Comprehensive Cytogenetic and Molecular Genetic Characterization of T-PLL Identifies for the First Time BCOR Mutations in a Lymphatic Disease.
Authors: Grossmann Vera , Kohlmann Alexander , Abáigar María , Kern Wolfgang , Haferlach Torsten , Schnittger Susanne , Haferlach Claudia 
Published: October 2019

Abstract
Abstract 2539

Introduction:
T-cell prolymphocytic leukemia (T-PLL) is a rare mature post-thymic T-cell neoplasm with aggressive clinical course and a median overall survival of less than one year. Due to the rareness of this disease (∼2% of cases of mature lymphocytic leukemias in adults) only few cases with cytogenetic and molecular genetic aberrations have been reported so far. However, almost 75% of T-PLL cases are reported to harbor chromosome 14 abnormalities involving the TCRA/D locus resulting in the aberrant activation of the proto-oncogenes TCL1A or MTCP1.

Aim:
Perform a comprehensive cytogenetic and molecular characterization of T-PLL.

Patient and Methods:
The cohort comprised 36 T-PLL cases diagnosed between 10/2005 and 07/2012 (23 male, 13 female patients). Median age was 71.0 yrs (range: 26.8–82.8 yrs). According to the WHO classification all T-PLL cases were characterized using immunophenotyping and cytomorphology. Patients were further investigated using chromosome banding analysis (CBA) (n=30), FISH for deletions of ATM (n=30), TP53 (n=29), and 13q (n=26), and CGH arrays (n=3, Human CGH 6×630K Whole-Genome Tiling Array, Roche NimbleGen, Madison, WI). Further, mutation analyses for BCOR and TP53 were performed using amplicon sequencing (Roche 454, Branford, CT).

Results:
Aberrant karyotypes were observed by CBA in 25/30 cases (83.3%). However, the 5 remaining cases with normal karyotype were due to insufficient proliferation of the T-PLL clone as gains, losses and rearrangements were detected in these 5 cases using FISH analyses. In more detail, combined CBA and FISH data revealed in 20/30 (66.7%) cases an inv(14)(q11q32)/t(14;14)(q11;q32)/TCRA/D-TCL1A (n=18) or t(X;14)(q27;q11)/TCRA/D-MTCP1 (n=2). Further, a gain of chromosome 8q and concomitant loss of 8p was observed in 13/30 (43.3%) cases. In addition, in 10/25 (40.0%) cases a 6q deletion and in 7/25 (28.0%) an 12p deletion were observed. Based on FISH data, deletions were detected of ATM in 19/30 (63.3%) cases, TP53 in 7/29 (24.1%), and 13q in 9/26 (34.6%) cases.
In addition, 3 cases were studied using array CGH. Hereby, an intragenic deletion in the BCOR gene was observed in one patient. BCOR is a BCL6 corepressor and located on chromosome Xp11.4. BCOR mutations were recently described in cases with AML. BCOR mutation frequency was determined at 3.8% in AML with normal karyotype and mutations were associated with shorter overall and event-free survival. The deletion in BCOR identified in one of our cases and the TP53 deletions in 7 T-PLL cases prompted us to screen 35 cases for molecular mutations in these two genes. Overall, BCOR mutations were detected in 5/35 (14.3%) patients and TP53 mutations in 4/35 (11.4%) cases. In total, 7 missense, one frame-shift and one nonsense mutations were found. Median mutation load was 90.0% for BCOR (range: 38–100%) and 80.0% (59–87%) for TP53.
Next, we performed correlation analyses between mutations in BCOR and TP53, rearrangements involving chromosome 14, deletions of ATM, TP53, 6q, 12p, and 13q and gain of 8q. Here, BCOR was not associated with any of these parameters. In contrast, TP53 mutations were accompanied in all 4 cases by TP53 deletions, while only 3/24 TP53 wild-type cases harbored a TP53 deletion (P=0.002). In addition, only one of 4 TP53 mutated cases harbored a chromosome 14 rearrangement while 18/25 (72%) TP53 wild-type cases did (P=0.105). In line with this result, TP53 deletions were also negatively associated with chromosome 14 rearrangements (2/7 vs 17/22, P=0.030). Further, all cases with a gain of 8q harbored a 14q rearrangement (13/13 vs 8/18 without gain of 8q, P=0.001).

Conclusions:
1. CBA, FISH and mutation analysis of TP53 and BCOR revealed genetic abnormalities in all 36 analyzed cases. 2. The most frequent abnormality involved the TCRA/D locus (14q11) (20/30; 66.7%) activating the proto-oncogenes TCL1A on chromosome 14q32 (90.0%) or MTCP1 on chromosome Xq28 (10.0%). 3. Deletions were detected for ATM (63.3%), TP53 (24.1%), 6q (40.0%), 13q (34.6%), 12p (28.0%), and a gain was detected for the long arm of chromosome 8 (43.3%). 3. In addition to the detection of TP53 mutations in 11.4%, BCOR mutations were observed for the first time in a lymphatic malignancy with a mutation frequency of 14.3%. 4. The prognostic relevance of such cytogenetic and molecular genetic aberrations has to be determined in T-PLL, given that in myeloid malignancies both BCOR and TP53 are associated with shorter OS.

Disclosures:
Grossmann: MLL Munich Leukemia Laboratory: Employment. Kohlmann:MLL Munich Leukemia Laboratory: Employment. Kern:MLL Munich Leukemia Laboratory: Equity Ownership. Haferlach:MLL Munich Leukemia Laboratory: Equity Ownership. Schnittger:MLL Munich Leukemia Laboratory: Equity Ownership. Haferlach:MLL Munich Leukemia Laboratory: Equity Ownership.