Prince Frederik of Luxembourg, the youngest son of Prince Robert of Luxembourg and Princess Julie of Nassau, has died at 22 years old after a lifelong battle with POLG mitochondrial disease, the family announced.

After gifting each of us with our farewells – some kind, some wise, some instructive – in true Frederik fashion, he left us collectively with a final long-standing family joke,” Prince Robert said in a statement posted to The POLG Foundation website, which Prince Frederik was a founder and creative director of. “Even in his last moments, his humour, and his boundless compassion, compelled him to leave us with one last laugh….to cheer us all up.”

According to the foundation website, POLG mitochondrial disease is a rare genetic disorder that “robs the body's cells of energy, in turn, causing progressive multiple organ dysfunction and failure.”

RELATED STORY | Princess Kate returns to public view after cancer diagnosis

Prince Robert said his son gathered their family into his room on Feb. 28 – which happened to be Rare Disease Day – to speak with all one last time.

“Frederik’s last question to me, prior to his other remarks was: “Papa, are you proud of me?” He had barely been able to speak for several days, so the clarity of these words was as surprising as the weight of the moment was profound. The answer was very easy, and he had heard it oh so many times…. but at this time, he needed reassurance that he had contributed all that he possibly could in his short and beautiful existence and that he could now finally move on.”

Prince Robert said his son “was born with a special capacity for positivity, joy and determination” and had “social skills like no other, an amazing sense of humour, an emotional intelligence and compassion that were off the charts, a sense of justice, fairness and decency that knows no bounds.”

Prince Frederik was diagnosed with POLG mitochondrial disease in 2016, according to his foundation’s website. He and his family formed the foundation as a way to help others suffering from the disease.