Mnemonic
A mnemonic for acute leukemia ages:
- ALL - adults Least likely ~ children (10 yrs)
- AML - adults Most likely (50 yrs)
Another:
- AML= (A)uer rods and (M)yeloperoxidase positive
Tip
Characteristic of CLL would be the “smudge cells”
Epidemiology
- Acute lymphoblastic leukemia
- Peak incidence: 2–5 years
- Most common malignant disease in children
- ∼ 80% of acute leukemias during childhood are lymphoblastic.
Etiology
Acute lymphoblastic leukemia (ALL)
- Genetic or chromosomal factors
- Down syndrome: Risk of ALL is, like that of AML, 10–20 times higher in patients with Down syndrome compared to the general population.
- Neurofibromatosis type 1
- Ataxia telangiectasia
Pathophysiology
Clinical features
- Oncologic emergencies can be the first sign of leukemia, e.g., an elderly patient presenting with priapism or DIC may have leukostasis (more common in AML than ALL)
Warning
The identification of DIC suggests APL, which is a medical emergency. Consult hematology and/or oncology immediately and transfer the patient to a critical care unit.
Diagnostics
Tip
MPO and LAP
- MPO (Myeloperoxidase)
- Primary marker for identifying cells of myeloid lineage
- Can help distinguish Acute Myeloid Leukemia (AML) from Acute Lymphoblastic Leukemia (ALL):
- AML cells are typically MPO positive
- ALL cells are MPO negative
- LAP (Leukocyte Alkaline Phosphatase)
- Enzyme present in mature neutrophils
- LAP score helps differentiate between various myeloproliferative conditions:
- Elevated in infections, inflammatory conditions, and polycythemia vera
- Low or absent in Chronic Myeloid Leukemia (CML)
- CML: All neutrophils are clonal (BCR-ABL1+), dysfunctional, and LAP-negative due to stem-cell-level dominance.
- AML: Residual neutrophils are derived from non-clonal, normal stem cells that escape leukemic transformation, retaining LAP activity.
- Particularly valuable in distinguishing CML from leukemoid reaction:
- CML: Low LAP score
- Leukemoid reaction: High LAP score
| Leukemia Type | MPO | LAP |
|---|---|---|
| AML | Positive (+) | Normal to Increased |
| CML | Positive (+) | Decreased (-) |
| ALL | Negative (-) | Normal to Increased |
| CLL | Negative (-) | Normal to Increased |
Tip
Some subtypes (especially M3, or APL) exhibit Auer rods
- Pink-red, rod-shaped granular cytoplasmic inclusion bodies in malignant immature myeloblasts or promyelocytes
- Myeloperoxidase (MPO) positive
Immunophenotype
- Immunohistochemistry
- ALL
- MPO negative
- Terminal deoxynucleotidyl transferase (TdT) positive
- TdT is a DNA polymerase involved in V(D)J recombination, which generates antigen receptor diversity during early lymphoid maturation.
- Periodic acid-Schiff (PAS): often positive
- AML
- MPO positive
- TdT negative
- PAS negative
- ALL
Genetic studies
- APL: t(15;17) translocation, producing PML-RARA
Differential diagnostics
Tip
Assessment of leukocyte alkaline phosphatase (an enzyme found in maturing neutrophils) can be used to distinguish marked leukocytosis due to leukemoid reaction from chronic myeloid leukemia (CML). Values are typically normal or increased in leukemoid reaction; in contrast, they are usually low in CML because the abnormal maturing neutrophils don’t bother to make these.
Treatment
Tip
In APL, the t(15;17) translocation and subsequent formation of the PML-RARA fusion gene can inhibit myeloblast differentiation under physiological levels of retinoic acid. High doses of ATRA (a vitamin A derivative) may induce myeloblast differentiation and promote remission.