Prognostic Significance of Molecular-Cytogenetic Abnormalities in Pediatric T-ALL Is Not Explained by Immunophenotypic Differences.

Pediatric T-cell acute lymphoblastic leukemia (T-ALL) is characterized by recurrent chromosomal rearrangements that may have prognostic significance. T-ALL cases with specific aberrations may arrest at specific T-cell development stages. Two classification systems were developed to classify T-ALL into developmental subgroups, i.e. the European Group for the Immunological Characterization of Leukemias (EGIL) and the T-cell receptor (TCR) classification system. In this study, we investigated the relationship between molecular-cytogenetic abnormalities and T-cell development stage. We investigated whether arrest at specific T-cell stages explains the prognostic significance of molecular-cytogenetic abnormalities. To this aim, we extensively studied 72 pediatric T-ALL cases by FISH and RQ-PCR for the presence of genetic abnormalities and expression of transcription factors, by PCR and sequencing for NOTCH1 mutations and by flow-cytometry to determine the T-cell receptor status as well as CD marker expression. The median clinical follow-up was 5 years. HOX11 rearranged cases were CD1 positive consistent with a cortical stage, but as 4 out of 5 cases lacked cytoplasmatic-beta expression, developmental arrest may precede beta-selection. HOX11L2 was especially confined to immature and pre-AB developmental stages, but 3 out of 17 HOX11L2 mature cases were restricted to the γδ-lineage. TAL1 rearrangements were restricted to the αβ-lineage with most cases being TCRαβ positive. NOTCH1 mutations were present in all molecular-cytogenetic subgroups without restriction to a specific developmental stage. The CALM-AF10 translocation as detected in 3 T-ALL patients was significantly associated with early relapse. TAL1 or HOX11L2 rearrangements were associated with trends to good and poor outcome, respectively. Cases with high TAL1 expression levels also demonstrated a trend towards good outcome, whereas cases with the lowest TAL1 levels had a poor outcome and were mostly HOX11L2 or CALM-AF10 positive. NOTCH1 mutations did not predict for poor outcome. Classification into T-cell developmental subgroups did not predict for outcome. In conclusion, the present study shows that differences in outcome for various molecular-cytogenetic subgroups cannot be attributed to differences in T-cell maturation stage.