High Clonal Evolution Rate in Relapsing Infant ALL Patients.

Abstract Immunoglobulin (Ig) and T-cell Receptor (TcR) gene rearrangements are used as patient-specific PCR targets for MRD detection in ALL. However, oligoclonality was reported in childhood ALL, and in those cases, the uncertainty of which clone is going to emerge at relapse may give rise to false negative MRD results. In particular, oligoclonality is a peculiar characteristic of Infant ALL (less than 1 year of age at diagnosis). So far, the prognostic relevance of MRD monitoring in Infant ALL has not been defined, and the assessment of oligoclonality and stability of PCR markers may have a profound impact in MRD monitoring. We successfully studied by PCR the frequency and stability of the currently used rearrangements of the Ig Heavy (IgH), Ig Kappa deleting element (IgK-Kde), TcR delta (TcRD), and TcR gamma (TcRG) gene rearrangements in 42/43 Infant ALL patients prospectively enrolled in Italy in the Interfant-99 International ALL protocol. Pro-B, common and pre-B ALL were 29 (67%), 8 (19%), and 6 (14%), respectively. Sixty-three percent of cases were prednisone good responders (PGR), 70% were MLL-rearranged, 44% were aged less than 6 months, and 21% of cases had more than 300.000 wbc at diagnosis. Overall, rearrangements of the IgH, IgK, TcRD, and TcRG genes were found in 91, 21, 40, and 21% of Infant ALL, respectively. The pattern of Ig/TcR rearrangements in immunophenotypic subgroups of Infant ALLs has been compared to the one of older children prospectively enrolled in Italy into the AIEOP-BFM ALL2000 protocol (n=649). While the overall frequency of IgH and TcRD are similar in the two age groups, IgK and TcRG are less rearranged in Infant cases, mainly due to the low percentage in the pro-B group (10% versus 41% for IgK VK-Kde, and 17% vs 36% for TcRG). In particular, none of the 29 pro-B cases showed rearrangement of the IgK intron. Sixteen Infant relapsed so far (37%), and 12 of them were successfully analyzed for Ig/TcR rearrangement at diagnosis and relapse. Ten were early (less than 18 months from diagnosis), and 2 were late relapses. None of the patients showed an identical Ig/TcR pattern at diagnosis and relapse; 7 (58%) had at least one, and 1 had at least 2 PCR targets preserved, while 4 cases (33%) presented completely different markers at relapse. This distribution was independent on age at diagnosis, site and time of relapse. Interestingly, we observed an increase of complete-IgH (from 10 alleles in 7 patients to 23 alleles in 11) and TcRG (from 4 alleles in 2 patients to14 alleles in 8) rearrangements at relapse. The stability of Ig/TcR markers was as it follows: IgH, 6/26; IgK, 3/5; TcRD, 1/9 and TcRG, 0/4. In conclusion, the oligoclonality feature of Infant ALL may potentially hamper the MRD predictivity in Infant ALL. An appropriate identification and selection of Ig/TcR MRD-PCR targets in Infant ALL is a crucial premise for obtaining clinically relevant MRD data and for preventing false-negative MRD results. Ig Kappa may be a preferential marker for MRD studies in Infant ALL, although its use is limited by the low frequency of the rearrangement. The leukemia-specific MLL genomic breakpoint may potentially overcome these limitations and improve the MRD analysis.