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The recessive disorder poikiloderma with neutropenia (PN) is caused by mutations in the C16orf57 gene that encodes the highly conserved USB1 protein. Here, we present the 1.1 Å resolution crystal structure of human USB1, defining it as a member of the LigT-like superfamily of 2H phosphoesterases. We show that human USB1 is a distributive 3'-5' exoribonuclease that posttranscriptionally removes uridine and adenosine nucleosides from the 3' end of spliceosomal U6 small nuclear RNA (snRNA), directly catalyzing terminal 2', 3' cyclic phosphate formation. USB1 measures the appropriate length of the U6 oligo(U) tail by reading the position of a key adenine nucleotide (A102) and pausing 5 uridine residues downstream.We show that the 3' ends of U6 snRNA in PN patient lymphoblasts are elongated and unexpectedly carry nontemplated 3' oligo(A) tails that are characteristic of nuclear RNA surveillance targets. Thus, our study reveals a novel quality control pathway in which posttranscriptional 3'-end processing by USB1 protects U6 snRNA from targeting and destruction by the nuclear exosome. Our data implicate aberrant oligoadenylation of U6 snRNA in the pathogenesis of the leukemia predisposition disorder PN.

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 a multisystem bone marrow failure syndrome characterized by a triad of mucocutaneous abnormalities and an increased predisposition to malignancy. X-linked DC is due to mutations in DKC1, while heterozygous mutations in TERC (telomerase RNA component) and TERT (telomerase reverse transcriptase) have been found in autosomal dominant DC. Many patients with DC remain uncharacterized, particularly families displaying autosomal recessive (AR) inheritance. We have now identified novel homozygous TERT mutations in 2 unrelated consanguineous families, where the index cases presented with classical DC or the more severe variant, Hoyeraal-Hreidarsson (HH) syndrome. These TERT mutations resulted in reduced telomerase activity and extremely short telomeres. As these mutations are homozygous, these patients are predicted to have significantly reduced telomerase activity in vivo. Interestingly, in contrast to patients with heterozygous TERT mutations or hemizygous DKC1 mutations, these 2 homozygous TERT patients were observed to have higher-than-expected TERC levels compared with controls. Collectively, the findings from this study demonstrate that homozygous TERT mutations, resulting in a pure but severe telomerase deficiency, produce a phenotype of classical AR-DC and its severe variant, the HH syndrome.

Myelodysplastic syndromes (MDS) comprise a diverse range of hematological conditions, all of which involve impaired or disturbed hematopoiesis, cytopenias, marrow dysplasia and the risk of progression to acute myeloid leukemia (AML). MDS and AML are believed to involve a multi-step process arising within bone marrow stem cells, but the specific defects responsible remain poorly understood. In cases of inherited, familial forms of the disease two genes have been implicated: the hematopoietic transcription factors CBFA2 (core binding factor a2) and CEBPA (CCATT box enhancer binding protein a) and these account for only a small number of the familial cases so far reported. The uncharacterized families represent an ideal resource for identifying the primary pathology of the disease. In this study we have screened 21 families where MDS/AML appears to be familial for mutations in the telomerase enzyme components TERC (telomerase RNA component) and TERT (telomerase reverse transcriptase). In two families, we have identified novel mutations in TERC and in another two we have identified mutations in TERT directly resulting in MDS/AML. Functional analysis has demonstrated that all mutations adversely impact telomerase activity in vitro and affected individuals have short telomeres (see Table 1 for summary). These results have several important implications: First, they identify a novel genetic pathway (defective telomere maintenance) that can lead to familial MDS/AML; second, they suggest that TERC and TERT act as tumour suppressors and; finally, they have implications for the biology, treatment and screening regimen for de novo cases of MDS/AML. Family Number of affected individuals Mutation Telomerase activity compared to wild type Age-adjusted telomere length 1 3 TERC c.212C>G 1% −4.62 2 5 TERC c.309G>T 4% 0.01 3 2 TERTc.1892G>A 0% −4.45 4 2 TERTc.2354C>T 11% −3.12 Table 1 – Summary of clinical and molecular analyses of four families with MDS/AML