The PARN, TOE1, and USB1 RNA deadenylases and their roles in non-coding RNA regulation.

The levels of non-coding RNAs (ncRNAs) are regulated by transcription, RNA processing, and RNA degradation pathways. One mechanism for the degradation of ncRNAs involves the addition of oligo(A) tails by non-canonical poly(A) polymerases, which then recruit processive sequence-independent 3' to 5' exonucleases for RNA degradation. This pathway of decay is also regulated by three 3' to 5' exoribonucleases, USB1, PARN, and TOE1, which remove oligo(A) tails and thereby can protect ncRNAs from decay in a manner analogous to the deubiquitination of proteins. Loss-of-function mutations in these genes lead to premature degradation of some ncRNAs and lead to specific human diseases such as Poikiloderma with Neutropenia (PN) for USB1, Dyskeratosis Congenita (DC) for PARN and Pontocerebellar Hypoplasia type 7 (PCH7) for TOE1. Herein, we review the biochemical properties of USB1, PARN, and TOE1, how they modulate ncRNA levels, and their roles in human diseases.

Impaired reproductive function and fertility preservation in a woman with a dyskeratosis congenita.

PURPOSE: To determine the impact of accelerated telomere shortening on the fertility parameters and treatment outcomes of a woman with dyskeratosis congenita (DKC). METHODS: A case study of the clinical data, blood, discarded oocytes, and arrested embryos of a woman with DKC and donated cryopreserved embryos from unaffected patients. Mean telomere length in blood cells was analyzed by flow cytometry-fluorescence in situ hybridization (flow-FISH) and qPCR. The load of short telomeres in blood cells was measured by universal single telomere length analysis (Universal STELA). The mean telomere length in embryos was analyzed by single-cell amplification of telomere repeats (SCATR) PCR. RESULTS: Comparison of clinical parameters revealed that the DKC patient had reduced anti-Mullerian hormone (0.3 vs 4.1 ± 5.7 ng/ML), reduced oocytes retrieved (7 vs 18.5 ± 9.5), reduced fertilization rate, and reduced euploidy rate relative to unaffected patients. Additionally, mean telomere length in DKC embryos were shorter than unaffected embryos. However, hormone treatment led to increased leukocyte telomere length, while the load of short telomeres was also shown to decrease during the course of treatment. CONCLUSIONS: We demonstrate for the first time the direct detrimental impacts of short telomeres on female fertility. We further demonstrate positive effects of hormone treatments for people with telomere disorders.

Refining the phenotype associated with biallelic DNAJC21 mutations.

Inherited bone marrow failure syndromes (IBMFS) are caused by mutations in genes involved in genomic stability. Although they may be recognized by the association of typical clinical features, variable penetrance and expressivity are common, and clinical diagnosis is often challenging. DNAJC21, which is involved in ribosome biogenesis, was recently linked to bone marrow failure. However, the specific phenotype and natural history remain to be defined. We correlate molecular data, phenotype, and clinical history of 5 unreported affected children and all individuals reported in the literature. All patients present features consistent with IBMFS: bone marrow failure, growth retardation, failure to thrive, developmental delay, recurrent infections, and skin, teeth or hair abnormalities. Additional features present in some individuals include retinal abnormalities, pancreatic insufficiency, liver cirrhosis, skeletal abnormalities, congenital hip dysplasia, joint hypermobility, and cryptorchidism. We suggest that DNAJC21-related diseases constitute a distinct IBMFS, with features overlapping Shwachman-Diamond syndrome and Dyskeratosis congenita, and additional characteristics that are specific to DNAJC21 mutations. The full phenotypic spectrum, natural history, and optimal management will require more reports. Considering the aplastic anemia, the possible increased risk for leukemia, and the multisystemic features, we provide a checklist for clinical evaluation at diagnosis and regular follow-up.

Anesthesia in a patient with dyskeratosis congenita presenting for urgent subtotal gastrectomy.

