MOLECULAR ANALYSIS OF MLL-1 AF4 RECOMBINATION IN INFANT ACUTE LYMPHOBLASTIC-LEUKEMIA/

We examined ten cases of acute lymphoblastic leukemia (ALL) in infants (less than 1 year of age) by RT-nested PCR for a MLL-1/AF4 rearrangem ent. Five patients revealed a positive result. The specific PCR produc t differed in size from approximately 380-670 bp indicating various sp licing variants in the MLL-1/AF4 rearrangement. Three patients had a f usion between exon 6 of the MLL-1 gene and codon 362 of the known AF4 cDNA sequence. Moreover, in two patients more than one specific PCR pr oduct was detected, possibly due to alternative splicing. In the first case, sequencing of these products revealed a hybrid mRNA consisting of MLI-1 exon 7 or exon 8, respectively, fused to the AF4 gene at codo n 348. In the second case with alternative splicing, again, exon 7 or 8 of the MLL-1 gene were fused to the AF4 gene as in case 1. The AF4 s equence involved in this patient, however, started at codon 362. The A F4 break was, therefore, identical to the three MLL-1/AF4 positive pat ients as described above. Moreover, we investigated all ten patients f or the reciprocal mRNA transcript AF4/MLL-1 by a similar PCR approach. In none of these patients, including the five MLL-1/AF4 positive case s was a specific PCR product obtained. However, in the MV411 cell line bearing a t(4;11), which served as a positive control in our MLL-1/AF 4-PCR assay, the reciprocal AF4/MLL-1 mRNA was detected. Our results i ndicate that a MLL-1/AF4 rearrangement occurs in about 50% of infants with ALL. In contrast, the reciprocal hybrid mRNA can only rarely be d etected, if at all.