C-Terminal Deletion Mutation of CD20 in B-Cell Malignancy Is Related to a Risk of Relapse and Resistance to Chimeric Anti-CD20 Antibody Therapy.

Introduction: Rituximab was the first chimeric antibody that is most frequently used for CD20-posistive B-cell lymphomas and gives better response and prognosis. However, resistance to rituximab is one of the important issues to be clarified in the increasing number of monoclonal antibody therapy, and we experienced the case whose lymphoma cells never expressed CD20 at the relapsed phase during rithuximab therapy. We investigated whether CD20 mutation is related to expression level of CD20, relapse or resistance to rituximab therapy, and prognosis.

Methods: We analyzed 50 patients with fresh or relapsed/ resistant B- cell lymphomas and DNA sequencing analysis of CD20 products from genomic PCR and RT-PCR was performed. CD20 mutants were subcloned by TA cloning, and tested CD20 expression after transfection to K562 cells.

RESULTS: Four types of CD20 mutations were found in 11 of 50 NHL patients (22.0%), which include C-terminal deletion(8.0%) extracellular domain (2.0%) transmembrane domain (2.0%) and early termination (10.0%). The group of C-terminal deletion mutations significantly showed lower CD20 expression (3.26, 95%CI = 0.09 to 6.89) than non-mutation (30.8, 95%CI = 22.4 to 39.2) (p < 0.05), whereas five patients with early termination mutation did not show significant difference of CD20 expression (19.5, 95%CI = 10.7 to 28.4) as compared with non-mutation (p > 0.05). Although there was no significant difference between the groups with non-mutation and C-terminal deletion mutation in CR rate (49% versus 25%, Fisher’s exact test; p = 0.6137), time to progression after rituximab therapy in C-terminal deletion mutation (7 months, 95%CI = 0 to 18 months) was significantly shorter than that in non-mutation (31 months, 95%CI = 18 to 44 months) by log-rank test (p = 0.0481).

Conclusion: The important mutations of CD20 gene related to shorter duration to progression disease after rituximab therapy were discovered. Especially, point mutations bearing during rituximab therapy should be examined at progression disease after partial remission.