The classification of CML, though not entirely straightforward, has recently been the subject of much revision by the World Health Organization (WHO).17 Patients with classical CML have a well-defined disease characterized by splenomegaly, leukocytosis, and the finding of a BCR-ABL fusion gene in all leukemic cells. The classical Ph chromosome is easily identifiable in 80% of CML patients; in a further 10% of patients, variant translocations may be "simple," involving chromosome 22 and a chromosome other than chromosome 9, or "complex," in which chromosome 9, 22, and additional chromosomes are involved. About 8% of patients with classical clinical and hematologic features of CML lack the Ph chromosome and are referred to as the cases of "Ph-negative CML." About half such patients have a BCR-ABL gene and are referred to as Ph-negative, BCR-ABL-positive cases. The remainder, perhaps less than 5% of patients with hematologically "acceptable" CML, are BCR-ABL negative, and some of these have mutations in the ras gene. These patients are usually classified as having Ph-negative, BCR-ABL-negative CML or atypical CML (sometimes also referred to as subacute myeloid leukemia), chronic myelomonocytic leukemia (CMML) or chronic neu-trophilic (CNL) (Table 16.2). Children may have a disease previously referred to as juvenile CML, and now juvenile myelomonocytic leukemia (JMML). Importantly, in none of these variants is there a Ph

Table 16.2 Classification of CML and its variants Classical Ph-positive CML

Ph-negative, bcr/abl-positive hematologically typical CML Ph-negative, bcr/abl-negative hematologically atypical CML Juvenile myelomonocytic leukemia Chronic myelomonocytic leukemia Chronic neutrophilic leukemias chromosome. Patients with atypical CML, CMML, and JMML usually have clinical and hematological features that suggest an overlap between CMPD and myelodys-plastic syndrome (MDS), and the contemporary classification of myelodysplastic/myeloproliferative diseases is appropriate (Table 16.3).

CNL is a rare disease, which is of considerable academic interest since it is sometimes associated with a Ph chromosome and a BCR-ABL related oncoprotein, the

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