Intricacies of CLL cytogenetic complexity

In this issue of Blood, Baliakas et al, on behalf of the European Research Initiative on CLL, present the largest retrospective analysis of the prognostic significance of karyotypic complexity, assessed by chromosome banding analysis (CBA) performed on stimulated cultures, in patients with chronic lymphocytic leukemia (CLL) and monoclonal B-cell lymphocytosis (MBL).¹  Their cohort contains >5000 CLL patients and ∼400 patients with MBL from 17 European institutions. Based on their results, the authors demonstrate prognostic heterogeneity among CLL cases that are karyotypically complex. They propose a hierarchical prognostic model that integrates complex karyotype, TP53 aberrations, and immunoglobulin heavy chain variable region gene (IGHV) somatic mutation status.

The landmark study published in 2000 by Döhner and coworkers demonstrated that 80% of CLL cases could be risk stratified based on the presence of 4 recurrent abnormalities detectable by fluorescence in situ hybridization (FISH) analysis performed on interphase (nondividing) nuclei: del(13)(q14.1), trisomy 12, del(11)(q22-23), and del(17)(p13.1).²  Patients with del(13q) as the sole abnormality had a good prognosis. Those with trisomy 12 or who lacked FISH-detectable abnormalities had an intermediate prognosis, and those with del(11q) had a relatively poor prognosis. Patients with del(17)(p13.1), the site of the TP53 tumor suppressor gene, had a particularly poor prognosis characterized by rapid disease progression, treatment resistance, and poor survival. Based on these results, and on the high yield and relative ease of performing FISH analysis on interphase nuclei compared with CBA performed on metaphases, FISH analysis using a panel of probes to the common recurrent abnormalities has become part of the routine clinical evaluation at the time of CLL diagnosis. However, a significant limitation of FISH is that it detects only those abnormalities to which the probes are directed; it cannot detect new abnormalities or karyotypic complexity.

Karyotypic complexity is defined by the International System for Human Cytogenetic Nomenclature as the presence of ≥3 numerical or structural abnormalities in a karyotype; unbalanced or balanced translocations are considered a single abnormality.³  Under standard culture conditions, most CLL cells, which are in the G0/G1 phase of the cell cycle, fail to divide. Newer methods, using different combinations of cytokines, CpG oligodeoxynucleotides, and mitogens, stimulate CLL cells to divide in culture and yield analyzable metaphases in up to 90% of samples.^4,5  Karyotypic complexity is associated with shorter treatment-free and overall survival (OS).^4-7 

Consistent with previous studies, the authors found that karyotypic complexity is generally associated with poor outcome. However, because their cohort was so large, the authors could assess the prognostic impact of the number and types of abnormalities in karyotypically complex cases (see figure). A major finding is that patients with ≥5 abnormalities exhibit a uniformly dismal prognosis regardless of clinical stage, TP53 aberration (deletion of the short arm of chromosome 17 and/or TP53 mutation), or IGHV somatic mutation status. The genomic abnormalities in these cases involve all chromosomes, presumably representing underlying genomic instability. In contrast, patients with 3 or 4 cytogenetic abnormalities (low or intermediate cytogenetic complexity, respectively) had better outcomes, with poor outcomes only in the presence of TP53 aberrations. The abnormalities in these cases tended to be the typical recurrent CLL abnormalities.

The authors described several additional important findings. First, not all patients with complex karyotypes have a poor prognosis. Cases with +12 and +19, in addition to other numerical and structural abnormalities, demonstrate distinctive clinicopathologic features and an indolent clinical course. Second, karyotypic complexity can occur early in the disease course in the absence of previous treatment. The vast majority of samples in this study were collected within 1 year of diagnosis before treatment, and karyotypic complexity was also identified in a small subset of patients with MBL. Finally, ∼5% of patients with either isolated del(13q) or lacking FISH abnormalities (so-called FISH-normal cases) are karyotypically complex and have a significantly shorter OS than patients with the same FISH findings, but without karyotypic complexity. The vast majority of these cases lack TP53 aberrations. Thus, CBA identifies a subset of high-risk patients who would otherwise be considered to have a good prognosis.

The latest International Workshop on Chronic Lymphocytic Leukemia (iwCLL) guidelines indicate that genetic risk stratification by FISH analysis for the common recurrent abnormalities and screening for TP53 mutation should be performed in all patients before treatment.⁸  The guidelines recognize that karyotypic complexity likely has adverse prognostic significance and consider stimulated CBA “desirable” in prospective clinical trials. The results of the current retrospective study have refined the prognostic impact of karyotypic complexity in patients with CLL, and have identified prognostically distinct subsets within the larger category of karyotypically complex CLL. The findings support the inclusion of stimulated CBA in prospective clinical trials to determine its potential contribution to routine clinical practice.