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Sex differences in a mouse model of radiation-induced cardiotoxicity

This study demonstrates that male BALB/cJ mice develop severe radiation-induced heart failure with reduced ejection fraction, inflammation, and fibrosis, whereas females exhibit a milder phenotype characterized by preserved ejection fraction and enhanced mitochondrial repair, highlighting significant sex differences in the pathogenesis of radiation-induced cardiotoxicity.

Original authors: Emily R. Whelan, Evan E. Perona, Jessica J. Fliess, S. Christian Kocsis, Gabriel J. Weigel, Elizabeth J. McCabe, Charwan Hamilton, Damian N. Di Florio, Varsini Balamurugan, Danielle J. Beetler, Logan

Published 2026-06-24

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Emily R. Whelan, Evan E. Perona, Jessica J. Fliess, S. Christian Kocsis, Gabriel J. Weigel, Elizabeth J. McCabe, Charwan Hamilton, Damian N. Di Florio, Varsini Balamurugan, Danielle J. Beetler, Logan P. Macomb, Max W. Strandes, Frances C. Wilson, Emma F. Murphy, Ashley A. Darakjian, Molly M. Watkins, Alayna M. Puls, Cameron J. Hartmoyer, Natalie Bonvie-Hill, Sami Khatib, Chris J. McLeod, Jordan C. Ray, Michael D. Story, Bradford S. Hoppe, DeLisa Fairweather

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: A Tale of Two Hearts

Imagine the heart as a busy city. This study looked at what happens to that city when it gets hit by a massive "bomb" of radiation (specifically, a high dose used to treat nearby cancers like lung or breast cancer). The researchers wanted to know: Does the city recover differently depending on whether it is run by a "male" or "female" administration?

To find out, they used two groups of mice (one male, one female) and gave them a single, powerful dose of radiation directly to their hearts. They checked the damage at three different times: 24 hours later (the immediate shock), 10 days later (the acute cleanup), and 35 days later (the long-term aftermath).

The Immediate Aftermath: The Bomb Goes Off

The Damage:
Right after the radiation hit, both male and female hearts suffered a "structural collapse."

DNA Breaks: Think of DNA as the city's master blueprint. The radiation snapped these blueprints. Interestingly, the female hearts actually showed more broken blueprints than the males at the 10-day mark. It was like the female city had a more visible pile of rubble initially.
Mitochondrial Blackout: The mitochondria are the heart's power plants. The radiation caused a blackout in both groups. The power plants stopped working efficiently, and the heart cells started looking like they had empty rooms (vacuoles) inside them.

The Divergence: How They Clean Up

This is where the story splits. Even though both groups started with similar damage, their cleanup crews reacted very differently.

The Female Heart: The Efficient Repair Crew

The Super-Tool: The female hearts activated a master switch called Pgc1α. You can think of Pgc1α as a "Chief Engineer" or a "Super-Tool" that knows exactly how to rebuild power plants.
The Result: Because the females turned on this switch, they were able to repair their mitochondria (power plants) much better than the males.
The Outcome: By day 35, the female hearts were stiff and a bit enlarged (like a city that has built extra walls to hold pressure), but the power was still on. They developed a condition called HFpEF (Heart Failure with Preserved Ejection Fraction). In plain English: The heart is stiff and has trouble filling up, but it still pumps blood out strongly. There was no major fire (inflammation) and no new concrete walls (fibrosis) blocking the streets.

The Male Heart: The Chaotic Cleanup

Missing the Tool: The male hearts did not turn on the Pgc1α "Chief Engineer." They couldn't fix their power plants effectively.
The Fire: Because the power plants were broken, the male hearts started a massive fire. This triggered a flood of immune cells (like an overzealous fire department) that swarmed the heart.
The Outcome: By day 35, the male hearts were a mess. They had:
- Inflammation: A raging fire (high levels of inflammatory markers).
- Fibrosis: The heart was covered in thick scar tissue (like concrete walls replacing the streets), making it hard to move.
- Dilated Cardiomyopathy: The heart became weak, floppy, and stretched out like a deflated balloon.
- HFrEF: Heart Failure with Reduced Ejection Fraction. The heart was too weak to pump blood out effectively.

The Vessels and Walls

The study also looked at the "roads" (blood vessels) and the "city walls" (heart muscle thickness).

Both sexes saw their roads get wider and their walls get thicker after the radiation.
However, by day 35, the males had significantly wider roads and much thicker walls than the females. It was as if the male city had expanded chaotically, while the female city stayed more contained.

The Summary Analogy

Imagine two houses hit by a hurricane (radiation).

The Female House: The roof is torn off, and the windows are shattered (DNA damage). But, the owner immediately calls a specialized repair crew (Pgc1α). They fix the power generator quickly. The house is a bit stiff and the walls are reinforced, but the lights stay on, and the house remains livable.
The Male House: The roof is also torn off. The owner tries to fix it but lacks the right tools. The broken generator causes a short circuit that starts a fire. The fire department arrives but causes more chaos, and the walls get covered in thick, hard cement (fibrosis). The house becomes weak, stretched out, and can no longer function properly.

The Bottom Line

The paper concludes that sex matters deeply in how radiation damages the heart.

Females suffered more initial DNA damage but had a better biological repair mechanism (Pgc1α) that saved them from the worst outcomes. They ended up with a "stiff but pumping" heart.
Males failed to repair their power plants, leading to inflammation, scarring, and a "weak and stretched" heart.

The researchers emphasize that this is a discovery in a mouse model and that more work is needed to understand exactly why the female "Chief Engineer" (Pgc1α) turns on while the male one stays off.

