Among children with acute lymphoblastic leukemia (ALL), which is the most common chromosomal abnormality associated with a favorable prognosis?

Explanation

## Chromosomal Abnormalities in Childhood ALL **Key Point:** The t(12;21) ETV6-RUNX1 fusion (formerly TEL-AML1) is the most common favorable cytogenetic abnormality in childhood ALL, occurring in 20–25% of cases and associated with excellent prognosis. ### Frequency and Prognostic Impact of Common Cytogenetic Abnormalities | Abnormality | Frequency | Prognosis | 5-Year EFS | Notes | |---|---|---|---|---| | t(12;21) ETV6-RUNX1 | 20–25% | Favorable | >90% | Most common favorable; standard-risk | | Hyperdiploidy (>50 chr) | 25–30% | Favorable | >85% | Especially if chr 4, 10 gain | | t(9;22) BCR-ABL | 3–5% | Unfavorable | 50–60% | Philadelphia chromosome; TKI responsive | | t(4;11) KMT2A-AFF1 | 2–3% | Unfavorable | 40–50% | Infant ALL; high-risk | | Hypodiploidy | 1–2% | Unfavorable | 30–40% | Very poor prognosis | | t(1;19) TCF3-PBX1 | 3–5% | Intermediate | 70–80% | Older children | ### ETV6-RUNX1 Fusion (t(12;21)) **High-Yield:** This is the single most common favorable cytogenetic abnormality in childhood ALL. **Clinical Pearl:** - Typically presents in children aged 2–5 years (the peak incidence group). - Often presents with lower white blood cell counts (<50,000/μL). - Responds excellently to standard chemotherapy protocols. - Rarely progresses to secondary AML. ### Prognostic Classification in Modern Trials **Mnemonic:** **FAVORABLE cytogenetics in ALL** = **ETV6-RUNX1, Hyperdiploidy, Low WBC** 1. **Favorable-risk ALL:** - t(12;21) ETV6-RUNX1 - Hyperdiploidy (>50 chromosomes, especially with gains of chromosomes 4 and 10) - Age 2–5 years - WBC <50,000/μL 2. **Unfavorable-risk ALL:** - t(9;22) BCR-ABL (Philadelphia chromosome) - t(4;11) and other KMT2A rearrangements (especially in infants) - Hypodiploidy (<45 chromosomes) - Age <1 year or >10 years - WBC >100,000/μL - MRD positivity at end of induction ### Why ETV6-RUNX1 is Most Common Favorable Abnormality - Occurs in ~20–25% of childhood ALL cases. - Creates a fusion protein that impairs normal hematopoietic differentiation but retains chemotherapy sensitivity. - Associated with B-cell precursor (BCP) ALL, the most common morphologic subtype. - Rarely found in adult ALL, making it a pediatric-specific favorable marker. **Warning:** Do not confuse t(12;21) with t(9;22). The Philadelphia chromosome t(9;22) is unfavorable and requires tyrosine kinase inhibitor therapy in addition to chemotherapy. [cite:Robbins 10e Ch 13]