Hallmark of Acute Lymphoblastic Leukemia (ALL)
The hallmark of ALL is recurrent chromosomal abnormalities and genetic alterations involving differentiation and proliferation of lymphoid precursor cells, which define the disease biologically and prognostically. 1, 2
Genetic and Molecular Hallmarks
The molecular signature of ALL is characterized by:
- Recurrent genetic alterations that are often cryptic by conventional cytogenetics but detectable by advanced molecular techniques 1
- Chromosomal rearrangements that serve as both diagnostic markers and prognostic indicators, governing risk stratification and treatment response 2, 3
- Copy number alterations (CNAs) detected in approximately 89% of cases, with two-thirds harboring ≥3 CNAs, particularly affecting genes like CDKN2A/B, IKZF1, and PAX5 4
- IKZF1 alterations (encoding the IKAROS transcription factor) are specifically a hallmark of high-risk B-progenitor ALL, including Philadelphia chromosome-positive and Ph-like subtypes 5
Immunophenotypic Hallmarks
The immunophenotype distinguishes ALL from other acute leukemias:
- TdT (terminal deoxynucleotidyl transferase) positivity is characteristic of ALL and expressed by all B- and T-cell progenitors, except mature B-ALL (Burkitt leukemia), while being negative in AML 6, 7
- B-lineage markers (CD19, CD22, cytoplasmic CD79a) define B-ALL, which accounts for 75-80% of cases 6, 7
- T-lineage markers (cytoplasmic CD3, surface CD7) characterize T-ALL, representing 20-25% of adult cases 6
Diagnostic Hallmarks
The diagnostic criteria that define ALL include:
- ≥20% bone marrow lymphoblasts on hematopathology review, though treatment protocols often use ≥25% to define leukemia 8, 9
- Lymphoid lineage commitment confirmed by comprehensive flow cytometric immunophenotyping, which cannot be reliably determined by morphology alone 7
Clinical Presentation Hallmarks
While not the molecular hallmark, ALL characteristically presents with:
- Bone marrow infiltration causing cytopenias (fatigue from anemia, bleeding from thrombocytopenia, infections from neutropenia) 8
- Bone and joint pain, which may be the only presenting symptom particularly in children 8
- Lymphadenopathy and organomegaly (splenomegaly/hepatomegaly in ~20% of patients) 8
Common Pitfall
A critical pitfall is attempting to distinguish ALL from AML based solely on morphology—comprehensive immunophenotyping is mandatory as lineage cannot be reliably determined by morphologic assessment alone, and this distinction is absolutely critical since treatment regimens are completely different and non-interchangeable 7.