Among the many new developments in B-cell lymphoma in recent years, the inhibition of Bruton tyrosine kinase represents a particularly notable achievement.
Non-Hodgkin Lymphoma (NHL) is 1 of the 10 most common malignancies, accounting for approximately 3% to 4% of cancer cases worldwide.1 Over 30 different subtypes of NHL have been identified.2 Of these, B-cell lymphomas make up the majority of cases in the United States and worldwide, accounting for approximately 85% of cases.3,4
The 5-year survival rate associated with B-cell lymphomas, which varies by subtype, ranges from 83% to 91% for patients with marginal zone lymphoma, and between 44% and 48% for those with plasma cell neoplasms.5 Some B-cell lymphomas, such as mantle cell lymphoma (MCL), are associated with a particularly grim prognosis due an aggressive disease course.5
Bruton tyrosine kinase (BTK) plays a central role in the development of several types of B-cell malignancies. Upon antigen binding to the B-cell receptor (BCR), a host of signaling mechanisms are activated, which eventually result in BTK activation.6 BTK is known to have a prominent role in B-cell development and in regulating BCR communication with the microenvironment.6 B-cell malignancies characteristically co-opt BCR signaling as a survival and proliferation mechanism via 1 or more known mechanisms: activating mutations in BCR signaling domains, antigen-dependent BCR activation, and/or ligand-independent, autonomous BCR pathway activation.6 The mechanism by which this process unfolds has relevance for how strongly BTK is involved.
Treatment of B-cell malignancies is largely based on chemotherapy-based regimens at induction. Although response is variable in subsequent lines of therapy, growth of subclonal cell populations that lead to recurrence and relapse is possible in some instances. Owing to the recognition of the central role of BTK in clonal B-cell survival, a new class of targeted therapies that inhibit BTK activity has emerged, demonstrating significant improvements in animal models, in vitro, in vivo, and in humans.6 The most mature data are associated with the BTK inhibitor ibrutinib, which has been studied in clinical trials of patients with MCL, chronic lymphocytic leukemia (CLL), diffuse large B-cell lymphoma, and Waldenström macroglobulinemia.
These studies have helped elucidate the multiple mechanisms by which BTK inhibition may lead to reduction in disease activity. Fundamentally, ibrutinib interferes with migration and adhesion of CLL and MCL cells, thereby permitting redistribution of the B-cell population.6 Correspondingly, continuous therapy with ibrutinib has been shown to result in normalization of lymphocyte counts and remissions in a majority of patients.6 Similar to experiences with other tyrosine kinase inhibitor therapy approaches utilized in other forms of cancers, both primary and secondary resistance to BTK inhibition has been noted. As a result, the potential role of second-generation BTK inhibitors in ibrutinib-resistant CLL and MCL, and in combinatorial approaches in other settings, is being evaluated.
Reviewing the epidemiology, etiology, and pathophysiology of MCL and CLL, with emphasis on the fundamental role of BTK, this article provides context and background for consid- eration of how the BTK inhibitors ibrutinib and acalabrutinib may impact treatment paradigms for these and potentially other B-cell malignancies.
