A Syndromic Intellectual Disability Disorder Caused by Variants in TELO2, a Gene Encoding a Component of the TTT Complex

Abstract

The proteins encoded by TELO2, TTI1, and TTI2 interact to form the TTT complex, a co-chaperone for maturation of the phosphatidylinositol 3-kinase-related protein kinases (PIKKs). Here we report six affected individuals from four families with intellectual disability (ID) and neurological and other congenital abnormalities associated with compound heterozygous variants in TELO2. Although their fibroblasts showed reduced steady-state levels of TELO2 and the other components of the TTT complex, PIKK functions were normal in cellular assays. Our results suggest that these TELO2 missense variants result in loss of function, perturb TTT complex stability, and cause an autosomal-recessive syndromic form of ID.

Figure 1

Clinical Phenotype of Individuals with TELO2 Variants (A) The pedigrees of families 1–4. The TELO2 genotype segregating in each family is shown. Individual II-1 in family 4 has an unaffected paternal half-sibling (not shown). (B) Current portraits of each affected individual. (C) Full body pictures of affected individuals in family 1. (D) Coronal view of chest CT angiogram of II-2 in family 1 showing an incomplete vascular ring with a right ascending aortic arch (RAoA), right common carotid artery (RCC), left common carotid artery (LCC), and descending aorta (DAo). There are also venous anomalies with a retroaortic left brachiocephalic vein (LBCv) that joins the azygous vein and enters the superior vena cava.

Figure 2

TELO2 Variants Affect Steady-State Levels of TTT Complex Components (A) A schematic of the TELO2 protein and the location of the missense variants identified in our study. (B) Evolutionary conservation of the amino acid residues altered by the missense variants in the indicated affected individuals (highlighted in yellow) and the surrounding TELO2 residues. The species for each sequence is listed on the left. The alignment was determined by ClustalW2 multiple protein alignment. (C) Immunoblot analysis of TTT complex components in extracts of primary fibroblasts in family 1 and family 2. The left panel shows decreased protein levels of TELO2, TTI1, and TTI2 in three affected siblings in family 1 (II-2, II-3, and II-4) and three control subjects (C.1, C.2, and C.3). Left lower panel is the quantification of TELO2, TTI1, and TTI2 levels in fibroblast extracts. The error bars show 1 SD around the mean determined in 3 independent experiments as measured by immunoblot analysis. The right panel shows that TELO2, TTI1, and TTI2 levels in extracts of skin fibroblast from the proband (II-2) from family 2 were reduced to about 17%, 18%, and 14% of these in controls in 3 independent experiments. The error bar shows 1 SD in three independent Western blots. (D) Immunoblot analysis of TTT complex in LCLs in family 1. Three controls LCL (C.1, C.2, and C.3) are shown. I-1 and I-2 are the heterozygous parents of the affected individuals in family 1. The three affected individuals in family 1 (II-2, II-3, and II-4) are shown. TELO2 protein levels were reduced to 31.8% of the mean of control subjects while the levels in the heterozygous parents are reduced to 74% of control subjects (p > 0.1, Student’s t test). Similarly, TTI1 and TTI2 proteins in the heterozygous parents of family 1 showed a decrease as compared to control levels which was not statistically significant (for TTI1, mean = 70%, p > 0.1; for TTI2, mean = 62%, p > 0.1, Student’s t test). In the affected individuals the level of TTI1 is reduced to 17% and TTI2 to 13% of control levels in LCL, respectively (p < 0.05, Student’s t test). The error bar shows 1 SD in 3 independent experiments. See also Figure S3.

Figure 3

Effect of Hsp90 Inhibition by 17AAG on Levels of TTT Complex Components and Selected PIKKs (A) Representative immunoblots of extracts from cultured skin fibroblasts of affected individuals in family 1 (II-2, II-3, and II-4) and three control subjects (C.1, C.2, and C.3) cultured in standard medium (left) or in medium with 1 μM 17AAG for 48 hr (right). (B) In standard medium (left), the steady-state levels of the TTT complex components and selected PIKKs expressed as percent of the mean of control levels are: TELO2 (33%, p < 0.05), TTI1 (18%, p < 0.05), TTI2 (51%, p = 0.06), ATM (77%, p > 0.1), PRKDC (78%, p > 0.1), ATR (76%, p > 0.1), and mTOR (77%, p > 0.1). In medium with 17AAG, the level of protein expression (right) expressed as percent of the mean of control levels are: TELO2 (12%, p < 0.05), TTI1 (32%, p = 0.056), TTI2 (19.6%, p < 0.05), ATM (34%, p < 0.05), PRKDC (56%, p < 0.05), ATR (87%, p > 0.1), and mTOR (111.8%, p > 0.1). The error bar indicates 1 SD in three independent experiments (Student’s t test).