POT1 mutations associated with long telomere length conferred a predisposition to a familial clonal haematopoiesis syndrome that was associated with a range of benign and malignant solid neoplasms. The risk of these phenotypes was mediated by extended cellular longevity and by the capacity to maintain telomeres over time.
The findings were presented at the Meeting of the Cold Springs Harbor Laboratory and simultaneously published by Dr. Mary Armanios of the Department of Oncology, Johns Hopkins University School of Medicine in Baltimore, MD, US, and colleagues on 4 May 2023 in The New England Journal of Medicine.
The length of telomeres predicts the onset of replicative senescence and functions as a mitotic clock. Telomeres shorten with cell division, and short dysfunctional telomeres signal DNA damage, which elicits a cellular response that leads to senescence or apoptosis. Germline loss-of-function mutations in genes that encode components of telomerase, the enzyme that synthesises new telomere repeats, cause a short telomere progeria that is marked by pulmonary and haematopoietic disease. Its most common manifestation is idiopathic pulmonary fibrosis. Long telomeres extend the replicative potential of cultured cells, which raises the possibility that mutations that prevent telomere shortening may influence specific aging phenotypes and disease risk.
The protein POT1 binds the single-stranded 3′ end of the telomere. It is a conserved, essential protein and protects telomeres from exonuclease degradation. POT1 has also been implicated in the regulation of telomerase-dependent elongation of the telomere. Some studies have suggested that it increases the processivity of telomerase, whereas others have identified a role of POT1 in the negative regulation of telomerase catalytic activity.
Heterozygous somatic POT1 mutations were initially identified in chronic lymphocytic leukaemia (CLL) cells, and germline mutations were later reported in families with isolated cancers, including melanoma, glioma, and CLL. However, the mechanisms underlying the risk of disease associated with heterozygosity for POT1 mutations are not understood; hypotheses have included telomere deprotection, genome instability, telomere elongation, and telomere shortening. Telomere shortening is a well-characterised cellular aging mechanism, and short telomere syndromes cause age-related disease. However, whether long telomere length is advantageous is poorly understood.
In this study, the investigators examined the clinical and molecular features of aging and cancer in persons carrying heterozygous loss-of-function mutations in the telomere-related gene POT1 and non-carrier relatives. They recruited participants for a Johns Hopkins University research study dedicated to understanding the role of telomeres in disease. Persons who were heterozygous for POT1 mutations were identified in a clinical setting and, along with their relatives, were invited to participate in the study. An initial group of 17 mutation carriers from 5 families, along with 21 of their non-carrier relatives, were first included in the study; subsequently, 6 mutation carriers from 3 additional families were recruited and included in validation studies.
A majority of the POT1 mutation carriers with telomere length evaluated (9 of 13) had long telomeres (>99th percentile). POT1 mutation carriers had a range of benign and malignant neoplasms involving epithelial, mesenchymal, and neuronal tissues in addition to B- and T-cell lymphoma and myeloid neoplasms. Five of 18 POT1 mutation carriers (28%) had T-cell clonality, and 8 of 12 (67%) had clonal haematopoiesis of indeterminate potential (CHIP).
A predisposition to clonal hematopoiesis had an autosomal dominant pattern of inheritance, as well as penetrance that increased with age; somatic DNMT3A and JAK2 hotspot mutations were common. These and other somatic driver mutations probably arose in the first decades of life, and their lineages secondarily accumulated a higher mutation burden characterised by a clocklike signature. Successive generations showed genetic anticipation (e.g. an increasingly early onset of disease). In contrast to non-carrier relatives, who had the typical telomere shortening with age, POT1 mutation carriers maintained telomere length over the course of 2 years.
The genetic data support the hypothesis that haploinsufficiency for POT1 facilitates telomerase-dependent telomere elongation, although the precise mechanisms by which telomerase repeat addition is enhanced in this context remains unclear. The study team determined that somatic driver mutations arise during the early decades of life and that their long-lived lineages sustain a high mutation burden, which shows clocklike signatures that are a hallmark of an extended replicative history. The loss of the tumour-suppressor mechanism of telomere shortening supports the expansion of clonal populations, which may explain the elevated risk of cancer.
JAK2 driver mutations have been inferred to occur during the perinatal period or in the first or second decade of life in older patients with overt myeloproliferative neoplasms. The study data support the hypothesis that long telomere length provides an advantage for the survival of these variant clones into adulthood. They also support a model wherein telomere-independent germline factors determine the risk of JAK2 mutagenesis, and replicative competence is necessary for sustaining the longevity of JAK2 variant clones. POT1 mutations were recently identified in 1% of patients with myeloproliferative neoplasms. Multiple genes regulate telomere length, and therefore variants in genes other than POT1 may account for the familial clustering of these disorders and their association with the risk of lymphoid neoplasms and solid tumours.
An increased incidence of clonal haematopoiesis has also been documented among persons with mendelian short telomere syndromes, but the spectrum of mutations is distinct, with somatic-reversion mutations that offset the inherited defect being most common. These observations, together with findings from this study, suggest that the germline genetic background influences the spectrum of mutations under clonal selection in haematopoietic compartments. Moreover, this study provides a mendelian context for understanding the biology that underlies population-based observations linking common variants near TERT, the gene encoding telomerase reverse-transcriptase, which are associated with long telomere length, with the risk of CHIP and nearly all solid tumours.
Described neoplastic predisposition presents a paradox at the intersection of aging and cancer biology. Long telomere length and increasing telomere length with age manifests as delayed replicative senescence. Simultaneously, it sustains clonality, which is typically associated with older age. The paradox is evident in the observations that both short and long extremes of telomere length appear to mediate two distinct age-associated disease phenotypes. Their contrast is illustrated in the predisposition to B- and T-cell lymphoproliferative and myeloproliferative disease in POT1 mutation carriers and the predisposition to B- and T-cell immunodeficiency and aplastic anaemia in persons with short telomere syndromes.
The authors commented that the syndrome reported is an archetype for a cellular pronearia (from the Greek nearós, meaning youth). Its phenotype would appear to affect several organ systems. The study team speculated that the relatively high burden of somatic variants in haematopoietic lineages may be similar to that of other tissues. They observed benign neoplasms, including cutaneous nevi, goiters, and uterine fibroids, in POT1 mutation carriers. The risk of these conditions has been linked to loci encompassing telomere genes; perhaps longer telomere length confers a risk of these neoplasms in addition to clonal haematopoiesis.
In an accompanied editorial article, Dr. George Vassiliou of the Department of Haematology, University of Cambridge in Cambridge, UK wrote that this study describes an association between germline mutations in POT and the development of clonal haematopoiesis. This condition represents the clonal expansion of a haematopoietic stem cell and its progeny, driven by somatic mutations in driver genes, such as DNMT3A, TET2, or JAK2, and is referred to as CHIP when the variant-allele frequency in blood DNA is 2% or higher.
POT1 mutations were previously shown to prevent the progressive shortening of telomeric DNA that normally occurs with each cell division and to be associated with cancers such as CLL, melanoma, and angiosarcoma. The discovery that they are strongly associated with CHIP suggests a critical role for telomere maintenance in CHIP development.
The extension of cellular replicative potential by means of maintenance of long telomeres prolongs the life span of a cell lineage and facilitates the acquisition of more somatic mutations. This increased number of somatic mutations is likely to be an important reason behind the increased risk of cancer among persons with POT1 variants. Normal telomere attrition may be particularly important for preventing the development of such cancers.
This work was supported by grants from the US National Institutes of Health, the S&R Foundation, the Commonwealth Foundation, and a gift from Godrej Industries.