Subclinical lung disease, macrocytosis, and premature graying in kindreds with telomerase (TERT) mutations

Abstract

Background: Mutations in the human gene encoding the protein component of telomerase (TERT) are the most common genetic defect in patients with familial idiopathic pulmonary fibrosis (IPF). The subclinical phenotypes of asymptomatic members of these families have not been evaluated with respect to TERT mutation status or telomere length.

Methods: We measured a variety of pulmonary, blood, skin, and bone parameters for 20 subjects with heterozygous TERT mutations (carriers) and 20 family members who had not inherited a TERT mutation (noncarriers) to identify the spectrum of phenotypes associated with mutations in this gene. The two groups were matched for sex, age, and cigarette smoking. Three TERT mutation carriers had IPF (IPF carriers). The rest of the carriers were apparently healthy (asymptomatic carriers) and were compared with the noncarriers.

Results: Asymptomatic carriers exhibited significantly lower diffusing capacity of lung for carbon monoxide (Dlco), impaired recruitment of Dlco with exercise, radiographic signs of lung fibrosis, and increased fractional lung tissue volume quantified by high-resolution chest CT scan than noncarriers. RBC and platelet counts were significantly lower, and the mean corpuscular volume and mean corpuscular hemoglobin concentration were significantly higher in carriers than in noncarriers. Carriers reported significantly earlier graying of hair than noncarriers. TERT mutation status is more accurately predicted by short telomere lengths than any of these measured phenotypes.

Conclusions: TERT mutation carriers exhibit early preclinical signs of lung fibrosis, bone marrow dysfunction, and premature graying. These clinical features and short telomere lengths characterize patients with germline TERT mutations.

Figure 1.

Telomere lengths of family members from TERT kindreds. Mean telomere lengths as measured by a quantitative polymerase chain reaction assay is shown for TERT mutation carriers who are healthy and asymptomatic (asymptomatic carriers,), TERT mutation carriers with IPF (IPF carriers, ■), and subjects without TERT mutations (noncarriers, ○). A, The telomere lengths of subjects are shown relative to the 50th percentile (center line) as well as the 10th and 90th percentiles for a previously described reference cohort of 195 unrelated healthy individuals aged 19 to 89 years (shaded region). B, Mean O-E age-adjusted telomere length for the control reference cohort, 20 subjects without TERT mutations [−], and 20 related subjects with TERT mutations [+].*P = .035. **P = 1.4 × 10⁻²². IPF = idiopathic pulmonary fibrosis; LN(T/S) = circulating leukocytes and telomere lengths; O-E = observed minus expected; TERT = human gene encoding the protein component of telomerase.

Figure 2.

Diffusion capacity measurements of subjects at rest and with exercise. A, % Predicted single-breath Dlco for subjects without TERT mutations (noncarriers, ○), TERT mutation carriers who are asymptomatic (asymptomatic carriers, ),and TERT mutation carriers with IPF (IPF carriers, ■). The results are graphed for all subjects. *Two-tailed Student t test P = .037 for asymptomatic carriers vs noncarriers. ***Two-tailed Student t test P = 9.1 × 10⁻⁵ for IPF carriers vs noncarriers. B-D, Recruitment of Dlco (B), Dmco (C), and Vc (D) with respect to cardiac output in noncarriers, asymptomatic carriers, and IPF carriers. The Dlco is expressed under standardized conditions of hemoglobin (14.6 mg/dL) and Pao₂ (120 mm Hg); Qc is the calculated cardiac output. The Vc was estimated by the Roughton-Forster method. The best-fit lines are as follow: Dlco std, 1.71 Qc + 10.40 (asymptomatic carriers, R² = 0.68); Dlco std, 1.17 Qc + 13.04 (noncarriers, R² = 0.69; P = .001); Dlco std, 1.24 Qc + 2.43 (IPF carriers, R² = 0.88); Dmco, 2.75 Qc + 28.85 (asymptomatic carriers, R² = 0.22); Dmco, 1.32 Qc + 34.64 (noncarriers, R² = 0.14; P = .05); Dmco, 1.80 Qc + 5.39 (IPF carriers, R² = 0.74); Vc, 5.75 Qc + 20.23 (asymptomatic carriers, R² = 0.65); Vc, 5.14 Qc + 23.07 (noncarriers, R² = 0.75; P > .5); and Vc, 3.70 Qc + 18.01 (IPF carriers, R² = 0.34). P values indicate a comparison of the slopes of individual regression lines between asymptomatic carriers and noncarriers. Dlco = diffusing capacity of lung for carbon monoxide; Dlco std = diffusing capacity of lung for carbon monoxide at standard conditions; Dmco = diffusing capacity of membrane for carbon monoxide; Vc = pulmonary capillary blood volume. See Figure 1 legend for expansion of other abbreviations.

