Lack of Prognostic Significance of the Conventional and Novel Prognostic Markers in Trisomy 12 Chronic Lymphocytic Leukemia (CLL)

Background. Trisomy 12 (tris12) is a recurrent cytogenetic abnormality in chronic lymphocytic leukemia (CLL), occurring in approximately 15-20% of cases, often as the unique cytogenetic alteration, that is usually considered a clonal driver lesion occurring early in CLL evolution. In the Dohner hierarchical categorization, tris12 CLL are identified as having an intermediate prognostic risk, although recent reports suggest a more complex and heterogeneous clinical behavior. Compared to CLL lacking this cytogenetic abnormality, tris12 CLL show more atypical morphology and immunophenotype, more frequent expression of the negative prognostic markers CD49d and CD38, and presence of NOTCH1 mutations and an unmutated (UM) IGHV gene status. The increased fraction of tris12 CLL carrying adverse prognostic features is in contrast to the intermediate clinical behavior associated with most tris12 CLL cases.

Aim. To perform a comprehensive evaluation of the clinical impact of the major genetic, immunogenetic and immunophenotypic prognostic markers in tris12 CLL.

Methods. The study was based on a multicenter series of tris12 CLL defined according to Dohner (n=283, including 73 cases also bearing del13q), and a comparison group (control) of 553 cases with either del13q (n=308) or without any cytogenetic abnormality (no del17p, del11q, tris12, del13q, n=245). Median follow-up of patients in the tris12 and control groups were 4 years (range 0-22) and 7 years (range 0-28), with 54% and 57% treated patients, and 18% and 15% deaths, respectively. Patient characterization included modified Rai stage, CD49d (CD49dhigh, ≥30% positive cells by flow cytometry), CD38 (CD38high, ≥30% positive cells by flow cytometry) and ZAP-70 (ZAP-70high, ≥20% positive cells by flow cytometry) expression, and IGHV mutational status (mutated, M, or UM according to the 2% cutoff). TP53, BIRC3, NOTCH1 andSF3B1 mutations were screened either at diagnosis or before therapy by NGS with at least 1000X coverage and 1% of sensitivity. Groups were compared by chi-square test; overall survival (OS) was computed from diagnosis to death or censored at last observation, and analyzed by Cox regression analysis.

Results. Comparing the tris12 and the control groups, median age was 64 years (range 30-92) vs 66 years (range 33-92), male gender 55% vs 56% (p=0.86), the modified Rai stage was early in 52% vs 54%, intermediate in 41% vs 42% and advanced in 7% vs 4% (p=0.20). As previously reported, tris12 CLL were characterized by a higher prevalence of cases expressing CD49d (85% vs 31%) and CD38 (62% vs 17%; all p<0.0001), and of UM IGHV cases (55% vs 25%, p<0.0001). Analysis of recurrent mutations highlighted a higher prevalence of NOTCH1 mutations (26% vs 8%, p<0.0001) and of BIRC3 mutations (21% vs 1%, p<0.0001) in tris12 vs control group CLL. Conversely, no differences were found in the fraction of cases with TP53 mutations (3% vs 4%, p=0.38) or SF3B1 mutations (7% vs 7%, p=0.89), and in cases expressing ZAP-70 (62% vs 52%, p=0.09). The impact of these features on OS was tested by univariate analysis: in tris12 CLL, only the UM IGHV gene status predicted shorter OS (HR=2.37, p=0.0063), while none of the other characteristics reaching statistical significance as OS predictors (CD49d HR=1.36, p=0.36; CD38 HR=0.42, p=0.052; ZAP-70 HR=3.12, p=0.07; TP53 HR=2.33, p=0.25; NOTCH1 HR=1.40, p=0.22; SF3B1 HR=2.05, p=0.17; BIRC3 HR=1.22, p=0.61). On the other hand, in the control cohort, a significantly higher HR was found for CD49d (HR 3.11, p<0.0001) and CD38 (HR 3.45, p<0.0001) expression, TP53 (HR 2.88, p=0.0026), NOTCH1 (HR 3.57, p<0.0001), and SF3B1 (HR 2.57, p=0.0038) mutations, as well as for the UM IGHV gene status (HR=2.81, p<0.0001), but not for ZAP-70 expression and BIRC3 mutations (HR=1.74 and HR=1.91, p=0.15 and p=0.37, respectively).

Conclusions. Mutational status of IGHV genes was the sole prognostic factor able to stratify OS in tris12 CLL. Despite the high frequency of NOTCH1 and BIRC3 mutations, as well as of CD49d and CD38 overexpression, these markers failed to convey a prognostic risk in tris12 CLL. The lack of a significant clinical impact for TP53 and SF3B1 mutations might be partly explained by the low number of mutated cases combined with a relative short follow up in our tris12 cohort. These findings are in keeping with the hypothesis of a different patho-biological mechanism occurring in tris12 CLL, which however remains to be fully elucidated.