Interphase FISH Analysis Reveals High Frequence of Partial 6q Deletions in Adult Acute Lymphoblastic Leukemia (ALL) with Normal or Failed Karyotype.

Karyotypic analysis in adult ALL has provided relevant insights in the prognostic significance of chromosome abnormalities, which in turn become of potential importance for the design of risk-adapted therapeutic strategies for newly diagnosed cases. Among structural karyotypic changes, partial deletions of the long arm of chromosome 6 (6q) are reported to be between 4 and 6% of cases of adult ALL by conventional cytogenetics and depict a subset of patients with strict correlation with T-lineage ALL and an unfavorable prognostic likelihood. Recent studies at the molecular level in childhood ALL have shown 6q deletions in up to 30% of the investigated cases. With the aim of more precisely defining the exact incidence of 6q deletions in adult ALL, we started to investigate by interphase FISH patients with normal or failed karyotype included in the GIMEMA 0496 and 2000 protocols. These trials contemplated a central handling of bone marrow samples of the enrolled patients at presentation. We identified the following bacteria-derived artificial chromosome (BAC) clones with a known map location within the most common deleted region of 6q: 90G9 (6q13), 12B13 (6q15), 465M14 (6q21), 81H16 (6q26), 91016 (6q27). BAC clones were obtained from the Children’s Hospital Oakland Research Institute (CHORI).

Twenty-seven consecutive ALL patients have so far been analyzed by interphase FISH, 16 with T- and 11 with B-cell phenotype. Eleven cases had a normal karyotype and 16 a failed cytogenetic analysis. Among the cases with normal karyotype, 2 cases with T-cell phenotype showed a deletion in the 6q21 and 6q15 regions, whereas in the group with failed cytogenetics 1 case with B- and 3 cases with T-cell phenotype harbored deletions spanning respectively the 6q13-15 (2), 6q13-q21 and 6q15-21 regions. The median percentage of nuclei with 6q deletion was 30% (range 7–75%). T-ALL deleted cases showed the absence of the most frequent fusion genes detectable with molecular analysis, whereas a BCR-ABL rearrangement was identified in the only B-lineage ALL deleted case. Overall, partial 6q deletions were observed in 22% of the cases analyzed and in 31% of cases with T-cell phenotype. The present study shows that the probe set we have utilized is a useful tool for providing accurate cytogenetic diagnosis of partial 6q deletions and indicates that this abnormality is a frequent karyotypic change in adult ALL, particularly in T-lineage cases where it appears to represent the most relevant genetic anomaly. Since this abnormality is underestimated by conventional cytogenetic analysis, interphase FISH for 6q deletions should be included in the routine genetic screening of the ALL patients with failed or normal cytogenetics