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 recombination has been demonstrated for both artificial substrates and endogenous genes, it is not known whether all hypomethylated targets are competent to rearrange or if other factors are required. We have created several artificial V(D)J recombination substrate transgenes whose methylation can be controlled by breeding into different genetic backgrounds. A transgene which contains the immunoglobulin heavy chain intronic enhancer rearranges efficiently in B lymphocytes when the transgene loci are unmethylated. When the same loci become methylated, upon breeding into a different mouse strain, no rearrangement can be detected. A similar transgene, but lacking the enhancer, also shows no evidence of V(D)J recombination when it is methylated. Even when this enhancerless transgene is hypomethylated, however, no V(D)J recombination can be detected in B lymphocytes. Thus, hypomethylation is required to permit V(D)J recombination but not all hypomethylated targets are capable of recombination. The results may indicate that the immunoglobulin enhancer is required for the assembly of factors involved in V(D)J recombination.