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CLL pathogenesis and management
(G>C, glycine-arginine) of IGLV3-21*01 mediated by somatic hypermutation confers autonomous BCR signal- ing.24 This change is present in 7-18% of CLL and seems responsible for the adverse outcome associated with the use of IGLV3-21 independently of the mutational status of the IGHV.24,25
Epigenetic studies have shown that, although both CLL subtypes are antigen-experienced, M-CLL keeps a methy- lation signature of germinal center-experienced cells (memory-like B cells), whereas U-CLL has a pre-germinal center, naïve-like methylation signature.5,26 Of note, these epigenetic studies also identified a third subtype with an intermediate profile made of cases with moderate IGHV mutation levels. All three epigenetic subsets have different usage of IGHV genes, stereotypes, genomic aberrations and clinical outcome (Table 1).27 Their prognostic rele- vance has been validated in retrospective cohorts and clin- ical trials.26–28 The intermediate epigenetic subtype may be more heterogeneous than initially thought since it includes most stereotype subset 2 cases with aggressive behavior whereas other cases may behave more indolent- ly. The understanding of the biological significance of this subtype requires further analysis.
The microenvironment in chronic lymphocytic leukemia
CLL cells are highly dependent on signals coming from
the microenvironment for proliferation and survival.
Tumor cells proliferate primarily in lymph nodes, and to
less extent in bone marrow,29 where they are in intimate
contact with extracellular matrix, T cells, nurse-like cells,
follicular dendritic cells and other stromal cells (Figure 2).
The interactions between CLL cells and this complex
microenvironment are mediated by a network of adhe-
sion molecules, cell surface ligands, chemokines,
cytokines, and their respective receptors. CLL cells organ-
ize their supportive inflammatory milieu and promote an
immunosuppressive microenvironment through different
mechanisms, such as secretion of soluble factors, cell-to-
cell contact, and release of extracellular vesicles (Figure 2).29,30
Environmental or self-antigens and homotypic interac- tions trigger BCR and Toll-like receptor (TLR) signaling, amplifying the response of CLL cells to other signals from the microenvironment and increasing the activation of anti-apoptotic and proliferation pathways.31,32 Genomic studies have identified recurrent mutations in genes regu- lating tumor cell-microenvironment interactions, which are already required for tumor cell growth. Thus, NOTCH1 mutations are dependent on the presence of Notch ligands in the microenvironment and activate processes such as cell migration, invasion and angiogene- sis.33,34 BCR and NOTCH1 pathways are functionally linked, mutually enhancing their activation.35 MYD88 mutations activate the NF-κB pathway in response to TLR ligands, increasing the cytokine release involved in recruiting stromal and T cells.36 Tumor cells also reconfig- ure the function of T- and myeloid-derived cells towards a leukemia-supportive and immunosuppressive microen- vironment.30,37 Thus, tumor cells reduce T-cell motility and the effector function of CD4+ cells while inducing CD8+-cell exhaustion38–41 and monocyte differentiation towards macrophages with protumoral functions (M2- like) and nurse-like cells.37
Many studies have confirmed the fundamental role of BCR activation for CLL pathogenesis.42 Several proteins,
including phosphatidylinositol 3 kinase (PI3K), Bruton tyrosine kinase (BTK) and spleen tyrosine kinase (SYK) are essential for BCR signal transduction.43 The effect of BCR-mediated signaling varies according to IGHV muta- tion status: M-CLL cells are generally driven towards anergy, whereas U-CLL cells are more directed towards cell growth and proliferation.44 Moreover, anergic cells normally retain a higher susceptibility to apoptosis unless anti-apoptotic proteins such as BCL2 are overexpressed, as is the case for CLL cells.45 Indeed, most major therapeu- tic advances occurring in the last decade are related to the inhibition of BCR and BCL2-mediated signaling.
Structural genomic aberrations
Initial chromosome banding analysis revealed that deletions or trisomies were relatively common but only observed in fewer than half of the patients.46 With the advent of fluorescent in situ hybridization (FISH), genom- ic aberrations were identified in more than 80% of patients, the more relevant being trisomy 12, 13q deletion [del(13q)], 11q deletion [del(11q)] and 17p deletion [del(17p)];47 and FISH became the gold standard for genomic evaluation in CLL. Later, the introduction of more effective mitogens expanded the use of chromo- some banding analysis in CLL and revealed other aberra- tions that could not be detected by FISH, including chro- mosome translocations in 20-35% of the cases.48 These translocations may occur in the context of complex karyotypes. The most common rearrangements involve 13q14, with multiple partners, and the IGH locus. The genes most commonly rearranged with IGH are BCL2 [t(14;18)(q32;q21)] (2% of cases, usually M-CLL);3,49 and BCL3 [t(14;19)(q32;q13)] or BCL11A [t(2;14)(p16;q32)] (<1% of cases, usually U-CLL with atypical features).50,51 Chromosomal microarray analysis has identified novel copy number alterations and also copy number neutral loss of heterozygosity.3,52 This latter is observed in 5% of patients, typically affects deleted regions such as 11q, 17p and 13q, and is associated with mutations of the target gene, particularly TP53.52,53 A novel use of both chromo-
Table 1. Clinical and biological characteristics of the three epigenetic sub- types in chronic lymphocytic leukemia.20,27,137
Methylation cell signature
Typical IG genes
Typical stereotype subset IGHV mutations
Mutated drivers
Clinical outcome27
Naïve-like
IGHV1, -5, -7 IGHD6-19 IGHJ4 IGKV1-39
1 Unmutated
NOTCH1 NFKBIE TP53
Aggressive; TTFT at 10 years = 97%
Intermediate
IGHV3-21 IGHJ6 IGLV3-21
2
Mutated or unmutated (around the 98% cutoff)
SF3B1
del(11q)
rarely TP53 Intermediate;
TTFT at 10
years = 38%
Memory-like
IGHV4-34 IGHD5-18 IGHJ6 IGKV2-30 4 Mutated
del(13q)
Indolent; TTFT at 10 years = 24%
IG: immunoglobulin; IGHV: immunoglobulin heavy-chain variable region; TTFT: time to first treatment; ND: not determined.
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