Chronic lymphocytic leukemia is heterogeneous in both molecular pathology and clinical course, and many questions regarding the most appropriate management of this disease remain unanswered. Researchers continue to investigate both biological and clinical parameters in an attempt to refine the staging systems and accurately determine the prognostic outcome for patients with chronic lymphocytic leukemia and to individualize treatment. We discuss the most common markers further on, but many other markers, including interleukin (IL)-6, IL-10, tumor necrosis factor, intra-cellular BCL-2, vascular endothelial growth factor, and other cytokines and enzymes, are being investigated for their potential prognostic significance.
Lymphocyte Doubling Time
The lymphocyte doubling time is calculated as the number of months it takes the absolute lymphocyte count to double in number. This factor has been confirmed as a prognostic indicator that is independent of stage in a variety of studies. After a 118-month
follow-up, median survival was 61 months in patients with an lymphocyte doubling time of less than 12 months, and median survival was not yet reached in patients with an lymphocyte doubling time of more than 12 months. According to published literature, this factor is subject to variation.
Serum Beta-2 Microglobulin
The serum marker beta-2 microglobulin correlates with tumor burden and disease stage in patients with chronic lymphocytic leukemia. A retrospective study from the M.D. Anderson Cancer Center (Houston, TX) found this serum marker to be the strongest predictor of five-year survival in a multivariate analysis that controlled for age, sex, and performance status. Similar results have been reported in other retrospective trials; however, a prospective trial did not find beta-2 microglobulin to be a significant predictor of survival in a multivariate analysis that controlled for stage and lymphocyte doubling time. Additional studies are required to further classify the role of beta-2 microglobulin in prognosis.
Immunoglobulin Mutation Status
The B-cell antigen receptor, or immunoglobulin, is a surface molecule that detects the presence of pathogenic antigens and activates an immune response. Its synthesis during B-cell development occurs via a process of gene rearrangement. Gene segments are selected and assembled in such a way that each B lymphocyte produces an immunoglobulinthat is unique to that cell. When the B lymphocyte encounters an antigen, the immunoglobulinmolecule is altered by a process known as somatic hypermutation, which makes the immunoglobulin more specific for the antigen, thus enhancing the immune response. The presence of mutations in an immunoglobulinis evidence of its encounter with antigen and a marker of B-cell maturity.
Chronic lymphocytic leukemia was initially thought to be a malignancy of antigen-naive B cells with unmutated immunoglobulin. Researchers subsequently found that a subset of chronic lymphocytic leukemia patients had mutated immunoglobulin, suggesting the cell of origin was more mature and had encountered antigen. In 1999, two studies simultaneously reported the prognostic significance of immunoglobulin mutation status in chronic lymphocytic leukemia. The studies demonstrated that patients with a mutated immunoglobulin status had significantly longer survival than those with unmutated immunoglobulin. Many studies have confirmed the prognostic significance of immunoglobulin mutation, and researchers have established correlations between immunoglobulin mutation status and the need for chemotherapy, the response to chemotherapy, and the risk of relapse after transplantation.
Serum Thymidine Kinase
Thymidine kinase is an enzyme involved in the DNA synthesis salvage pathway, a process required for the synthesis of new DNA precursors in dividing cells. Present in dividing cells and absent in resting cells, it is a marker of proliferation. A recent study demonstrated the relationship between serum thymidine kinase level and mutation status: a level greater than 15 U/L was a strong predictor of unmutated immunoglobulin genes. Other studies have described a relationship between thymidine kinase level and lymphocyte doubling time, lymphocyte count, and beta-2 microglobulin. This enzyme has also been shown to be a significant predictor of survival and response to treatment. Commercial assays are available for the measurement of this enzyme. Additional large-scale studies are required to determine its prognostic importance in multivariate analyses.
CD38. CD38 is a cell-surface molecule expressed by mature B cells. Its expression on chronic lymphocytic leukemia cells has been shown to have prognostic significance in univariate and multivariate analyses, which have included clinical-stage, cytogenetic abnormalities and beta-2 microglobulin levels. CD38 expression has been associated with unmutated immunoglobulin status, though results are discordant in approximately 30% of cases. Whether CD38 has prognostic significance in patients with known immunoglobulin mutation status is controversial. This marker may also vary during the course of disease. CD38 is reportedly a useful and easily measured prognostic marker, but it is limited by differences in expression over time for individual patients and is not a surrogate for immunoglobulin mutation status.
ZAP70. ZAP70 is a signaling molecule involved in transducing intracellular signals from the T-cell receptor to the nucleus in T lymphocytes. This molecule was found to be aberrantly expressed in malignant chronic lymphocytic leukemia B cells that have unmutated immunoglobulin genes. The aberrant expression of ZAP70 in this subset of chronic lymphocytic leukemia cells was discovered by microarray gene chip expression profiling and confirmed by protein analysis. It was also found to be elevated in chronic lymphocytic leukemia cells that are CD38-positive. A flow cytometric assay that could identify ZAP70 expression is under investigation. Although promising, additional studies are needed to further define the prognostic value of this marker in relation to other chronic lymphocytic leukemia markers.
As many as 80% of chronic lymphocytic leukemia patients have identifiable cytogenetic abnormalities, although these problems are more common in patients with advanced disease than in those with early-stage disease. No single specific abnormality or gene has been consistently correlated with the pathogenesis of chronic lymphocytic leukemia, and any relationship between the genetic changes and the evolution of the disease has yet to be fully elucidated.
The most common cytogenetic changes found in chronic lymphocytic leukemia are deletions in chromosomes 11, 13, and 17, and the presence of an additional chromosome 12. Deletions in chromosome 17 are associated with p53 oncogene inactivation. These chromosomal abnormalities have prognostic significance (TABLE. Common Cytogenetic Changes in Chronic Lymphocytic Leukemia and Their Significance) and affect treatment decisions in hospitals where the technology is available to perform the analysis.