Leukemia Adult Chronic
Chronic Lymphocytic Leukemia
Chronic lymphocytic leukemia (CLL) is a type of cancer that starts from white blood cells (called lymphocytes) of the bone marrow. It then invades the blood. In time, it can also invade other parts of the body, including the lymph nodes, liver, and spleen. Another chronic leukemia, called chronic myeloid leukemia or chronic myelogenous leukemia (CML), also starts in the bone marrow, but in a different type of blood cell. Chronic myeloid leukemia is discussed in a separate ACS document. Leukemia is different from other types of cancer that start in organs such as the lungs, colon, or breast and then spread to the bone marrow. These cancers that start elsewhere and then spread to the bone marrow are not leukemia.
What Are the Risk Factors for Chronic Lymphocytic Leukemia?
A risk factor is anything that increases a person's chance of getting a disease such as cancer. Risk factors for cancer are lifestyle-related, environmental, or genetic (inherited). Lifestyle-related risk factors for some types of cancer include smoking, a low-fiber diet, or exposing skin to strong sunlight. However, there are no known lifestyle-related risk factors for CLL. In general, there are no certain environmental risk factors for chronic lymphocytic leukemia.
Environmental risk factors are influences in our surroundings such as radiation, chemicals, and infections. High-dose radiation exposure (such as being a survivor of an atomic bomb blast or nuclear reactor accident) increases the risk of developing chronic myeloid leukemia, but not chronic lymphocytic leukemia. Recently, some studies have suggested that chemicals used in farming, such as herbicides and insecticides, may increase the risk of CLL. Also, studies have linked exposure to Agent Orange, an herbicide used during the Vietnam War, to CLL. The only inherited risk factor known for chronic leukemia is that first-degree relatives (parents, siblings, or children) of CLL patients have a two-to-fourfold increased risk for this cancer.
What Causes Chronic Lymphocytic Leukemia?
The development of normal human cells mostly depends on the information contained in the cells’ chromosomes. Chromosomes are large molecules of DNA. DNA is the chemical that carries the instructions for nearly everything our cells do. We usually resemble our parents because they are the source of our DNA. However, DNA affects more than the way we look. During the past few years, scientists have made great progress in understanding how certain changes in DNA can cause normal bone marrow cells to become leukemia cells. Some genes (packets of our DNA) contain instructions for controlling when our cells grow and divide. Certain genes that promote cell division are called protooncogenes. Others that slow down cell division or cause cells to die at the right time are called tumor suppressor genes.
Cancers can be caused by DNA mutations (gene defects) that turn on protooncogenes or turn off tumor suppressor genes. Every time a cell prepares to divide into two new cells, it must duplicate all of the DNA in its chromosomes. This process is not perfect, and errors can occur that may affect genes within the DNA. Fortunately, cells have repair enzymes that proofread DNA. But some errors may slip past, especially if the cells are growing rapidly. Each human cell contains 23 pairs of chromosomes. In chronic lymphocytic leukemia (CLL), there is often a deletion -- that is, loss of a part of a chromosome. Often this occurs in chromosome 13, but other chromosomes such as 11 and 17 can also be affected. Sometimes there is an extra chromosome 12. Other less common abnormalities are also found.
Although these abnormalities are important in CLL, scientists still don’t know exactly how they lead to leukemia. We do know that normal B-lymphocytes are programmed to grow and multiply when they encounter a foreign protein called an antigen. Scientists think that CLL begins because the B-lymphocytes continue to multiply without restraint after they have reacted to an antigen. Why this happens is not yet known. Sometimes a person inherits DNA abnormalities from a parent that greatly increase the risk of developing certain types of cancer. But these inherited mutations very rarely cause CLL. DNA mutations related to CLL usually occur during the person’s lifetime rather than having been inherited before birth.
Can Chronic Lymphocytic Leukemia Be Prevented?
Although many types of cancer can be prevented by lifestyle changes to avoid certain risk factors, there are no known lifestyle-related risk factors for chronic lymphocytic leukemia. Since most chronic lymphocytic leukemia patients have no known risk factors, there was no way to prevent their leukemias from developing.
Credit: National Institute of Health.