T-ALL cytogenetics/genomics

Notch pathway signaling. Notch pathway signaling is commonly activated by NOTCH1 and FBXW7 gene variants in T-ALL, and these are the most commonly altered genes in pediatric T-ALL.[162,167] NOTCH1-activating gene variants occur in approximately 50% to 60% of T-ALL cases, and FBXW7-inactivating gene variants occur in approximately 15% of cases. Approximately 60% of T-ALL cases have Notch pathway activation by variants in at least one of these genes.[168,169] The prognostic significance of NOTCH1 and FBXW7 variants may be modulated by genomic alterations in RAS and PTEN. The French Acute Lymphoblastic Leukaemia Study Group (FRALLE) and the Group for Research on Adult Acute Lymphoblastic Leukemia reported that patients having altered NOTCH1 or FBXW7 and wild-type PTEN and RAS constituted a favorable-risk group (i.e., low-risk group), while patients with PTEN or RAS variants, regardless of NOTCH1 and FBXW7 status, have a significantly higher risk of treatment failure (i.e., high-risk group).[170,171] In the FRALLE study, the 5-year disease-free survival rate was 88% for the genetic low-risk group of patients and 60% for the genetic high-risk group of patients.[170] However, using the same criteria to define the genetic risk group, the Dana-Farber Cancer Institute consortium was unable to replicate these results. They reported a 5-year EFS rate of 86% for genetic low-risk patients and 79% for the genetic high-risk patients, a difference that was not statistically significant (P = .26).[169]

Chromosomal translocations. Multiple chromosomal translocations have been identified in T-ALL that lead to deregulated expression of the target genes. These chromosome rearrangements fuse genes encoding transcription factors (e.g., TAL1, TAL2, LMO1, LMO2, LYL1, TLX1, TLX3, NKX2-I, HOXA, and MYB) to one of the T-cell receptor loci (or to other genes) and result in deregulated expression of these transcription factors in leukemia cells.[162,163,172-176] These translocations are often not apparent by examining a standard karyotype, but can be identified using more sensitive screening techniques, including FISH or PCR.[163] Variants in a noncoding region near the TAL1 gene that produce a super-enhancer upstream of TAL1 represent nontranslocation genomic alterations that can also activate TAL1 transcription to induce T-ALL.[165] Translocations resulting in chimeric fusion proteins are also observed in T-ALL.[170]

Chromosomal translocations. Multiple chromosomal translocations have been identified in T-ALL that lead to deregulated expression of the target genes. These chromosome rearrangements fuse genes encoding transcription factors (e.g., TAL1, TAL2, LMO1, LMO2, LYL1, TLX1, TLX3, NKX2-I, HOXA, and MYB) to one of the T-cell receptor loci (or to other genes) and result in deregulated expression of these transcription factors in leukemia cells.[162,163,172-176] These translocations are often not apparent by examining a standard karyotype, but can be identified using more sensitive screening techniques, including FISH or PCR.[163] Variants in a noncoding region near the TAL1 gene that produce a super-enhancer upstream of TAL1 represent nontranslocation genomic alterations that can also activate TAL1 transcription to induce T-ALL.[165] Translocations resulting in chimeric fusion proteins are also observed in T-ALL.[170]

A NUP214::ABL1 fusion has been noted in 4% to 6% of T-ALL cases and is observed in both adults and children, with a male predominance.[177-179] The fusion is cytogenetically cryptic and is seen in FISH on amplified episomes or, more rarely, as a small homogeneous staining region.[179] T-ALL may also uncommonly show ABL1 fusion proteins with other gene partners (e.g., ETV6, BCR, and EML1).[179] ABL tyrosine kinase inhibitors, such as imatinib or dasatinib, may demonstrate therapeutic benefits in this T-ALL subtype,[177,178,180] although clinical experience with this strategy is very limited.[181-183]

