Improved Outcome of CLL Patients with Leukemic Clones Expressing Mutated IGHV May Not Be Due to An Inability to Bind (auto)Antigen In Vivo

CLL patients with mutated IGHVs (M-CLL) have clinical outcomes better than patients with unmutated BCRs (U-CLL). It is believed that this difference stems from the fact that somatic mutations have altered the structure of the BCR such that it no longer binds stimulatory (auto)antigens and therefore cannot deliver trophic signals to the leukemic cells. We sought to determine if this belief could be corroborated by analyzing the types (silent vs. conservative vs. non-conservative) and locations (FRs v CDRs) of mutations that occurred in IGHVs of M-CLL clones and then comparing the time to first therapy (TTFT) in patients with different IGHV features.

Using IGMT software, we analyzed the IGHV sequences of 1560 cases and characterized their mutations in several respects: first, if IGHV mutations altered amino acid structure (silent vs. replacement); second, if mutations occurred in CDRs (antigen binding domains) or FRs (scaffolds of the BCR); third, if mutations were conservative or non-conservative (as determined by charge, hydropathy, size). TTFT for patients was correlated with various combinations of the above parameters. Differences in TTFT were estimated by the method of Kaplan and Meier; differences were assessed using the log rank test.

First we compared TTFT in patients who had only silent mutations (n=32) to those with no mutations (n=563) and found a trend toward longer TTFT in patients with silent mutations (3.0 vs. 3.6 yrs, p=0.09). Because the statistical significance of this result was likely influenced by the small numbers of clones with only silent mutations, we expanded the group to include IGHVs with replacement mutations of only the conservative type (n=83) since these would be unlikely to havemajor impact on BCR structure. Comparison of this group with those with no mutations for differences in TTFT was significant (3.0 vs. 3.9 yrs, p=0.0007). Furthermore, comparisons of patients with silent plus conservative mutations in FRs only (n=123) versus patients with no mutations showed a clear difference in TTFT (3.0 vs. 4.1, respectively; (p<0.003). In both of the above instances, non-conservative replacement mutations do not have a significantly greater impact on TTFT than the silent plus conservative combination.

It is assumed that M-CLL patients have a clinical course superior to U-CLL patients because IGHV somatic mutations change the structure of the M-CLL BCR such that it no longer binds stimulatory antigens. For this theory to be correct, only replacement mutations and in particular non-conservative replacement mutations that would alter amino structure of the IGHV/D/J rearrangements would have relevance. Silent mutations, which by definition do not change amino acid structure, should be irrelevant in this regard, and conservative mutations, which substitute amino acids with similar chemical properties to the original amino acid, would be unlikely to result in major changes in the BCR. Therefore, the fact that the combination of only silent plus conservative mutations, which probably would not result in changes in BCR structure and therefore antigen binding, is associated with a lengthened TTFT militates against a loss of antigen binding as a reason for improved clinical course. Therefore, we suggest that another explanation for the enhanced survival of M-CLL patients should be sought. One possibility is that the cells that have the capacity to mutate their IGHVs are different in origin and/or biologic properties from those that cannot, and these account for the differences in survival between M- and U-CLL patients.

(This work was supported NIH grant PO1-CA81534 of the CLL Research Consortium.)

Byrd: Genzyme Corporation: Research Funding. Kipps: GlaxoSmithKline: Research Funding.