Chromosomal breakage analysis in dyskeratosis congenita peripheral blood lymphocytes

Modelling acute myeloid leukemia (AML): What's new? A transition from the classical to the modern

Acute myeloid leukemia (AML) is a heterogeneous malignancy affecting myeloid cells in the bone marrow (BM) but can spread giving rise to impaired hematopoiesis. AML incidence increases with age and is associated with poor prognostic outcomes. There has been a disconnect between the success of novel drug compounds observed in preclinical studies of hematological malignancy and less than exceptional therapeutic responses in clinical trials. This review aims to provide a state-of-the-art overview on the different preclinical models of AML available to expand insights into disease pathology and as preclinical screening tools. Deciphering the complex physiological and pathological processes and developing predictive preclinical models are key to understanding disease progression and fundamental in the development and testing of new effective drug treatments. Standard scaffold-free suspension models fail to recapitulate the complex environment where AML occurs. To this end, we review advances in scaffold/matrix-based 3D models and outline the most recent advances in on-chip technology. We also provide an overview of clinically relevant animal models and review the expanding use of patient-derived samples, which offer the prospect to create more "patient specific" screening tools either in the guise of 3D matrix models, microphysiological "organ-on-chip" tools or xenograft models and discuss representative examples.

Dyskeratosis congenita: a literature review

Dyskeratosis congenita is a rare hereditary disease that occurs predominantly in males and manifests clinically as the classic triad of reticulate hyperpigmentation, nail dystrophy and leukoplakia. It increases the risk of malignancy and other potentially lethal complications such as bone marrow failure, lung and liver diseases. Mutations in 19 genes are associated with dyskeratosis congenita, and a fifth of the pathogenic mutations are found in DKC1, the gene coding for dyskerin. This review aims to address the clinical and genetic aspects of the disease.

[The study of genetic instability in patients with Dyskeratosis congenital]

目的

研究先天性角化不良症（DC）患者细胞遗传不稳定性、分析其可能的机制及其与造血衰竭严重程度的相关性。

方法

应用彗星试验检测4例DC患者、29例Fanconi贫血患者和24名正常志愿者的外周血淋巴细胞，比较各组彗星头部DNA百分比（HeadDNA%）、彗星尾部DNA百分比（TailDNA%）、尾矩（TM）、Olive尾矩（OTM）和彗星细胞率（CCP），结合丝裂霉素C（MMC）试验和骨髓造血细胞常规染色体核型结果进行分析。

结果

①4例DC患者均未发现克隆性异常染色体核型。②DC组的TM (6.77±0.90）、OTM(6.19±0.80）和CCP[（46.00±5.03）%]均明显高于正常对照组[0.61±0.49、0.66±0.42、（5.91±3.19）%]，差异均有统计学意义（P值均<0.05），与Fanconi贫血组[7.81±3.58、6.65±2.21、（56.03±13.47）%]比较差异无统计学意义（P值均≥0.05）。③在MMC 80 µg/L条件下DC组畸变细胞率明显低于Fanconi贫血组[（21.00±3.16）%对（31.97±6.33）%，P=0.003]。④DC患者的CCP、OTM、TailDNA%、TM与其外周血HGB水平和PLT、中性粒细胞绝对计数无明显相关性（P>0.05）。

结论

DC患者细胞遗传不稳定性显著增高，而DNA损伤修复机制基本正常；DC遗传不稳定程度与已经发生的造血衰竭严重程度无相关。

RTEL1: functions of a disease-associated helicase

DNA secondary structures that arise during DNA replication, repair, and recombination (3R) must be processed correctly to prevent genetic instability. Regulator of telomere length 1 (RTEL1) is an essential DNA helicase that disassembles a variety of DNA secondary structures to facilitate 3R processes and to maintain telomere integrity. The past few years have witnessed the emergence of RTEL1 variants that confer increased susceptibility to high-grade glioma, astrocytomas, and glioblastomas. Mutations in RTEL1 have also been implicated in Hoyeraal-Hreidarsson syndrome, a severe form of the bone-marrow failure and cancer predisposition disorder, dyskeratosis congenita. We review these recent findings and highlight its crucial link between DNA secondary-structure metabolism and human disease.

