Immunoglobulin heavy chain variable region gene and prediction of time to first treatment in patients with chronic lymphocytic leukemia: Mutational load or mutational status? Analysis of 1003 cases

To the Editor: The mutation status of the immunoglobulin heavy chain variable region gene (IGHV) is a well-recognized prognostic indicator in patients with chronic lymphocytic leukemia (CLL). The occurrence of IGHV somatic hypermutations (SHM) in CLL cells and the predictive value of its presence or absence have been recognized in patients with CLL for roughly 20 years. The cut-off value of a 2% deviation from (or ≤ 98% identity with) the corresponding IGHV germline sequence has been used in the initial studies to distinguish “mutated” (M-CLL) from “unmutated” (UM-CLL) cases. Based on this cut-off value, it was determined that unmutated cases had a significantly shorter time to first treatment (TTFT) and outcome. This cut-off value was selected based on the consideration that differences of IGHV genes from the germline of up to 2% might have been related also to allele polymorphisms within the immunoglobulin loci. These somewhat conservative parameters were also suggested by the relative scanty number of sequence data available at that time. The frequency of IGHV gene polymorphisms was reported to be 0.0005% by a subsequent study suggesting that a cut-off value of 99.99% homology to the germline would be sufficient to distinguish truly unmutated from mutated cases. Some of the subsequent studies indicated that a 2% cut-off value was adequate to distinguish 2 subgroups of patients with a different prognostic expectation while others, virtually in equal numbers, proposed different cut-off values. Despite these discrepancies, the 2% cut-off value is generally adopted in the current clinical practice. Recently, Jain et al. demonstrated in patients treated with the fludarabine, cyclophosphamide, and rituximab regimen that their subsequent course and survival was greatly influenced by the amount of mutations present in the IGHV gene segment. Thus, the higher levels of mutation corresponded to a better outcome, pointing to the potential prognostic relevance of the percentage of IGHV gene mutation as a continuous rather than a dichotomized variable. The particular study design adopted by Jain et al. raises the question of whether their criteria for mutation evaluation have a universal value or apply only to patients treated with that particular protocol, as pointed out by Gocke and Richardson in a recent commentary. We assessed the prognostic significance of the extent of SHM on the TTFT in a large cohort of 467 newly diagnosed Binet stage A patients from several Italian institutions who were prospectively enrolled in the O-CLL protocol (training cohort, median follow-up 84 months; clinicaltrial.gov identifier: 115 NCT00917540). The same analysis was performed on a Mayo Clinic validation cohort of 536 cases (median follow-up of 78 months). Further details and data analyses are described in the Supporting Information Appendix. Among the 141 cases classified as UM-CLL in the O-CLL protocol based on the ≤2% mutation cut-off point, 94 (67%) did not present any mutation (truly unmutated cases), 31(22%) presented a 1% mutation and 16 (11%) presented a 2% mutation (Figure 1A). No significant differences were noted in the TTFT of these 3 groups of patients (Figure 1B). The remaining 326 cases, classified as mutated in the O-CLL protocol, had a more even distribution among the different mutation categories according to criteria similar to those employed by Jain et al. (Figure 1A). However, no significant differences were observed in terms of risk to be treated in the groups of mutated cases stratified according to the extent of SHM (Figure 1C). Similar results were observed in the 536 cases of the Mayo Clinic cohort (Figure 1D-F), which provided a further validation of the O-CLL data. Thus, a ≥ 98% homology between the IGHV genes of CLL clones and their normal counterpart represents a valid discriminant to distinguish groups with a different prognostic likelihood, without a need for a further stratification based upon the extent of IGHV gene mutation. With the aim of further validating this result, a Cox multivariable analysis was performed introducing into the model a minimal number of variables which represented a significant discriminant for TTFT in the Cox univariate analysis—ie, Rai stage (H.R. 2.5, C.I. 2.0-3.2, P < .0001), β2M (H.R. 2.3, C.I. 1.8-2.9, P < .0001)— together with the extent of IGHV mutation leading to the stratification of CLL cases into different groups. This analysis was carried Received: 23 June 2018 Accepted: 28 June 2018