Pathophysiology

Chronic lymphocytic leukemia (CLL) is amalignant lymphoproliferative disorder characterized by the abnormal clonal expansion of mature CD5+ B cells that slowly accumulate in the peripheral blood, bone marrow, and lymphoid tissues.¹Although CLL patients are often asymptomatic at presentation, the progressive accumulation of B cells leads to leukocytosis, lymphadenopathy, hepatosplenomegaly, bone marrow failure, recurrent infections, and possibly autoimmune diseases (for example, hemolytic anemia) over time.² The clinical course of CLL is highly varied, with some patients experiencing rapid disease progression and death a few months after diagnosis and other patients presenting with indolent disease and prolonged survival.¹ CLL cells have a characteristic immunophenotype with co-expression of CD5, CD23, CD19 and weak surface membrane immunoglobulin levels and CD20 expression.

CLL is characterized by the accumulation of long-lived clonal lymphocytes that are resistant to apoptosis. This clonal expansion is largely the result of defects in apoptotic pathways, such as the overexpression of BCL-2 anti-apoptotic proteins in CLL cells.¹ The intrinsic mitochondrial pathway of apoptosis proceeds when molecules sequestered between the outer and inner membranes of the mitochondria are released to the cytosol by mitochondrial outer membrane permeabilization (MOMP). The intrinsic apoptotic pathway is often dysregulated in relapsed/refractory CLL due to a deficiency in pro-apoptotic proteins such as TP53 and overexpression of anti-apoptotic proteins such as BCL-2.³

BCL-2 is a member of the BCL-2 family of regulatory proteins that either induce or inhibit apoptosis.⁴ The balance between pro-survival and pro-death BCL-2 proteins is a major factor in determining if cells undergo apoptosis in response to cell stress. Ongoing research has demonstrated that disruption of BCL-2 leads to cell death. Some cancers are dependent on BCL-2 for survival and BCL-2 inhibitors can trigger apoptosis, even in del(17p) CLL.^3,5 BCL-2 is also involved in the development of resistance to chemotherapeutic agents, further stressing the importance of targeting this gene as well as other genetic abnormalities with novel targeted agents in order to improve patient outcomes.^5,6

Biomarkers

Studies show that over 80% of CLL patients have genetic abnormalities: 14%-40% have del(13q), 10%-32% del(11q), 11%-18% trisomy 12, 3%-27% del(17p), and 2%-9% del(6q). The frequencies of each abnormality range depend on the stage of the disease and whether or not the disease is resistant to conventional therapy.⁷ For example, del(17p) is identified in 5%-10% of CLL patients at initial diagnosis and in up to 40% of patients with relapsed disease after fludarabine-based therapy.⁸ In a recent classification based on chromosomal changes, CLL has been divided into three risk categories: low risk in patients with a normal karyotype or 13q deletion, intermediate risk in patients with del(11q), trisomy 12 or del(6q), and high risk in patients with del(17p) or complex karyotype.⁹A study assessing the median survival of CLL patients with specific chromosomal abnormalities found the shortest survival with del(17p) (32 months), followed by del(11q) (79 months), trisomy 12 (114 months), normal karyotype (111 months), and del(13q) as the sole abnormality (133 months).¹⁰

Among these abnormalities, deletion of the long arm of chromosome 17, which involves TP53 deletion, is associated with the worst prognosis. Patients with del(17p) are subject to rapid progression of disease, lack of response to therapy, short response duration, and short overall survival. The risk of death in patients with TP53 mutations is 13 times that of patients lacking the mutation, due to the possible involvement of this protein in the pathogenesis of more aggressive forms of CLL.¹¹ In addition, the clinical relevance of the number of cells displaying del(17p) has been demonstrated in CLL patients. The presence of >20% of cells with loss of TP53 has been associated with an adverse prognosis, whereas patients with <20% of cells with TP53 loss had a prognosis similar to CLL patients overall.⁹

Deletion of chromosome 17p causes loss of one allele of the tumor suppressor TP53, a gene responsible for DNA repair, cell-cycle arrest, and apoptosis in response to genetic abnormalities.¹² TP53 and del(17p) mutations are associated with poorer overall response rates and shorter progression-free and overall survival.¹³ The majority of CLL patients are initially highly responsive to chemotherapy, which commonly acts by inducing apoptosis via TP53.¹⁴ In up to half of relapsed or refractory patients, TP53 mutations are acquired through the selective pressure of chemotherapy and clonal evolution, a key feature of cancer progression.¹⁵ Currently, most laboratories monitor TP53 status by identifying 17p deletions; however, 23%-45% of patients with a TP53 abnormality have a gene mutation rather than a chromosomal deletion.¹²

An additional biomarker with prognostic value is the mutational status of the immunoglobulin heavy chain variable region (IGHV). The B-cell response to antigen is mediated through the B-cell receptor (BCR), a highly variable receptor that is distinct to each B cell. The diversity of the BCR is generated through the rearrangement of V, D, and J gene segments and the introduction of somatic mutations during the creation of the heavy and light chains of the receptor.² The BCR in a normal mature B cell has a highly unique antigen-binding site due to this process of random gene rearrangement and the probability of two independent B cells with the same BCR is exceptionally small. However, approximately 30% of patients with CLL express similar, if not identical, BCRs with “stereotyped” features, suggesting the recognition of a similar antigen may be involved in the pathogenesis of CLL.¹ In addition, IGHV is classified as either mutated or unmutated in CLL patients, with unmutated IGHVs showing poorer survival and more aggressive disease.^1,2 The median progression-free survival of patients with unmutated IGHV genes ranges between 1 to 5 years, significantly shorter than the 9.2 to 18.9 years seen in patients with mutated IGHV genes. Similarly, the median overall survival for unmutated IGHV is 3.2 to 10 years and ranges between 17.9 and 25.8 years for patients with mutated IGHV.^16,17 This difference is partly due to differences in protein expression, telomere length, and the acquisition of detrimental chromosomal abnormalities between unmutated and mutated IGHV CLL cells.²

Clinical Staging¹⁸

There are two widely accepted staging methods for use in both patient care decisions and clinical trials: the Rai system and the Binet system.

Rai Staging System

• Low-risk disease (Stage 0): lymphocytosis with leukemia cells in the blood and/or marrow (lymphoid cells >30%)
• Intermediate-risk disease (Stage I or II): lymphocytosis, enlarged nodes in any site, and splenomegaly and/or hepatomegaly
• High-risk disease (Stage III): disease related anemia (hemoglobin levels <110 g/L [11g/dL])
• High-risk disease (Stage IV): thrombocytopenia (platelet count <100 x 10⁹/L)

Binet Staging System

Staging is dependent on number of areas involved and the presence of anemia or thrombocytopenia.

Areas of involvement considered for staging:

1. Head and neck, including the Waldeyer ring (this counts as one area, even if more than one group of nodes is enlarged)
2. Axillae (involvement of both axillae counts as one area)
3. Groins, including superficial femorals (involvement of both groins counts as one area)
4. Palpable spleen
5. Palpable liver (clinically enlarged)

• Stage A: hemoglobin 100 g/L [10 g/dL] or more and platelets 100 x 10⁹/L or more and up to 2 of the above areas involved
• Stage B: hemoglobin 100 g/L [10 g/dL] or more and platelets 100 x 10⁹/L or more and 3 or more areas of nodal or organ enlargement
• Stage C: All patients with hemoglobin less than 100 g/L [10 g/dL] and/or platelet count less than 100 x 10⁹/L, irrespective of organomegaly

References

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