MCL is a rarely occurring but clinically aggressive entity that accounts for approximately 2% to 10% of all NHLs.1,7,8 Annual incidence of MCL in the United States is estimated to be approximately 0.51 to 0.55 per 100,000 persons, which is similar to rates observed with marginal zone lymphoma, lymphoplasmacytic lymphoma, and Burkitt lymphoma.1 MCL is more common among males and individuals over the age of 60 and rarely occurs among those individuals who are younger than 30 years.1 Some studies have suggested a rising incidence in the past 2 decades, although the contribution of improved diagnosis may artificially inflate the number of cases. As MCL was only formally categorized as a B-cell NHL subtype in 1992,9 there is limited historical data for comparison.1
The underlying cause of MCL has not been fully elucidated. The contribution of various environmental factors (ie, infectious entities, autoimmune disease, and/or lifestyle) and family history of hematopoietic malignancies have each been associated with increased risk but not confirmed.1 Although body mass index, history of cigarette smoking, and alcohol consumption are generally associated with an increased risk of NHL, these have not been implicated as risk factors for MCL.1,2 Meanwhile, occupational exposure to pesticides and solvents has likewise been noted to contribute to B-cell lymphoma pathogenesis, although these risk factors have not been extensively studied in the context of MCL.1,2
There is suggestive evidence that immune suppression and viral infection may increase the risk of developing NHL, but these have been incompletely studied in individuals with MCL.2 By contrast, other findings suggest that MCL disease burden is not increased in individuals with compromised immune systems.1 Smaller studies have noted a link between infection with Borrelia burgdoferi and an increased risk of MCL but not other NHL subtypes; however, these findings have not been confirmed in other studies.2
Although the majority of typical morphology of MCL consists of small- to medium-sized cells with irregular nuclei, dense chromatin, and unapparent nucleoli, a blastoid variant, characterized by high mitotic rate and aggressive proliferation, has been described.7 Additionally, numerous morphologic variants have been recognized to occur, several of which may overlap with and mimic other malignancies, including CLL, marginal zone lymphomas, large B-cell lymphomas, and blastic hematologic proliferations.7 Nevertheless, identification of the hallmark translocation t(11;14)(q13;q32) by fluorescence in situ hybridization (FISH) or detection of cyclin D1 overexpression by immunohistochemistry (IHC) is considered crucial for making a confirmed diagnosis.8 It should be noted that rare cases of D1-negative MCL have been described in the literature2; in such cases, identification of secondary genomic alterations associated with MCL, such as SOX11 transcription factor, may be beneficial.7
Molecular Characteristics and Importance of the Microenvironment
Although the clinical course of MCL is similar to other NHL subtypes, MCL has a distinct molecular profile relative to other NHL subtypes. MCL has been called a “paradigm of cell cycle dysregulation."8 A translocation of the cyclin D1 (CCND1) gene at chromosome 11q13 to the immunoglobulin heavy chain gene (IGH) at chromosome 14q32 is present in the majority of cases.8 This pathognomonic t(11;14) (q13;q32) chromosomal translocation leads to constitutional overexpression of the protein cyclin D1, a cell cycle regulator that promotes transition from the G1 to S phase of the cell cycle.8 Overexpresson of cyclin D1 may also overcome the suppressor effect of retinoblastoma 1 (RB1) and the cell cycle inhibitor p27.2 As a result, if left unchecked, the rate of MCL proliferation increases over time; although, the specific mechanisms and pathways that drive this process are incompletely understood.8 It may be the case that a milieu of secondary chromosomal and molecular alterations that regulate the cell cycle and senescence (ie, BMI1, INK4a, acute renal failure, cyclin-dependent kinase 4, and RB1) and interfere with the cellular response to DNA damage (ie, ataxia telangi- ectasia mutated [ATM], checkpoint kinase 2, and tumor protein [TP53]) may be involved, while the role of epigenetic factors has not been conclusively ruled out.2
In addition to intrinsic tumor growth-promoting factors, signaling from the tumor microenvironment may be consequential for MCL growth and dissemination. Extranodal manifestations, apparent in more than 90% of patients, and a typically high prevalence of circulating MCL cells suggests a high propensity for systemic dissemination.10 As B-cell lymphoma and leukemia cells maintain the same capacity for trafficking and homing as normal cells, stromal cell-derived chemokine gradients are likely highly influential in this process.10 Additionally, signaling pathways deriving from the tumor microenvironment may also contribute to the development of drug resistance.10
