Analysis of the V(D)J recombination efficiency at lymphoid chromosomal translocation breakpoints

Chromosomal translocations and deletions are among the major events that in itiate neoplasia. For lymphoid chromosomal translocations, misrecognition b y the RAG (recombination activating gene) complex of V(D)J recombination is one contributing factor that has long been proposed. The chromosomal trans locations involving LMO2 (t(11;14)(p13;q11)), Ttg-1 (t(11;14)(p15;q11)), an d Hox11 (t(10;14)(q24;q11)) are among the clearest examples in which it app ears that a D or J segment has synapsed with an adventitious heptamer/nonam er at a gene outside of one of the antigen receptor loci. The interstitial deletion at 1p32 involving SIL (SCL-interrupting locus)/SCL (stem cell leuk emia) is a case involving two non-V(D)J sites that have been suggested to b e V(D)J recombination mistakes. Here we have used our human extrachromosoma l substrate assay to formally test the hypothesis that these regions are V( D)J recombination misrecognition sites and, more importantly, to quantify t heir efficiency as V(D)J recombination targets within the cell. We find tha t the LMO2 fragile site functions as a 12-signal at an efficiency that is o nly 27-fold lower than that of a consensus 12-signal. The Ttg-1 site functi ons as a 23-signal at an efficiency 530-fold lower than that of a consensus 23-signal. Hox11 failed to undergo recombination as a 12. or 23-signal and was at least 20,000-fold less efficient than consensus signals. SIL has be en predicted to function as a 12-signal and SCL as a 23-signal. However, we find that SH, actually functions as a 23-signal. These results provide a f ormal demonstration that certain chromosomal fragile sites can serve as RAG complex targets, and they determine whether these sites function as 12- ve rsus 23-signals. These results quantify one of the three major factors that determine the frequency of these translocations in T-cell acute lymphocyti c leukemia.