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.

Dyskeratosis congenita (DC) is characterized by multiple features including mucocutaneous abnormalities, bone marrow failure and an increased predisposition to cancer. It exhibits marked clinical and genetic heterogeneity. DKC1 encoding dyskerin, a component of H/ACA small nucleolar ribonucleoprotein (snoRNP) particles is mutated in X-linked recessive DC. Telomerase RNA component (TERC), the RNA component and TERT the enzymatic component of telomerase, are mutated in autosomal dominant DC, suggesting that DC is primarily a disease of defective telomere maintenance. The gene(s) involved in autosomal recessive DC remains elusive. This paper describes studies aimed at defining the genetic basis of AR-DC. Homozygosity mapping in 16 consanguineous families with 25 affected individuals demonstrates that there is no single genetic locus for AR-DC. However, we show that NOP10, a component of H/ACA snoRNP complexes including telomerase is mutated in a large consanguineous family with classical DC. Affected homozygous individuals have significant telomere shortening and reduced TERC levels. While a reduction of TERC levels is not a universal feature of DC, it can be brought about through a reduction of NOP10 transcripts, as demonstrated by siRNA interference studies. A similar reduction in TERC levels is also seen when the mutant NOP10 is expressed in HeLa cells. These findings identify the genetic basis of one subtype of AR-DC being due to the first documented mutations in NOP10. This further strengthens the model that defective telomere maintenance is the primary pathology in DC and substantiates the evidence in humans for the involvement of NOP10 in the telomerase complex and telomere maintenance.

Dyskeratosis congenita (DC) and its phenotypically severe variant, Hoyeraal-Hreidarsson syndrome (HHS), are multisystem bone-marrow-failure syndromes in which the principal pathology is defective telomere maintenance. The genetic basis of many cases of DC and HHS remains unknown. Using whole-exome sequencing, we identified biallelic mutations in RTEL1 , encoding a helicase essential for telomere maintenance and regulation of homologous recombination, in an individual with familial HHS. Additional screening of RTEL1 identified biallelic mutations in 6/23 index cases with HHS but none in 102 DC or DC-like cases. All 11 mutations in ten HHS individuals from seven families segregated in an autosomal-recessive manner, and telomere lengths were significantly shorter in cases than in controls (p = 0.0003). This group had significantly higher levels of telomeric circles, produced as a consequence of incorrect processing of telomere ends, than did controls (p = 0.0148). These biallelic RTEL1 mutations are responsible for a major subgroup (∼29%) of HHS. Our studies show that cells harboring these mutations have significant defects in telomere maintenance, but not in homologous recombination, and that incorrect resolution of T-loops is a mechanism for telomere shortening and disease causation in humans. They also demonstrate the severe multisystem consequences of its dysfunction.

Dyskeratosis congenita is a premature aging syndrome characterized by muco-cutaneous features and a range of other abnormalities, including early greying, dental loss, osteoporosis, and malignancy. Dyskeratosis congenita cells age prematurely and have very short telomeres. Patients have mutations in genes that encode components of the telomerase complex (dyskerin, TERC, TERT, and NOP10), important in the maintenance of telomeres. Many dyskeratosis congenita patients remain uncharacterized. Here, we describe the analysis of two other proteins, NHP2 and GAR1, that together with dyskerin and NOP10 are key components of telomerase and small nucleolar ribonucleoprotein (snoRNP) complexes. We have identified previously uncharacterized NHP2 mutations that can cause autosomal recessive dyskeratosis congenita but have not found any GAR1 mutations. Patients with NHP2 mutations, in common with patients bearing dyskerin and NOP10 mutations had short telomeres and low TERC levels. SiRNAmediated knockdown of NHP2 in human cells led to low TERC levels, but this reduction was not observed after GAR1 knockdown. These findings suggest that, in human cells, GAR1 has a different impact on the accumulation of TERC compared with dyskerin, NOP10, and NHP2. Most of the mutations so far identified in patients with classical dyskeratosis congenita impact either directly or indirectly on the stability of RNAs. In keeping with this effect, patients with dyskerin, NOP10, and now NHP2 mutations have all been shown to have low levels of telomerase RNA in their peripheral blood, providing direct evidence of their role in telomere maintenance in humans.GAR1 ͉ bone marrow failure ͉ telomeres

Dyskeratosis congenita (DC) is an inherited syndrome exhibiting marked clinical and genetic heterogeneity. It is characterized by multiple features including mucocutaneous abnormalities, bone marrow failure and an increased predisposition to cancer. Three genetic subtypes are recognized: X-linked recessive DC bears mutations in DKC1, the gene encoding dyskerin, a component of H/ACA small nucleolar ribonucleoprotein particles; autosomal dominant (AD) DC has heterozygous mutations in either TERC or TERT, the RNA and enzymatic components of telomerase, respectively, and autosomal recessive DC in which the genes involved remain largely elusive. Disease pathology is believed to be a consequence of chromosome instability because of telomerase deficiency due to mutations in DKC1, TERC and TERT; in patients with DKC1 mutations, defects in ribosomal RNA modification, ribosome biogenesis, translation control or mRNA splicing may also contribute to disease pathogenesis. The involvement of telomerase complex components in X-linked and AD forms and the presence of short telomeres in DC patients suggest that DC is primarily a disease of defective telomere maintenance. Treatment is variable and complicated by the development of secondary cancers but, being a monogenic disorder, it could potentially be treated by gene therapy. DC overlaps both clinically and genetically with several other diseases including Hoyeraal-Hreidarsson syndrome, aplastic anaemia and myelodysplasia, among others and its underlying telomeric defect has implications for a broader range of biological processes including ageing and many forms of cancer.