Telomere Replication Stress Induced by POT1 Inactivation Accelerates Tumorigenesis

Abstract

Genome sequencing studies have revealed a number of cancer-associated mutations in the telomere-binding factor POT1. Here, we show that when combined with p53 deficiency, depletion of murine POT1a in common lymphoid progenitor cells fosters genetic instability, accelerates the onset, and increases the severity of T cell lymphomas. In parallel, we examined human and mouse cells carrying POT1 mutations found in cutaneous T cell lymphoma (CTCL) patients. Inhibition of POT1 activates ATR-dependent DNA damage signaling and induces telomere fragility, replication fork stalling, and telomere elongation. Our data suggest that these phenotypes are linked to impaired CST (CTC1-STN1-TEN1) function at telomeres. Lastly, we show that proliferation of cancer cells lacking POT1 is enabled by the attenuation of the ATR kinase pathway. These results uncover a role for defective telomere replication during tumorigenesis.

Keywords: ATR; POT1; cutaneous T cell lymphoma; replication stress; telomere; thymic lymphoma.

Figure 1.. Conditional Deletion of POT1a and p53 in Mouse CLP Cells Leads to Aggressive Thymic Lymphomas

(A) Flow cytometry analysis of CD4 (y axis) and CD8a (x axis) on thymocytes from mice of the indicated genotypes at approximately 16 weeks of age. (B) Survival curve of mice of the indicated genotypes: POT1^Cd2 (n = 20), p53^Cd2 (n = 18), POT1p53^Cd2 (n = 18), and Wild-Type (n = 25) (this cohort includes hCd2iCRE negative mice, as well as hCd2-iCRE positive not containing floxed alleles for POT1 or p53); survival curves of p53^Cd2 and POT1p53^Cd2 mice were compared using the log rank test (p < 0.01). (C) Representative images of thymus and spleen from mice of the indicated genotypes. (D)Quantification of thymus (top) and spleen size (bottom) from wild-type (WT), POT1^Cd2, p53^Cd2, and POT1p53^Cd2 mice. The bars represent mean ± SD (n = 10 mice/genotype). The numbers within the graph indicate the mean value. (E)Histological analysis of liver and lung tissues from p53^Cd2 and POT1p53^Cd2 mice that succumbed to tumor burden. The tissue sections were stained with H&E, as well as anti-CD3 antibody.

Figure 2.. Exacerbated Genome Instability in Tumors Derived from POT1p53^Cd2 Mice

(A)Circos plots illustrating the inter-chromosomal fusion transcripts detected by RNaseq analysis of p53^Cd2 and POT1p53^Cd2 thymic lymphomas. The connecting curves represent the chromosomal location of the genes involved in translocations. The chromosome number and name of the genes involved in translocation are indicated in the figure. Note that a complete list of translocations identified is also reported in Figure S2. (B) Graph depicting the number of inter- and intra-chromosome fusions in tumors isolated from p53^Cd2 and POT1p53^Cd2 mice. (C) Representative metaphase spreads from thymic lymphomas with the indicated genotype. The telomeres are in green (peptide nucleic acid [PNA] probe) andthe chromosomes in red (DAPI). The highlighted chromosomes in the right images show examples of telomeres fusions (white arrow) and fragile telomeres (red arrow). (D) Quantification of chromosome end-to-end fusions and fragile telomeres detected on metaphase spreads derived from primary thymocytes of the indicatedgenotypes. The bars represent mean values of three independent experiments with SDs. (E) SKY analysis of tumor-derived thymocytes from POT1p53^Cd2 mice displaying extensive chromosome rearrangements. (F) Aberrant mitotic figures in thymic lymphoma sections from POT1p53^Cd2 mice stained with H&E.

Figure 3.. POT1 F62V and K90E Localize to the Telomeres and Induce Telomere Elongation

(A) Schematic of the POT1/POT1a protein domains, highlighting the F62V and K90E mutations, which map to conserved residues in the OB fold of the protein. (B) Representative EMSA to determine the DNAbinding affinity of purified WT and variant POT1 proteins to a radioactively end-labeled telomeric primer (5ˊ-TTAGGG)₃. (C) Data from (B) plotted as a function of fractionbound versus protein concentration. The mean KD values of three independent experiments are reported in the graph. (D) Southern blot analysis to monitor telomerelength in HT1080 cells overexpressing WT and mutant POT1 proteins over several PDs. The median telomere length in each lane was calculated using the Telometric 1.2 software (Grant et al., 2001). The elongation rate for each condition was subsequently computed from the differences in median telomere length between lanes.

