Reply: Telomerase Makes Connections between Pulmonary Fibrosis and Emphysema

Abstract

From the Authors: I appreciate the opportunity to respond to the letter by Nunes and colleagues, which reports a family with a mutation in the telomerase reverse transcriptase gene, TERT, and several individuals with emphysema or the combined pulmonary fibrosis emphysema (CPFE) phenotype. The authors’ observations follow on a study from our group that reported idiopathic pulmonary fibrosis, emphysema, and the CPFE phenotype in related individuals who shared a loss-of-function mutation in the telomerase RNA gene, TR (1). Our clinical report was in the context of observations in telomerase null mice that we had shown developed emphysema after chronic cigarette smoke exposure. We concluded that short telomere length is a genetic susceptibility factor for emphysema in mice and in some human cases (1). Mutations in the essential telomerase genes, TERT and TR, underlie familial pulmonary fibrosis in up to one-fifth of cases and are its most prevalent genetic cause (2, 3). In these families, pulmonary fibrosis manifests as an autosomal dominant disorder with age-dependent penetrance. In some cases, pulmonary fibrosis is one of several manifestations of a telomere syndrome that includes bone marrow failure, cryptogenic liver cirrhosis, enteropathies, and certain malignancies (4). Mutations in the X-linked telomerase component DKC1 also cause pulmonary fibrosis, underscoring the important role of telomerase dysfunction in pulmonary fibrosis pathogenesis (5). The letter by Nunes and colleagues raises the question as to whether telomere syndromes are a cause of CPFE. Although it is still early, given that there are a number of cases now, the answer seems to be yes. However, I would like to make a case for using the term “telomere syndromes” in reference to telomere disorders rather “telomeropathy.” Our group prefers the syndromic terminology because in individuals with telomerase mutations, there is often evidence of multiple organs affected outside the lung. The reference to “telomere syndromes” therefore reflects the fact that these patients do not have isolated lung disease but have systemic telomere maintenance defects that manifest in predictable patterns that are relevant for clinical decisions (4). For example, extrapulmonary degenerative disease may cause only mild blood count abnormalities de novo, but cytopenias may be significant and associated with morbidity in the lung transplant setting where myelosuppressive drugs are used. Telomerase mutation carriers often also have an increased predilection for rare medication-related toxicities. Such observations are the topic of ongoing work and highlight the importance of recognizing this entity in patients with lung disease. I would finally like to highlight the opportunities that human genetics opens in considering the classification of lung disease. Although idiopathic interstitial pneumonias and emphysema have distinct physiologic and pathological classifications, it is clear from recent genetic studies that telomerase mutations are sufficient to provoke a broad spectrum of lung pathologies (reviewed in Reference 2). These often occur in the same individual, suggesting they represent differences in histology rather than pathophysiology per se. Such observations are important as we witness the twenty-first century revolution in genetic medicine. In this era, molecular medicine holds the promise to inform our understanding of disease mechanisms while opening possibilities for appreciating clinical patterns that may not have been imagined previously. The fact that telomerase mutations predispose to pulmonary fibrosis, emphysema, and the combined entity exemplifies such an insight because the mortality burden for these disorders is particularly devastating and because new approaches are very much needed. One thus wonders whether a day will come when we will refer to “telomere-mediated lung disease” rather than “emphysema” or “idiopathic pulmonary fibrosis” as the cause of a patient’s dyspnea, and hopefully, offer such a patient telomere-directed therapies.