[Dyskeratosis congenita: an update]

Dyskeratosis congenita is a rare inherited bone marrow failure characterized by excessively short telomeres in highly proliferative tissues. These abnormalities are due to disturbance of the telomere maintenance machinery. The clinical presentation is characterized by skin pigmentation, nail dystrophy, and mucosal leukoplakia. All these mucocutaneous features are rare in childhood: they usually appear between 5 and 10 years of age. In young children, the initial presentation can associate bone marrow failure and neurological or ocular problems: Hoyeraal-Hreidarsson and Revesz syndromes, respectively. Clinical progression of the disease can lead to aplastic anemia (86% of all patients) and to pulmonary or hepatic complications. These patients also have an increased risk of cancer. Diagnosis is often suspected on bone marrow failure with no clinical or biological abnormalities compatible with Fanconi anemia diagnosis. The telomere length study can be helpful for diagnosis in case of aplastic anemia in children before studying gene mutations. Until now, 6 genes (DKC1, TERT, TERC, NOLA2, NOLA3, TINF2) have been identified in dyskeratosis congenita. Transmission of the disease can be autosomal recessive, autosomal dominant, or X-linked. In half of the cases, the genetic abnormality is unknown. Treatment of DC has to be adapted to each patient, from symptomatic or androgenic treatment to hematopoietic stem cell transplantation.

Dyskeratosis Congenita: a historical perspective

"Dyskeratosis Congenita (DC) also known as Zinsser-Engman-Cole syndrome is a rare multi-system bone marrow failure syndrome characterised by mucocutaneous abnormalities and an increased predisposition to cancer". This is a common definition of DC but how did this definition arise? The aim of this review is to follow the development of DC and associated diseases from its first reported description in the early 20th century to the current understanding of the genes involved and its pathophysiology in 2007 in a chronological order. Although this review is not intended to be an exhaustive citation of the literature available it does provide a summary of the key developments, citing particularly the earlier reports of each development.

Allogeneic stem cell transplantation in a patient with dyskeratosis congenita after conditioning with low-dose cyclophosphamide and anti-thymocyte globulin

Bone marrow failure is the major cause of early mortality in patients with dyskeratosis congenita (DC); early trials with conventional conditioning regimens were associated with remarkable chronic morbidity and mortality, and the optimal conditioning regimen for these patients remains undetermined. We report a case of a child afflicted with DC who underwent related full HLA-matched stem cell transplant (SCT) using a regimen of low dose cyclophosphamide and antithymocyte globulin (ATG). The regimen was well tolerated and associated with no significant short-term toxicity.

[Nosologic discussion between Fanconi disease and congenital dyskeratosis: 1 case of congenital bone marrow aplasia]

Based on a case report of aplastic anemia associated with malformation, we discuss the diagnostic criteria and the nosologic problem between the 2 principal aplastic anemia accompanied with malformation: Fanconi disease and dyskeratosis congenita.

CASE REPORT

A 19-year-old girl, issued from a third degree consanguineous marriage, was admitted because of anemic and hemorrhagic syndrome. Physical examination showed several malformations: microphtalmia, brownish spots, generalized hyperpigmentation and ungueal dystrophy without mucosal leucoplasia. Statural and ponderal retardation were noted. On the hemogram there was a pancytopenia and on biopsy, the bone marrow was desertic. The caryotype performed on peripheral blood lymphocytes after sensibilisation with mitomycin C revealed chromosomal instability aspects. Based on these clinical and biological features, the diagnosis of hereditary aplastic anaemia was retained. The patient was given norethandrolone. She died 3 months later by septic shock.

DISCUSSION

Coexistence of aplastic anemia with a malformative syndrome suggests most probably an hereditary form of aplastic anemia. Fanconi anemia is the most frequent. It associates characteristic anomalies of the face, with microphtalmia, brownish spots, statural and ponderal retardation, and thumb anomalies. Ungueal dystrophy, mucosal leucoplasia are almost pathognomonic of congenital dyskeratosis. When the malformative syndrome is not characteristic, the cytogenetic study may also fail to make the differential diagnosis, as was the situation in our case.

