Chronic Lymphocytic Leukemia

The B-Cell Receptor in Chronic Lymphocytic Leukemia

At the time when chronic lymphocytic leukemia is diagnosed, the population of B cells with a common immunophenotype by flow cytometry also carries and expresses a clonal, rearranged immunoglobulin heavy and light chain gene. Somatic hypermutation can be present, with up to 15% or more variations from germline. Although the degree of somatic hypermutation is a continuum across chronic lymphocytic leukemia, the striking clinical observation is that the disease’s behavior and outcome vary tremendously based on a dichotomous cutoff of 2% variation from germline. Patients whose chronic lymphocytic leukemia carries a mutated immunoglobulin heavy chain variable sequence that is more than 2% different from germline, called mutated IGHV, have an indolent course, sometimes not requiring therapy at all. By comparison, patients whose immunoglobulin heavy chain variable sequence is not mutated, defined as 2% or less different than germline and called unmutated IGHV, have higher risk disease. The biologic basis for this prognostic difference is unclear. The gene expression profile of both mutated and unmutated cells resembles a memory B cell and is more similar to each other than to any other normal or malignant B cell. Chronic lymphocytic leukemia with an unmutated immunoglobulin heavy chain variable sequence shows increased activation of the B-cell receptor, which drives survival and proliferation and is likely associated with increased clonal evolution over time. Most of the recurrently mutated cancer genes in chronic lymphocytic leukemia occur predominantly in cells with an unmutated immunoglobulin heavy chain variable sequence. In contrast, although chronic lymphocytic leukemias with mutated immunoglobulin heavy chain variable sequences have B cell receptors, activation is at a lower level, and these chronic lymphocytic leukemias are much less likely to have high risk cytogenetics and somatic mutations. Within a subset, chronic lymphocytic leukemia has similar clinical behavior, and some subsets have been associated with particular somatic mutations, thereby suggesting the possibility of codevelopment or crosstalk between B-cell receptor stimulation/selection and genetic abnormalities.

Chromosomal Abnormalities

Chronic lymphocytic leukemia generally has a stable karyotype with few or no abnormalities, particularly at the time of diagnosis. In contrast to many other hematologic malignancies, balanced translocations are rare in chronic lymphocytic leukemia, which more commonly is associated with unbalanced deletions or amplifications but also usually has a quite stable karyotype. By fluorescence in situ hybridization (FISH) on interphase cells, the most common abnormalities in chronic lymphocytic leukemia are deletion 13q14, deletion 11q22, deletion 17p13, and trisomy 12. Deletion 13q14, which is the most common abnormality in chronic lymphocytic leukemia, is seen in about 55% of patients and is the only abnormality that has been associated with favorable prognosis. In general, however, the prognosis of patients with deletion 13q14, trisomy 12, or normal cytogenetics follows what would be expected based on their IGHV mutation status. The highest risk abnormality is deletion 17p13, which deletes one copy of the TP53 gene and is associated with the poorest prognosis. The second highest risk is deletion 11q22, which deletes one copy of the ATM gene. Both of these mutations are strongly associated with unmutated immunoglobulin heavy chain variable sequence, but mutated immunoglobulin heavy chain variable sequence mitigates the prognostic impact of the FISH abnormality. Other findings (e.g., deletion 6q22, trisomy 8, 8q gain, and 8p deletion) are relatively rare, especially at diagnosis, and have generally been associated with more aggressive disease. Cases with more than three to five abnormalities are associated with worse outcomes, but this information is not routinely used for determining prognosis.

Recurrent Somatic Mutations

By next generation sequencing, chronic lymphocytic leukemia has a relatively low mutation rate. Unlike Waldenstrom macroglobulinemia ( MYD88 ; Chapter 173 ) and hairy cell leukemia ( BRAF ), chronic lymphocytic leukemia has no recurrent defining somatic mutation. The most common mutations in chronic lymphocytic leukemia occur in only 15 to 20% of patients and affect the TP53 gene, NOTCH1 , SF3B1 , and ATM . Many other cancer genes are mutated in a few percent of cases, thereby making the disease genetically very heterogeneous. Some mutations can be clustered into affected pathways that are thought to be activated in most patients, either genetically or otherwise, and that may act cooperatively to induce disease.

Epigenetics

Methylation patterns in chronic lymphocytic leukemia correspond to the developmental pattern of the antecedent normal B cell and do not vary over the course of the disease. The analysis of methylation patterns identifies three groups (low, intermediate, and high programmed chronic lymphocytic leukemia) that correspond to unmutated immunoglobulin heavy chain variable sequence, a mix of unmutated and mutated immunoglobulin heavy chain variable sequence, and mutated immunoglobulin heavy chain variable sequence.

Methylation can also silence genes that are sometimes deleted in chronic lymphocytic leukemia. For example, the most common recurrent genetic event in chronic lymphocytic leukemia is deletion 13q14, which removes the miR-15 and - 16 genes that target BCL-2 . When they are deleted, BCL-2 is overexpressed. Interestingly, chronic lymphocytic leukemias that do not carry deletions of 13q14 typically have hypermethylation of that region, and the hypermethylation effectively silences miR-15 and -16 . This example of alternative mechanisms resulting in the same key pathogenetic event is believed to apply to other pathways in chronic lymphocytic leukemia (e.g., NOTCH1 can be activated both mutationally or through stimulation by the microenvironment).

Immune Dysfunction

Patients with chronic lymphocytic leukemia display significant defects in immune function, including hypogammaglobulinemia and functional T-cell defects. Hypogammaglobulinemia can be present at diagnosis and progresses as the burden of disease increases. T cells show evidence of chronic stimulation, with exhaustion that also worsens with an increasing burden of disease. The number of T cells is often increased, but the cells are dysfunctional. Regulatory T cells are also increased. These defects do not always translate into infections in individual patients, but collectively patients with chronic lymphocytic leukemia have a substantially increased risk of infection.

Disease-Related Complications