Dyskeratosis congenita (DC) is a multisystem bone marrow failure syndrome characterized by a triad of mucocutaneous abnormalities and a predisposition to cancer. The genetic basis of DC remains unknown in more than 60% of patients. Mutations have been identified in components of the telomerase complex (dyskerin, TERC, TERT, NOP10, and NHP2), and recently in one component of the shelterin complex TIN2 (gene TINF2). To establish the role of TINF2 mutations, we screened DNA from 175 uncharacterised patients with DC as well as 244 patients with other bone marrow failure disorders. Heterozygous coding mutations were found in 33 of 175 previously uncharacterized DC index patients and 3 of 244 other patients. A total of 21 of the mutations affected amino acid 282, changing arginine to histidine (n = 14) or cysteine (n = 7). A total of 32 of 33 patients with DC with TINF2 mutations have severe disease, with most developing aplastic anaemia by the age of 10 years. Telomere lengths in patients with TINF2 mutations were the shortest compared with other DC subtypes, but TERC levels were normal. In this large series, TINF2 mutations account for approximately 11% of all DC, but they do not play a significant role in patients with related disorders. This study emphasises the role of defective telomere maintenance on human disease.

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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.

Hoyeraal-Hreidarsson syndrome represents a severe variant of dyskeratosis congenita (Zinsser-Cole-Engman syndrome). This X-linked recessive, progressive, multisystemic disorder reported so far in 12 pedigrees is characterised by intrauterine growth retardation, microcephaly, cerebellar hypoplasia, mental retardation, progressive combined immune deficiency and aplastic anaemia. Mutations in the DKC1gene on Xq28 have been identified in the X-linked form of dyskeratosis congenita and in some Hoyeraal-Hreidarsson syndrome patients. We report on two sibs and two other unrelated patients with the striking clinical features of Hoyeraal-Hreidarsson syndrome. Noticeably, all four had early digestive problems, with chronic, bloody diarrhoea and feeding problems causing one of the most difficult problems in the supportive treatment of this uniformly lethal condition. Pathological changes in the proliferative compartment of the digestive mucosa included alterations of the glandular architecture and focal rarefaction of the glands. This aspect seems consistent with altered telomerase function associated with a dyskerin mutation which may decrease the proliferative capacity of digestive epithelial cells. A missense mutation 146 C-->T (Thr49Met) in the DKC1gene was found in two unrelated patients, whereas mutation screening was negative for one single case. The absence of mutations of the DKC1gene in patients with Hoyeraal-Hreidarsson syndrome emphasises the probable implication of one or more other loci.

Key Points• Telomerase variants in patients with bone marrow failure syndromes are difficult to categorize as diseasecausing or otherwise.• DC can derive from triallelic mutations in 2 telomerase genes and epigenetic-like inheritance of short telomeres.Dyskeratosis congenita (DC) and related diseases are a heterogeneous group of disorders characterized by impaired telomere maintenance, known collectively as the telomeropathies. Disease-causing variants have been identified in 10 telomere-related genes including the reverse transcriptase (TERT) and the RNA component (TERC) of the telomerase complex. Variants in TERC and TERT can impede telomere elongation causing stem cells to enter premature replicative senescence and/or apoptosis as telomeres become critically short. This explains the major impact of the disease on highly proliferative tissues such as the bone marrow and skin. However, telomerase variants are not always fully penetrant and in some families disease-causing variants are seen in asymptomatic family members. As a result, determining the pathogenic status of newly identified variants in TERC or TERT can be quite challenging. Over a 3-year period, we have identified 26 telomerase variants (16 of which are novel) in 23 families. Additional investigations (including family segregation and functional studies) enabled these to be categorized into 3 groups: (1) disease-causing (n 5 15), (2) uncertain status (n 5 6), and (3) bystanders (n 5 5). Remarkably, this process has also enabled us to identify families with novel mechanisms of inheriting human telomeropathies. These include triallelic mutations, involving 2 different telomerase genes, and an epigenetic-like inheritance of short telomeres in the absence of a telomerase mutation. This study therefore highlights that telomerase variants have highly variable functional and clinical manifestations and require thorough investigation to assess their pathogenic contribution. (Blood. 2015;126(2):176-184)

A gene causing Dyskeratosis Congenita (DC), a rare genetic disorder associated with bone marrow failure, has been mapped to chromosome Xq28, but autosomal inheritance of the disease has also been reported. We have investigated the pattern of X-inactivation in the peripheral blood of carriers of DC using the methylation-sensitive Hpa II site in the androgen receptor gene (HUMARA). In 5 different families in which the inheritance of DC appears to be X-linked, all 16 carriers showed skewed X-inactivation patterns. These cases indicate that, in the hematopoiesis of heterozygous females, cells expressing the normal DC allele have a growth advantage over cells that express the mutant allele. In 7 other families with sporadic cases of DC or with an uncertain pattern of inheritance, both skewed and normal patterns of X-inactivation were observed. In these families or where crucial family members are unavailable, the study of X-inactivation patterns will add to linkage analysis in providing information about carrier status.

The inherited bone marrow (BM) failure syndromes are a diverse group of disorders characterized by BM failure usually in association with one or more extra-hematopoietic abnormality. The BM failure, which can involve one or more cell lineages, often presents in the pediatric age group. Furthermore, some children initially labelled as having "idiopathic aplastic anemia" or "myelodysplasia" represent cryptic presentations of these syndromes. Significant advances in the genetics of these syndromes have been made with more than 100 disease genes identified. These advances have given insights into normal hematopoiesis and how this is disrupted in patients with BM failure. They have also provided important information on fundamental biological pathways: DNA repair-Fanconi anemia (FA) genes; telomere maintenance-dyskeratosis congenita (DC) genes; ribosome biogenesis-Shwachman Diamond syndrome and Diamond-Blackfan anemia genes. Additionally, as these disorders are usually associated with extra-hematopoietic abnormalities and an increased risk of cancer they have provided insights into human development and the genesis of cancer. In the clinic, genetic tests stemming from the recent advances are facilitating diagnosis especially when clinical features may not be sufficient to make an accurate classification. Hematopoietic stem cell transplantation using fludarabine based protocols has improved outcomes significantly particularly for patients with FA and DC. Management of some of the other complications, such as cancer, remains a challenge. Recent studies suggest the possibility of new and potentially more efficacious therapies. This includes renewed focus on hematopoietic gene therapy and drugs (TGF-b inhibitors for FA and PAPD5 inhibitors for DC) that target disease specific defects.

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