Immunophenotypic Profile and Recurrent Somatic Mutations in 131 Chronic Lymphocytic Leukemia Patients: Low Expression of CD25 in NOTCH1-Mutated Cases

Background: We and others have reported on the impact of recurrent somatic mutations not only in the multistep pathogenetic process, but also in the clinical heterogeneity of chronic lymphocytic leukemia (CLL) patients. Immunophenotyping, as part of the diagnostic workout, is used for assessing clonality, as a differential diagnosis tool, and to examine the expression of molecules associated with a worse prognosis. Recently, NOTCH1 mutations have been linked to low CD20 levels in CLL and with a relative resistance to anti-CD20 immunotherapy in vitro. But to date, there is limited information on the correlation between cell surface marker expression and the presence of somatic mutations in CLL. The aim of this study was to evaluate potential associations between an extended phenotypic panel and the mutational status of 13 recurrently mutated genes in CLL detected by deep sequencing.

Patients and Methods: To this end, we performed targeted NGS sequencing of blood samples, collected at diagnosis, from 131 CLL patients. Every patient underwent, at baseline, a flow cytometry characterization with a panel including (sIg)λ, (sIg)κ, CD19, CD5, CD11b, CD81, CD10, CD79b, CD29, CD38, FMC7, CD22, CD45, CD103, CD11c, CD25, ZAP70, CD11a, and CD24. We designed a TruSeq Custom Amplicon panel (Illumina, Inc. San Diego, CA, USA) containing 13 genes and covering 28.099 bases. The average amplicon size was 238 base pairs and ~ 99.1% of the regions were covered on both strands. Paired-end sequencing (2x250 bp) was performed with MiSeq v2.2 chemistry, and a mean depth of 998 reads/base within the regions of interest was obtained. Raw data were analyzed with IlluminaonJboard Real Time Analysis (RTA v.2.4.60.8) software and MiSeq Reporter.

Results: With a median age of 68 y.o. (range, 33-95) and a slight male predominance, the median follow up time of our cohort was 43 months (24-104). We found that 47/131 (35%) patients harbored at least one mutation, with NOTCH1 (n = 13, 10%), ATM (n = 10, %), TP53 (n = 8, %), and SF3B1 (n = 8, 5.5%), as the most frequently mutated genes. Those patients with a NOTCH1 mutation showed a lower CD25 expression (25 mean fluorescence intensity units (MFIu)) than those without a mutation (45 MFIu), p=0.001. In addition, a higher expression of CD5 (265 vs. 219 MFIu, p= 0.02), of the monoclonal light chain (90.5 vs. 58.6 MFIu, p=0.03), and a higher percentage of CD38+ cells in the CD19+CD5+ compartment (37% vs. 19%, p=0.006) were significantly associated with the presence of, at least, one somatic mutation. We could not validate the recently reported association between the presence of NOTCH1 mutations and a low expression of CD20. In our cohort, the MFI expression in NOTCH1 mutated and non-mutated patients was 176 and 135 units, respectively (p=0.2) In the multivariate Cox analysis, the presence of a somatic variant in TP53 and a higher percentage of positive CD38 cells in the tumour population showed both a worse overall survival and shorter time to first treatment. The independence of these two variables was also supported by not finding a significative difference percentage of CD38 positive cells between TP53 mutated and non mutated cases (p=0.5).

Conclusions: The associations described herein suggest potential pathogenic pathways in CLL, in particular the CD25-NOTCH1 axis, with a significative inferior expression of CD25 when activating NOTCH1 mutations are present. The relationship found between these two variables, with an inversed direction to that found in physiological conditions, has also been shown in the setting of NOTCH1-mutated acute lymphoblastic leukemia, emerging as a potential targetable pathway in this subset of CLL patients.