TINF2, a Component of the Shelterin Telomere Protection Complex, Is Mutated in Dyskeratosis Congenita.
Authors: Savage Sharon A. , Giri Neelam , Baerlocher Gabriela M. , Orr Nick , Lansdorp Peter M. , Alter Blanche P. 
Published: November 2019

Abstract
Dyskeratosis congenita (DC) is an inherited bone marrow failure syndrome characterized by the triad of abnormal nails, lacey reticular skin pigmentation, and oral leukoplakia. Patients with DC are at high risk of developing aplastic anemia, myelodysplastic syndrome and leukemia. Diagnosis of DC is challenging due to variability of the triad and heterogeneous clinical findings such as pulmonary and liver disease, avascular necrosis, esophageal or urethral stenosis and development delay. The unifying feature in DC is exceedingly short telomere lengths and defects in telomere biology. Gene mutations have been identified in DKC1 (X-linked), TERC and TERT (dominant, AD) and NOP10 (recessive), but approximately 60% of DC patients lack a known mutation. We identified a non-consanguineous family with AD DC and no mutations in DKCI, TERC or TERT. The first DC cases were monozygotic twin brothers (now deceased). Telomere length was determined by flow-FISH on the twins’ children (6 affected, 4 unaffected), wives, their 8 siblings and parents. Despite variable clinical phenotypes, the 6 affected individuals (1 female, 5 male) all had very short telomere lengths (<1st%ile for age). Unaffected relatives had normal telomere lengths. A single nucleotide polymorphism (SNP) genome-wide linkage screen (Human Linkage IVb, Illumina, Inc) was conducted using telomere length <1st%ile as the affected phenotype. SNPLINK was used to remove SNPs in linkage disequilibrium (D′=0.7, R2=0.4). Data were analyzed with GeneHunter under a parametric, AD, rare, highly-penetrant disease model. Evidence favoring linkage was found in a 17 megabase (Mb) region on chromosome 2p and a 3.1 Mb region on chromosome14q (LOD score=2.62 at both sites). Bi-directional sequence analysis of the two best candidate genes, DDX1 (2p) and TINF2 (14q) was conducted to identify mutations. A novel mutation, K280E, in TINF2 (protein name TIN2) was identified in the 6 living, affected family members but not in the 8 unaffected relatives, suggesting inheritance from the affected fathers. There were no mutations in DDX1. TINF2 was sequenced in 8 additional, unrelated DC probands without DKCI, TERC or TERT mutations and 7 with known mutations. An R282H mutation was present in 3 unrelated DC probands (1 Hoyeraal-Hreidarsson, 1 Revesz Syndrome). Another DC patient had an R282S mutation. TINF2 mutations were not present in unaffected relatives, DC probands with mutations in DKC1, TERC or TERT, or 298 controls. The mutation prevalence in DC probands represented in our cohort of patients with DC are DKC1 (18.8%), TERC (18.8%), TERT (6.2%), and TINF2 (31.2%). As a component of shelterin, the protein complex that stabilizes telomeres, TIN2 is highly evolutionarily conserved. It serves as a bridge between the three primary telomere DNA-binding proteins, TRF1, TRF2 and POT1 (via TPP1). In silico analyses predict that K280E, R282H and R282S are deleterious mutations. Functional studies are underway to characterize possible effects of these mutations on shelterin protein interactions. By focusing on telomere length as the affected phenotype, instead of the heterogeneous clinical features present in DC patients, we identified mutations in TINF2 and further validated telomere length as a diagnostic test for DC. This study demonstrates that TINF2 is the 5th gene mutated in DC, the 1st in Revesz Syndrome, and the 1st shelterin complex gene mutated in human disease.