Leukemia

Leukemia is a potentially fatal disease of the blood-forming tissues. The term leukemia means “white blood” and arose from the discovery of extremely large numbers of white blood cells in the blood of certain persons; counts as high as 500,000 per cubic millimetre and even 1,000,000 per cubic millimetre may be found in some instances. There are two main varieties of leukemia: myelogenous, or granulocytic, and lymphocytic. These terms refer to the types of cell that are involved. Each of these types is further subdivided into acute and chronic categories, referring to the duration of the untreated disease. Before the advent of modern chemotherapy, patients with acute leukemia usually died within weeks or months of the first manifestations of the disease. The life span of patients with chronic leukemia is now measured in years.

The cause of leukemia is not known, but researchers have identified certain risk factors that increase a person’s chance of developing the disease. Ionizing radiation is a leukemia-inducing agent. Survivors of the atomic bomb explosions in Hiroshima and Nagasaki during World War II, pioneering radiologists who used inadequately shielded apparatus, and certain patients receiving a particular kind of radiation are known to have developed leukemia with a frequency far exceeding that of the general population. Noteworthy is the fact that almost all radiation-induced leukemia has been of the myelogenous variety. The prolonged administration of radiomimetic drugs used in cancer chemotherapy, termed alkylating agents, is also associated with an increased risk of developing leukemia. Some evidence suggests that certain industrial chemicals, notably benzene, may cause leukemia. Genetic factors may lead to an increased frequency of leukemia in certain selected instances. This is suggested by evidence that shows the higher probability for acute leukemia occurring in both identical twins if one is affected as compared with both fraternal twins under the same conditions as well as by the frequency of development of acute myelogenous leukemia in children with Down syndrome, a condition in which there is a recognized chromosome defect.

Advances in molecular genetics have greatly increased the understanding of leukemia. The disease seems to arise from a genetic change (mutation) in an early progenitor, or stem cell, in the bone marrow. The mutant cell passes the genetic change on to all of its progeny, thus giving rise to a clone of leukemic cells. In many cases of leukemia, the mutation is detectable by analysis of the chromosomes of leukemic cells. A well-studied abnormality of this type, the Philadelphia chromosome, occurs in almost all cases of chronic myelogenous leukemia. The chromosomal aberrations affect genes that influence vital aspects of cell growth and function. These genes, the oncogenes, may themselves be mutated or their regulation may be abnormal. The entire process, beginning with the mutation, usually involves many steps that culminate in a cell with the malignant attributes of a leukemic cell. This concept of the mechanism of leukemia has influenced treatment of the disease, which aims to eliminate the mutant clone and all of its progeny by chemotherapy or bone marrow transplantation (see below).

Leukemia primarily involves the bone marrow, but the lymph nodes and spleen may also be affected. Changes also take place in the leukocytes, red blood cells, and platelets, with consequent anemia and bleeding manifestations. There may be weakness and an increased tendency to become fatigued because of anemia, in addition to hemorrhages into the skin, nosebleeds, or gum bleeding due to a decrease in the number of platelets. Leukemia is diagnosed by examination of the blood, supplemented in most instances by examination of the bone marrow.

Acute leukemia is marked by the presence in the blood of immature cells normally not present. In acute lymphocytic anemia (ALL), most frequently seen in children, the cells are immature forms of the lymphatic series of cells. In acute myelogenous leukemia (AML), the predominant cells are the youngest recognizable precursors (myeloblasts) of the neutrophils of the blood. In a third and the least common variety, acute monocytic leukemia, the immature cells appear to be precursors of the monocytes of the blood. Myelogenous and monocytic leukemia occur more commonly in adults and adolescents than in young children. In general, acute leukemia occurs in young persons, but no age group is exempt. The total white cell count usually is increased but not uncommonly is normal or lower than normal (leukopenic). In such cases, abnormal immature cells may nevertheless be seen in the blood. In all forms of acute leukemia, the typical cells are found in abundance in the bone marrow.

Chronic myelogenous leukemia (CML) is characterized by the appearance in the blood of large numbers of immature white blood cells of the myelogenous series in the stage following the myeloblast, namely, myelocytes. The spleen becomes enlarged, anemia develops, and the affected person may lose weight. The platelets may be normal or increased in number, abnormally low values being found only in the late stages of the disease or as an unintended result of therapy. The disease is most commonly encountered in persons between 30 and 60 years of age. With treatment the leukocyte count falls to normal, anemia is relieved, and the size of the spleen is greatly reduced. When the leukocyte count rises again, treatment is reinstituted. Such cycles of treatment, remission, and relapse with rise of leukocyte count can be repeated many times, but a stage ultimately is reached when treatment no longer is effective. The disease then often terminates in a form resembling acute leukemia (blastic crisis). There is considerable variation in the duration of the disease. Although in various series the average life span has been about 3 years, many affected persons live in good general condition for 5 to 10 years and sometimes longer.

