Connective tissue growth in a mouse model of Kosaki overgrowth syndrome is limited by STAT1

This study demonstrates that a mouse model of Kosaki overgrowth syndrome driven by a constitutively active PDGFRb mutation recapitulates human disease phenotypes and reveals that STAT1 signaling acts as a critical limiting factor that opposes PDGFRb-driven overgrowth and fibrosis.

The Gist

The Big Picture: A Broken "Growth Switch"

Imagine your body has a master control panel for growth and repair. One of the most important buttons on this panel is called PDGFRb. Think of this button as a "Growth and Repair Switch" for your connective tissues (like skin, bone, and tendons).

Normally, this switch only turns on when it receives a specific signal (like a message saying, "Hey, we need to build some bone here"). Once the job is done, the switch turns itself off.

Kosaki Overgrowth Syndrome (KOGS) is a rare human disease caused by a tiny typo in the genetic code for this switch. Specifically, a mutation called P584R (or P583R in mice) jams the switch in the "ON" position. Because the switch never turns off, the body gets confused and starts building too much bone, growing too tall, and developing weird skin issues.

The Experiment: Building a Mouse Model

The scientists wanted to understand why this happens and if they could fix it. Since they couldn't experiment on human patients, they built a "mouse model."

They took a mouse and genetically edited it so that its "Growth Switch" (PDGFRb) had the same jammed "ON" mutation found in human KOGS patients.

• The Result: These mice looked normal at birth. But as they grew, they started showing the exact same problems as human patients: they got very heavy, their bones grew too long, their skulls fused prematurely (craniosynostosis), and their skin became dangerously thin and fragile, losing all its fat. They also developed a strange, recurring problem where their organs would prolapse (fall out), likely because their connective tissue was too weak to hold everything in place.

The Mystery: Why isn't the body stopping the growth?

If the "Growth Switch" is stuck on, why doesn't the body just shut it down? The scientists suspected there might be a "Safety Brake" in the system that tries to counteract the stuck switch.

They looked inside the cells of these mice and found a surprise. The jammed switch wasn't just turning on the "Build" signals; it was also accidentally turning on a protein called STAT1.

Think of STAT1 as a Security Guard or a Brake Pedal.

• In many cancers, STAT1 acts as a tumor suppressor (it stops cells from growing out of control).
• In this specific scenario, the scientists found that STAT1 was trying to do its job: it was activating "Interferon" signals (a type of alarm system) to tell the cells, "Hey, stop growing so fast!"

The Twist: Removing the Brake Makes it Worse

To test if STAT1 was actually the "Safety Brake," the scientists created a new group of mice. These mice had the jammed "Growth Switch" (KOGS mutation) PLUS the "Safety Brake" (STAT1) was completely removed.

What happened?
The scientists expected that removing the brake might help, or at least not make things worse. Instead, the opposite happened.

• The mice grew even bigger: Their bones became massive, their spines curved (scoliosis), and their skulls were even more deformed.
• The skin changed: Instead of being thin and fragile, the skin became thick, hard, and scarred (like a keloid), but it still lacked fat.

The Conclusion:
The "Safety Brake" (STAT1) was actually holding the monster in check. Even though the KOGS mutation was jamming the growth switch, STAT1 was fighting back, trying to limit the damage. When the scientists removed STAT1, the "Growth Switch" had no one to stop it, and the overgrowth went into overdrive.

The "JAK" Confusion: A New Discovery

Usually, when cells talk to each other to activate STAT proteins, they use a middleman messenger called JAK. It's like sending a letter through a post office.

The scientists discovered something unique about this specific mutation: The KOGS mutation doesn't need the post office.

• The jammed PDGFRb switch activates STAT1 directly, bypassing the usual JAK middleman.
• Why does this matter? Many drugs (like Ruxolitinib) work by blocking the "post office" (JAK) to stop the alarm. But because this mutation bypasses the post office, those drugs might not work for KOGS patients. The scientists found that blocking JAK didn't stop the STAT1 activation in these mice, but blocking the PDGFRb switch itself did.

Summary: What Does This Mean for Humans?