Collectively, recognition of the important role of cell signaling in MCL cell survival and proliferation has led to the discovery of new therapeutic targets, with the goal of disrupting crosstalk between tumor cells and the microenvironment. Two distinct strategies have emerged: blockade of homing receptors to mobilize cells from protective tissue niches and targeting signaling molecules that are activated via interactions with the microenvironment.10 Regarding the latter, mounting evidence indicates that B-cell antigen receptor (BCR)—associated kinases are abundantly overexpressed in MCL and play a prominent role in disease maintenance and progression.10,11 A downstream consequence of antigen-BCR binding is the activation of spleen tyrosine kinase (SYK) and the SRC-family kinases (Lyn, Fyn, Blk, or Lck); in turn, the SRC-family kinases then activate BTK.10,11 The integral role of BTK in normal B-cell activity is highlighted by X-linked agammaglobulinemia, a genetic disorder charac- terized by a BTK loss-of-function mutation that is associated with deficient mature B-cell populations and antibody production.11 Correspondingly, targeted agents that have interfered with these prominent BCR signaling pathways and sequester neoplastic B cells from BCR signaling influences, have demonstrated an ability to induce normalization of peripheral lymphocyte counts, leading to clinical remission in the majority of patients.6,10
The MCL International Prognostic Index (MIPI), which is derived based on age and evaluation of performance status, lactate dehydrogenase (LDH) level, and leukocyte count, is a widely used clinical tool for prognostic stratification.7,13,14 Low-risk MIPI scores are associated with a 5-year median overall survival (OS) of 60%, whereas the median OS among those with intermediate- and high-risk scores is approximately 51 and 29 months, respectively.13 An evaluation of 958 patients with MCL (median age, 65 years; range, 32 to 87 years) treated upfront in the MCL Younger or MCL Elderly clinical trials confirmed the clinical utility of MIPI scoring as a prognostic tool.15 MIPI low, intermediate, and high-risk groups were 83%, 63%, and 30% in the 5-year OS rates.15 Intermediate and high MIPI scores were also each correlated with higher hazard ratios for OS and predicted time to treatment failure. A simplified version of the MIPI, as well as 1 that also accounts for Ki-67 proliferation index (MIPI-b) have also been described.
Significance of Tumor Cell Proliferation
High tumor cell proliferation may be predictive of poor prognosis. Some evidence suggests that MCL tumor proliferation is the only prognostic factor that has consistently demonstrated an ability to identify high-risk cases with weak progression-free survival (PFS) and OS.8 Several other biologic parameters believed to have prognostic value have been evaluated, although none has demonstrated significance as an independent variable.16
The Ki-67 index, a marker for tumor cell proliferation, has consistently been shown to be predictive and prognostic in various clinical trials.17 Biologically, Ki-67 is a cellular marker strictly associated with proliferation. In the context of MCL, the Ki-67 index, defined as the percentage of Ki-67-positive lymphoma cells on histopathological slides, has been validated in retrospective analyses of pretreatment specimens has been correlated with the level of cyclin D1 expression.17 Notably, the Ki-67 index varies by MCL subtypes, with blastic and pleomorphic MCL demonstrating higher values indicative of increased proliferative status.17 Correspondingly, these entities are associated with worse clinical outcomes. Correlations between high expression of Ki-67 measured at baseline in patients with MCL and poor clinical outcomes has been suggested, and various treatment trials indicate that Ki-67 status may be useful for risk stratification.8 Hoster and colleagues demonstrated that Ki-67 ≥30% predicts shorter OS and TTF compared with <30%, suggesting that Ki-67 evaluation may be a useful metric for individualized thera- peutic approaches.18
Although the Ki-67 index is validated as independent prognostic indicator, it appears to have greater value when combined with MIPI score in the modified MIPI-b index. In clinical studies, the MIPI-b has shown utility for discriminating patients with good and dismal prognosis.15 At the current time, data are limited on the role of Ki-67 as a prognostic index among individuals with relapsed or refractory MCL, patients treated for advanced-stage disease. However, Determann and colleagues evaluated the role of the Ki-67 index as a prognostic index for advanced-stage MCL. Among patients treated with CHOP (cyclophosphamide, doxorubicin, vincristine, and prednisone) or R-CHOP (rituximab-CHOP), significantly different OS was noted among individuals with a Ki-67 index <10%, 10% to <30%, and ≥ 30%.19
High-proliferation status may have consequences for the outcomes of treatment. Intuitively, interfering in the cell cycle would seem a rational approach to abate cell proliferation. For this reason, the chemotherapy agent cytarabine is recommended for use as a backbone to aggressive treatment in induction and in later lines of therapy. Cytarabine is known to interfere with DNA synthesis during the S phase. However, results from clinical trials in patients with MCL and high-proliferation status with this agent have been less than promising.8 The MCL5 study was aborted after a regimen of high-dose cytarabine-containing induction followed by autologous stem cell transplant demonstrated limited efficacy, suggesting that cytarabine may be insufficient to overcome high proliferation status. Yet, there still may be a role for this agent in combinatorial regimens. For example, as ibrutinib blocks the survival signal oTf the BCR pathway, there is strong rationale that an ibrutinib-cytarabine regimen would be synergistic in slowing cell proliferation, providing benefit for this subpopulation of high-risk patients.8 Due to low reproducibility of assessing Ki-67 status and a lack of established break points for stratifying patients, it is not recommended to use Ki-67 index alone for clinical decision making because of interindividual variability in Ki-67 scores 8.