Technical Summary: Sex Differences in a Mouse Model of Radiation-Induced Cardiotoxicity

Problem Statement
Radiation therapy for thoracic malignancies (e.g., breast and lung cancer) carries a risk of developing radiation-induced heart disease (RIHD) months to years post-treatment. RIHD manifests as cardiomyopathy, vascular damage, fibrosis, and heart failure. While numerous mouse models exist to study RIHD pathogenesis, most utilize specific strains (often C57BL/6) or single sexes, and to the authors' knowledge, no prior studies have systematically examined sex differences in RIHD within a mouse model. Furthermore, clinical data presents conflicting views on sex-specific prevalence and mortality, necessitating a controlled translational model to elucidate biological mechanisms driving these differences.

Methodology
The study utilized adult BALB/cJ mice (8–10 weeks old), a strain selected for its susceptibility to radiation-induced cardiomyopathy compared to resistant C57BL/6 strains.

Experimental Design: Male and female mice were randomly assigned to receive a single whole-heart dose of 22 Gray (Gy) radiation via a C5 collimator or a sham control (0 Gy, anesthesia only).
Timepoints: Assessments were conducted at three distinct intervals: 24 hours (innate cardiotoxicity), Day 10 (acute cardiotoxicity), and Day 35 (chronic cardiotoxicity/cardiomyopathy).
Radiation Delivery: Using an X-Rad SmART irradiator, radiation was focused superior to the heart apex and inferior to the atrial appendages to minimize lung collateral damage, confirmed via dose-volume histograms (DVH).
Assessments:
- Histology & Immunohistochemistry: H&E, Masson's trichrome, and Picrosirius red staining were used to evaluate inflammation, vacuolation, and collagen deposition. γH2A.x staining quantified double-stranded DNA (dsDNA) breaks.
- Echocardiography: Baseline (Day -1) and endpoint (Day 35) transthoracic echocardiography measured Left Ventricular Ejection Fraction (LVEF), volumes, diameters, fractional shortening (FS), and Global Longitudinal Strain (GLS).
- Molecular Analysis: qRT-PCR assessed immune cell markers (CD45, CD11b, F4/80, etc.). Bulk RNA sequencing (RNAseq) with Weighted Gene Co-Expression Network Analysis (WGCNA) was performed at Day 10 to identify gene pathways. ELISA measured protein levels of Pgc1α and IL-1β.

Key Results
The study observed distinct sex-specific trajectories in the progression of RIHD:

Acute DNA Damage (Day 10): Both sexes exhibited radiation-induced dsDNA breaks (γH2A.x). However, females demonstrated significantly greater DNA damage than males ( $p < 0.001$ ).
Vascular and Cardiac Remodeling:
- Vessels: Radiation increased vessel area and wall thickness in both sexes at Day 10. By Day 35, males exhibited significantly larger vessel areas and thicker walls compared to females ( $p < 0.001$ ).
- Hypertrophy: Males developed cardiac hypertrophy (increased body weight/tibia length ratio) by Day 10, which persisted through Day 35. Females did not show this hypertrophic response.
Mitochondrial Dysfunction and Repair (Day 10):
- RNAseq revealed that both sexes downregulated mitochondrial metabolic pathways (e.g., oxidative phosphorylation) and upregulated cellular repair pathways.
- Crucially, females upregulated Pgc1α (a master regulator of mitochondrial biogenesis) at Day 10 ( $p = 0.0001$ ), whereas males did not. This suggests females possess a superior capacity for mitochondrial repair.
Chronic Outcomes (Day 35):
- Males: Developed Heart Failure with Reduced Ejection Fraction (HFrEF), characterized by reduced LVEF ( $\le 50\%$ ), increased ventricular volumes (dilated cardiomyopathy), and significant cardiac inflammation. Histology and qPCR confirmed a mixed inflammatory infiltrate (macrophages, T cells, mast cells) and elevated IL-1β, leading to extensive myocardial fibrosis. The phenotype was identified as inflammatory dilated cardiomyopathy (iDCM).
- Females: Developed Heart Failure with Preserved Ejection Fraction (HFpEF), with LVEF averaging ~65%. They exhibited no significant cardiac inflammation, fibrosis, or dilation. While they had initial mitochondrial damage, the upregulation of Pgc1α appeared to mitigate long-term structural damage.

Key Contributions

First Translational Sex-Difference Model: This is the first study to demonstrate that BALB/c mice exposed to whole-heart radiation develop distinct sex-specific phenotypes, with males progressing to iDCM/HFrEF and females to HFpEF.
Mechanistic Insight: The study identifies Pgc1α upregulation in females as a potential protective mechanism against radiation-induced mitochondrial dysfunction, contrasting with the inflammatory cascade (IL-1β driven) observed in males.
Phenotypic Divergence: The research highlights that despite similar initial insults (DNA breaks, vascular damage), the downstream pathological outcomes diverge significantly based on sex, with males suffering more severe structural and functional decline.

Significance and Claims
The authors claim this work provides the first translational evidence of sex differences in RIHD in mice. They posit that the differential expression of Pgc1α drives the divergence in outcomes: females recover mitochondrial function, avoiding inflammation and fibrosis, while males fail to upregulate this regulator, leading to immune recruitment, fibrosis, and HFrEF.

The paper concludes that these findings underscore the necessity of considering sex as a biological variable in RIHD research. It suggests that the clinical presentation of RIHD may mimic other cardiovascular conditions (e.g., atherosclerosis or idiopathic DCM) and that understanding these sex-specific mechanisms is vital for identifying early biomarkers and developing targeted therapies. The authors emphasize that further research is required to fully elucidate the biological drivers of these differences.