Low Expression Of BCL11B Predicts Poor Overall Survival In Adult Standard Risk T-ALL

Although risk stratification, detection of minimal residual disease (MRD) and implementation of novel therapeutic agents have improved outcome in acute lymphoblastic leukemia (ALL), survival in adult T-ALL patients still remains unsatisfactory. Therefore, new prognostic markers and potential therapeutic targets are warranted. BCL11B, a key player in normal T-cell development, has recently gained interest due to its high mutation rate (9-16%) in T-ALL. We investigated the frequency of BCL11B mutations, expression levels and the prognostic value in a large uniformly treated cohort of adult T-ALL.

We analyzed bone marrow (BM) samples of 201 adult T-ALL patients sent to the reference laboratory of the German Multicenter Study Group for Adult ALL (GMALL). BCL11B expression was determined in 195 patients by qRT-PCR, BCL11B mutations were assessed in 178 patients by Sanger sequencing of exon 4 (including all 6 zink finger [ZF] domains). Low expression of BCL11B was defined by expression levels in the lowest quartile (BCL11Blow), high expression by levels in the three remaining quartiles (BCL11Bhigh). Samples had previously been characterized for expression of BAALC, IGFBP7, MN1, WT1, GATA3, ERG as well as for the mutations status of NOTCH1, WT1, and TCR rearrangements. Clinical data were available for 169 patients enrolled on the GMALL trials. We generated BCL11B associated gene expression profiles (GEP) derived from an independent set of 86 T-ALL patients enrolled in the Microarrays Innovations in LEukemia multicenter study.

BCL11B was aberrantly expressed in adult T-ALL with significantly higher expression levels in thymic compared to early T-ALL (0.6 vs. 0.3, P=0.01). Expression of genes associated with a prognostic impact (BAALC, IGFBP7, ERG) or/and T-cell stage dependent expression profile (GATA3, IGFBP7) showed that BCL11Blow (n=49) had higher MN1 (1.6 vs, 0.3, P=0.01), IGFBP7 (1.3 vs. 0.5, P=0.02), and lower GATA3 expression (2.1 vs. 5.7, P<0.01) compared to BCL11Bhigh patients (n=146). This maturation stage specific expression of BCL11B was stressed by a higher rate of TCR rearrangement in BCL11Bhigh patients (73% vs. 27%, P=0.005) and further underlined by the BCL11B derived GEP linking low BCL11B to an immature molecular signature characterized by high expression of BAALC, IGFBP7, FLT3, CD34. Regarding clinical characteristics, low BCL11B expression was associated with a poor prognosis (5-year overall survival (OS): low 35% (n=40) vs. high 53% (n=129), P=0.02) in the overall T-ALL cohort. In the standard risk group of thymic T-ALL (n=102), BCL11Blow identified patients with an unexpected poor outcome (5-year OS: 20%, n=18) compared to BCL11Bhigh (62%, n=84, P<0.001). In addition, BCL11Blow thymic T-ALL patients showed a lower remission rate (5 years: 38% vs. 72%, P=0.02).

BCL11B mutations were found in 24 of the 178 (13.5 %) T-ALL patients. In 9 patients, mutations resulted in frame shifts, whereas the remaining, except for a single one, missense mutations were located in the ZF domains: three resulting in introduction of a stop codon. BCL11B mutations were enriched in the mature immunophenotype (thymic: 20%, mature: 8%, early: 3%, P=0.03). No differences were observed in mRNA levels for BAALC, IGFBP7, MN1, WT1, GATA3, ERG, but patients with BCL11B mutations were less frequently assigned to the BCL11Blow group compared to those with high expression (5% vs. 95%, P=0.02). Regarding the clinical characteristics, BCL11B mutations had no prognostic impact regarding OS, neither in the total T-ALL cohort (5-year OS: 56% vs. 48%, n.s.) nor in the thymic T-ALL subgroup (5-year OS: 65% vs. 51%, n.s).

Our data confirmingly show a high frequency of BCL11B mutations (13.5%) in the so far largest cohort of adult T-ALL patients. As loss of function mutations were restricted to functional ZF domains and recurrently occurred in thymic T-ALL, these data stress a potential pathogenetic role of BCL11B as T-cell specific transcription factor. Importantly, low expression was associated with poor prognosis; in particular in the standard risk group of thymic T-ALL, BCL11Blow is a novel marker that identifies patients with an unacceptable poor prognosis. These findings might help to improve risk stratification in a significant proportion of adult T-ALL patients, which fail to standard therapy despite the favourable immunophenotypic characteristics.

No relevant conflicts of interest to declare.