Higher-order connections between stereotyped subsets: implications for improved patient classification in CLL

Abstract

Chronic lymphocytic leukemia (CLL) is characterized by the existence of subsets of patients with (quasi)identical, stereotyped B-cell receptor (BcR) immunoglobulins. Patients in certain major stereotyped subsets often display remarkably consistent clinicobiological profiles, suggesting that the study of BcR immunoglobulin stereotypy in CLL has important implications for understanding disease pathophysiology and refining clinical decision-making. Nevertheless, several issues remain open, especially pertaining to the actual frequency of BcR immunoglobulin stereotypy and major subsets, as well as the existence of higher-order connections between individual subsets. To address these issues, we investigated clonotypic IGHV-IGHD-IGHJ gene rearrangements in a series of 29 856 patients with CLL, by far the largest series worldwide. We report that the stereotyped fraction of CLL peaks at 41% of the entire cohort and that all 19 previously identified major subsets retained their relative size and ranking, while 10 new ones emerged; overall, major stereotyped subsets had a cumulative frequency of 13.5%. Higher-level relationships were evident between subsets, particularly for major stereotyped subsets with unmutated IGHV genes (U-CLL), for which close relations with other subsets, termed "satellites," were identified. Satellite subsets accounted for 3% of the entire cohort. These results confirm our previous notion that major subsets can be robustly identified and are consistent in relative size, hence representing distinct disease variants amenable to compartmentalized research with the potential of overcoming the pronounced heterogeneity of CLL. Furthermore, the existence of satellite subsets reveals a novel aspect of repertoire restriction with implications for refined molecular classification of CLL.

Graphical abstract

Figure 1.

The IGHV gene repertoire of stereotyped CLL is clearly distinct from the general cohort. Cases expressing the IGHV1-69 and IGHV3-21 genes were significantly overexpressed in the stereotyped fraction of CLL compared with the heterogeneous group (16.4% vs 12.5% and 7.9% vs 4.8%, respectively). On the other hand, cases carrying the IGHV3-23 showed the opposite trend (4.6% in the stereotyped fraction vs 7.6% in the general cohort). Finally, the frequency of IGHV4-34 expressing cases was high in both groups (8.8% vs 8.7%).

Figure 2.

Cases assigned to major stereotyped subsets represent a significant portion of CLL. Twenty-nine different subsets were identified in the present study containing a minimum of 60 cases (0.2% of the cohort) and were defined as major. The relative size of each major subset is indicated in the graph. Their basic immunogenetic information is given in supplemental Table 4; all sequences assigned to each major subset are listed in supplemental Table 5. Altogether, major subsets comprised in total 4098 of 30 413 rearrangements that corresponds to the 13.5% of the cohort.

Figure 3.

Frequency of BcR immunoglobulin stereotypy in the CLL mutational subgroups. The majority of cases carrying BcR immunoglobulin sequences belonging to U-CLL were assigned to stereotyped subsets compared with approximately one-third of cases from M-CLL.

Figure 4.

The impact of satellite subsets on the relative size of major stereotyped subsets. More pronounced increases were evident in major subsets comprising U-CLL cases, whereas most M-CLL major subsets were not significantly affected. Characteristic examples concern subsets #1, #2 and #8 that correlate with dismal prognosis. On the other hand, subset #148B showed the highest increase in the category of M-CLL subsets.

Figure 5.

Major subset #2 and its close immunogenetic relatives (satellite subsets). Stereotyped subset #2 had 8 satellite subsets with the most frequent being subset #169, also characterized as major. The remaining satellite subsets were minor and cumulatively accounted for 44 BcR immunoglobulin sequences. Subset #2 satellite full names are the following: #164 (Sat-1), #2B (Sat-2), V3-30|9|1 (Sat-3), V3|11|42 (Sat-4), V3-74|8|1 (Sat-5), V3-48|7|1 (Sat-6), and V3|10|25 (Sat-7).

Figure 6.

Patients assigned to major stereotyped subsets and their satellites exhibit consistent clinical profiles. Consistent clinical outcomes were observed between patients belonging to subset #2 and subset #169, its main satellite, with no statistical difference regarding: (A) time-to-first-treatment (TTFT) (P = .98) and (B) overall survival (OS) (P = .23). Similarly, patients from stereotyped subset #1 and #99, its main satellite, showed highly similar (C) TTFT (P = .45), and (D) OS (P = .65).