Dyskeratosis congenita is a rare and complex congenital disease that may complicate surgical treatment and impact anesthetic care. We present the perioperative management of a patient with severe pancytopenia, respiratory dysfunction, and oral leukoplakia who presented for urgent surgery for removal of a gastric hemorrhagic malignant tumor. Important issues in the management of this patient include choice of anesthetic technique, correction of pancytopenia (thrombocytopenia in particular), judicious perioperative fluid management to avoid dilutional coagulopathy, antibiotic prophylaxis, and strict aseptic technique. Careful management of a potentially difficult airway and a higher likelihood of respiratory insufficiency further complicate patient care. Knowledge of this rare disease process and its potential impact on anesthetic management is paramount for safe perioperative patient care.

Telomere dysfunction and tumor suppression responses in dyskeratosis congenita: balancing cancer and tissue renewal impairment.

Dyskeratosis congenita (DC) encompasses a large spectrum of diseases and clinical manifestations generally related to premature aging, including bone marrow failure and cancer predisposition. The major risk factor for DC is to carry germline telomere-related mutations - in telomerase or telomere shelterin genes - which results in premature telomere dysfunction, thus increasing the risk of premature aging impairments. Despite the advances that have been accomplished in DC research, the molecular aspects underlying the phenotypic variability of the disease remain poorly understood. Here different aspects of telomere biology, concerning adult stem cells senescence, tumor suppression and cancer are considered in the context of DC, resulting in two translational models: late onset of DC symptoms in telomere-related mutations carriers is a potential indicator of increased cancer risk and differences in tumor suppression capacities among the genetic subgroups are (at least partial) causes of different clinical manifestations of the disease. The limitations of both models are presented, and further experiments for their validation, as well as clinical implications, are discussed.

Dyskeratosis congenita.

Dyskeratosis congenita (DC) is a multisystem inherited syndrome exhibiting marked clinical and genetic heterogeneity. In its classic form, it is characterized by mucocutaneous abnormalities, BM failure, and a predisposition to cancer. BM failure is the principal cause of premature mortality. Studies over the last 15 years have led to significant advances, with 8 DC genes (DKC1, TERC, TERT, NOP10, NHP2, TIN2, C16orf57, and TCAB1) having been characterized. Seven of these are important in telomere maintenance either because they encode components of the telomerase enzyme complex (DKC1, TERC, TERT, NOP10, NHP2, and TCAB1) or the shelterin complex (TINF2). DC is therefore principally a disease of defective telomere maintenance and patients usually have very short telomeres. The genetic advances have led to the unification of DC with several other disorders, including the severe multisystem disorders Hoyeraal-Hreidarsson and Revesz syndromes, as well as a subset of patients with aplastic anemia, myelodysplasia, leukemia, and idiopathic pulmonary fibrosis. This wide spectrum of diseases ranging from classic DC to aplastic anemia can be regarded as disorders of defective telomere maintenance-"the telomereopathies." These advances have increased our understanding of normal hematopoiesis and highlighted the important role of telomerase and telomeres in human biology. They are also facilitating the diagnosis (especially when presentation is atypical) and management of DC.

Dyskeratosis congenita: a combined immunodeficiency with broad clinical spectrum--a single-center pediatric experience.

BACKGROUND: Dyskeratosis Congenita (DKC) is a syndrome characterized by immunodeficiency, bone marrow failure, somatic abnormalities, and cancer predisposition resulting from defective telomere maintenance. The immunologic features of DKC remain under diagnosed and under treated despite the fact that immunodeficiency is a major cause of premature mortality in DKC. METHODS: This study undertook a retrospective review of 7 DKC patients diagnosed at the Children's Hospital of Philadelphia. In parallel, we reviewed previously reported immunologic findings in DKC patients. RESULTS: Immunologic abnormalities (lymphopenia, low B-cell numbers, hypogammaglobulinemia, and decreased T-cell function) were the most frequent laboratory findings at initial presentation, preceding the development of significant anemia or thrombocytopenia. Recurrent sinopulmonary or opportunistic infections were present in 6/7 patients. Infant-onset patients had more severe immunologic and somatic features (particularly severe enteropathy). CONCLUSION: In DKC, development of immunologic abnormalities can precede bone marrow failure, highlighting the importance of proper immunodeficiency management to minimize morbidity and premature mortality in this disease.