Other Prognostic Indicators
Other metrics for gauging proliferation status have been proposed. The utility of the mitotic index was assessed in 2 retrospective analyses.8 In these studies, mitotic index was strongly correlated with the KI-67 index and OS. More recently, the development of gene expression to profile messenger ribonucleic acid as an index of proliferation in bulk tissue may overcome some inherent limitations of measuring other biomarkers of proliferation status. However, further validation studies are needed to standardize testing protocols and to address technical challenges in ribonucleic acid extraction.8 In addition to proliferative status, histology may also be useful for understanding the clinical prognosis. Diffuse and nodular MCL are each associated with poor OS and incomplete response to treatment, while the mantle zone variant typically has a similar prognosis to other low-grade lymphomas.2 Additionally, cytologic variants have been identified. MCL cells are most often an intermediate size with irregular nuclei, although blastoid variants, believed to be faster growing and associated with poorer survival outcomes, are usually intermediate to large size with dispersed chromatin.2
Treatment Consideration and Options
Diagnosis of MCL is based on a biopsy of a lymph node, tissue, bone marrow, or blood phenotype.14 Immunophenotyping based on the results of an IHC panel that evaluates the following parameters may be helpful in directing treatment decisions (note: parenthetical represents typical immunophenotype): CD20 (+), CD3 (+), CD5 (+), cyclin D1 (+), CD10 (+/−), DC21, CD23 (+/−), BCL2, BCL6, Ki-67 (where proliferation fraction <30% in lymph nodes is associated with a more favorable prognosis) (Table 1).12 Additionally, cell surface marker analysis by flow cytometry may be useful to identify the following: CD19, CD20, CD5, CD23, CD10.12 The results of such testing and the clinical examination are useful for staging patients, with subsequent utility for directing therapy approaches.
C/A/P indicates chest, abdomen, pelvis; CBC; complete blood count; CCND1, cyclin D1; CT, computed tomography; IHC, immunohistochemistry; LDH, lactate dehydrogenase; MCL, mantle cell lymphoma; MUGA, multiple gated acquisi- tion; NCCN, National Comprehensive Cancer Network; PET, positron emission tomography.
As noted previously, MCL generally follows a moderately to highly aggressive clinical course, which suggests a poor prognosis for most patients. The 5-year survival rate in patients with MCL is between 50% and 70%.2 The poor prognosis of MCL suggests a role for aggressive treatment,20 although several factors contribute to an overall complexity in directing therapy. For example, due to the relative rarity of MCL, treatment recommendations are largely based on phase II data and earlier.14 In addition, clinical response is heterogeneous, with some patients exhibiting robustimprovement. Others succumb rapidly to their disease.14 Presently, chemotherapy is recommended as induction therapy. A variety of chemotherapy-based regimens have been described in the literature and are typically categorized as aggressive or less aggressive therapy (Table 2).12 Response rates, however, are limited, with duration estimated to be approximately 1.5 to 3 years and a median OS of 3 to 6 years.14 Recent developments in drug development offer to significantly improve these outcomes. Of note, the BTK inhibitor ibrutinib has demonstrated impressive results in clinical trials among individuals with relapsed disease, and it is currently being studied in combinatorial approaches and as induction therapy.20 Results from a phase III study showed that the PFS was 14.6 months among patients on ibrutinib, compared with 6.2 months for temsirolimus; additionally, ibrutinib was associated with a more favorable safety profile.21 Based on these findings, ibtrunib may be considered the new standard of care in this setting.20
Guidelines from the National Comprehensive Cancer Network (NCCN) suggest that BTK inhibitors should be used as second- line options after either short- or extended-duration response to chemotherapy regimens (Table 3).12
Role of Deferred Initial Therapy
NCCN indicates National Comprehensive Cancer Network; NORDIC, dose-inten- sified induction immunochemotherapy with rituximab + cyclophosphamide, vincris- tine, doxorubicin, prednisone; RBAC, rituximab, bendamustine, cytarabine; RCHOP, rituximab, cyclophosphamide, doxorubicin, vincristine, prednisone; RDHAP, rituximab, dexamethasone, cytarabine, cisplatin; RDHAX rituximab, dexametha- sone, cytarabine, oxaliplatin; VRCAP, bortezomib, rituximab, cyclophosphamide, doxorubicin, and prednisone.
