Involvement of the NOTCH1 and NOTCH2 Genes in B-Cell Lymphomagenesis.

The NOTCH pathway is a key regulator of developmental choices, differentiation and function throughout the hematopoietic system. An aberrant activation of this pathway may underlie leukemogenesis, as shown in T-ALL cases with the rare t(7;9)(q35;q34) targeting NOTCH1 or with mutation(s) of NOTCH1 found in approximately 50% of T-ALL cases. We report here evidence for NOTCH1 and NOTCH2 rearrangements in B-cell lymphoma. Involvement of NOTCH1 was identified by the presence of dic(9;14)(q34;q32) and trisomy 12 in a case of chronic lymphocytic leukemia (CLL) at the time of transformation, and by st(9;22)(q34;q11) accompanied by t(14;18)(q32;q21)/IGH-BCL2 and t(8;14)(q24;q32)/IGH-CMYC in a follicular lymphoma (FL). FISH analysis of dic(9;14) and t(9;22) demonstrated the respective involvement of the IGH and IGL loci, and mapped both 9q34 breakpoints to the 5′ end of NOTCH1. Western blot analysis with cleaved Notch1 (Val1744) antibody was performed in both cases and hyperactive NOTCH1 signaling was found in the first case only. This particular case displayed in addition a mutation in the PEST domain of NOTCH1. No mutations, neither in the PEST, nor in HD domain of NOTCH1 (2 domains frequently mutated in T-ALL) were found in the FL case. FISH analysis of 30 additional lymphoma cases documented by structural aberrations of 9q33q34 revealed a normal status of NOTCH1.

NOTCH2 rearrangement was identified by FISH in 1 out of the 3 leukemia/lymphoma cases we could collect with t(1;14)(p13;q32). This particular CLL case showed a 3-way translocation, t(1;14;18)(p13;q32;q21), targeting also BCL2. So far, this patient did not show any clinical features of CLL transformation.

We hypothesize that IG-translocations affecting NOTCH genes in B-cell malignancies contribute to the lymphomagenesis by deregulating the transcription of these genes resulting in an aberrant ligand-independent NOTCH signaling. Mutation in the PEST domain of NOTCH1 as found in one case with dic(9;14) may further contribute to the process by stabilization of the aberrant product. Further molecular and immunohistochemical investigations of these cases are in progress.