An epigenetic mechanism of azole tolerance facilitates acquired antifungal resistance in Aspergillus fumigatus

This study identifies the epigenetic regulator IngB in *Aspergillus fumigatus* as a key factor controlling azole tolerance, demonstrating that its loss facilitates the rapid emergence of acquired antifungal resistance through an epistatic mechanism.

The Gist

Imagine a fortress under siege. The attackers are powerful drugs called azoles, designed to stop a dangerous mold fungus (Aspergillus fumigatus) from building its essential walls (a substance called ergosterol). Usually, if the walls are weak, the fortress falls. But sometimes, the fungus doesn't just fall; it learns to survive the attack, and eventually, it becomes completely immune.

This paper tells the story of how a specific "security guard" inside the fungus, when removed, accidentally teaches the fungus how to survive the siege and then how to build an impenetrable shield.

Here is the breakdown of the discovery using simple analogies:

1. The Security Guard: IngB

Inside the fungus, there is a protein called IngB. Think of IngB as a foreman or a traffic controller in a factory. Its job is to make sure the factory produces the right amount of building materials (specifically, the materials needed to build the fungal cell wall) and that the workers don't get confused.

2. The "Tolerance" Phase: The Stubborn Survivor

The researchers decided to fire the foreman (they created a fungus without the ingB gene).

• The Result: The factory didn't stop working, but it got messy. The workers started panicking and shifting their priorities. Instead of building the standard walls, they started hoarding resources for a different emergency (iron starvation).
• The Surprise: When the drug (the siege engine) was brought in, the normal fungus (with the foreman) died quickly. But the "fired-foreman" fungus didn't die. It grew very slowly, but it survived at drug levels that should have killed it.
• The Analogy: Imagine a soldier who is told to stop fighting and hide. The normal soldier gets captured immediately. The "fired-foreman" soldier is so confused and desperate that they hide in a bunker the enemy didn't expect. They aren't immune yet (they are still weak), but they are tolerant. They can endure the attack long enough to wait for a chance to fight back.

3. The "Resistance" Phase: The Mutant Super-Strain

This is the most critical part of the story. The researchers kept the "tolerant" fungus alive and kept hitting it with high doses of the drug.

• The Normal Fungus: If you hit the normal fungus with a high dose, it just dies. It has no way to adapt.
• The Tolerant Fungus: Because the "fired-foreman" fungus was already struggling and surviving in the bunker, it had time to make a desperate, lucky mistake. It developed a new mutation in a gene called UmpA.
• The Analogy: Think of the "tolerant" fungus as a person stuck in a burning building. They are coughing and weak (tolerance), but they are still alive. Because they are alive, they have time to find a fire escape and learn how to use it. Suddenly, they aren't just surviving the fire; they are immune to it. The new mutation (UmpA) acted like a master key that unlocked a door to total resistance.

4. The "UmpA" Mystery: The Broken Garbage Truck

The new mutation happened in a gene called UmpA.

• What it does: UmpA is like a garbage truck that cleans up broken proteins inside the cell.
• What went wrong: The mutation broke the garbage truck.
• Why this helps the fungus: In a normal cell, the garbage truck quickly removes damaged parts. But when the truck is broken, the cell gets messy. The researchers suspect this messiness actually confuses the drug or makes the fungus change its DNA faster, allowing it to accidentally stumble upon a way to become immune to the drug. It's like a broken garbage truck causing a traffic jam that accidentally blocks the enemy's path.

5. Why This Matters

• The Trap: Doctors often see a patient who isn't getting better, even though lab tests say the drug should work. This is because the fungus is in the "tolerant" phase (hiding in the bunker).
• The Danger: If doctors keep using the same drug on a "tolerant" fungus, they are accidentally giving it time to evolve into a "resistant" super-strain.
• The Lesson: The paper suggests that we need to find ways to kill the "tolerant" fungus before it has time to evolve. If we can stop the fungus from hiding in the bunker, it never gets the chance to find the fire escape.

Summary

The researchers found that removing a specific protein (IngB) makes the fungus tolerant (able to survive the drug without dying immediately). This tolerance is dangerous because it acts as a stepping stone. It gives the fungus the time and the chaotic environment it needs to accidentally evolve into a resistant super-strain (via the UmpA mutation).

In short: Tolerance isn't just a pause; it's a training ground for resistance. If we want to stop drug-resistant super-fungi, we need to stop them from learning how to survive the first hit.

Technical Summary

1. Problem Statement

• Clinical Context: Aspergillus fumigatus is a leading cause of invasive fungal infections. While azoles (e.g., voriconazole) are the frontline treatment, therapeutic failures occur even with isolates that appear susceptible in standard laboratory tests (MIC-based).
• The Knowledge Gap: There is a significant lack of understanding regarding antifungal tolerance (the ability of a population to survive/grow at drug concentrations above the MIC without a change in the MIC itself) in pathogenic molds. Unlike bacteria, where tolerance is known to precede resistance, the evolutionary link between tolerance and acquired resistance in A. fumigatus remains elusive.
• Objective: To identify the molecular regulators of azole tolerance in A. fumigatus and determine if this tolerance phenotype acts as a stepping stone to acquired drug resistance.

