Justin Bieber mourns 6-year-old cancer victim Avalanna, his 'angel'

Justin Bieber has turned to Twitter to express his sorrow over the death of Avalanna Routh -- a 6-year-old who was one of his biggest fans and became a Web sensation, dubbed "Mrs. Bieber," because of her love for the pop star.

Bieber, 18, typically uses his Twitter account to promote upcoming performances, or his new "Believe" album, or to retweet the likes of Kim Kardashian and members of his Bieber Army. But Bieber's most recent tweets to his more than 28 million followers tell a different story -- underscoring just how hard-hit he was by news of the death of the little girl from Merrimac, Mass.

Avalanna was suffering from atypical teratoid rhabdoid tumor -- a rare cancer typically referred to as AT/RT. She was smitten by anything related to Bieber, so much so that the staff at the Dana-Farber/Children’s Hospital Cancer Center threw Avalanna a wedding -- to Bieber -- last year. There was cake, flowers, a singer.

Soon, there was a Facebook page, "Get Avalanna to Meet Justin Bieber." It caught the attention of the pop star, who met and became friends with the little girl who then earned the nickname Mrs. Bieber.

Wednesday night, Bieber was tweeting congrats to singer Carly Rae Jepson over her new album, plus thanks to his Japanese fans. Moments later came another tweet -- and several after that -- that struck a starkly different chord (and might have made those who considered him just another self-absorbed pop star to rethink their opinion).

-- "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."