Dyskeratosis Congenita

Dyskeratosis congenita (DC) is a rare inherited multi-system disorder. Although DC is classically characterized by mucocutaneous features, the vast majority of patients develop hematologic abnormalities, and in its occult form the disease can present as aplastic anemia. The gene responsible for the X-linked form of the disease encodes a protein involved in ribosome biogenesis and in stabilizing the telomerase complex, while the autosomal dominant form is caused by mutations in the core RNA component of telomerase. It has been suggested that DC is primarily a disease of defective telomere maintenance. Premature shortening of telomeres resulting in a limited proliferative potential of stem cells would explain the pathology observed in DC, as the affected tissues are those that require constant renewal.

Dyskeratosis congenita: its link to telomerase and aplastic anaemia

Dyskeratosis congenita (DC) is an inherited bone marrow failure syndrome exhibiting considerable clinical and genetic heterogeneity. X-linked recessive, autosomal dominant and autosomal recessive forms are recognised. The gene mutated in X-linked DC (DKC1) encodes a highly conserved nucleolar protein called dyskerin. Dyskerin associates with the H/ACA class of small nucleolar RNAs which are important in guiding the conversion of uracil to pseudouracil in ribosomal RNA. Dyskerin also associates with the RNA component of telomerase (hTR) which is important in the maintenance of telomeres. Mutations in hTR were recently demonstrated in patients with autosomal dominant DC and in a subset of patients with aplastic anaemia (AA) but without other diagnostic features of DC. This discovery demonstrates that both DC and a subset of AA are due to a defect in telomerase. The link between DC and AA and in turn to defective telomerase suggests that treatments directed at correction of telomerase activity might benefit DC/AA patients who do not respond to conventional therapy.

Treatment of dyskeratosis congenita with granulocyte-macrophage colony-stimulating factor and erythropoietin

Dyskeratosis congenita (DC) is a rare inherited disorder characterized by reticulate skin pigmentation, nail dystrophy, mucosal leucoplakia, and bone marrow failure. Pancytopenia is difficult to manage in patients with this disorder. We describe a 13-month-old-boy who presented with reticulate skin lesions, paleness, and hepatosplenomegaly. Anemia and leukopenia developed by the age of 43 months. The patient was treated with granulocyte-macrophage colony-stimulating factor (GM-CSF) (5 microg/kg/d, subcutaneously) for 19 months and erythropoietin (150 U/kg 3 days in a week, subcutaneously) for 8 months, with excellent neutrophil and hemoglobin response. Recurrent infections were not developed after starting GM-CSF, and packed red blood cell transfusion was not given to the patient after starting erythropoietin. GM-CSF combined with erythropoietin may be used in the treatment of bone marrow failure in patients with DC without an HLA-identical donor.

Telomerase in the human organism

The intent of this review is to describe what is known and unknown about telomerase in somatic cells of the human organism. First, we consider the telomerase enzyme. Human telomerase ribonucleoproteins undergo at least three stages of cellular biogenesis: accumulation, catalytic activation and recruitment to the telomere. Next, we describe the patterns of telomerase regulation in the human soma. Telomerase activation in some cell types appears to offset proliferation-dependent telomere shortening, delaying but not defeating the inherent mitotic clock. Finally, we elaborate the connection between telomerase misregulation and human disease, in the contexts of inappropriate telomerase activation and telomerase deficiency. We discuss how our current perspectives on telomerase function could be applied to improving human health.

Dyskeratosis congenita: report of a case

Dyskeratosis congenita is a rare multisystem condition involving mainly the ectoderm. It is characterized by a triad of reticular skin pigmentation, nail dystrophy and leukoplakia of mucous membranes. Oral and dental abnormalities may also be present. Complications are a predisposition to malignancy and bone marrow involvement with pancytopenia. The case of a 14-year-old girl is described who presented with several of the characteristic systemic features of this condition, together with the following oral features: hypodontia, diminutive maxillary lateral incisors, delayed dental eruption, crowding in the maxillary premolar region, short roots, poor oral hygiene, gingival inflammation and bleeding, alveolar bone loss, caries and a smooth atrophic tongue with leukoplakia. Although this condition is rare, dental surgeons should be aware of the dental abnormalities that exist and the risk of malignant transformation within the areas of leukoplakia.

