Role of the 3’ heavy chain enhancers in germline transcription and switch recombination (88.1)

Abstract

Abstract Class switch recombination (CSR) is preceded by germline transcription and then mediated by a DNA recombination event. To study germline transcription and CSR we utilize transgenic mice with a 230 kb bacterial artificial chromosome that includes a rearranged VDJ gene, the entire heavy chain C region locus, and all known 3’ enhancers. In the intact transgene, germline transcription of and CSR to the γ, ϵ, and α heavy chain genes are regulated in parallel to the endogenous genes. We have reported that deletion of a 28 kb segment, including the 3’ enhancers HS3A, HS1,2, HS3B, and HS4, results in a reduction of CSR to all heavy chain genes to about 1% of the level of the intact transgene. We have derived four new versions of the heavy chain transgene: with 1 kb deletions of (i) HS1,2 and HS3B, (ii) HS1,2, HS3B, and HS4, (iii) HS3A, HS1,2, and HS3B, and (iv) HS3A, HS1,2, HS3B, and HS4. Comparing levels of CSR in these deletion variants to intact transgenes, we find that deletion of more enhancers results in a reduction in CSR for more heavy chain genes, and the reduction is more severe for a given heavy chain gene. Nevertheless, different enhancers have different effects on different heavy chain genes. Finally, there are enhancer deletions for which the reduction in CSR is much greater than the reduction in germline transcription. This suggests that the 3’ enhancers have a role in CSR that is distinct from the enhancement of germline transcription.