Dysfunctional telomeres and hematological disorders

Abstract

Telomere biology disorders, which are characterized by telomerase activity haploinsufficiency and accelerated telomere shortening, most commonly manifest as degenerative diseases. Tissues with high rates of cell turnover, such as those in the hematopoietic system, are particularly vulnerable to defects in telomere maintenance genes that eventually culminate in bone marrow (BM) failure syndromes, in which the BM cannot produce sufficient new blood cells. Here, we review how telomere defects induce degenerative phenotypes across multiple organs, with particular focus on how they impact the hematopoietic stem and progenitor compartment and affect hematopoietic stem cell (HSC) self-renewal and differentiation. We also discuss how both the increased risk of myelodysplastic syndromes and other hematological malignancies that is associated with telomere disorders and the discovery of cancer-associated somatic mutations in the shelterin components challenge the conventional interpretation that telomere defects are cancer-protective rather than cancer-promoting.

Keywords: DNA damage response; Hematopoietic stem cells; Myelodysplastic syndrome; Telomere biology disorders.

Figure 1. The shelterin complex

The shelterin complex is a 6-protein, telomere-specific complex consisting of TRF1, TRF2, TIN2, RAP1, TPP1, and POT1. TRF1 and TRF2 bind to the double-stranded TTAGGG repeats, whereas POT1 recognizes the single-stranded telomeric overhang. The shelterin complex safeguards the chromosome ends from nuclease degradation and aberrant activation of the DNA damage response and regulates telomerase access to telomere ends. TRF2 represses the activation of the ATM-dependent DNA damage response and NHEJ, while POT1 (together with RAP1) prevents the activation of ATR-dependent signaling and suppresses HR.

Figure 2. Telomere shortening and genomic instability

In the absence of telomerase, the linear nature of eukaryotic chromosomes results in telomere shortening during every cell division. Even cells that maintain telomerase activity, such as stem cells, undergo telomere shortening as they age. Unprotected telomeres engage DNA damage response (DDR) pathways that activate replicative senescence and/or apoptosis. Cells with defective p53 or p16/RB pathway function, which deactivates DNA-damage signaling, continue to divide. As many telomeres become critically short owing to the prolonged cell proliferation, uncapped telomeres can undergo end-to-end fusion and recombination, which leads to extensive genomic instability (i.e., telomere crisis). Eventually telomerase reactivation or activation of the ALT pathway, which restores telomere function, limits genomic instability and allows cells to escape the anti-proliferative barrier imposed by telomere dysfunction.

Figure 3. Hematopoietic phenotypes induced by telomere dysfunction

A) H&E-stained section of a BM biopsy specimen from a representative patient with a germline inactivating mutation of TERT and critically short telomeres who developed MDS. The black arrowhead indicates areas of BM aplasia and the red arrowhead indicates MDS-associated hypercellularity. B) H&E-stained sections of BM biopsy specimens from representative G0 (left panel) and G5 (right panel) mice. C) Flow cytometry analysis of the Lineage⁻Sca1⁺c-Kit⁺ (LSK) stem cell and Lineage⁻c-Kit⁺ (LK) progenitor cell compartment (left panel) of representative aged G0 (upper panel) and G5 (bottom panel) mice. Flow cytometry analysis of the long-term (LT)-HSC and multipotent progenitor (MPP) populations in the LSK compartment (middle panel) and the megakaryocyte-erythroid progenitor (MEP), granulocyte-macrophage progenitor (GMP), and common myeloid progenitor (CMP) populations in the LK compartment (right panel). D) Representative telomere-FISH and anti-γH2AX immunofluorescence in Lineage⁻Sca1⁺c-Kit⁺CD34⁻CD135⁻ LT-HSCs sorted from 2-month-old G0 (left panel) and G5 (right panel) mice (telomere: red; anti-γH2AX: green; co-localization: yellow).

Figure 4. Proposed model of mutant POT1–mediated genomic instability in cancer(Pinzaru et al., 2016)

A) Wild type POT1 binds the single-stranded telomeric overhang by its N-terminal OB fold domain and restricts the access of telomerase to telomeric DNA (the red blind-ended arrow indicates POT1-mediated inhibition of telomerase access to telomere and the inhibition of telomere replication). B) During telomere replication, POT1-TPP1 enhances telomerase processivity. The CST complex acts as a polymerase-alpha primary accessory factor that enables the replication restart after fork stalling at the G-rich telomeric repeats (Stewart et al., 2012) (green arrow indicate telomere replication restart). C) Mutant POT1 impairs the recruitment of the CST complex to the telomere ends during telomere replication resulting in persistent telomere elongation by telomerase (the green arrow indicates telomere replication). D) In the absence of the CST complex at telomeres the replication machinery frequently stalls at the G-rich telomeric repeats (the red blind-ended arrow indicates stalling of the replication). Stalling of the replication machinery at the G-rich telomeric repeats activates the ATR-dependent DNA damage response that leads to fragile telomeres and telomere dysfunction and eventually culminates in the end-to-end fusions of chromosomes.