Long Distance Cyclin D1 Activation in B Cell Malignancies: Role of RNA Polymerase II.

Translocations involving the immunoglobulin heavy chain gene locus (IgH) are common in B cell malignancies. One common target gene is cyclin D1, which is deregulated in most patients with mantle cell lymphoma (MCL) and approximately 15–20% of patients with multiple myeloma (MM). Cyclin D1 is not expressed in normal lymphocytes; cyclin D1 expression in B cell malignancies is deregulated by IgH translocations and insertions. We have shown that the cyclin D1 locus, including the promoter and upstream regions, are acetylated and DNA hypomethylated in both malignant and nonmalignant B cells (Liu et al, Blood in press). Using chromatin immunoprecipitation (ChIP) assays, we have found that RNA polymerase II (Pol II) is a) present at the cyclin D1 promoter b) located at regions far upstream of the cyclin D1 gene and c) present at 3′ Cα IgH regulatory elements only in cyclin D1 expressing malignant B cells. Pol II is present at the translocation breakpoint in a MCL cell line, but we do not find Pol II present continuously from the IgH regulatory regions to the cyclin D1 promoter. Confirmatory RT-PCR analyses also do not demonstrate continous evidence of RNA transcripts extending from the IgH regulatory elements to the cyclin D1 promoter. Mutants derived from gene targeting experiments that have lost the translocated chromosome show Pol II binding only at the Eu intronic enhancer, consistent with known promoter acivity and sterile transcripts originating at this location. Our data suggest that Pol II may play a important role in initiating long distance cyclin D1 deregulation by translocated IgH sequences. Analogous to the mouse β-globin gene locus, a polymerase transfer (LPT) model may be invoked, where other proteins such as CTCF effect Pol II transfer from 3′ Cα IgH regulatory sequences to the cyclin D1 promoter and initiate cyclin D1 transcription.