Identification of an alternative CD20 transcript variant in B-cell malignancies coding for a novel protein associated to rituximab resistance

Human CD20 is a B-cell lineage-specific marker expressed by normal and leukemic B cells from the pre-B to the plasmacell stages and is a target for rituximab (RTX) immunotherapy. A CD20 reverse transcriptase-polymerase chain reaction (PCR) on B-cell lines cDNA yielded a short PCR product (Delta CD20) corresponding to a spliced mRNA transcript linking the exon 3 and exon 7 ends. We established here that this novel, alternatively spliced CD20 transcript is expressed and detectable at various levels in leukemic B cells, lymphoma B cells, in vivo tonsil-or in vitro CD40L-activated B cells, and Epstein-Barr virus (EBV)-transformed B cells, but not in resting CD19(+)-or CD20(+)-sorted B cells from peripheral blood or bone marrow of healthy donors. The truncated CD20 sequence is within the reading frame, codes a protein of 130 amino acids (similar to 15-17 kDa) lacking large parts of the 4 transmembrane segments, suggesting that Delta CD20 is a nonanchored membrane protein. We demonstrated the translation into a Delta CD20 protein which is associated with the membrane CD20 protein and showed its involvement in RTX resistance. Study of patient samples before and after RTX resistance or escape confirms our in vitro findings. (Blood. 2010;115:2420-2429)