Mutations in the telomere capping complex in bone marrow failure and related syndromes

Abstract

Dyskeratosis congenita and its variants have overlapping phenotypes with many disorders including Coats plus, and their underlying pathology is thought to be one of defective telomere maintenance. Recently, biallelic CTC1 mutations have been described in patients with syndromes overlapping Coats plus. CTC1, STN1 and TEN1 are part of the telomere-capping complex involved in maintaining telomeric structural integrity. Based on phenotypic overlap we screened 73 genetically uncharacterized patients with dyskeratosis congenita and related bone marrow failure syndromes for mutations in this complex. Biallelic CTC1 mutations were identified in 6 patients but none in either STN1 or TEN1. We have expanded the phenotypic spectrum associated with CTC1 mutations and report that intracranial and retinal abnormalities are not a defining feature, as well as showing that the effect of these mutations on telomere length is variable. The study also demonstrates the lack of disease-causing mutations in other components of the telomere-capping complex.

Figure 1.

Segregation and relative position of mutations in CTC1 in patients with DC and associated phenotypes. (A) Pedigrees associated with CTC1 mutations. Filled symbols indicate affected individuals. Identified mutations at the protein level are shown below each genotyped individual. “WT” denotes wild type with no mutation. (B) Schematic structure of CTC1 protein showing the relative location of all reported mutations. The black boxes show the predicted oligosaccharide-binding (OB) folds; the N-terminal 700aa region has been shown to be involved in DNA binding whereas the C-terminal region is involved in STN1 binding. (Number) indicates the number of families in which each mutation has been observed. Novel mutations identified in this study in red. Mutations seen in this study and previously reported in blue. Mutations seen in other reports in black.

Figure 2.

Telomere lengths are not significantly short in our patients with CTC1 mutations as measured by two different methods. (A) Telomere lengths measured by MMqPCR expressed as a T/S ratio in bone marrow failure affected patients, heterozygous parents with CTC1 mutations and control individuals. There was no significant difference when either of the sample groups was compared with controls, either as the whole control set or when segregated out to match according to age. Black squares indicate patients with biallelic CTC1 mutations; black triangles heterozygous parents of patients with CTC1 mutations; open diamonds controls; black circles patients with TERC mutations. (B) Telomere length measured by Southern blotting Genomic DNA was digested with BamH1 and analyzed by Southern blot using a 0.75% agarose gel and the subtelomeric probe pTelBam8. [*] Telomere lengths were measured as the size of the fragment of peak signal intensity and adjusted to exclude the 7.8Kb of subtelomeric DNA. Sizes were determined with reference to the same standards run on each gel using Image Quant software. In both graphs, patients with known mutations in TERC are included as examples of short telomeres. Black square indicates patient F2: II-1; black star patient F3: II-1; open diamonds controls; black circles patients with TERC mutations.