Recurrent Mutations Of NOTCH Genes In Follicular Lymphoma

Follicular lymphoma (FL) is one of the most common malignant lymphomas. The t(14;18)(q32;q21) is found in about 80% of these cases and is regarded to play a principal role in their lymphomagenesis. However, the molecular mechanisms involved in the development and progression of these tumors are not fully understood. Although the clinical course is usually indolent, FL is sometimes refractory to chemotherapy and 25-35% of cases transform to aggressive lymphomas. This histological transformation is usually associated with a rapid progressive clinical course with around one year overall survival. Until now, several molecular mechanisms of the transformation have been described. P53 mutations, CDKN2A deletions and MYC translocations have been associated with FL transformation but some cases do not carry any of those genetic alterations suggesting that other mechanisms may also play a role. NOTCH signaling pathway is associated with various physiologic cellular processes such as cell proliferation, differentiation and death. Gain-of function mutations of NOTCH1 or NOTCH2 have been reported in acute T-cell lymphoblastic leukemia/lymphoma, chronic lymphocytic leukemia (CLL), mantle cell lymphoma (MCL), splenic marginal zone lymphoma (SMZL) and in most cases have been associated with transformation and a more aggressive biological behavior of the tumor. The presence of NOTCH1 or 2 mutations in FL and its possible relationship to tumor transformation have not been previously studied. Almost all NOTCH 1 or 2 mutations in CLL, MCL and SMZL are truncating mutations of the PEST domain and result in the elimination of degradation signals of NOTCH proteins. This prolongs the half-life of intracellular subunits leading to the activation of NOTCH signaling. In this study we have investigated the mutational status of NOTCH1 and NOTCH2 in 114 FL using the conventional Sanger’s sequencing method. These analyses covered the whole PEST domain and most of the TADD domain. NOTCH1 and NOTCH2 mutations were identified in five and two cases, respectively, (total 7/114, 6.1%). All mutations predicted for truncated protein in the PEST domain and were identical to those identified in other B cell lymphoid neoplasms. Pathologically, four out of the seven cases (57%) with NOTCH mutation had a morphological grade 3. A simultaneous DLBCL component was observed in four cases, three with grade 3 and one grade 2. One additional FL grade 2 subsequently transformed to DLBCL one year after the initial diagnosis. No MYC translocation or TP53 mutations were observed in the 5 FL/DLBCL with NOTCH mutations studied. Transformation to DLBCL histologically confirmed (simultaneously or subsequently) was significantly more frequent in NOTCH mutated (5/7, 71%) than unmutated FL (29/106, 27%)(P=0.025). BCL6 and BCL2 were positive in five out of six cases (83%) while CD10 was positive in three cases (50%). Only one of the NOTCH mutated cases had the t(14;18)(q32;q21) and two carried t(3q27) rearrangements. The frequency of t(14;18) in NOTCH mutated cases, therefore, is significantly lower than that in the NOTCH unmutated cases (14% vs. 72%, respectively; P=0.008). Clinically, extra-nodal involvement was found in six out of the seven (86%) NOTCH mutated cases and spleen involvement in four. Splenic involvement in the NOTCH mutated cases was significantly more frequent than that in the NOTCH unmutated cases (57 vs. 16%, respectively; P=0.027). Interestingly, all NOTCH mutated cases arose in female patients while no sex deviation was identified in the NOTCH unmutated cases (female proportion: 48%) (P=0.013). These results suggests that NOTCH mutations are uncommon in FL but may occur in a small subset of female patients with particular tumors characterized by the absence of the t(14;18), splenic involvement, and frequent transformation to DLBCL.