Hypomethylation is necessary but not sufficient for V(D)J recombination within a transgenic substrate

Although an inverse correlation between CpG methylation and V(D)J recombina tion has been demonstrated for both artificial substrates and endogenous ge nes, it is not known whether all hypomethylated targets are competent to re arrange or if other factors are required. We have created several artificia l V(D)J recombination substrate transgenes whose methylation can be control led by breeding into different genetic backgrounds. A transgene which conta ins the immunoglobulin heavy chain intronic enhancer rearranges efficiently in B lymphocytes when the transgene loci are unmethylated. When the same l oci become methylated, upon breeding into a different mouse strain, no rear rangement can be detected. A similar transgene, but lacking the enhancer, a lso shows no evidence of V(D)J recombination when it is methylated. Even wh en this enhancerless transgene is hypomethylated, however, no V(D)J recombi nation can be detected in B lymphocytes. Thus, hypomethylation is required to permit V(D)J recombination but not all hypomethylated targets are capabl e of recombination. The results may indicate that the immunoglobulin enhanc er is required for the assembly of factors involved in V(D)J recombination. (C) 2000 Elsevier Science Ltd. All rights reserved.