Over-expression of activation-induced cytidine deaminase in TCL1 mice leads to the development of IGHV-mutated and -unmutated CLL clones that resemble unique subsets of human CLL

YAN XIAO-YEN; MORANDE PABLO; EZRA B KOLITZ; CRYSTAL D GRANT; GAUTAM NAYYAR; OPPEZZO PABLO; CHIORAZZI NICHOLAS

Sydney

Workshop; XVI International Workshop on Chronic Lymphocytic Leukaemia; 2015

Somatic hypermutation (SHM) and class-switch recombination (CSR) are critical physiologicevents in an effective normal B-cell immune response, and both are initiated by activationinducedcytidine deaminase (AID). In CLL, IGHV clonal mutations correlate strongly withbetter clinical outcomes. E-T-cell leukemia-1 (TCL1) transgenic (Tg) mice are a valuablemodel of CLL. However because SHM and CSR occur rarely in these animals, they mimiconly IGHV-unmutated CLL and do not provide an understanding of the roles of SHM andCSR in disease evolution. To address these issues, we developed two new TCL1 strains byinterbreeding mice over-expressing AID in all cells (Eμ-TCL1xActin-AID) or only in Blymphocytes (Eμ-TCL1xVκ-AID). B-cell clonal expansions were identified in spleen cellsfrom 22 TCL1 and 33 TCL1xAID Tg (10 Eμ-TCL1xActin-AID and 23 Eμ-TCL1xVκ-AID)mice at 10-20 months of age by amplifying cDNAs by PCR using consensus FR and IgM,IgG, and IgA primers. DNA sequences were compared to murine germline IGHVs andIGHV-D-J rearrangements by IMGT V-Quest. Because there were no major differences inthe parameters listed below for the two TCL1xAID Tg mouse strains, data were combined.Monoclonal/oligoclonal expansions were detected in all TCL1 mice; these used only Hchains. Similar expansions were detected in 26 of 33 TCL1xAID mice; each animal bore anIgM+ clone and 7 also an IgG+ clone. IGHV use did not differ significantly between IgM+TCL1 and IgM+ TCL1xAID clones. Approximately 50% used VH1-55, VH11-2, or VH12-3,some of which encoded stereotyped anti-phosphatidylcholine antibodies. IgM+ TCL1 clonesexhibited a mutation frequency of 0.05%, which was considerably less than that inTCL1xAID mice (0.47% for IgM+ and 3.0% for IgG+ clones). Mutations localized 10 timesmore frequently in AID hotspots than coldspots. However, SHM did not affect all clonesequally. Mutation frequency in VH12-3 and VH11-2 clones was only 0.38% (range: 0-1.9%),and no mutations were detected in VH1-55 clones. None of these genes were found in IgGexpressingclones. Notably, in only 2 of 9 instances was the same IGHV-D-J rearrangementfound in IgM+ and IgG+ clones; these used VH5 genes. For the remaining 7, only the IgG+version was detected; all but one of these used a VH1 gene. Also, within the IgG-only group,IGHV1-47 was used by 2 different clones that were highly mutated (8.9%). Thus, overexpressionof AID in TCL1 mice led to markedly increased SHM and CSR. However, SHMwas not equivalent for all IGHVs since despite AID over-expression, certain genes appearedresistant to major increases in SHM and CSR. This property resembles some human CLLIGHVs that rarely develop SHMs or undergo CSR despite the B-cell's ability to synthesizeAID (e.g., IGHV1-69). AID overexpression also led to IgG+ clones for which an IgMprecursor was not found. This resembles those human stereotyped CLL clones that are onlyfound as IgGs (e.g., stereotyped subsets 4 and 8). Finally, the two new TCL1xAID mousestrains described provide new models to study IGHV-mutated and IGHV-unmutated CLLand represent novel tools to evaluate the role of AID in leukemic progression.