MCL indicates mantle cell lymphoma; NCCN, National Comprehensive Cancer Network; PEPC, prednisone, etoposide, procarbazine, cyclophosphamide; RBAC, rituximab, bendamustine, cytarabine; RCHOP, rituximab, cyclophosphamide, doxo- rubicin, vincristine, prednisone; VRCAP, bortezomib, rituximab, cyclophosphamide, doxorubicin, and prednisone.
Although MCL typically exhibits an aggressive course, roughly 10% to 15% of patients have indolent disease that may not warrant treatment.7 Patients with indolent disease are typically SOX11− (immunoglobulin heavy chain variable region [IGHV]-mutated) and present with leukemic non-nodal CLL-like splenomegaly, low tumor burden, and Ki-67 proliferation fraction <10%.12 Deferredinitial treatment is not likely to affect OS in these patients.22 In a retrospective review of 440 patients with MCL identified in the British Columbia Cancer Agency Lymphoid Cancer Database, Abrisqueta and colleagues noted that good performance status, no symptoms classically associated with lymphoma (such as fever, fatigue, and extreme weight loss), low LDH, nonbulky disease, nonblastoid morphology, and lower Ki-67 values were associated with delayed initiation of treatment, with over 80% of patients observed longer than 12 months. Of note, OS was 72 months in the observation group (n = 75; 17%) compared with 52.5 months (n = 365; 83%) among those who received immediate treatment and treatment decision was not associated with OS.23
Potential for Personalized Therapy
As in other areas of oncology, there is concerted effort to develop reliable prognostic tools to facilitate the ability to offer personalized therapy.7,8 No validated strategies exist to date for stratifying patients at baseline before induction, although the potential role of measuring minimal residual disease as an indication of the potential for relapse has been widely studied. MRD is defined as the minimal traceable persistence of lymphoma cells following a successful treatment.24 It has been suggested that lymphoma cells remaining after a line of treatment may receive signals from accessory cells that promote survival and/or drug resistance, creating a microenvironment conducive to disease reactivation.10
At the current time, a lack of standardized testing protocols and metrics present challenges for incorporating MRD into clinical practice.24 Nevertheless, MRD may have play several presumptive roles in MCL, including an indication of the success of different induction regimens, to predict disease recurrence, for stratifying patients after treatment, and, potentially, an indication that preemptive treatment may be warranted due to high-risk for relapse.24
Currently, real-time quantitative polymerase chain reaction targeting the IGH locus or the t(11;14) translocation is considered the gold standard for MRD assessment in MCL due to high sensitivity, quantitative results, and standardized techniques.25 Multiparameter flow cytometry and amplicon-based high throughput sequencing have been suggested as alternative methods of evaluating MRD status, although neither has been evaluated in large, prospective clinical trials.25
Some important limitations of MRD should be acknowledged. MRD has been studied extensively among individuals after receiving high-intensity treatment, although data is limited on the impact of low-intensity treatment on affecting molecular remission.25 Second, at the current time, MRD may be more useful as an endpoint in clinical trials. There is insufficient data to discern its role in routine clinical care of patients.25 There is also a lack of clinical trial data regarding the use of MRD status to direct subsequent lines of therapy, and MRD status has not been studied in the context of targeted treatments, such as ibrutinib.24
CLL is the most common leukemia occurring in adults in Western countries, accounting for approximately 25% of all new cases of leukemia.26,27 It is characterized by the accumulation of small, typically monoclonal and functionally incompetent malignant B-cell clones.28 CLL is often associated with a similar entity called small lymphocytic leukemia (SLL); in many classification systems, the term CLL is used when the disease occurs mainly in the blood, SLL describes the same entity that is primarily nodal in presentation. For the purposes of this review, CLL and SLL will be considered together.
