Feasibility of an easily applicable method of ZAP-70 measurement in chronic lymphocytic leukemia in the routine flow cytometry setting: A methodological approach

Detection methods of ZAP-70 in chronic lymphocytic leukemia

The clinical course of patients with chronic lymphocytic leukemia (CLL) is highly heterogeneous, with some patients experiencing rapid disease progression and others living for decades without requiring treatment. Immunoglobulin heavy chain variable region (IGHV) mutation status is a powerful prognostic factor in patients with CLL. The presence or absence of IGHV mutation status is currently the gold-standard prognostic factor, but this technique is labor-intensive and costly. The expression of ζ-chain-associated protein kinase 70 kDa (ZAP-70), detected by flow cytometry, has been served as a potential surrogate for the absence of IGHV mutation status since 2003. Given the current problems with the standardization of ZAP-70 assessment by flow cytometry, this chapter reviews the analysis of ZAP-70 expression by flow cytometry and the relative experimental conditions using the associated publications for "ZAP-70" and "detection method" on Medline since 2003. Thus, mean fluorescence intensity (MFI) ratio method yielded a more reproducible and easily adaptable method to routine use in the clinical diagnostic laboratory. In conclusion, though ZAP-70 is a valuable prognostic factor in CLL, detection methods of ZAP-70 remain to be fully standardized.

Prognostic impact of ZAP-70 expression in chronic lymphocytic leukemia: mean fluorescence intensity T/B ratio versus percentage of positive cells

BACKGROUND

ZAP-70 is an independent negative prognostic marker in chronic lymphocytic leukemia (CLL). Usually, its expression is investigated by flow cytometric protocols in which the percentage of ZAP-70 positive CLL cells is determined in respect to isotypic control (ISO-method) or residual ZAP-70 positive T cells (T-method). These methods, however, beside suffering of an inherent subjectivity in their application, may give discordant results in some cases. The aim of this study was to assess the prognostic significance of these methods in comparison with another in which ZAP-70 expression was evaluated as a Mean-Fluorescence-Intensity Ratio between gated T and CLL cells (T/B Ratio-method).

METHODS

Cytometric files relative to ZAP-70 determination according to the three readouts were retrospectively reviewed on a cohort of 173 patients (test set), all with complete clinical and biological prognostic assessment and time-to-treatment (TTT) available. Findings were then validated in an independent cohort of 341 cases from a different institution (validation set).

RESULTS

The optimal prognostic cut-offs for ZAP-70 expression were selected at 11% (ISO-method) or 20% of positive cells (T-method), as well as at 3.0 (T/B Ratio-method) in the test set; these cut-offs yielded 66, 60 and 73 ZAP-70+ cases, respectively. Univariate analyses resulted in a better separation of ZAP-70+ vs. ZAP-70- CLL patients utilizing the T/B Ratio, compared to T- or ISO-methods. In multivariate analyses which included the major clinical and biological prognostic markers for CLL, the prognostic impact of ZAP-70 appeared stronger when the T/B-Ratio method was applied. These findings were confirmed in the validation set, in which ZAP-70 expression, evaluated by the T- (cut-off = 20%) or T/B Ratio- (cut-off = 3.0) methods, yielded 180 or 127 ZAP-70+ cases, respectively. ZAP-70+ patients according to the T/B Ratio-method had shorter TTT, both if compared to ZAP-70- CLL, and to cases classified ZAP-70+ by the T-method only.

CONCLUSIONS

We suggest to evaluate ZAP-70 expression in routine settings using the T/B Ratio-method, given the operator and laboratory independent feature of this approach. We propose the 3.0 T/B Ratio value as optimal cut-off to discriminate ZAP-70+ (T/B Ratio less than 3.0) from ZAP-70- (T/B Ratio more/equal than 3.0) cases.

Validation of an LC-MS based approach for profiling histones in chronic lymphocytic leukemia

The in vitro evaluation of histones and their PTMs has drawn substantial interest in the development of epigenetic therapies. The differential expression of histone isoforms may serve as a potential marker in the classification of diseases affected by chromatin abnormalities. In this study, protein profiling by LC and MS was used to explore differences in histone composition in primary chronic lymphocytic leukemia (CLL) cells. Extensive method validations were performed to determine the experimental variances that would impact histone relative abundance. The resulting data demonstrated that the proposed methodology was suitable for the analysis of histone profiles. In 4 normal individuals and 40 CLL patients, a significant decrease in the relative abundance of histone H2A variants (H2AFL and H2AFA/M*) was observed in primary CLL cells as compared to normal B cells. Protein identities were determined using high mass accuracy MS and shotgun proteomics.