Loss of NFAT2 expression results in the acceleration of clonal evolution in chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) can be defined as a clonal expansion of B cells with stereotypic BCRs. Somatic hypermutation of the BCR heavy chains (IGVH) defines a subgroup of patients with a better prognosis. In up to 10% of CLL cases, a transformation to an aggressive B cell lymphoma (Richter's syndrome) with a dismal prognosis can be observed over time. NFAT proteins are transcription factors originally identified in T cells, which also play an important role in B cells. The TCL1 transgenic mouse is a well-accepted model of CLL. Upon B cell-specific deletion of NFAT2, TCL1 transgenic mice develop a disease resembling human Richter's syndrome. Whereas TCL1 B cells exhibit tonic anergic BCR signaling characteristic of human CLL, loss of NFAT2 expression leads to readily activated BCRs indicating different BCR usage with altered downstream signaling. Here, we analyzed BCR usage in wild-type and TCL1 transgenic mice with and without NFAT2 deletion employing conventional molecular biology techniques and next-generation sequencing (NGS). We demonstrate that the loss of NFAT2 in CLL precipitates the selection of unmutated BCRs and the preferential usage of certain VDJ recombinations, which subsequently results in the accelerated development of oligoclonal disease.