Classification of Pediatric Myeloid Malignancies
Table 2. Acute Leukemias of Ambiguous Lineage According to the WHO Classification of Tumors of Hematopoietic and Lymphoid Tissuesa continued...
Molecular abnormalities associated with an unfavorable prognosis include the following:
- Chromosomes 5 and 7: Chromosomal abnormalities associated with poor prognosis in adults with AML include those involving chromosome 5 (monosomy 5 and del(5q)) and chromosome 7 (monosomy 7).[26,38] These cytogenetic subgroups represent approximately 2% and 4% of pediatric AML cases, respectively, and are also associated with poor prognosis in children.[29,38,63,64,65] In the past, patients with del(7q) were also considered to be at high risk of treatment failure and data from adults with AML support a poor prognosis for both del(7q) and monosomy 7. However, outcome for children with del(7q), but not monosomy 7, appears to be comparable to that of other children with AML.[30,65] The presence of del(7q) does not abrogate the prognostic significance of favorable cytogenetic characteristics (e.g., inv(16) and t(8;21)).[26,65,66]
- Chromosome 3 (inv(3)(q21;q26) or t(3;3)(q21;q26)) and EVI1 overexpression: The inv(3) and t(3;3) abnormalities involving the EVI1 gene located at chromosome 3q26 are associated with poor prognosis in adults with AML,[26,38,67] but are very uncommon in children (<1% of pediatric AML cases).[29,40,68]
- FLT3 mutations: Presence of a FLT3-ITD mutation appears to be associated with poor prognosis in adults with AML, particularly when both alleles are mutated or there is a high ratio of the mutant allele to the normal allele.[70,71]FLT3-ITD mutations also convey a poor prognosis in children with AML.[56,72,73,74,75,76] The frequency of FLT3-ITD mutations in children is lower than that observed in adults, especially for children younger than 10 years, for whom 5% to 10% of cases have the mutation (compared with approximately 30% for adults).[74,75,77] A longer length of the ITD segment of FLT3-ITD has been reported to be associated with a poorer outcome.
Presence of the FLT3-ITD mutation is strongly associated with the microgranular variant (M3v) of APL and with hyperleukocytosis.[73,79,80] It remains unclear whether FLT3 mutations are associated with poorer prognosis in patients with APL who are treated with modern therapy that includes all-trans retinoic acid.[80,81,82,83]
Activating point mutations of FLT3 have also been identified in both adults and children with AML,[70,74,84] though the clinical significance of these mutations is not clearly defined. FLT3-ITD and point mutations occur in 30% to 40% of children and adults with APL.[73,79,81,82] The prognostic significance of this mutation in APL is unclear, although a mutant to wild type allelic ratio of greater than or equal to 0.5 may be associated with a worse outcome.