Chromosomal translocations. Multiple chromosomal translocations have been identified in T-ALL that lead to deregulated expression of the target genes. These chromosome rearrangements fuse genes encoding transcription factors (e.g., TAL1, TAL2, LMO1, LMO2, LYL1, TLX1, TLX3, NKX2-I, HOXA, and MYB) to one of the T-cell receptor loci (or to other genes) and result in deregulated expression of these transcription factors in leukemia cells.[162,163,172-176] These translocations are often not apparent by examining a standard karyotype, but can be identified using more sensitive screening techniques, including FISH or PCR.[163] Variants in a noncoding region near the TAL1 gene that produce a super-enhancer upstream of TAL1 represent nontranslocation genomic alterations that can also activate TAL1 transcription to induce T-ALL.[165] Translocations resulting in chimeric fusion proteins are also observed in T-ALL.[170]

Gene fusions involving SPI1 (encoding the transcription factor PU.1) were reported in 4% of Japanese children with T-ALL.[184] Fusion partners included STMN1 and TCF7. T-ALL cases with SPI1 fusions had a particularly poor prognosis; six of seven affected individuals died within 3 years of diagnosis of early relapse.

Chromosomal translocations. Multiple chromosomal translocations have been identified in T-ALL that lead to deregulated expression of the target genes. These chromosome rearrangements fuse genes encoding transcription factors (e.g., TAL1, TAL2, LMO1, LMO2, LYL1, TLX1, TLX3, NKX2-I, HOXA, and MYB) to one of the T-cell receptor loci (or to other genes) and result in deregulated expression of these transcription factors in leukemia cells.[162,163,172-176] These translocations are often not apparent by examining a standard karyotype, but can be identified using more sensitive screening techniques, including FISH or PCR.[163] Variants in a noncoding region near the TAL1 gene that produce a super-enhancer upstream of TAL1 represent nontranslocation genomic alterations that can also activate TAL1 transcription to induce T-ALL.[165] Translocations resulting in chimeric fusion proteins are also observed in T-ALL.[170]

BCL11B is a zinc finger transcription factor that plays a dual role as a transcription activator and repressor. It is known to play a critical role in T-cell differentiation. In T-ALL, the BCL11B gene is involved in a t(5;14)(q35;q32) translocation where a distal BCL11B enhancer drives aberrant expression of TLX3 (or NKX2-5).[185] In the process of donating its enhancer, one allele of BCL11B is inactivated. However, the resulting haploinsufficient state itself may also play a role in tumor pathogenesis. The role of BCL11B as a tumor suppressor gene is supported by the finding that about 16% of patients have T-ALL that harbors deletions or missense variants.[162,186] As described in the sections for early T-cell precursor (ETP) and T/myeloid mixed phenotype acute leukemia (T/M MPAL), BCL11B may also be leukemogenic through overexpression.

Chromosomal translocations. Multiple chromosomal translocations have been identified in T-ALL that lead to deregulated expression of the target genes. These chromosome rearrangements fuse genes encoding transcription factors (e.g., TAL1, TAL2, LMO1, LMO2, LYL1, TLX1, TLX3, NKX2-I, HOXA, and MYB) to one of the T-cell receptor loci (or to other genes) and result in deregulated expression of these transcription factors in leukemia cells.[162,163,172-176] These translocations are often not apparent by examining a standard karyotype, but can be identified using more sensitive screening techniques, including FISH or PCR.[163] Variants in a noncoding region near the TAL1 gene that produce a super-enhancer upstream of TAL1 represent nontranslocation genomic alterations that can also activate TAL1 transcription to induce T-ALL.[165] Translocations resulting in chimeric fusion proteins are also observed in T-ALL.[170]

Other recurring gene fusions in T-ALL patients include those involving MLLT10, KMT2A, NUP214, and NUP98.[162,166]

Ploidy.

Ploidy.

Recurrent abnormalities in chromosome number are much less common in T-ALL than in B-ALL. One study included 2,250 pediatric patients with T-ALL who were treated in Associazione Italiana di Ematologia e Oncologia Pediatrica/Berlin-Frankfurt-Münster protocols. The study found that near tetraploidy (DNA index, 1.79–2.28 or 81–103 chromosomes), observed in 1.4% of patients, was associated with favorable disease features and outcomes.[187]