Justin Bieber tweets about death of 6-year-old fan known as Mrs. Bieber

Justin Bieber is heartbroken today after one of his youngest fans, 6-year-old Avalanna Routh – who was better known as Mrs. Bieber – died of cancer. The "Boyfriend" singer tweeted a sweet message about the Boston girl on Wednesday afternoon, writing, "just got the worst news ever. one of the greatest spirits i have ever known is gone. please pray for her family and for her. RIP Avalanna. i love you." About an hour later, he wrote, “please show respect to her and her family. say a prayer for her and for them. i miss her.”

Routh gained national attention last year when she pretended to marry Bieber in a wedding ceremony staged by the doctors and nurses at the Boston hospital where she was being treated. The day before Valentine's Day, the singer himself flew her and her parents to meet him in New York, where they spent an afternoon playing board games and just hanging out. The Biebs even let the cutie play with his famous hair! At the time he tweeted, "That was one of the best things i have ever done. She was AWESOME! Feeling really inspired now!" and used the hashtag Mrs. Bieber. Routh appeared onstage with Bieber at the Apollo Theater, and when he appeared on "Today" in June, he said that the two continued to keep in touch via iChat and FaceTime.

Sadly, Routh was sick for most of her life. Dr. Charles Robert of the Dana-Farber Cancer Institute, where Routh was treated, told the Associated Press that she was diagnosed with a rare teratoid rhabdoid tumor – only about 30 of them are diagnosed in the U.S. each year – when she was just 18 months old. She "responded to initial treatment for quite a while, but the cancer kept coming back and ultimately she was no longer responsive" and died at her Merrimac, Massachusetts, home, Robert said. The Dana-Farber Cancer Institute issued a statement about Routh's death, which read, "She was a very courageous young person who lived her life with grace and determination. By generously sharing her story, she raised awareness worldwide about atypical teratoid rhabdoid tumors and articulated the need for greater research of this rare cancer."

Routh's family also posted a beautiful goodbye message on Twitter. "Our darling Avalanna went to Heaven this morning," they wrote. "Oh Avalanna, the brightest star — you took our hearts with you, our greatest Love."

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