In contrast to acute leukemia, the chronic leukemias are slowly developing conditions and may be present for years while causing minimal symptoms. The two main forms of chronic leukemia seen in adults are chronic myeloid leukemia (CML) and chronic lymphocytic leukemia (CLL).
Chronic myeloid leukemia is characterized by the accumulation of an abnormal clone of cells of the neutrophil lineage (a type of white blood cell), in various stages of differentiation, in the bone marrow, the peripheral blood, and the spleen, which typically becomes massively enlarged. In the chronic phase of CML there are few symptoms, but this phase inevitably gives way, after a period of two to five years, to an acute phase or blast crisis, which resembles acute leukemia and carries a grave prognosis. CML cells invariably carry a chromosomal translocation affecting chromosomes 9 and 22, which is known as the Philadelphia chromosome; this translocation results in the fusion of two genes, BCR and ABL. The presence of the BCR-ABL fusion appears to be necessary and sufficient for the development of CML. Currently, the mainstays of therapy for CML are interferon alpha, a natural substance that can induce long-term remissions in this disease, and allogeneic bone marrow transplantation. A recently developed drug that specifically inhibits the action of BCR-ABL shows tremendous promise as a CML treatment.
In chronic lymphocytic leukemia, there is accumulation of a clone of abnormal lymphocytes. This initially causes no symptoms, but over time there is progressive accumulation of the malignant cells in the lymph nodes, bone marrow, and spleen. Anemia and thrombocytopenia result, with symptoms of fatigue and bruising or bleeding. Normal lymphocytes are suppressed, and susceptibility to infections is increased. CLL runs an indolent course, and often no treatment is needed for several years following diagnosis. No cure for CLL is currently available, and treatment comprises supportive measures, such as red cell transfusion, and chemotherapy with oral or intravenous agents to control bulky disease in lymph nodes and spleen.