We previously reported that mutations of CD20 gene were found in patients with B-cell non-Hodgkin's lymphoma, and we proposed that C-terminal deletion mutations of CD20 might be involved in relapse/resistance after rituximab containing therapy. Most of the patients that had mutation in the C-terminal leagion were diagnosed as CD20 negative by immunohistochemistry using L26 monoclonal antibody. L26 recognizes the cytoplasmic region of CD20 molecules, but no more detailed information about its epitope had been reported. So at first we determined the binding site of L26 antibody on CD20 protein. Then we developed novel diagnostic antibodies that recognize wide variety of CD20 molecular subtypes including those having mutations.
To determine the epitope of L26 antibody, we established six sub-lines expressing various length of C-terminal truncated CD20 using an originally CD20 negative myeloma cell line. Then we carried out epitope-mapping using these cell lines. To detect comprehensive CD20 molecules including that having mutation in C-terminal region, we developed antibodies that recognize near the amino terminus of CD20 molecules (CD20N antibody). CD20N antibody is the only monoclonal antibody that recognizes N-terminal region of CD20 so far. Using these antibodies, we screened the specimens of the cases diagnosed as CD20 negative determined by L26-based immunohistochemistry.
The epitope-mapping revealed that L26 recognizes near the C-terminus of CD20. This suggested that most of CD20 molecules with the C-terminal deletion mutation or frame-shift mutation could not be recognized by L26. Then we screened previously diagnosed specimens and found several cases that having the cells stained by our novel antibody but not by L26. Genetic analysis revealed that all these cells had a mutation in the C-terminal cytoplasmic region of CD20. One of these cases, we successfully analyzed the phenotype of lymphoma cells with mutated CD20 in detail using cryopreserves living specimens. In this case, a frame shift mutation occurred due to one base nucleotide deletion, resulting in the translation of peptide of another reading frame of 41 amino acids with premature stop at the amino acid position 250. Interestingly, mutant CD20 molecule expressed adjacent to the plasma membrane, but rituximab could not bind to these cells. DNA sequencing study about genome and mRNA of CD20 gene suggested that the lymphoma cells of this patient had one normal and one mutated CD20 allele.
The C-terminal region of CD20 may undertake a pivotal role in presentation of the large loop where the rituximab binding site locates. Thus, deletion or frame-shift mutation of CD20 in C-terminal cytoplasmic region impairs the antigenicity against rituximab and it may cause resistance to rituximab therapy. The resistance caused by gene mutation thought to be irreversible. And it should be discriminated from transient downregulation of antigen expression. We propose here that immunohistochemical screening using CD20N antibody is very rapid and effective screening stategy that find out irreversible rituximab resistant cases.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.