Telomere status in chronic lymphocytic leukemia with TP53 disruption

Abstract

In chronic lymphocytic leukemia (CLL), telomere dysfunction is associated with poor outcomes. TP53 is involved in cellular responses to dysfunctional telomeres, and its inactivation is the strongest adverse prognostic factor for CLL. Given the biological relationship between TP53 and telomeres, and their prognostic value, it is important to improve our understanding of the impact of TP53 alterations on telomeres. We performed a comprehensive study of the deletions and mutations of the TP53 gene and telomere parameters, including hTERT and the shelterin complex, in 115 CLL patients. We found that any type of TP53 alteration was associated with very short telomeres and high hTERT expression, independently of other biological CLL features. Patients with disrupted TP53 showed telomere deletions and chromosomal end-to-end fusions in cells with complex karyotypes. TP53 disruption was characterized by downregulation of shelterin genes. Interestingly, low expression of POT1, TPP1 and TIN2 was also found in some patients with wild-type TP53 and had an adverse impact on progression-free survival after standard genotoxic therapy. In conclusion, we have demonstrated that patients with disrupted TP53 have severe telomere dysfunction and high genomic instability. Thus, the telomeric profile could be tested as a biomarker in CLL patients treated with new therapeutic agents.

Keywords: TP53; chronic lymphocytic leukemia; hTERT; shelterin; telomere.

Figure 1. TP53 loss and mutation are equally associated with telomere shortening and hTERT overexpression in chronic lymphocytic leukaemia (CLL)

Mean telomere length A. and hTERT expression B. in patients with wild-type (wt) TP53 and in patients with either del(17p) or TP53 mutations (mut) in the absence of del(17p) (Kruskal–Wallis H test). The impact of TP53 disruption (dis) was independent of IGHV status and lymphocyte doubling time (LDT), as shown in the multiple logistic regression, with an association between shorter (<0.925, mean) telomeres and higher (>55.5, mean) hTERT C. This is illustrated by comparing telomere lengths in patients with mutated and unmutated IGHV D. and hTERT expression to patients with long and short LDTs E. Bars correspond to the standard error of the mean.

Figure 2. Pantelomeric FISH of representative metaphases from wild-type (wt) TP53

A. and disrupted TP53 B. TP53 disruption was associated with telomeric fusions that led to dicentric chromosomes (arrows) and frequent loss of telomeric signals (one of them is shown with an arrowhead). A duplication of a telomeric signal is shown with an asterisk.

Figure 3. Low shelterin-gene expression in CLL patients with disrupted TP53 and in patients with adverse prognostic factors

A. Hierarchical clustering of 115 CLL patients according to TP53 status, telomere length and expression levels of hTERT and shelterin genes (TRF1, TRF2, POT1, TPP1, RAP1, TIN2). Patients with disrupted TP53 (red) are clustered into a distinct group with short telomeres, high hTERT and low levels of shelterin genes. B. Mean expression levels of shelterin genes in different CLL subgroups according to TP53 status, IGHV mutation profile, CD38 expression, lymphocyte doubling time (LDT) and Binet stage (H test). Bars correspond to the standard error of the mean.

Figure 4. Telomere length and shelterin expression predict progression-free survival (PFS) in patients with wild-type (wt) TP53

A. Kaplan–Meier estimates of PFS in the whole patient population as a function of TP53 status and the wild-type TP53 cohort according to telomere length B. POT1 C. TPP1 D. and TIN2 E. levels. The optimal cut-off values were determined using recursive partitioning. CLL patients with short telomeres and low POT1, TPP1 and TIN2 levels had significantly shorter PFSs. F. Corresponding hazard ratios (HR) and 95%-confidence intervals (95%CI) are presented. When adjusted for telomere length and IGHV mutation profile, low POT1, TPP1 and TIN2 expression remained significant predictors for a shorter PFS.