TELOMERE LENGTH AND POT1 MUTATIONS IN CLL: INCIDENCE, ASSOCIATIONS AND CLINICAL IMPACT IN THE COMPLEMENT 1 TRIAL

Background

POT1 mutations in CLL have been reported to be functionally relevant by a dominant negative effect, leading to uncapping of the telomeric ends, thereby enabling telomerase to aberrantly elongate the telomeres, causing fusion events and chromosomal aberrations. However, their impact on telomere length and outcome has not been established in a clinical trial cohort.

Aims
To study the impact of POT1 mutations on telomere length and disease outcome in a clinical trial cohort.

Methods
We assessed telomere length associations with disease characteristics, especially with that of POT1 mutations in the Complement 1 (OMB110911) trial (1^st line chlorambucil (Chl) vs. ofatumumab-Chl (O-Chl)) in patients considered inappropriate for fludarabine-based therapy. Telomere length was analysed using quantitative PCR from baseline samples of 368 patients (82.3%) with available DNA and this cohort was representative of the full trial population with regard to baseline characteristics. Validation of the technique was done using terminal restriction fragment length analysis (TRF) (R²=0.859, P<0.001) in an independent control sample set (n=18) and 6 of these samples were included in every batch as controls.

Results

Analysis of telomere length associations was performed by dichotomizing the cohort based on the median telomere length (4.53kb). No significant association of telomere length was found with the clinical characteristics age, sex, Binet stage  and ECOG status while short telomeres were significantly associated with high WBC count (P<0.001), high β2-MG (P<0.001), and high CIRS score (P<0.01).

As previously reported in a similar trial (CLL8, Jebaraj et al., ASH 2013), short telomeres were significantly associated with other adverse prognostic factors namely, unmutated IGHV (P<0.001), 17p- (P<0.04) and 11q- (P<0.001). Also gene mutations in NOTCH1 (P<0.001), SF3B1 (P<0.02), TP53 (P<0.02), ATM (P<0.001) and BIRC3 (P<0.01) were all significantly associated with short telomeres.

POT1 mutations were found in 7.6% (n=28 of 368 cases) cases in Complement1 and the mutations were predicted to be dominant negative in function. POT1 mutations were not significantly associated with any of the clinical characteristics or genomic aberrations. Interestingly, there was also no significant association of telomere length with POT1 mutations (4.76 kb vs. 5.34 kb, P=0.68).

At a median follow-up of 31.7 months, there were 245 (67%) events for progression free survival (PFS) and 60 (16.3%) events for overall survival (OS). Incidence of POT1 mutations (Chl: n=16, O-Chl: n=12, P=0.70) and short telomeres (Chl: n=95, O-Chl: n=89, P=0.83) were balanced in both treatment arms. Analysis of the impact of POT1 mutations showed no significant association with remission rate, PFS and OS. In contrast, short telomeres were found to be significantly associated with reduced overall response (72.0% vs. 87.6%, P<0.001), shorter PFS (HR=1.98, P<0.001, Fig. 1) and OS (HR=1.88, P=0.02).

To evaluate their independent prognostic impact, we performed multivariable analysis by Cox regression including as variables the treatment arms, 11q-, +12q, 17p-, mutation status of IGHV, TP53, NOTCH1, SF3B1, BIRC3, MYD88, ATM, FBXW7, POT1 and telomere length. For PFS, O-Chl (HR 0.43, P<0.001), 17p- (HR 3.01, P=0.001), NOTCH1 (HR 1.45, P=0.04),TP53 (HR 1.78, P=0.04 and telomere length (HR 1.71, P=0.001) were identified as independent factors, while for OS, only 17p- had an adverse prognostic impact (HR 3.90, P<0.01).

Summary
In the Complement1 trial short telomeres were found to be significantly associated with various adverse clinical and biological prognostic disease characteristics and poor outcome, including PFS and OS, but had no association with POT1mutations. There was no association between POT1 mutations and clinical outcome.

Keyword(s): Chronic lymphocytic leukemia, Telomere length

www.eha.org