2. Methodology

The study employed a multidisciplinary approach combining genetics, transcriptomics, phenotypic assays, and in vivo modeling:

• Strain Generation: Used CRISPR-Cas9 to generate a deletion mutant ($\Delta$ingB) and a reconstituted strain ($\Delta$ingBRec) in the reference background AF293.
• Phenotypic Assays:
  • MIC Determination: Standardized CLSI microdilution assays.
  • Tolerance Assays: Solid agar plate growth assays at sub-MIC and supra-MIC azole concentrations.
  • Biofilm Susceptibility: Quantified biomass reduction in biofilms treated with azoles.
  • Metabolic Profiling: Measured siderophore (TAFC) production and tested susceptibility to other ergosterol pathway inhibitors (statins, terbinafine).
• Transcriptomics: RNA-seq to compare gene expression between $\Delta$ingB and Wild Type (WT).
• Selection Experiments: Exposed WT, $\Delta$ingB, and reconstituted strains to high azole concentrations to observe the emergence of resistance.
• Genomics: Whole-genome sequencing (WGS) of evolved resistant colonies to identify mutations.
• In Vivo Model: Murine model of invasive pulmonary aspergillosis (immunocompromised mice) treated with voriconazole to assess fungal burden and survival.

3. Key Contributions

1. Identification of IngB: Discovered ingB (encoding an Inhibitor of Growth domain protein) as a novel epigenetic regulator of azole tolerance.
2. Tolerance-to-Resistance Link: Demonstrated that azole tolerance is not merely a survival state but a permissive background that accelerates the acquisition of stable, heritable resistance mutations.
3. Novel Resistance Gene (umpA): Identified a frameshift mutation in umpA (a 20S proteasome maturation protein) as a driver of high-level azole resistance, a mechanism previously uncharacterized in A. fumigatus.
4. Metabolic Mechanism: Elucidated that loss of ingB rewires metabolism, shunting precursors from ergosterol biosynthesis toward siderophore production (iron starvation response), thereby reducing the drug target (ergosterol) availability.

4. Detailed Results

A. Loss of ingB Confers Azole Tolerance

• MIC vs. Tolerance: The $\Delta$ingB strain maintained wild-type MICs for voriconazole, itraconazole, and posaconazole. However, on solid agar, $\Delta$ingB exhibited significant growth at concentrations above the MIC (supra-MIC), whereas WT growth was completely inhibited.
• Biofilms and In Vivo: $\Delta$ingB biofilms were significantly less susceptible to azoles than WT biofilms. In a murine model, mice infected with $\Delta$ingB and treated with voriconazole showed a significantly higher fungal burden (only a 2.5-fold reduction) compared to WT-infected mice (16-fold reduction), confirming tolerance in vivo.

B. Mechanism: Metabolic Rewiring and Epigenetic Regulation

• Transcriptomics: Loss of ingB (a component of the Nua3 histone acetyltransferase complex) resulted in the differential expression of ~12% of the genome.
  • Downregulated: Genes in the ergosterol biosynthesis pathway (e.g., erg13A) and mevalonate pathway.
  • Upregulated: Genes involved in the iron starvation response (siderophore biosynthesis: sidI, sidF) and stress response (HSPs, glutathione S-transferases).
• Metabolic Trade-off: The data supports a hypothesis where ingB loss triggers a constitutive iron starvation signal. This forces the fungus to divert acetyl-CoA and mevalonate precursors away from ergosterol synthesis toward siderophore production. Reduced ergosterol flux likely lowers the efficacy of azoles (which target Cyp51 in the ergosterol pathway).
• Validation: $\Delta$ingB showed tolerance to statins (HMG-CoA reductase inhibitors) and increased TAFC siderophore production.

C. Tolerance Drives Acquired Resistance

• Selection Experiment: When exposed to high voriconazole concentrations:
  • WT: Failed to grow; no resistant colonies emerged.
  • $\Delta$ingB: Rapidly acquired resistance, forming colonies with distinct morphologies (compact, reddish, increased mycelium).
• Mutation Identification: WGS of resistant $\Delta$ingB colonies revealed a frameshift mutation (Ser55fs) in Afu5g10740, identified as umpA (a conserved 20S proteasome maturation factor).
• Validation of umpA:
  • Creating a $\Delta$umpA mutant in the WT background conferred azole resistance.
  • Restoring the WT umpA allele in the resistant colony restored susceptibility.
  • The umpA mutation alone conferred resistance, but the combination of ingB loss and umpA mutation (in the evolved strain) resulted in a specific epistatic interaction that allowed growth at very high drug concentrations (4x MIC).

5. Significance and Implications

• Paradigm Shift: This study challenges the view that tolerance and resistance are separate phenomena in fungi. It provides evidence that tolerance is a prerequisite for the rapid evolution of resistance in A. fumigatus under azole pressure.
• Clinical Relevance: The identification of ingB variants in longitudinal isolates from patients with Chronic Granulomatous Disease (CGD) suggests that tolerance-promoting mutations may arise during chronic infections, eventually leading to treatment failure via resistance.
• Therapeutic Strategy: Targeting the mechanisms of tolerance (e.g., the metabolic rewiring or the epigenetic regulator IngB) could prevent the emergence of resistance, offering a new strategy to preserve the efficacy of existing azole antifungals.
• Novel Biology: The discovery of umpA as a resistance factor links proteasome function and genomic stability (via the hypermutator phenotype hypothesis) to antifungal resistance, opening new avenues for research into fungal stress adaptation.

Conclusion: The paper establishes that the loss of the epigenetic regulator ingB creates a tolerant state in A. fumigatus via metabolic rewiring. This tolerant state acts as an evolutionary bridge, facilitating the rapid acquisition of resistance mutations (specifically in umpA) that would not occur in a susceptible wild-type background.