There are approximately 20,940 new diagnosis of CLL each year in the United States and it is responsible for approximately 4510 deaths.29 Incidence is estimated to be between <1 and 5.5 per 100,000 people.26 In Europe, the incidence rate is 5.87 per 100,000 population per year among men and 4.01 per 100,000 population among women.30 The median age at diagnosis of CLL is 64 to 70 years and it is rare in children.26,27,29 The increased prevalence of CLL in Western societies versus Asian nations supports the hypothesis that genetic influences play a role in disease development.26 Furthermore, in a study among Japanese individuals who immigrated to the Hawaiian Islands, no increased risk of CLL was noted compared with native populations, implicating the of role of genetics in disease development. Because no single environmental factor contributing to increased risk for CLL has been identified, This notion is further substantiated.26 Additionally, studies have noted a higher risk of developing CLL among first-degree family members of individuals with CLL.31 A higher occurrence of monoclonal B-cell lymphocytosis (MBL), a clinical entity believed to precede CLL in a majority of cases, has also been noted to occur among first-degree family members of patients with CLL.32
Genetic Features of Chronic Lymphocytic Leukemia
Genetic influences play a prominent role in CLL pathogenesis, both in etiology, as well as later in the disease course to influence recurrence. Several chromosomal abnormalities have been identified in patients with CLL. Additionally, mutations in IGHV, while not implicated in CLL development or proliferation, may help to explain more aggressive disease activity in some individuals.
CLL Pathogenesis: Associations With MBL and Implications of Clonal Diversity
The pathogenesis of CLL is understood to be a complex process driven by a variety of intrinsic factors under the influence of mechanisms inherent to the microenvironment. Analogous to the manner in which monoclonal gammopathy of uncertain significance is known to precede multiple myeloma, the vast majority to CLL, if not all cases, are preceded by MBL.33 Overall, MBL occurs in 3% to 10% of the population, and only a small fraction (between 1% to 2% per year) will transform to CLL.33 Notably, MBL shares similar epidemiologic features (ie, predominance in older males of Caucasian descent) and exhibits similar molecular characteristics as CLL (ie, 13q and 7p deletion, and trisomy 12).33 Although the specific mechanisms by which MBL may transform to CLL are unclear, the most likely scenarios are: MBL develops under the influence of B-cell antigenic stimulation, gene mutations, and cytogenetic abnormalities, epigenetic modification may play a role, the process is supported by signaling pathways derived from the microenvironment. Subsequently, in a minority of individuals with MBL, additional events lead to a conversion to CLL with several probable influences playing a role. Genetic mutations harbored by subclones, which become more pronounced as primary clones are eliminated, have been suggested as a potential source of selective pressure on the clonal B-cell population. In addition, whether or not transformation occurs may depend on the presence of passenger mutations that foster the survival of the emerging subclone.33
The Role of Chromosomal Abnormalities
Currently, FISH is considered the gold standard for assessing chromosomal variations, although the emergence of next-gener- ation sequencing (NGS) methods may help redefine the role of genetic testing in CLL.34 Chromosomal abnormalities occur in approximately 80% of patients with CLL and may have importance for prognosis and for directing therapeutic strategies.34 The most commonly identified chromosomal abnormalities are deletions on 13q, 11q, 17p, or 6q.34 Gains of entire chromosomes (eg, trisomy 12, which occurs in approximately 10% to 20% of cases), are less frequent, and often appear as the unique cytogenic alteration.34 Of these, 11q and 17p deletions are associated with the poorest clinical outcome.34
Overall, 13q are typically small and monoallelic.34 Of these, deletion of the 13q14 region is the most common and is present in roughly 50% of patients with CLL.34 Relative to other known chromosomal abnormalities in CLL, 13q deletions are considered lower risk.34 However, biallelic losses have been found to occur in approximately 30% of CLL patients with a 13q deletion.34 Large biallelic losses involving RB1 have been associated with shorter OS in some studies.34
