Prince Frederik of Luxembourg, the youngest son of Prince Robert of Luxembourg and Princess Julie of Nassau, died in Paris at age 22 on March 1—a day after Rare Disease Day—of a rare genetic disease known as POLG.

His father announced his passing on March 7 in a statement posted on the website of the POLG Foundation, which Frederik cofounded in 2022 and was the creative director of.

“Frederik fought his disease valiantly until the very end,” Robert wrote. “His indomitable lust for life propelled him through the hardest of physical and mental challenges.”

Frederik—who is survived by his parents; his brother, Alexander; his sister, Charlotte; his cousins, Charly, Louis, and Donall; his brother-in-law, Mansour; his aunt and uncle, Charlotte and Mark; and his dog, Mushu—was remembered as a resilient fighter and a headstrong advocate. “Frederik knows that he is my Superhero,” Robert wrote. “Part of his superpower was his ability to inspire and to lead by example.”

He even expressed gratitude for having his disease, Robert wrote. “He felt that there were so many amazing people that he would never have known had it not been for his disease,” and once told a friend, “‘Even though I’ll die from it … and even if my parents do not have the time to save me, I know that they will be able to save other children.’”

What is POLG?
POLG disease is a mitochondrial disorder caused by inherited mutations in the POLG gene—which is critical to the processes of replicating cells’ genetic material and DNA repair. The disease, which has no cure, affects multiple organs, including the brain, nerves, muscles, and liver, and symptoms can be wide-ranging and debilitating. “One might compare it to having a faulty battery that never fully recharges, is in a constant state of depletion and eventually loses power,” Robert wrote.
It’s also difficult to diagnose. While it’s one of the most common inherited mitochondrial diseases, affecting around 1 in 10,000 people, it’s considered a rare disease. Molecular genetic testing for the mutation exists, and the disease can also be detected through brain imaging to look for associated brain changes and electroencephalogram (EEG) testing, but the range in symptoms and lack of public awareness around the disease means it can be hard for even physicians to identify. Frederik was diagnosed at age 14.

Doug Turnbull, a professor of neurology at Newcastle University and a member of the POLG Foundation’s scientific advisory board, described POLG deficiency as “the worst” of all mitochondrial diseases. “It is so relentlessly progressive, attacking so many different systems with sadly the same conclusion.”

The disease can impact vision, mobility, and speech, and it can be deadly—life expectancy ranges from three months to 12 years from the onset of the disease.