Identification of an Alternative CD20 Transcript Variant in B Cell Malignancies Coding for a Novel Protein Associated to Rituximab Resistance
Blood. 2010 Jan 20; Epub ahead of print, C Henry, M Deschamps, P-S Rohrlich, J-R Pallandre, J-P Rémy-Martin, M Callanan, A Traverse-Glehen, C GrandClément, F Garnache-Ottou, R Gressin, E Deconinck, G Salles, E Robinet, P Tiberghien, C Borg, C Ferrand
ABSTRACT
Human CD20 is a B-cell lineage-specific marker expressed by normal and leukemic B cells from the preB to the plasma-cell stages and is a target for rituximab (RTX) immunotherapy. A CD20 reverse-transcriptase-polymerase chain reaction on B-cell lines cDNA yielded a short PCR product ( ΔCD20) corresponding to a spliced mRNA transcript linking the exon3 and exon7 ends. We established here that this novel, alternatively spliced CD20 transcript is expressed and detectable at various levels in leukemic, lymphoma, in vivo tonsil- or in vitro CD40L-activated B cells, in 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 (~15-17 kD) lacking large parts of the four transmembrane segments, suggesting that ΔCD20 is a non-anchored membrane protein. We demonstrated the translation into a ΔCD20 protein which is associated with the membrane CD20 protein and showed its involvement in RTX resistance. Study of patient's samples before and after RTX-resistance or -escape confirms our in vitro findings.
