Mutation and Expression of Fbx4 in Mantle Cell Lymphoma: A Molecular Study of 31 Cases
YC Liu, L Xi, TA Pham, TH Pham, S Pittaluga, M Raffeld. NCI, NIH, Bethesda, MD
Background: Fbx4 was recently identified as the E3 ubiquitin ligase in the protein complex targeting cyclin D1 for degradation. Mutations in Fbx4 are reported to cause nuclear accumulation of cyclin D1. These mutations are associated with oncogenic transformation in the context of cyclin D1 expression, and are reported to occur in approximately 14% of cyclin D1 expressing human esophageal carcinomas. Mantle cell lymphoma (MCL) is characterized by dysregulated expression of cyclin D1 as a result of the t(11;14). Nonetheless, overexpression of cyclin D1 alone has consistently been shown to be insufficient for tumor induction, leading to a search for secondary cooperating genetic alterations. Since mutational inactivation of Fbx4 has been shown to cooperate with cyclin D1 in oncogenic transformation, we wished to determine whether this mechanism might play a role in MCL lymphomagenesis.
Design: 31 confirmed cases of MCL were retrieved from the archives. A subset of cases with frozen material (n=3) were evaluated for the expression of Fbx4 by real-time RT-PCR in comparison with purified normal B cells. PCR and subsequent sequencing of the dimerization domain of Fbx4 were performed on DNA extracted from paraffin tissue blocks in all 31 cases. Multiple sets of primers were designed to cover the first exon and part of the second exon, essential for dimerization, in which the reported mutations occurred.
Results: No significant difference in Fbx4 expression was noted between normal B cells and the subset of MCL (n=3), indicating that downregulation of Fbx4 is not likely to contribute significantly to maintaining high cyclin D1 levels in MCL. One potential mutation (G9R) in exon 1, which has not been reported previously, was identified by sequencing in 1/31 samples (3.23%). No other mutations were identified.
Conclusions: The low frequency of mutation (<5%) and normal expression levels of Fbx4 indicate that Fbx4 alterations do not play a significant role in maintaining the high levels of cyclin D1 seen in MCL. The functional phenotype of the single potential mutation G9R merits further study.
Monday, March 22, 2010 1:00 PM
Poster Session II # 164, Monday Afternoon