Figure 4.. POT1 Mutations Elicit an ATR Response and Lead to Telomere Fragility

(A)IF-FISH staining for the DNA damage factor 53BP1 (green) at telomere DNA (red) in HT1080 cells overexpressing Myc-tagged WT POT1, as well as POT1-F62Vand POT1-K90E. (B) Quantification of TIFs in HT1080 cells in the absence (white bars) and in the presence of ATR inhibitor (gray bars 1 mM ATRi and black bars 5 μM ATRi) for 6 hr. The bars represent the mean of three independent experiments (n = 100 nuclei per sample) ±SD (p value derived using student’s t test) (two-tailed). (C) CRISPR/Cas9 gene targeting was used to create single base substitutions to the POT1 locus in RPE-1 cells and to generate clonally derived cells with the K90E mutation. IF-FISH was used to denote the TIFs response in cells expressing mutant POT1.

Figure 5.. POT1 Mutations Lead to Replication Fork Stalling at Telomere DNA

(A) SMARD analysis of telomeric DNA from MEFs overexpressing WT and mutant POT1a. The telomeric DNA molecules of variable lengths were identified bytelomeric FISH (TelC; blue) with incorporated IdU and CldU detected with fluorescent antibodies (red and green, respectively). The molecules are representative of a normal replication profile. (B) Representative molecules that exhibit stalled replication forks at the junction between subtelomeres and telomeres. (C) Quantification of stalled replication forks in MEFs overexpressing WT and mutant POT1a. The average of two independent experiments ±SEM is shown. (D) Quantification of TIFs for cells with the indicated genotype. There were >2 independently isolated clones that were analyzed for each genotype. The barsrepresent the mean of three independent experiments (n = 100 nuclei per sample) ±SD (p value derived using student’s t test) (two-tailed). (E) Quantification of fragile telomeres on metaphase spreads from HT1080 cells expressing WT and POT1 variants. The mean of at least three independent experiments ±SD (n > 4,000 total analyzed telomeres) (p value derived using student’s t test) (two-tailed) are shown. (F) Quantification of fragile telomeres on metaphase spreads from tumor-derived POT1p53^Cd2 thymocytes overexpressing WT and POT1a mutants. The mean of three independent experiments ±SD (p value derived using student’s t test) (two-tailed) are shown.

Figure 6.. F62V and K90E Mutations Impact CST Function at Telomeres

(A) Quantification of TIFs in HT1080 cells overexpressing WT and mutant POT1 and treated with shRNA against CTC1 (gray bars), STN1 (black bars), or control (white bars). The mean of three independent experiments (n > 100 nuclei per sample) ±SD (p value derived using student’s t test) (two-tailed) are shown. (B) Quantification of fragile telomeres on metaphase spreads from HT1080 cells with the indicated treatments. The mean of three independent experiments ±SD (n > 2,500 total analyzed telomeres) (p value derived using student’s t test) (two-tailed) are shown. (C) Telomere-ChIP from S phase synchronized TEN1^3XFLAG HeLa 1.3 cells expressing WT and mutant POT1. The telomeric DNA was blotted and detected with a telomere specific probe. The graph represents the % of input quantification of the telomere ChIP. The values were normalized to the control sample expressing WT POT1. The mean of five experiments ±SD (p value derived using student’s t test) (two-tailed, unequal variance) are shown. (D) Quantification of fragile telomeres on metaphase spreads from HT1080 cells overexpressing WT and mutant POT1 in the absence (white bars) or presenceof overexpressed TEN1 (gray bars). The values were normalized to WT samples with no exogenous CST proteins. The mean of three independent experiments (n > 4,500 total analyzed telomeres) ±SD (p value derived using student’s t test) (two-tailed) are shown.

Figure 7.. Attenuation of the DNA Damage Response following POT1 Inhibition

(A) RNA-seq analysis of three p53^Cd2 and four independent POT1p53^Cd2 thymic lymphomas. (B) Western blot analysis of phosphorylated-Chk1 (pCHK1) and CHK1. The lysates prepared from primary thymocytes isolated from mice of the indicated genotypes were treated with UV (100 J/m²) and harvested 90 min after. (C) Bar graph displaying percentage of TIF positive heterozygous (F62V/+) and homozygous (F62V/F62V) mouse ES cells, comparing levels of TIFs in early (whitebars) and late passage (gray bars) cells. The mean of three independent experiments (n > 100 nuclei per sample) ±SD are shown. (D) Model depicting the proposed mechanism by which POT1 mutations shape tumor progression. POT1 inhibition impairs the function of CST at telomerespossibly by destabilizing the interaction between STN1 and TEN1. This then induces replication-dependent telomere defects (telomere fragility, ATR-dependent signaling, telomere elongation, and chromosome end-to-end fusions), which in turn triggers chromosomal instability. Ultimately, attenuation of the DDR enables the proliferation of cancer cells, leading to more aggressive tumors.