Filmmaker Mei Fa Tan reflected on her experience working with Luxembourg’s Prince Frederik before his death at the age of 22. “[He was] so interested in so many topics,” Tan told Us Weekly on Wednesday,

Commissioned by Frederik’s mother, Princess Julia of Nassau, to helm an educational short film about the disease with Frederik at its center, director Mei Fa Tan said she was left “inspired” by the prince but also found it “painful to watch” him struggle with his health.

P rince 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.