specific chemicals called ligands (like Amphiregulin or AREG).
• These ligands are like gasoline. They are secreted by the cancer cells, float around, and hit the gas pedal (EGFR) even harder, telling the cell to speed up.

The "Amphiregulin" Hero: Among all the gasoline types, Amphiregulin (AREG) is the most powerful. The researchers found that if you cut off the supply of AREG, the cancer cells stop growing, almost as if you had removed p63 and p73 entirely.

Why This Matters for Patients

This discovery is a game-changer for treating skin cancer for a few reasons:

1. It's a Team Effort: You can't just target p63 or just p73. Because they work so closely together, you have to think about the whole team.
2. The "Gasoline" Strategy: Since p63 and p73 are pumping out so much "gasoline" (AREG), maybe we don't need to attack the cancer cells directly. Instead, we could block the gas pedal or stop the production of the gasoline.
3. Better Predictions: Doctors might be able to look at a patient's tumor and see how much "gasoline" (AREG) is being produced. If there is a lot, that patient might respond very well to drugs that block EGFR (like Cetuximab), because the cancer is totally dependent on that signal.

Summary in One Sentence

This paper reveals that two proteins, p63 and p73, form a powerful, inseparable team in skin cancer that acts like a master switch, turning on a "gas pedal" (via a chemical called Amphiregulin) that drives the tumor to grow, suggesting that blocking this specific chemical signal could be a new way to stop the cancer.

Technical Summary

1. Problem Statement

Cutaneous Squamous Cell Carcinoma (SCC) is a complex malignancy driven by UV-induced mutations and characterized by the dysregulation of master transcription factors, particularly the p53 family members (p53, p63, p73).

• The Gap: While p63 (specifically the $\Delta$Np63 isoform) is well-established as a central driver of squamous lineage identity and oncogenesis in SCC, the functional role of its paralog p73 in SCC remains poorly understood.
• The Question: Do p63 and p73 function redundantly, or do they regulate distinct transcriptional programs? How do they interact mechanistically to sustain tumor growth, and what are their specific downstream targets?

2. Methodology

The study employed an integrative multi-omics approach combined with functional validation in vitro and in vivo:

• Clinical and Model Systems: Analysis of human tissue microarrays (Normal skin, Actinic Keratosis, SCC), mouse chemically-induced skin carcinogenesis models (DMBA/TPA), and patient-derived SCC cell lines.
• Genetic Perturbation: Depletion of p63, p73, and specific isoforms using siRNA and shRNA in SCC cells and primary human keratinocytes (HK).
• Transcriptomics: RNA-seq to profile global gene expression changes upon factor depletion.
• Epigenomics: ChIP-seq to map genome-wide binding sites of p63 and p73, identifying co-occupancy and motif enrichment. Integration with H3K27Ac data to define active enhancers.
• Protein Interaction: Co-immunoprecipitation (Co-IP) and Proximity Ligation Assays (PLA) to verify physical heteromeric complex formation.
• Functional Assays:
  • In vitro: BrdU incorporation (DNA synthesis), flow cytometry (cell cycle), clonogenic assays, and tumorsphere formation (anchorage-independent growth).
  • Signaling: ELISA for ligand secretion, Western blot for phosphorylated EGFR/ERK/AKT, and rescue experiments with recombinant ligands.
  • In vivo: Subepidermal injection of SCC cells into NOD/SCID mice to assess tumorigenicity.

3. Key Contributions & Findings

A. Expression and Isoform Landscape

• Upregulation: Both p63 and p73 are significantly upregulated in premalignant (AK) and malignant (SCC) skin lesions compared to normal skin.
• Isoform Specificity:
  • p63: $\Delta$Np63 is the dominant isoform in both normal and malignant keratinocytes.
  • p73: Contrary to previous assumptions in other cancers, SCCs do not express canonical TAp73 or $\Delta$Np73. Instead, p73 expression is sustained by a 5'-truncated transcript initiating at exon 4, which lacks the N-terminal transactivation domain.

B. Essentiality for Tumorigenesis

• Depletion of either p63 or p73 in SCC cells leads to:
  • Severe suppression of DNA synthesis (G1/S arrest).
  • Reduced clonogenicity and tumorsphere formation.
  • No significant increase in apoptosis, indicating that their loss primarily affects proliferation and survival mechanisms rather than triggering cell death directly.

C. Mechanistic Interplay: Convergent and Divergent Programs

• Physical Interaction: p63 and p73 form stable heterotetrameric complexes in SCC cells.
• Genomic Co-occupancy: ChIP-seq reveals extensive co-binding at distal enhancer elements. Approximately 94% of p73 peaks overlap with p63 peaks.
• Transcriptional Output:
  • Convergent (Shared): Both factors cooperatively drive core proliferation programs (E2F, MYC targets) and repress inflammatory/apoptotic pathways.
  • Divergent (Factor-Specific):
    • p63: Preferentially reinforces epithelial lineage identity, differentiation programs, and NOTCH signaling.
    • p73: Specifically contributes to DNA replication, repair, centrosome organization, and ciliogenesis.

D. The EGFR Ligand Feed-Forward Loop

• Direct Regulation: p63 and p73 co-regulate multiple EGFR ligands, including Amphiregulin (AREG), Epiregulin (EREG), HBEGF, and TGFA.
• AREG Dominance: AREG is the most abundantly expressed ligand and acts as the primary functional mediator.
• Rescue Experiments: Depletion of p63 or p73 reduces EGFR phosphorylation. Crucially, supplementation with recombinant AREG restores ERK/AKT signaling and rescues proliferation defects in p63-depleted cells, confirming that the p63/p73-EGFR axis is a critical dependency.
• In Vivo Validation: Depletion of p63, p73, or AREG individually abolishes tumor formation in mouse xenograft models.

4. Significance and Clinical Implications

• Mechanistic Insight: The study redefines the role of p73 in SCC, moving it from a potential tumor suppressor (as seen in some contexts) to a critical oncogenic co-driver that cooperates with p63 via heterotetramer formation.
• Therapeutic Vulnerability: The identification of the p63/p73 $\rightarrow$ EGFR Ligand (specifically AREG) $\rightarrow$ EGFR axis provides a mechanistic explanation for the efficacy of EGFR inhibitors (e.g., Cetuximab) in SCC.
• Biomarker Potential: The findings suggest that assessing EGFR ligand expression (rather than just EGFR levels) could better predict patient response to anti-EGFR therapies, particularly in PD-1 refractory advanced SCC.
• Broader Relevance: The convergence on EGFR ligand regulation by p63/p73 may extend to other squamous malignancies, such as pancreatic cancer, where p63 drives squamous differentiation and AREG is highly upregulated.

Conclusion

This paper establishes that p63 and p73 function as a cooperative, heteromeric transcriptional complex in cutaneous SCC. While they share a vast genomic landscape to drive core proliferation, they exert factor-specific biases to maintain lineage identity and DNA integrity. Their most critical shared output is the transcriptional upregulation of EGFR ligands, particularly Amphiregulin, creating a self-sustaining autocrine loop essential for tumor maintenance. This highlights the p63/p73-EGFR axis as a promising therapeutic target for advanced skin cancers.