1. The Model Works: These mice perfectly mimic the human disease, giving doctors a real lab to study the problem.
2. STAT1 is a Double-Edged Sword: In this disease, STAT1 is actually trying to help by limiting the overgrowth. It's not the villain; it's the hero trying to hold back the flood.
3. Treatment Implications: Since STAT1 is trying to stop the overgrowth, we shouldn't try to block it. Instead, we need to find drugs that specifically turn off the jammed PDGFRb switch.
4. New Symptoms: The study also highlighted that these patients might suffer from weak connective tissue leading to prolapses (organs falling out), which wasn't fully appreciated before.

In a nutshell: The researchers found a mouse with a "stuck growth button." They discovered that the body's natural "brake" (STAT1) was trying to stop the car from speeding, but the brake was being overridden. When they removed the brake, the car crashed. This tells us that to treat the disease, we need to fix the stuck button, not break the brake.

Technical Summary

1. Problem Statement

Kosaki Overgrowth Syndrome (KOGS) is an ultra-rare genetic disorder caused by activating mutations in the PDGFRB gene (encoding Platelet-Derived Growth Factor Receptor beta). The most common mutation is P584R (human) in the juxtamembrane domain, which leads to constitutive receptor activation.

• Clinical Phenotype: Patients exhibit increased linear growth, craniosynostosis (premature fusion of skull sutures), scoliosis, and thin, hyperelastic skin with hypertrophic scarring.
• Knowledge Gap: Due to the rarity of patients, the molecular mechanisms driving these specific phenotypes and the role of potential modifier genes remain poorly understood. Previous mouse models (e.g., Pdgfrb^D849V) caused lethal autoinflammation rather than the connective tissue overgrowth seen in KOGS, making it difficult to model the human disease accurately.
• Hypothesis: The authors sought to determine if a mouse model expressing the human-equivalent P583R mutation would recapitulate KOGS phenotypes and to investigate the role of STAT1, a signaling molecule known to be activated by PDGFRB, in modulating these phenotypes.

2. Methodology

The study employed a combination of genetic engineering, phenotypic characterization, molecular signaling analysis, and proteomics.

• Mouse Model Generation:
  • Created a conditional knock-in mouse line (Pdgfrb^LSL-P583R) carrying the mouse equivalent of the human P584R mutation.
  • Crossed with Sox2-Cre to drive ubiquitous expression of the mutant receptor from early embryogenesis.
  • Generated double-mutant mice (Stat1^-/- Pdgfrb^+/P583R) to test the role of STAT1.
• Phenotypic Characterization:
  • Macroscopic: Monitored body weight, skeletal length, and incidence of prolapses (penile/rectal).
  • Skeletal Analysis: Used whole-mount skeletal staining (Alcian Blue/Alizarin Red), micro-CT (µCT) for 3D reconstruction, and histological analysis to assess bone volume, trabecular/cortical bone metrics, and craniosynostosis.
  • Dermal Analysis: Histology (Masson's trichrome, Picrosirius red) and immunostaining (Perilipin 1) to assess skin thickness, collagen deposition, and adipocyte loss (lipodystrophy).
• Molecular Signaling Analysis:
  • Western Blotting: Analyzed phosphorylation levels of PDGFRB, PLCγ, Akt, Shp2, ERK, and STATs (1, 2, 3, 5) in serum-starved newborn dermal fibroblasts (NDFs).
  • Inhibitor Studies: Used Ruxolitinib (JAK1/2 inhibitor) and Imatinib (PDGFRB kinase inhibitor) to determine the dependency of STAT activation on JAK kinases.
• Omics Approaches:
  • Phosphoproteomics: Performed LC-MS/MS on NDFs using TMT labeling to identify differentially expressed proteins (DEPs) and phosphopeptides (DPPs).
  • Bioinformatics: Used Ingenuity Pathway Analysis (IPA) and Gene Set Enrichment Analysis (GSEA) to map signaling pathways.
• Gene Expression: qPCR to quantify Interferon Stimulated Genes (ISGs).

