Pre-pre-B acute lymphoblastic leukemia: high frequency of alternatively spliced ALL1-AF4 transcripts and absence of minimal residual disease during complete remission

Abstract

We used the polymerase chain reaction (PCR) to detect ALL1-AF4 rearrangements, the molecular hallmark of t(4;11) in a series of 46 pre- pre-B (CD19+, CD24+, CD10/CD20/cylgM/sIgM-) acute lymphoblastic leukemias (ALL). Eighteen patients (39%) exhibited fusion transcripts including 4 of 12 children and 14 of 34 adults. This genetic defect was associated with hyperleukocytosis (median leukocyte count 176 x 10(9)/L) and expression of myeloid-associated antigens (CDw65+). In contrast, only two patients from a group of 67 common (CD19/CD10+, cylgM/sIgM-) and pre-B ALLs (CD19/cylgM+, CD10 +/-, sIgM-) showed ALL1- AF4 mRNA. All PCR-positive cases showed multiple amplification products representing alternative splicing events. Moreover, reciprocal der (4)- derived AF4-ALL1 transcripts were observed in 65% of the cases analyzed. Eight of the 18 pre-pre-B ALL patients with an ALL1-AF4 recombination are currently in complete continuous remission for up to 54 months (median, 26 months). Twelve remission samples were available from seven cases, and all of them lacked evidence of minimal residual disease. Overall this study documents a similarly high incidence of ALL1-AF4 recombinations in children (infants excluded) and adults with pre-pre-B ALL and demonstrates the decline of the leukemic cell clone below the detection level of PCR in a remarkable proportion of patients under intense treatment protocols.

Clinical relevance of residual disease monitoring by polymerase chain reaction in patients with ALL‐1/AF‐4 positive‐acute lymphoblastic leukaemia

“…In fact, all five patients in CCR were found persistently RT-PCR negative. These data confirm the earliest evidence of the disappearance of the ALL-1/AF-4 chimaeric transcript in t(4;11) ALL long-term survivors Janssen et al, 1994). Although usually regarded as an aggressive disease with an overall poor prognosis, these observations suggest that ALL-1/ AF-4-positive ALL is, at least in some instances, a potentially curable leukaemia.…”

“…Using this approach, the ALL-1/AF-4 hybrid was amplified in diagnostic samples of all patients carrying a cytogenetically detectable t(4;11), whereas patients tested in long-term complete remission (CR) after treatment were found PCR negative . This latter observation has been confirmed recently by Janssen et al (1994) in a larger series of t(4;11) ALLs. In both studies, however, t(4;11) ALL patients were tested only once during remission, and no longitudinal monitoring of MRD has so far been reported.…”

“…Two cases of our series (nos 2 and 10, Table I) had more than one mRNA species at diagnosis. ALL-1 breakpoints in intron 6 were also frequently found in an adult ALL cohort by Janssen et al (1994) (10/16 cases studied), whereas the same investigators reported a higher frequency of alternatively spliced mRNA in these patients. This latter discrepancy might be due to different patient selection and the small number of cases in our series.…”

Exon/intron structure of the human AF‐4 gene, a member of the AF‐4/LAF‐4/FMR‐2 gene family coding for a nuclear protein with structural alterations in acute leukaemia

“…8 kb transcript was named FelC. The majority of t(4;11) translocation breakpoints mapped to date (Downing et al, 1994;Hilden et al, 1993;Corral et al, 1993;Janssen et al, 1994) interrupt the AF-4 cDNA sequence (FelB) after nucleotide 1038. The 2 .…”

“…Chromosomal breakpoints have been mapped in introns 3, 4 and rarely in intron 5 of the AF-4 gene Chen et al, 1993;Corral et al, 1993;Janssen et al, 1994). However, all primer sets used in published studies bind to the 3 0 part of exon 3 and to exons 6, 7 or 8.…”