Transallelic Effects at the Cyclin D1 Loci in Mantle Cell Lymphoma.

Cyclin D1 is a gene commonly deregulated in B cell malgnancies. Translocation between chromosomes 11 and 14 (t;14) are found in most patients with mantle cell lymphoma (MCL) and 15–20% of patients with multiple myeloma (MM). Gene targeting experiments in cyclin D1 overexpressing MCL and MM were carried out and genetic variants isolated which had lost the IgH translocated or inserted chromosomes. These clones no longer expressed cyclin D1 but expressed high levels of cyclin D3. Analysis of DNA methylation patterns in these clones has 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 cyclin D1 locus is densely DNA methylated. Reintroduction of the translocated chromosome by somatic cell hybrid fusion induced transhypomethylation in trans. 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, suggesting that 3′ Cα IgH sequences are sufficient for this effect. 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. Allelic ChIP studies showed the presence of CTCF and nucleophosmin on both the translocated and normal chromosome 11(s). Tethering of the translocated and nontranslocated chromosomes by CTCF/nucleophosmin at the nucleolus provides a mechanism for pairing and long distance DNA transhypomethylation.