T-cell ALL is defined by expression of the T cell–associated antigens (cytoplasmic CD3, with CD7 plus CD2 or CD5) on leukemic blasts. T-cell ALL is frequently associated with a constellation of clinical features, including the following:[17,28,61]
- Male gender.
- Older age.
- Mediastinal mass.
With appropriately intensive therapy, children with T-cell ALL have an outcome approaching that of children with B-lineage ALL.[17,28,31,32,61]
There are few commonly accepted prognostic factors for patients with T-cell ALL. Conflicting data exist regarding the prognostic significance of presenting leukocyte counts in T-cell ALL.[6,28,29,30,31,32,33,34] The presence or absence of a mediastinal mass at diagnosis has no prognostic significance. In patients with a mediastinal mass, the rate of regression of the mass lacks prognostic significance.
Cytogenetic abnormalities common in B-lineage ALL (e.g., hyperdiploidy) are rare in T-cell ALL.[76,77]
Multiple chromosomal translocations have been identified in T-cell ALL, with many genes encoding for transcription factors (e.g., TAL1, LMO1 and LMO2, LYL1, TLX1/HOX11, and TLX3/HOX11L2) fusing to one of the T-cell receptor loci and resulting in aberrant expression of these transcription factors in leukemia cells.[76,78,79,80,81,82] These translocations are often not apparent by examining a standard karyotype, but are identified using more sensitive screening techniques, such as fluorescence in situ hybridization (FISH) or polymerase chain reaction. High expression of TLX1/HOX11 resulting from translocations involving this gene occurs in 5% to 10% of pediatric T-cell ALL cases and is associated with more favorable outcome in both adults and children with T-cell ALL.[78,79,80,82] Overexpression of TLX3/HOX11L2 resulting from the cryptic t(5;14)(q35;q32) translocation occurs in approximately 20% of pediatric T-cell ALL cases and appears to be associated with increased risk of treatment failure, although not in all studies.
Notch pathway signaling is commonly activated by NOTCH1 and FBXW7 gene mutations in T-cell ALL.NOTCH1-activating gene mutations occur in approximately 50% of T-cell ALL cases, and FBXW7 inactivating gene mutations occur in approximately 15% of cases, with the result that approximately 60% of cases having Notch pathway activation by mutations in at least one of these genes. The prognostic significance of Notch pathway activation by NOTCH1 and FBXW7 mutations in pediatric T-cell ALL is not clear, as some studies have shown a favorable prognosis for mutated cases,[84,85,86] while other studies have not shown prognostic significance for the presence of NOTCH1 and/or FBXW7 mutations.[87,88,89]
A NUP214–ABL1 fusion has been noted in 4% to 6% of T-cell ALL cases and is observed in both adults and children with a male predominance.[90,91,92] The fusion is cytogenetically cryptic and is seen in FISH on amplified episomes or more rarely, as a small homogeneous staining region. T-cell ALL may also uncommonly show ABL1 fusion proteins with other gene partners (e.g., ETV6, BCR, and EML1).ABL tyrosine kinase inhibitors, such as imatinib or dasatinib, may have therapeutic benefit in this T-cell ALL subtype,[90,91,93] although clinical experience with this strategy is very limited.[94,95,96]
Early T-cell precursor ALL
Early T-cell precursor ALL, a distinct subset of childhood T-cell ALL, was initially defined by identifying T-cell ALL cases with gene expression profiles highly related to expression profiles for normal early T-cell precursors. The subset of T-cell ALL cases, identified by these analyses represented 13% of all cases and they were characterized by a distinctive immunophenotype (CD1a and CD8 negativity, with weak expression of CD5 and coexpression of stem cell or myeloid markers). Detailed molecular characterization of early T-cell precursor ALL showed this entity to be highly heterogeneous at the molecular level, with no single gene affected by mutation or copy number alteration in more than one-third of cases. Compared with other T-ALL cases, the early T-cell precursor group had a lower rate of NOTCH1 mutations and significantly higher frequencies of alterations in genes regulating cytokine receptors and RAS signaling, hematopoietic development, and histone modification. The transcriptional profile of early T-cell precursor ALL shows similarities to that of normal hematopoietic stem cells and myeloid leukemia stem cells. Retrospective analyses have suggested that this subset has a poorer prognosis than other cases of T-cell ALL.[35,98,99] However, further study in larger patient cohorts is needed.
Studies have found that the absence of biallelic deletion of the TCRgamma locus (ABGD), as detected by comparative genomic hybridization and/or quantitative DNA–polymerase chain reaction, was associated with early treatment failure in patients with T-cell ALL.[100,101] ABGD is characteristic of early thymic precursor cells, and many of the T-cell ALL patients with ABGD have an immunophenotype consistent with the diagnosis of early T-cell precursor phenotype.