Deletion of the 11q23 region, the most common 11q deletion that is detected in 5% to 20% of CLL patients, may affect several genes, such as ATM, RDS, FRDX1, RAB39, CUL5, ACAT, NPAT, KDELC2, EXPH2, MRE11, and BIRC3.34 ATM mutations are the most common and are present in almost all cases. Under normal conditions, ATM plays a prominent role in recognition of and response to DNA damage, and in phosphorylating proteins that help regulate cell-cycle checkpoints, nuclear localization, gene transcription and expression, response to oxidative stress, apoptosis, nonsense-mediated decay, etc.35 In the presence of DNA breaks, the ATM kinase stalls the cell cycle, thereby affording opportunity for repair rather than passing missense or erroneous information to daughter cells.35 The absence of these critical DNA repair mechanisms, as in the case of mutated ATM, permits unregulated proliferation of B-cell clones. Correspondingly, patients with 11q deletions and those who lack physiologic ATM activity have a poor prognosis and response to initial treatment and shorter time to treatment failure, remission duration, and OS after standard chemotherapy.34
Deletions on 17p, which occur in approximately 3% to 8% of treatment-naïve patients with CLL, and more frequently, among individuals with relapsed/refractory CLL, have been associated with the poorest prognosis relative to other known chromosomal abnormalities in CLL, a possible consequence of the resulting loss of the TP53 gene.34 The tumor-suppressing TP53 gene is located on the short arm of chromosome 17 and its activity is abrogated via 17p deletions. Monallelic inactivation of TP53 is found in 75% of patients with CLL harboring a 17p deletion. Biallelic inactivation of TP53 is associated with signif- icantly poorer OS, PFS, and response rate.34 Nevertheless, even monoallelic inactivation of TP53 may be sufficient for clonal selection and can predict poor response to alkylating agents and purine analogs.34 The latter suggests the need for novel therapeutic approaches, including potential BTK inhibitors. In addition to affecting TP53 in almost all cases, 17p deletions may also affect a number of genes that regulate apoptosis, cell cycle regulation, and B-cell receptor signaling, including leading to an underexpression of cyclin D3, B-cell lymphoma 2 (BCL2), SYK, ATM, T-cell lymphomas, phosphoinositide 3-kinase (PI3K), CCND1, AID, and overexpression of MYC, P2, and AICL.34 Recent advances in genetic sequencing methods have contributed to a greater understanding of the molecular biology of CLL.
For example, whole-genome sequencing studies demonstrate that deletions in 17p and 11q are heterogeneous and occur both in the early and late stages of CLL, which suggests they may each have prominent roles in affecting driver mutations in clonal and subclonal CLL populations.36 In the latter review, the authors proposed a model in which passenger events accumulate in the cell prior to transfor- mation, eventually leading to a founding mutation (ie, a driver mutation) occurring in a cell that fosters subsequent proliferation. Potential driver genes in this early clonal phase include del(13q), myeloid differentiation primary response 88, and trisomy 12. Later, subclonal mutations expand due to the accrual of favorable intrinsic characteristics (eg, pro-prolif- eration and mechanisms to avoid apoptosis) and under the influence of external pressures (eg, interclonal competition and therapy). The most relevant subclonal driver mutations include ATM, TP53, and recurrent aphthous stomatitis mutations. Ultimately, this would suggest that mutations selec- tively affecting B cells may be more consequential for disease initiation while more generic cancer drives may influence disease progression.36
The Role of IGHV Mutations
Whereas several genetic mutations associated with CLL are known to be either directly involved in CLL pathogenesis or implicated in development of MBL and/or conversion to CLL, mutations in the IGHV gene found in CLL cells reflect the stage of maturation of their parental B cell but play no direct role in CLL development.28 Somatic mutations arising in IGHV are a natural part of affinity maturation of antibodies. During this process, B cells located in germinal centers within lymph nodes experience hypermutation in their immunoglobulin variable region genes and selection during an immune response. Correspondingly, CLL cells harboring mutated IGHV display similar immunoglobulins profiles as normal B-cells (although they are not identical). By contrast, CLL cells that express an unmutated IGHV are progeny of B-cells that had not undergone differentiation, and may be less immune competent. This may explain why unmutated IGHV status has been associated with more aggressive disease behavior.