Accelerated hematopoietic stem cell aging in a mouse model of dyskeratosis congenita responds to antioxidant treatment.

Mutations in DKC1, encoding telomerase associated protein dyskerin, cause X-linked dyskeratosis congenita (DC), a bone marrow (BM) failure, and cancer susceptibility syndrome. Decreased accumulation of telomerase RNA resulting in excessive telomere shortening and premature cellular senescence is thought to be the primary cause of disease in X-linked DC. Affected tissues are those that require constant renewal by stem cell activity. We previously showed that in Dkc1(Δ15) mice, which contain a mutation that is a copy of a human mutation causing DC, mutant cells have a telomerase-dependent proliferative defect and increased accumulation of DNA damage in the first generation before the telomeres are short. We now demonstrate the presence of the growth defect in Dkc1(Δ15) mouse embryonic fibroblasts in vitro and show that accumulation of DNA damage and levels of reactive oxygen species increase with increasing population doublings. Treatment with the antioxidant, N-acetyl cysteine (NAC), partially rescued the growth disadvantage of mutant cells in vitro and in vivo. Competitive BM repopulation experiments showed that the Dkc1(Δ15) mutation is associated with a functional stem cell defect that becomes more severe with increasing age, consistent with accelerated senescence, a hallmark of DC hematopoiesis. This stem cell phenotype was partially corrected by NAC treatment. These results suggest that a pathogenic Dkc1 mutation accelerates stem cell aging, that increased oxidative stress might play a role in the pathogenesis of X-linked DC, and that some manifestations of DC may be prevented or delayed by antioxidant treatment.

Disruption of telomerase trafficking by TCAB1 mutation causes dyskeratosis congenita.

Dyskeratosis congenita (DC) is a genetic disorder of defective tissue maintenance and cancer predisposition caused by short telomeres and impaired stem cell function. Telomerase mutations are thought to precipitate DC by reducing either the catalytic activity or the overall levels of the telomerase complex. However, the underlying genetic mutations and the mechanisms of telomere shortening remain unknown for as many as 50% of DC patients, who lack mutations in genes controlling telomere homeostasis. Here, we show that disruption of telomerase trafficking accounts for unknown cases of DC. We identify DC patients with missense mutations in TCAB1, a telomerase holoenzyme protein that facilitates trafficking of telomerase to Cajal bodies. Compound heterozygous mutations in TCAB1 disrupt telomerase localization to Cajal bodies, resulting in misdirection of telomerase RNA to nucleoli, which prevents telomerase from elongating telomeres. Our findings establish telomerase mislocalization as a novel cause of DC, and suggest that telomerase trafficking defects may contribute more broadly to the pathogenesis of telomere-related disease.

Disease-specific hematopoietic cell transplantation: nonmyeloablative conditioning regimen for dyskeratosis congenita.

Dyskeratosis congenita (DC) is characterized by reticular skin pigmentation, oral leukoplakia and abnormal nails. Patients with DC have very short telomeres and approximately one-half have mutations in telomere biology genes. A majority of patients with DC develop BM failure (BMF). Hematopoietic cell transplantation (HCT) represents the only known cure for BMF in DC, but poses significant toxicities. We report six patients who underwent allogeneic HCT with a novel nonmyeloablative conditioning regimen specifically designed for DC patients. Graft sources included related PBSCs (1), unrelated BM (2) and unrelated double umbilical cord blood (3). Complete donor engraftment was achieved in five of six patients. One patient had initial autologous hematopoietic recovery, which was followed by a second transplant that resulted in 88% donor chimerism. With a median follow-up of 26.5 months, four patients are alive, three of whom were recipients of unrelated grafts. We conclude with this small study that encouraging short-term survival can be achieved with HCT in patients with DC using a preparative regimen designed to promote donor engraftment and minimize life-threatening disease-specific complications such as pulmonary fibrosis. Long-term follow-up will be crucial with respect to individualized patient care with each of the transplanted individuals.