Chronic lymphocytic leukemia (CLL) occurs most often in people over age 50 and worsens gradually over time. It is mainly characterized by an increase in the number of lymphocytes in the blood and bone marrow, often accompanied by more or less generalized enlargement of lymph nodes and the spleen. In the majority of CLL cases, the affected lymphocytes are B cells, the cancerous lineage of which becomes locked in a state of proliferation, resulting in the increased production of genetically identical cancer cells. In rare CLL cases, the malignancy affects the other type of lymphocyte, the T cell. Affected persons may go for many years without treatment and without any other manifestations. There may be no anemia and no loss of weight. Life span in this disease is measured in terms of 5 to 15 years, occasionally even longer. Two events mark a change in the state of relative good health. One is the development of anemia, sometimes hemolytic in type, often accompanied by some decrease in the number of platelets. The other is impairment of immune mechanisms, resulting in great susceptibility to bacterial infections.

Treatment differs according to the type of leukemia. Consequently, proper classification of the leukemia is the first step, once the diagnosis of leukemia has been made. A number of drugs are used for the treatment of leukemia; and, though now less frequently than before chemotherapy was available, so are various forms of radiation. All therapeutic agents used for CLL are myelotoxic (i.e., they injure all the cells of the bone marrow, normal cells as well as leukemic cells). Their mode of action is through direct damage to the dividing stem cell (unspecialized cell from which specialized cells develop) or by slowing or cessation of cell division (mitosis). These effects may be accomplished (1) by antimetabolites, drugs that interfere with the synthesis of deoxyribonucleic acid (DNA), a constituent of the chromosomes in the cell nucleus; (2) by blocking DNA strand duplication through the binding of drugs such as nitrogen mustard with the base groups of DNA; (3) by disruption of the mitotic spindle during mitosis; or (4) by interfering with the formation or functioning of ribonucleic acid (RNA), which is manufactured in the cell nucleus and plays an essential role in the production of protein and in other cell functions. Drugs with different modes of action are often combined, especially in the treatment of acute leukemia. In the process of treatment, anemia may increase; the body defenses, through the decrease in the number of neutrophils, may be impaired, and the platelets may be greatly reduced in number. Anemia can be treated with blood transfusions, and serious reductions in platelets can be met for a time with platelet transfusions.

Other therapies, such as immunotherapy, in which immune defenses are deployed to fight cancer cells, are under investigation for CLL, as well as for other forms of leukemia. One such therapy utilized a virus designed to stimulate the expression on a patient’s own T cells of receptor molecules known as chimeric antigen receptors that were capable of binding to specific proteins found only on the surface of malignant B cells and that activated the T cells to kill the B cells. T cells removed from the patient’s blood were infected with the virus and then infused back into the patient. Two out of three patients given the treatment experienced remission of their cancers for more than one year, and some of the engineered T cells persisted as memory T cells, indicating that the cells likely would be able to kill new cancer cells should the cancer return. Although more testing was needed and the therapy was associated with tumour lysis syndrome (a sometimes severe metabolic disturbance caused by the dying off of massive numbers of cancer cells), it was hailed at the time of its report in 2011 as a major breakthrough in the treatment of leukemia.

Acute lymphocytic leukemia is more successfully treated than are other forms of acute leukemia; prolonged remissions and even cures can be brought about in children with the disease. Certain drugs are used to bring about remission; if the remission is complete, the patient becomes well, and no signs of the disease are demonstrable in the blood or bone marrow; drugs other than those used to induce remission often are more useful in maintaining the remission than the remission-inducing drugs.

Acute myelogenous leukemia and acute monocytic leukemia are less effectively treated by available drugs than is acute lymphocytic leukemia. Nevertheless, new and aggressive forms of chemotherapy can induce lengthy remissions of the disease. Transplantation of normal bone marrow, often from histocompatible siblings, following total radiation of the patient to destroy all normal bone marrow cells as well as the leukemic cells, has shown promise. Although the treatment is arduous and complex, cures of acute lymphocytic and acute myelogenous forms of leukemia are possible with bone marrow transplantation.

Chronic myelogenous leukemia is treated with the drugs hydroxyurea or busulfan in daily doses until the leukocyte count has returned to normal. Treatment then is interrupted until the leukocyte count has risen to about 50,000 cells per cubic millimetre, when treatment is resumed. This can be repeated many times, and thus the affected person is maintained in good health for years. Not infrequently, the intervals between treatments are six months or longer. Busulfan, however, like other antileukemic agents, can injure the bone marrow, and other adverse effects may occur. Other drugs and radiation therapy also have been used but are somewhat less valuable than hydroxyurea or busulfan. In suitable young patients, bone marrow transplantation has been used successfully in chronic myelogenous leukemia.

In its early stages, chronic lymphocytic leukemia seems best untreated, as long as anemia is not present or glandular enlargement is not too troublesome. None of the current treatments are curative. The high leukocyte counts in themselves are not harmful. When there is severe anemia, however, or when the platelet count is very low and bleeding manifestations are severe, adrenocorticosteroid hormones are often given.