Justin Bieber has taken to Twitter to mourn the death of one of his biggest fans. 

Avalanna Routh, the 6-year-old Merrimac girl that has become better known across the country as "Mrs. Bieber", passed away this morning.

A note posted on Avalanna's Twitter  page Wednesday morning read, "Our darling Avalanna went to Heaven this morning. Oh Avalanna, the brightest star - you took our hearts with you, our greatest Love."

After hearing the news, Bieber tweeted, "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."

He also tweeted: "RIP Avalanna. I love you" and asked fans to "please show respect to her and her family. say a prayer for her and for them. i miss her."

The six-year-old was diagnosed in 2006 with a rare brain cancer and had been getting treatment at the Dana-Farber Cancer Institute, according to Fox 25 news. The singer followed up by posting a website devoted to families with children battling the rare cancer.

Dennis and Callahan of the sports morning radio show on WEEI, who met Avalanna while working for the Jimmy Fund, paid tribute to Avalanna and her family on this morning's broadcast.

Avalanna's story gained national attention after friends started a Facebook and Twitter campaign to get the girl to meet the love of her life. The hashtag "#MrsBieber" even trended across the country on Twitter.

After learning of Routh's love for him, Justin Beiber made her dreams come true in any way he could. Routh and Bieber got together with her family for a play wedding between the two at the Jimmy Fund Clinic where she also  travelled to New York City in February. In June, Bieber brought Routh on stage with him while performing at the Apollo Theater.

After the February trip, Avalanna's mother said that Bieber was "just a dream." She continued saying: "We were Bieber fans before, but there is now a whole new respect for this kid that did such a great job with her and was just so graceful in doing it."

The young girl's mother said the visit was "a moment" and that it was "important at this point to look at the bigger picture. I want Avalanna's legacy to be more."

The Routh family has been working to raise awareness and funds for researching atypical teratoid rhabdoid tumors in order to find a cure.