Recent progress in dyskeratosis congenita.

Dyskeratosis congenita (DC) is an inherited disease associated with nail dystrophy, abnormal skin pigmentation, oral leukoplakia, bone marrow failure and a predisposition to cancer. DC is a disease of defective telomere maintenance and patients with DC have very short telomeres. To date, mutations in six genes of telomerase and telomere components have been identified in patients with DC. Recently, mutations in telomerase and telomere components were also identified in patients with aplastic anemia, pulmonary fibrosis, and liver diseases who did not have mucocutaneous manifestations. These findings imply that defective telomere maintenance may cause not only classical DC but also a broad spectrum of diseases previously thought to be idiopathic, and have led to a new concept of diseases, termed "syndromes of telomere shortening". An understanding of the role of telomeres in these diseases is indispensable for diagnosis, genetic counseling and clinical management.

[Molecular diagnosis and therapeutic measures in patients with dyskeratosis congenita]. / Dyskeratosis congenita: molekuláris diagnosztika és terápiás lehetoségek.

Dyskeratosis congenita is a rare genetically heterogeneous disorder characterized by bone marrow failure and premature ageing. Current knowledge on clinical manifestations, molecular pathomechanisms, diagnostic criteria and therapeutic possibilities of patients with dyskeratosis congenita are described. Mutation analysis of the gene encoding for dyskerin revealed the c.IVS2-5C>G splice site mutation. The importance of early diagnosis in order to prevent severe invasive infections and non-infectious complications is emphasized. Family screening is important to identify carriers as prenatal genetic diagnosis conveys great benefits for family planning.

Syndromes of telomere shortening.

Telomeres and telomerase were initially discovered in pursuit of questions about how the ends of chromosomes are maintained. The implications of these discoveries to age-related disease have emerged in recent years with the recognition of a group of telomere-mediated syndromes. Telomere-mediated disease was initially identified in the context of dyskeratosis congenita, a rare syndrome of premature aging. More recently, mutations in telomerase components were identified in adults with idiopathic pulmonary fibrosis. These findings have revealed that the spectrum of telomere-mediated disease is broad and includes clinical presentations in both children and adults. We have previously proposed that these disorders be collectively considered as syndromes of telomere shortening. Here, the spectrum of these disorders and the unique telomere genetics that underlies them are reviewed. I also propose broader clinical criteria for defining telomere-mediated syndromes outside of dyskeratosis congenita, with the goal of facilitating their diagnosis and highlighting their pathophysiology.

Diminished telomeric 3' overhangs are associated with telomere dysfunction in Hoyeraal-Hreidarsson syndrome.

BACKGROUND: Eukaryotic chromosomes end with telomeres, which in most organisms are composed of tandem DNA repeats associated with telomeric proteins. These DNA repeats are synthesized by the enzyme telomerase, whose activity in most human tissues is tightly regulated, leading to gradual telomere shortening with cell divisions. Shortening beyond a critical length causes telomere uncapping, manifested by the activation of a DNA damage response (DDR) and consequently cell cycle arrest. Thus, telomere length limits the number of cell divisions and provides a tumor-suppressing mechanism. However, not only telomere shortening, but also damaged telomere structure, can cause telomere uncapping. Dyskeratosis Congenita (DC) and its severe form Hoyeraal-Hreidarsson Syndrome (HHS) are genetic disorders mainly characterized by telomerase deficiency, accelerated telomere shortening, impaired cell proliferation, bone marrow failure, and immunodeficiency. METHODOLOGY/PRINCIPAL FINDINGS: We studied the telomere phenotypes in a family affected with HHS, in which the genes implicated in other cases of DC and HHS have been excluded, and telomerase expression and activity appears to be normal. Telomeres in blood leukocytes derived from the patients were severely short, but in primary fibroblasts they were normal in length. Nevertheless, a significant fraction of telomeres in these fibroblasts activated DDR, an indication of their uncapped state. In addition, the telomeric 3' overhangs are diminished in blood cells and fibroblasts derived from the patients, consistent with a defect in telomere structure common to both cell types. CONCLUSIONS/SIGNIFICANCE: Altogether, these results suggest that the primary defect in these patients lies in the telomere structure, rather than length. We postulate that this defect hinders the access of telomerase to telomeres, thus causing accelerated telomere shortening in blood cells that rely on telomerase to replenish their telomeres. In addition, it activates the DDR and impairs cell proliferation, even in cells with normal telomere length such as fibroblasts. This work demonstrates a telomere length-independent pathway that contributes to a telomere dysfunction disease.

