Long Distance Trans DNA Hypomethylation in Cyclin D1 Deregulated B Cell Malignancies.

Cyclin D1 is a common target gene in B cell malignancies. Cyclin D1 is deregulated by translocation in most patients with mantle cell lymphoma (MCL) and 15–20% of patients with multiple myeloma (MM). Cyclin D1 is not expressed in normal lymphocytes. Gene targeting experiments in cyclin D1 overexpressing MCL and MM were carried out and genetic variants isolated which had lost the translocated or inserted 11:14 chromosomes. These clones no longer expressed cyclin D1 but expressed high levels of cyclin D3. Analysis of DNA methylation patterns in these clones demonstrated that the translocated chromosome exerts a long distance trans DNA hypomethylating effect on the normal chromosome (transvection) at the cyclin D1 locus and at least 100 kilobases upstream. Thus, in the absence of the translocated chromosome, the normal chromosome is densely DNA methylated. This long distance trans hypomethylating effect was also demonstrated in a MM cell line (U266) which contains an insertion rather than a translocation of IgH sequences. Combined FISH/ immunofluorescent antibody labelling studies have shown the presence of both the normal and translocated chromosome 11’s at the outer nucleolar membrane, where they are tethered by the proteins CTCF and nucleophosmin, as demonstrated using chromatin immunoprecipitation assays. Tethering of the translocated and nontranslocated chromosomes by CTCF/nucleophosmin provides a mechanism for pairing and long distance DNA transhypomethylation.