Mantle Cell Lymphoma: An Epigenetic Disease of Cyclin D1 Deregulation Involving Nucleophosmin, CTCF and Small RNA’s.

Translocations involving the immunoglobulin heavy chain gene locus (IgH) are common in B cell malignancies. The molecular mechanisms of long distance gene activation by IgH regulatory elements in B cell malignancies are unknown. One common target gene is cyclin D1, which is deregulated in most patients with mantle cell lymphoma (MCL) and 20–30% of patients with multiple myeloma (MM). Cyclin D1 is known not to be expressed in normal lymphocytes, yet is deregulated by IgH translocations and insertions. Chromatin immunoprecipitation (ChIP) assays demonstrated the presence of the CCCTC-binding factor (CTCF) at both the cyclin D1 promoter and 3′ IgH regulatory regions only in cyclin D1 expressing malignant B cell lines containing IgH translocations or insertions. The nucleolar protein nucleophosmin (NPM), a newly identified CTCF interaction partner, exhibited a similar distribution in ChIP assays. Combined FISH/immunofluorescence assays in MCL cell lines and patient samples have demonstrated the presence of the cyclin D1/IgH translocated locus at the nucleolus specifically in cells with the t(11;14). Short hairpin (Sh) RNA mediated NPM knockdown results in a specific growth arrest only in cells with translocated chromosomes. Cell growth in variant MCL cells and B lymphocytes is not altered. The presence of small RNA’s and binding of the protein Dicer at the translocation breakpoint has been demonstrated. ShRNA knockdown of dicer results in significant reduction of cyclin D1 protein in MCL cells. Tethering of the cyclin D1 promoter to the 3′ IgH regulatory regions at the nucleolar membrane by CTCF/nucleophosmin provides a mechanism for spatial juxtaposition of the cyclin D1 promoter and 3′ IgH regulatory elements that facilitates deregulated expression of the cyclin D1 gene. These results also demonstrate the participation of small RNA’s and the RNA interference pathway in the deregulated expression of cyclin D1 in MCL.