Cancer‐associated POT1 mutations lead to telomere elongation without induction of a DNA damage response

Abstract

Mutations in the shelterin protein POT1 are associated with chronic lymphocytic leukemia (CLL), Hodgkin lymphoma, angiosarcoma, melanoma, and other cancers. These cancer-associated POT1 (caPOT1) mutations are generally heterozygous, missense, or nonsense mutations occurring throughout the POT1 reading frame. Cancers with caPOT1 mutations have elongated telomeres and show increased genomic instability, but which of the two phenotypes promotes tumorigenesis is unclear. We tested the effects of CAS9-engineered caPOT1 mutations in human embryonic and hematopoietic stem cells (hESCs and HSCs, respectively). HSCs with caPOT1 mutations did not show overt telomere damage. In vitro and in vivo competition experiments showed the caPOT1 mutations did not confer a selective disadvantage. Since DNA damage signaling is known to affect the fitness of HSCs, the data argue that caPOT1 mutations do not cause significant telomere damage. Furthermore, hESC lines with caPOT1 mutations showed no detectable telomere damage response while showing consistent telomere elongation. Thus, caPOT1 mutations are likely selected for during cancer progression because of their ability to elongate telomeres and extend the proliferative capacity of the incipient cancer cells.

Germline POT1 Deregulation Can Predispose to Myeloid Malignancies in Childhood

“…Compared to the overexpression model, the primary fibroblasts harboring heterozygous POT1 p.Q199* displayed strong telomeric elongation. This is in line with published data from human embryonic stem cells (hESC), in which cancer-associated POT1 mutations did not trigger DNA damage responses but led to longer telomeres [ 21 ]. This telomere elongation is further confirmed by numerous in vitro POT1 knockout and overexpression models [ 5 , 21 , 22 , 23 ].…”

“…Moreover, previous reports on POT1 germline variants in other cancers connect POT1 deregulation with telomere elongation [ 26 , 27 ]. Therefore, our findings suggest that germline POT1 haploinsufficiency causes abnormally long telomeres, which might generate a susceptible cell population with extended proliferative capacity and the potential to acquire additional mutations required for malignant transformation [ 21 ]. Likewise, long telomeres are more fragile and pose an increased risk for genomic instabilities, favoring cancer progression [ 18 ].…”

“…Likewise, long telomeres are more fragile and pose an increased risk for genomic instabilities, favoring cancer progression [ 18 ]. POT1 is mostly expressed in stem and progenitor cells as depicted by scRNA-Seq data and human hematopoietic stem cells (HSCs) harboring CRISPR-Cas9-induced heterozygous POT1 stop-gain mutations do not display fitness disadvantages in vivo [ 21 ]. Therefore, the precursor cells of the hematopoietic system could be directly affected by POT1 aberrations, which is in line with the clinical phenotype observed in the patient.…”

Telomeric replication stress: the beginning and the end for alternative lengthening of telomeres cancers

“…More recently, it has been shown that telomeric replication stress caused by POT1 dysfunction enables telomeres to travel along nuclear actin filaments to the nuclear periphery, resulting in MiDAS at telomeres in telomerase-positive cells [ 94 , 95 ]. Consistent with replication stress-induced telomere lengthening, POT1 mutations have been identified in a growing number of cancer types, including chronic lymphocytic leukaemia (CLL), melanoma and sarcoma, and are associated with longer telomere lengths [ 96 ]. These discoveries demonstrate the capacity for localized replication stress to promote the repositioning of telomeres and the engagement of telomerase.…”

Genetic and Epigenetic Inheritance at Telomeres

“…The Q94E mutation has been found in individuals from families with high levels of melanoma and has also been identified as a recurrent somatic mutation that occurs during development of chronic lymphocytic leukemia [187,190]. Expression of several cancer-associated POT1 variants leads to telomere elongation and sometimes to increased telomere fragility [188,191]. The long somatic telomeres caused by human germline POT1 mutations could impede the ability of senescence to suppress tumor development but may also affect DNA damage signaling and recombination at telomeres [192,193].…”