DECREASED FREQUENCY OF REARRANGEMENT DUE TO THE SYNERGISTIC EFFECT OFNUCLEOTIDE CHANGES IN THE HEPTAMER AND NONAMER OF THE RECOMBINATION SIGNAL SEQUENCE OF THE V-KAPPA GENE A2B, WHICH IS ASSOCIATED WITH INCREASED SUSCEPTIBILITY OF NAVAJOS TO HAEMOPHILUS-INFLUENZAE TYPE-B DISEASE

Navajos and genetically related populations have a 10-fold increased i ncidence of Haemophilus influenzae type b (Hib) disease compared with control populations. The V kappa gene A2 is used to encode the majorit y of anti-Hib Abs, and these are the highest affinity anti-Hib Abs, Na vajos carry a different allele of the A2 gene segment (A2b) that is de fective in its ability to undergo V-S recombination, The A2b allele ha s only three nucleotide changes from the commonly occurring A2a allele , two of which could potentially affect its ability to recombine, In t his study we used two independent in vitro assays to test whether the nucleotide change found in the A2b promoter and/or in the A2b recombin ation signal sequence (RSS) might be responsible for thr! decrease in recombination frequency observed in vivo, Using a luciferase reporter gene assay, we found no significant difference between A2a and A2b pro moter activities. However, the competition recombination substrate ass ay showed a 4.5-fold reduction in the relative frequency of recombinat ion of the A2b RSS compared with A2a, We show that this decreased freq uency is due to a synergistic effect of the unique nucleotide change p resent in the heptamer of the A2b RSS and the shared nucleotide change present in the nonamer of both A2b and A2a, This in vitro relative fr equency of rearrangement is not significantly different from that obse rved in vivo; therefore, the A2b RSS is probably the factor associated with the increased susceptibility to Hib disease among individuals ca rrying the A2b allele.