Dyskerin localizes to the nucleolus and its mislocalization is unlikely to play a role in the pathogenesis of dyskeratosis congenita

Mutations in the DKC1 gene are responsible for causing the bone marrow failure syndrome, dyskeratosis congenita (DKC; OMIM 305000). The majority of mutations identified to date are missense mutations and are clustered in exons 3, 4 and 11. It is predicted that the corresponding protein dyskerin is a nucleolar phosphoprotein which functions in both pseudo-uridylation and cleavage of precursor rRNA. Dyskerin contains multiple putative nuclear localization signals (NLSs) at the N-terminus (KKHKKKKERKS) and C-terminus [KRKR(X)(17)KKEKKKSKKDKKAK(X)(17)-KKKKKKKKAKEVELVSE]. By fusing dyskerin with the enhanced green fluorescent protein (EGFP) and by following a time course of expression in mammalian cell lines, we showed that full-length dyskerin initially localizes to the nucleoplasm and subsequently accumulates in the nucleoli. A co-localization to the coiled bodies was observed in some cells where dyskerin-EGFP had translocated to the nucleoli. Analysis of a series of mutant constructs indicated that whereas the most C-terminal lysine-rich clusters [KKEKKKS-KKDKKAK(X)(17)KKKKKKKKAKEVELVSE] influence the rate of nucleoplasmic and nucleolar accumulation, the KRKR sequence is primarily responsible for the nuclear import. Nucleolar localization was maintained when either the N- or C-terminal motifs were mutated, but not when all NLSs were removed. We conclude that the intranuclear localization of dyskerin is accomplished by the synergistic effect of a number of NLSs and that the nucleolar localization signals are contained within the NLSs. Further, examination of dyskerin-EGFP fusions mimicking mutations detected in patients indicated that the intracellular mislocalization of dyskerin is unlikely to cause DKC.

Analysis of epitope-tagged forms of the dyskeratosis congenital protein (dyskerin): identification of a nuclear localization signal

The X-linked form of the bone marrow failure syndrome Dyskeratosis congenital is caused by mutations in dyskerin, a 514 amino acid protein that is presumed to play a role in ribosome biogenesis. Here we report that dyskerin tagged with the human immunoglobulin epitope localizes to nuclei of transfected HeLa and COS-1 cells. A carboxyl-terminal domain consisting of amino acids 467-475 and encoding KKEKKKSKK is both necessary and sufficient to mediate nuclear entry. Immunoglobulin-tagged dyskerin did not interact with the Fanconi anemia group A protein, FANCA. These results suggest a nuclear role for dyskerin. Moreover, hematopoietic failure observed in both Dyskeratosis congenital and the most common type of Fanconi anemia is unlikely to have a common mechanism resulting from abnormal physical interactions between the respective gene products of these disorders.

X-linked dyskeratosis congenita is predominantly caused by missense mutations in the DKC1 gene

Dyskeratosis congenita is a rare inherited bone marrow-failure syndrome characterized by abnormal skin pigmentation, nail dystrophy, and mucosal leukoplakia. More than 80% of patients develop bone-marrow failure, and this is the major cause of premature death. The X-linked form of the disease (MIM 305000) has been shown to be caused by mutations in the DKC1 gene. The gene encodes a 514-amino-acid protein, dyskerin, that is homologous to Saccharomyces cerevisiae Cbf5p and rat Nap57 proteins. By analogy to the homologues in other species, dyskerin is predicted to be a nucleolar protein with a role in both the biogenesis of ribosomes and, in particular, the pseudouridylation of rRNA precursors. We have determined the genomic structure of the DKC1 gene; it consists of 15 exons spanning a region of 15 kb. This has enabled us to screen for mutations in the genomic DNA, by using SSCP analysis. Mutations were detected in 21 of 37 additional families with dyskeratosis congenita that were analyzed. These mutations consisted of 11 different single-nucleotide substitutions, which resulted in 10 missense mutations and 1 putative splicing mutation within an intron. The missense change A353V was observed in 10 different families and was shown to be a recurring de novo event. Two polymorphisms were also detected, one of which resulted in the insertion of an additional lysine in the carboxy-terminal polylysine domain. It is apparent that X-linked dyskeratosis congenita is predominantly caused by missense mutations; the precise effect on the function of dyskerin remains to be determined.