Ribosomal dysfunction and inherited marrow failure.

Impairment of ribosome biogenesis or function characterizes several of the inherited bone marrow failure syndromes: Diamond-Blackfan anaemia, dyskeratosis congenita (DC), Shwachman-Diamond syndrome and cartilage-hair hypoplasia. These syndromes exhibit overlapping but distinct clinical phenotypes and each disorder involves different aspects of ribosomal biogenesis. The clinical characteristics of each syndrome are briefly reviewed. Molecular studies of ribosome biogenesis and function in each of these syndromes are discussed. Models of how impairment of ribosomal pathways might affect haematopoiesis and tumorigenesis are explored.

[Dyskeratosis congenita, a disease caused by defective telomere maintenance]. / La dyskératose congénitale: une maladie méconnue due à un maintien défectueux des télomères.

Dyskeratosis congenita (DC), also called Zinsser-Cole-Engman syndrome, is a rare, often fatal, inherited disease described for the first time at the dermatological level by Zinsser in 1906. It is a very polymorphous disease at the clinical level, with several modes of inheritance. Several clinical symptoms of the disease can appear after a latency period. These features render DC particularly difficult to diagnose. Mutations of several genes can cause DC, four of them having been identified so far. However, for a majority of patients, the affected gene has not been found. Remarkably, all identified genes (DKC1, hTERC, hTERT, and NOP10) encode components of telomerase, all required for telomere length maintenance. DC is thus a unique clinical model for the study of the roles of telomerase and telomeres. Moreover, proteins encoded by the DKC1 and NOP10 genes are also components of so-called box H/ACA RNPs required for ribosome synthesis and pre-mRNA processing. Alterations of these processes could contribute to the symptoms of DC patients carrying mutations in DKC1 or NOP10.

Telomerase reverse transcriptase haploinsufficiency and telomere length in individuals with 5p- syndrome.

Telomerase, which maintains the ends of chromosomes, consists of two core components, the telomerase reverse transcriptase (TERT) and the telomerase RNA (TERC). Haploinsufficiency for TERC or TERT leads to progressive telomere shortening and autosomal dominant dyskeratosis congenita (DC). The clinical manifestations of autosomal dominant DC are thought to occur when telomeres become critically short, but the rate of telomere shortening in this condition is unknown. Here, we investigated the consequences of de novo TERT gene deletions in a large cohort of individuals with 5p- syndrome. The study group included 41 individuals in which the chromosome deletion resulted in loss of one copy of the TERT gene at 5p15.33. Telomere length in peripheral blood cells from these individuals, although within the normal range, was on average shorter than in normal controls. The shortening was more significant in older individuals suggesting an accelerated age-dependent shortening. In contrast, individuals with autosomal dominant DC due to an inherited TERC gene deletion had very short telomeres, and the telomeres were equally short regardless of the age. Although some individuals with 5p- syndrome showed clinical features that were reminiscent of autosomal dominant DC, these features did not correlate with telomere length, suggesting that these were not caused by critically short telomeres. We conclude that a TERT gene deletion leads to slightly shorter telomeres within one generation. However, our results suggest that several generations of TERT haploinsufficiency are needed to produce the very short telomeres seen in patients with DC.