Relationship between idiopathic hypereosinophilic syndrome, eosinophilic leukemia, and systemic mastocytosis.

Chronic eosinophilic leukemia is a neoplastic condition with persistent eosinophilia as the major hematological abnormality and with the eosinophils being part of the neoplastic clone. Some cases can be recognized by traditional hematological criteria, but many can be recognized only when a clonal cytogenetic or molecular genetic abnormality is demonstrated. A range of cytogenetic and molecular genetic abnormalities has been recognized, including both those seen in other myeloid malignancies (such as trisomy 8, monosomy 7, and 20q-) and those that are particularly linked to eosinophil differentiation (such as rearrangements of PDGFRB, FGFR1, and PDGFRA, the latter with formation of a FIP1L1-PDGFRA fusion gene). The discovery of the FIP1L1-PDGFRA fusion gene has led to the recognition that many patients who would previously have been regarded as having idiopathic hypereosinophilia actually have chronic eosinophilic leukemia. The same fusion gene has also been found in patients with hypereosinophilia and atypical bone marrow mast cells but whether this syndrome should be regarded as a variant of eosinophilic leukemia or as a variant of systemic mastocytosis remains to be established.