Abstract
Introduction: Follicular lymphoma (FL) is the second most prevalent non-Hodgkins lymphoma (NHL) in the United States. Although the clinical course is usually indolent, about 50% of low-grade FL transform to an aggressive lymphoma within 15 years of diagnosis. Somatic hypermutation (SHM) is a normal process in B cells which targets the immunoglobulin locus and introduces point mutations and small insertions/deletions. In diffuse large B cell lymphoma, the SHM misfires and aberrantly targets multiple loci, including some proto-oncogenes (PIM-1, PAX-5, RhoH/TTF and C-MYC).
Hypothesis: We hypothesized that acquired mutations in the same proto-oncogenes could be responsible for the clinical transformation of follicular lymphoma.
Materials and methods: The clinical database of the Department of Pathology, Washington University School of Medicine, St. Louis was searched to find patients with serial biopsies and the diagnosis of either stable (sFL) or transformed follicular lymphoma (tFL). Thirty-five (35) formalin-fixed paraffin-embedded (FFPE) tissue samples from 17 patients were identified. Eleven of seventeen patients had transformed to large cell lymphoma and six had stable low-grade follicular lymphoma. To exclude the possibility of a second unrelated lymphoma the common clonal origin of the original and follow-up lymphoma was confirmed by comparing immunoglobulin heavy and/or light chain variable gene sequences. Previously identified mutational hot spots in four genes, BCL-6, PAX-5, RhoH/TTF and C-MYC, were amplified and sequenced directly.
Results: Fifty-five mutations were found in the four genes sequenced; all patients had at least one mutation. The number of mutations was similar in both the low grade and the transformed specimens overall, and most paired specimens had identical mutations or a single mutation change over time. Of the mutations identified, only mutations in Myc exon 2 correlated with the transformation. Acquired missense mutations were detected in the trans-activation domain of Myc in two of the eleven transformed FL specimens but none of the 24 low-grade follicular lymphoma specimens. The first case had two mutations: Pro57 to Ser57 and Glu39 to Asp39. The second case had a single Glu39 to Asp39 change. One of the mutations (Pro57 to Ser57) is close to a functionally critical phosphorylation site, Thr58, and has been reported in both Diffuse large B cell and Burkitt’s lymphomas. The other mutation (Glu39 to Asp39) has also been reported in both Burkitts and DLBCL cases, and is not reported as a normal variant.
Conclusion: There is not an accumulation in the overall number of SHM with transformation as the frequency of SHM is similar in low-grade FL and transformed FL. However, mutations in the trans-activation domain in c-Myc, caused by an aberrant SHM, may play a role in the transformation of low-grade FL to a diffuse, large cell lymphoma in a subset of patients.
Somatic Hypermutation Analysis of 17 serial Follicular Lymphoma cases (35 specimens) Gene Name % Specimens with SHM Mutation frequency per 100 bp Number of unique single base pair substitutions Number of unique deletions or insertions Number of substitutions targeting G+C vs A+T NA = Not applicable BCL-6 63% 0.14 25 3 16 vs 9 PAX-5 43% 0.06 7 2 7 vs 0 Rho/TTF 17% 0.03 8 2 1 vs 7 C-MYC 31% 0.02 6 1 6 vs 0 All genes 83% NA 47 8 31 vs 17
Analysis with antiidiotype antibody of a patient with chronic lymphocytic leukemia and a large cell lymphoma (Richter's syndrome)