A 6-year-old girl with newly diagnosed acute lymphoblastic leukemia undergoes cytogenetic analysis. Which chromosomal abnormality is the most common favorable prognostic marker found in childhood ALL?

Explanation

## Cytogenetics and Prognosis in Childhood ALL **Key Point:** Hyperdiploidy (>50 chromosomes) is the most common favorable cytogenetic abnormality in childhood ALL, found in approximately 25–30% of cases and associated with excellent prognosis. ### Cytogenetic Abnormalities in Childhood ALL | Abnormality | Frequency | Prognosis | 5-Year EFS | Mechanism | |---|---|---|---|---| | Hyperdiploidy (>50 chr) | 25–30% | **Favorable** | >90% | Increased drug sensitivity; trisomy of chromosomes 4, 10, 17 | | t(12;21) ETV6-RUNX1 | 20–25% | **Favorable** | >90% | Fusion transcription factor; common in 2–5 year-olds | | t(9;22) Philadelphia | 3–5% | **Unfavorable** | <50% | BCR-ABL tyrosine kinase; requires TKI therapy | | t(4;11) KMT2A-MLLT1 | 2–3% | **Unfavorable** | 40–60% | Affects infants and very young children; poor response | | Complex karyotype | 5–10% | **Unfavorable** | <40% | Multiple structural abnormalities; chemotherapy resistance | **High-Yield:** Hyperdiploidy and t(12;21) together account for approximately 50% of childhood ALL and are associated with the best outcomes. These are the most frequently tested favorable prognostic markers. ### Why Hyperdiploidy is Favorable 1. **Increased drug sensitivity:** Extra chromosomes may enhance cellular uptake of chemotherapeutic agents. 2. **Gene dosage effects:** Trisomy of specific chromosomes (especially 4, 10, 17) confers growth disadvantage and chemosensitivity. 3. **Lower relapse risk:** Hyperdiploidy is associated with lower minimal residual disease (MRD) at end of induction therapy. **Mnemonic:** **HYPE** = **H**yperdiploidy, **Y**oung age (2–5 years), **P**erfect response to induction, **E**xcellent prognosis. ### Unfavorable Cytogenetics - **t(9;22) Philadelphia:** BCR-ABL fusion; requires tyrosine kinase inhibitor (TKI) therapy in addition to chemotherapy; historically very poor prognosis, now improved with TKIs. - **t(4;11) KMT2A-MLLT1:** Affects infants (<1 year); associated with early relapse and poor chemotherapy response. - **Complex karyotype:** Multiple structural abnormalities; indicates high-risk disease requiring intensified therapy. **Clinical Pearl:** Modern risk stratification also incorporates minimal residual disease (MRD) by flow cytometry at day 15 and end of induction. MRD >0.01% is a strong independent adverse prognostic factor, sometimes overriding cytogenetics. [cite:Robbins 10e Ch 7]