What causes CLL?

Chronic lymphocytic leukemia (CLL) is the most common leukemia in the U.S. Moreover, the incidence rates of this disease appear to be on the rise. Genetic factors contribute to the development of this disease.1 First-degree relatives of patients with CLL are significantly higher risk for having this disease than is the general population.2  Afflicted individuals within such families often present at a younger age than most patients with CLL, suggesting that genetic factors in familial CLL contribute to early leukemogenesis.3  In addition, environmental factors also have been implicated. In 2003, a committee of the US Institute of Medicine concluded that a positive association exists between exposures to herbicides used as defoliants from 1962 to 1971 during the Vietnam war and the risk of developing CLL4.  The principal defoliant used by the US military forces in Vietnam was a mixture of 2,4-dichlorophenoxyacetic acid (2,4-D), 2,4,5-trichlorophenoxyacetic acid (2,4,5,-T) (Agent Orange), and one form of dioxin, TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin), present as contaminant.  However, other herbicides and insecticides also have been implicated in epidemiology studies performed on rural farming communities5.

An apparent increase in incidence of this disease also could be due in part to enhanced methods of early diagnosis, as diagnosis now is being made at earlier stages and in younger patients.6  In one recent study, 3.5% of healthy persons over age 40 were found to have blood lymphocytes with the phenotype of CLL B cells that had apparent restriction in the use of immunoglobulin light-chains and immunoglobulin heavy-chain variable region gene subgroups.7  Whether these cell populations represent a very early stage of indolent CLL remains speculative.  Nevertheless, due to the advancing age of the U.S. population, the overall incidence rates of each stage of overt CLL appears to be increasing over time.6,8   Although the notion of an apparent increase in the incidence of CLL has been challenged by a more recent analysis of data from the Surveillance, Epidemiology, and End Results (SEER) program,9 this same study observed a notable lack of improvement in survival for patients with CLL when comparing the period 1974-1983 with 1984-1993.

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