3. Key Results

A. Recapitulation of KOGS Phenotypes

The Pdgfrb^+/P583R mice successfully modeled human KOGS:

• Overgrowth: Mice exhibited progressive weight gain and increased skeletal length (tibias, forelimbs) starting at 3 weeks.
• Craniosynostosis: Premature fusion of cranial sutures and skull overgrowth were observed by 15 weeks.
• Skin Pathology: Progressive dermal atrophy and lipodystrophy (loss of dermal fat) occurred, mimicking the thin, fragile skin of KOGS patients.
• Novel Findings: The model developed ectopic bone in tendons (heterotopic ossification) and a high incidence of penile/rectal prolapses (100% penetrance by 30 weeks), likely due to connective tissue weakness.

B. Signaling Mechanisms

• Constitutive Activation: The P583R mutation caused ligand-independent phosphorylation of PDGFRB and downstream mediators (PLCγ, Shp2, Akt1).
• STAT Activation: There was significant hyperphosphorylation of STAT1, STAT2, STAT3, and STAT5.
• JAK-Independence: Crucially, STAT phosphorylation in P583R cells was not blocked by Ruxolitinib (JAK inhibitor) but was blocked by Imatinib. This indicates that mutant PDGFRB phosphorylates STATs directly or via a JAK-independent mechanism, potentially bypassing normal negative feedback loops (e.g., SOCS proteins).
• Proteomics: Phosphoproteomics confirmed upregulation of interferon signaling pathways and identified STAT1 as a central hub.

C. The Dual Role of STAT1

The study revealed a paradoxical, dual role for STAT1 in the context of PDGFRB-driven overgrowth:

1. In P583R mice (STAT1 present): STAT1 activation drives an interferon response (ISG upregulation) which correlates with skin thinning and lipodystrophy.
2. In P583R/Stat1-/- mice (STAT1 absent):
   • Exacerbated Overgrowth: Deletion of STAT1 led to more severe skeletal overgrowth (increased bone volume, severe craniosynostosis, scoliosis) and keloid-like dermal fibrosis (thickened skin with dense collagen).
   • Mechanism: Removing STAT1 relieved an inhibitory brake on pro-growth pathways. Western blots showed that Stat1 deletion in the P583R background significantly increased ERK1/2 phosphorylation and slightly increased Akt1 and STAT5 phosphorylation.
   • Normalization of ISGs: The interferon gene signature was normalized, confirming STAT1 as the driver of the inflammatory/ISG phenotype.

4. Key Contributions

1. First KOGS Mouse Model: Established a conditional knock-in mouse model (Pdgfrb^P583R) that faithfully recapitulates the connective tissue overgrowth, craniosynostosis, and skin atrophy of human KOGS, overcoming the lethality issues of previous models.
2. Discovery of STAT1 as a Modifier: Identified STAT1 not just as a pro-inflammatory mediator, but as a tumor-suppressor-like inhibitor of connective tissue overgrowth in this context. Its activity limits the severity of skeletal and dermal hyperplasia.
3. JAK-Independent STAT Activation: Demonstrated that mutant PDGFRB activates STATs independently of JAK kinases, suggesting a mechanism of resistance to standard JAK-inhibitor therapies for the overgrowth phenotype.
4. Phenotypic Expansion: Documented novel features of PDGFRB mutations, including heterotopic ossification in tendons and severe connective tissue weakness leading to prolapses.

5. Significance

• Therapeutic Implications: The findings suggest that while JAK inhibitors might reduce inflammation, they may not address (and could potentially worsen) the connective tissue overgrowth in KOGS if STAT1's anti-growth function is compromised. Therapeutic strategies may need to target the PDGFRB kinase directly (e.g., Imatinib) or downstream ERK pathways.
• Disease Mechanism: The study clarifies that KOGS pathogenesis involves a complex balance between pro-growth signaling (ERK/Akt) and anti-growth/anti-inflammatory signaling (STAT1). The "overgrowth" phenotype is the result of PDGFRB hyperactivation overcoming the suppressive effects of STAT1.
• Model Utility: This mouse model provides a robust platform for testing potential therapies for KOGS and understanding the molecular basis of other PDGFRB-related disorders (e.g., Penttinen syndrome, infantile myofibromatosis).

In conclusion, the paper demonstrates that STAT1 activity opposes PDGFRB-driven overgrowth and fibrosis. While STAT1 activation causes the skin thinning and lipodystrophy seen in KOGS, its absence unleashes severe skeletal and dermal overgrowth, highlighting STAT1 as a critical gene modifier in PDGFRB-related diseases.