Figure 3.

High-resolution CT (HRCT) scans of subjects without TERT mutations (noncarriers) and those with TERT mutations (asymptomatic carriers and IPF carriers). Inspiratory HRCT scans were binned into four different categories based on radiographic evidence of reticular changes. The categories are indicated from left to right as follows: group 1, no reticular changes; group 2, minimal radiographic densities (scans included in this category had evidence of apical, single-level, or scattered peripheral or peribronchial reticulations; parenchymal bands; irregular linear opacities; or mild ground glass opacities); group 3, increased radiographic densities (scans included in this category had evidence of multilobe peripheral or peribronchial reticulations, subpleural lines, or mild ground glass opacities; they did not display honeycombing or any findings inconsistent with UIP); and group 4, consistent with UIP (scans in this category had features typical of IPF with peripheral and basal-predominant reticulations with or without honeycombing). The total number of subjects in each category is indicated beneath a representative HRCT scan radiograph and in graphical form. UIP = usual interstitial pneumonia. See Figure 1 legend for expansion of other abbreviations.

Figure 4.

Fractional tissue volume (FTV) was calculated from HRCT scans of the chest as described in the “Materials and Methods” section. A, Axial HRCT images obtained at supine end inspiration from a subject without a TERT mutation (noncarrier, left), an asymptomatic subject with a TERT mutation (asymptomatic carrier, middle), and a TERT mutation carrier with IPF (IPF carrier, right). Corresponding color maps show the distribution of FTV in the same images. B, Three-dimensional color maps show the topographical surface distribution of FTV in three orientations. The three coordinate axes are x, subject’s right to left; y, posterior to anterior; and z, cephalad to caudal. The highest FTV is seen in the posterior, peripheral regions of the lower lobes. See Figure 1 and 3 legends for expansion of abbreviations.

Figure 5.

Supine expiratory total lung FTV for subjects without TERT mutations (noncarriers, ○) and those with TERT mutations with or without IPF (IPF carriers [■] and asymptomatic carriers [], respectively). A, The results are graphed for all subjects (left), those who had never smoked (middle), and those who were either current or past smokers (right). *Two-tailed Student t test P = .024. ***Two-tailed Student t test P = 7.4 × 10⁻⁴. The one-tailed Student t test P values analyzing the subgroups are 0.10 (n.s.), 0.034 (†), and 2.9 × 10⁻³ (†††). B, Expiratory total lung FTV vs FVC % predicted. The best-fit line is drawn (R² = 0.256, P = .001). C, Expiratory total lung FTV vs Dlco % predicted. The best-fit line is drawn (R² = 0.227, P = .002). See Figure 1, 2, and 4 legends for expansion of abbreviations.

Figure 6.

Blood counts and quantitative phenotypes of subjects without TERT mutations (noncarriers, ○) and those with TERT mutations with or without IPF (IPF carriers [■] and asymptomatic carriers [], respectively). A, RBC counts (millions/mL) for carriers are significantly lower than for family member control subjects. **P = 2.9 × 10⁻³. B, MCV (fL) is significantly higher for carriers than for noncarriers. ***P = 6.6 × 10⁻⁸. C, Platelet counts (thousands/μL) for carriers are lower than for noncarriers. *P = .032. D, RBC count (millions/μL) vs observed minus expected age-adjusted telomere length for all subjects. The best-fit line is drawn (R² = 0.153, P = .012). E, MCV (fL) vs observed minus expected age-adjusted telomere length for all subjects. The best-fit line is drawn (R² = 0.294, P = .0003). F, Dlco % predicted vs observed minus expected age-adjusted telomere length for all subjects. The best-fit line is drawn (R² = 0.170, P = .008). G, Expiratory total lung FTV vs observed minus expected age-adjusted telomere length for all subjects. The best-fit line is drawn (R² = 0.230, P = .002). MCV = mean corpuscular volume. See Figure 1, 2, and 4 legends for expansion of other abbreviations.