Episomal vectors to monitor and induce somatic hypermutation in human Burkitt-Lymphoma cell lines

Somatic hypermutation (SHM) occurs at a specific B-cell differentiation stage, during the germinal centre reaction, and provides a means to diversify and shape the antibody repertoire of the adaptive immune system. Burkitt-Lymphoma (BL) is a germinal centre derived B-cell malignancy. Presumably deregulation of the somatic hypermutation- and/or class switch recombination process causes a translocation between the myc-locus and one of the Ig-loci, which is characteristic for BL. We show here that we developed an episomal-based vector system to monitor and induce AID-dependent somatic hypermutation in human cell lines. Exemplarily, we utilized this system to investigate the hypermutation status of various BL cell lines. Analysis of the influence of Ig-specific cis-regulatory elements for the mutability of a green fluorescent protein (GFP) reporter transgene revealed, that the presence of Ig-enhancers is required for an efficient targeting of a constitutively transcribed GFP transgene. In contrast, a transcriptional inducible GFP transgene was hypermutated by overexpression of activation-induced cytidine deaminase (AID), even in the absence of Ig-specific sequences. This observation verifies that overexpression of AID in non hypermutating BL cell lines as well as the expression of endogenous AID in the hypermutating BL cell line Raji can overcome the target restriction of AID.