B-PRECURSOR ACUTE LYMPHOBLASTIC-LEUKEMIA 3RD COMPLEMENTARITY-DETERMINING REGIONS PREDOMINANTLY REPRESENT AN UNBIASED RECOMBINATION REPERTOIRE - LEUKEMIC TRANSFORMATION FREQUENTLY OCCURS IN FETAL LIFE

To determine whether the ALL (acute lymphoblastic leukemia) CDR3 (thir d complementarity-determining region) repertoire represents the recomb ination repertoire, or shows evidence of selectional processes inheren t to normal B cell differentiation or malignant transformation, we ana lyzed 68 ALL CDR3 regions and included 127 previously published sequen ces in the analyses. We found no evidence of selection prior to malign ant transformation as recombination was random with 1/3 ''in frame'' a nd 2/3 ''out of frame'' joinings and usage of all three D reading fram es was observed. D and J(H) gene segments were predominantly umutated which allowed a detailed analysis of gene usage and rearrangement char acteristics. J(H)4 and J(H)6 usage (both 32.2%) was significantly diff erent (p = 0.005) from that observed in peripheral B lymphocytes. D ge ne family usage roughly represented D gene family size with the except ion of the D-XP and D-A/K family which were over- and underrepresented (p = less than or equal to 0.05), respectively. D-D fusions were foun d in 26.2% of CDR3 regions. If less stringent criteria were applied DI R homology was found in 40/65 sequences, suggesting the frequent invol vement of DIR gene segments in human CDR3 formation. The rearranged D genes were evenly distributed over the D locus, suggesting that D reco mbination is a predominantly random process, independent of physical l ocation at the locus. Also, there was no correlation between J(H) gene usage and physical location of the rearranged D gene segment, which e xcludes a major contribution of the DJ(H) replacement recombination me chanism. In 36.1% of CDR3 regions N-nucleotides at the DJ(H) junction were absent. This frequency is higher than observed for peripheral B l ymphocytes. It is suggested that for a number of ALL the initial trans formational event took place in early fetal life. We conclude that ALL CDR3 sequences show no evidence of selection prior to malignant trans formation, nor of extensive changes subsequent to malignant transforma tion.