Aberrant Somatic Hypermutation of Follicular Lymphoma Transformed To Diffuse Large B-Cell Lymphoma.

Follicular Lymphoma (FL) accounts for approximately 20–30% of all malignant lymphomas and the frequency of histological transformation (HT) into diffuse large B-cell lymphomas (DLBCL) varies from 10% to 70%. Many genetic lesions have already been described in histological transformation, but a mechanism of genome-wide instability during histological transformation (HT) has not been reported. We have shown that the somatic hypermutation process (SHM) physiologically aimed at mutating the immunoglobulin variable gene (IgV) aberrantly targets multiple proto-oncogenes in >50% of DLCBL (Pasqualucci et al., Nature 412:341, 2001). Consequently, multiple mutations are introduced in the 5′ region of genes including known proto-oncogenes such as PIM-1, PAX-5, Rho/TTF and c-MYC. To further investigate whether aberrant somatic hypermutation (ASHM) also occurs in histological transformation of follicular lymphoma in DLBCL we studied the mutational profile of these genes in a total of 26 cases consisting of 10 paired samples of follicular lymphoma together with the corresponding DLBCL and 6 single cases of transformed DLBCL. Genomic DNA from histologically confirmed, macrodissected tissue obtained from formalin fixed and paraffin embedded tissue or cryoconserved samples was directly sequenced. Mutations in one or more genes were detected in 6 of 10 (60%) cases of pre-HT follicular lymphoma and in 13 of 16 (81.2%) post-HT cases. Two ore more genes were affected in 1 of 10 (10%) of FL and in 7 of 16 (43.7%) cases with DLBCL. Mutations in PIM-1 occurred in 3 of 10 (30%) cases of follicular lymphoma and in 9 of 16 (56.2%) in DLBCL. For PAX-5, the distribution of the mutated cases between FL and DLBCL was 2 of 10 (20%) and 7 of 16 (43.7%), for RhoH/TTF 2 of 10 (20%) and 3 of 16 (18.7%) and for c-MYC none of 10 (0%) FL and 2 of 16 (12.5%) DLBCL. A total of 38 single base pair substitutions were found in 19 cases, 10 sequence variants in 6 FL cases and 28 sequence variants in 13 DLBCL cases. The mutations were of somatic origin and share features of the IgV SHM process including bias for transition over transversion, elevated ratio of G+C over A+T substitutions and restriction to the first 1-2Kb from the promoter initiation site. The mean mutation frequency in mutated follicular lymphoma was with 0.024 ×10⁻²/bp 1.4 fold lower compared to 0.032 ×10⁻²/bp in the transformed DLBCLs. Further in PIM-1 and c-MYC some of the mutations were found to affect coding exons, leading to amino acid exchanges, thus potentially altering gene function. These data support a role of aberrant SHM in the histological transformation of FL to overt DLBCL.