At the current time, IGHV mutation status is not a validated criterion for selecting therapy or for directing treatment decisions. However, regardless of IGHV mutation status, CLL cells characteris- tically exhibit a limited repertoire of immunoglobulin molecules.28 The latter likely reflects that IGHV genes that have limited somatic mutation activity and heavy-light chain combinatorial diversity develop a survival advantage in CLL clonal populations. In turn, this limited immunoglobulin diversity suggests that CLL B cells are selected based on the binding activity of their expressed surface immunoglobulin, which directly implicates the B-cell receptor signaling pathway in CLL pathogenesis.28
Patient Experience and Considerations for Treatment
CLL is commonly discovered incidentally after a presentation of elevated lymphocyte count, although fatigue and difficulty exercising are also common complaints at the time of initial presentation.26 Approximately 5% to 10% of patients with CLL exhibit symptoms classically associated with lymphoma, such as unintentional and significant weight loss, persistent high- grade fever, night sweats, and extreme fatigue.37 Common signs include lymphadenopathy, splenomegaly, hepatomegaly, and skin infiltration, although virtually any lymphoid tissue may be involved.37
A confirmed diagnosis is made based on hematopathology review and flow cyometry to detect for a number of parameters.38 Diagnosis of CLL depends on the presence of monoclonal B lymphocytes ≥5 109/L in peripheral blood, while SLL requires lymphadenopathy and/or splenomegaly with B lymphocytes ≤5 109/L in peripheral blood with confirmation via histopathology evaluation of lymph node biopsy. Immunophenotyping should be directed to detect the following cell surface markers (note: typical immunophenotype shown in parenthetical): κ/λ, CD19 (+), CD20 (dim), CD5 (+), CD23 (+), and CD10 (−); the following are included if flow cytometry are used: cytospin for cyclin D1 (−) or FISH for t(11;14), which helps rule out MCL. Several treatment regimens for CLL have been described.
Overall, 3 prevalent strategies are used: targeting of proteins on the cell (usually chemotherapy approaches), targeting pathways involved in disease activity (ie, BTK and PI3K inhibitors), and targeting the microenvironment (ie, with checkpoint inhib- itors). Considerations for choice of agent and approach depend on a number of factors, including patient age, stage of disease, prior exposure to CLL therapy, mutational status (including presence or absence of 17p or TP53 mutations), and presence or absence of comorbidities.28,38 Early treatment with chemotherapy or chemoimmunotherapy may improve long-term outcomes in first-line settings, although results are variable overall, and the rise of next generation targeted agents that are less toxic may increase the need for early intervention.39
The Role of BTK Inhibitors
Antigen engagement of the BCRs induces downstream activation of 2 classes of tyrosine kinases: the SRC-family kinases and SYK.10 The former activates BTK, which is known to play a prominent role in B-cell development and regulate BCR communication with the microenvironment. BTK activation has been implicated in the pathogenesis of CLL and MCL, and it is potentially active in other malignancies as well.6,40 In the context of CLL, malignant cells take advantage of the BTK pathway as a survival and proliferation mechanism.6 This revelation has led to the development of targeted agents that interfere with BCR signals, including SYK, PI3K, and BTK inhibitors. The BTK inhibitor ibrutinib is 1 example, and it has already gained indications in relapsed/refractory MCL and CLL.
In the context of CLL, BTK inhibition yields compartment shift of malignant B cells from the tissues into the blood that results in transient lymphocytosis and eventual lymph node shrinkage.10 Sequestering neoplastic B cells from BCR and other microenvironment prosurvival signaling mechanisms leads to normalization of peripheral lymphocyte counts and disease remission in the majority of treated patients.6,10 Both primary and secondary resistance to the BTK inhibitor ibrutinib have been described, leading to the development of second-generation BTK inhibitors that overcome such resistance mechanisms and which may have a role in combinatorial therapy.
Both MCL and CLL are complex malignancies with distinct characteristics and features. However, in both, the BTK pathway plays a prominent role in disease development. Both MCL and CLL cells take advantage of the BTK pathway for cell survival and proliferation. To date, these malignancies have been associated with generally unfavorable outcomes, with a particularly grim prognosis for patients with MCL. Novel targeted therapeutic approaches that target the BTK signaling pathway offer to significantly improve the treatment paradigm in MCL and CLL, and emerging evidence suggests that BTK inhibition may also be beneficial in the context of other B-cell malignancy subtypes.