The molecular mechanisms of cyclin D1 gene deregulation by immunoglobulin regulatory sequences in B cell malignancies

Abstract

Cyclin D1 expression is deregulated by chromosome translocation in mantle cell lymphoma and a subset of multiple myeloma. The epigenetic mechanisms involved in this long-distance gene deregulation were studied. The cyclin D1 promoter was hypomethylated and hyperacetylated in expressing cell lines and patient samples. Domains of hyperacetylated histones and hypomethylated DNA extended over 120 kilobases upstream of the cyclin D1 gene. Interestingly, hypomethylated DNA and hyperacetylated histones were also located at the cyclin D1 promoter but not upstream regions in cyclin D1 nonexpressing, nontumorigenic B and T cells. The findings of complete cyclin D1 promoter demethylation were surprising given the presence of both the translocated or inserted and the normal chromosome 11. The cyclin D1 locus and sequences 90 kilobases upstream were methylated in genetic variants that had lost the translocated or inserted but retained one or more copies of normal chromosome 11. These observations indicate a trans demethylating effect of the translocated chromosome containing the immunoglobulin heavy chain (IgH) locus that resembles transvection in Drosophila and paramutation in maize. RNA polymerase II (Pol II) binding was demonstrated at the cyclin D1 promoter, upstream sequences, and 3' IgH regulatory regions only in malignant B cell lines with deregulated expression of the cyclin D1 gene. These results suggest Pol II bound at IgH regulatory sequences can activate the cyclin D1 promoter by either a long-range polymerase transfer or tracking mechanism. The observation that the DNA binding protein CTCF concurrently binds to the cyclin D1 promoter region and IgH regulatory elements only in the cyclin D1(+) MCL cell lines but not in the cyclin D1(-) EBV-transformed B lymphocytes suggests that CTCF may be crucial in the long distance interaction between IgH regulatory elements and the cyclin D1 gene locus. I propose that, in cyclin D1 expressing B cell malignancies, after chromosomal translocation, concurrent binding of Pol II, CTCF, and other transcription factors at the IgH regulatory regions and cyclin D1 gene promoter juxtaposes these two loci, affecting cis and trans DNA hypomethylation and histone hyperacetylation, and initiating deregulated expression of cyclin D1.