|Home | About | Journals | Submit | Contact Us | Français|
Idiopathic pulmonary fibrosis (IPF) is the most common manifestation of telomere-mediated disorders. Germline mutations in the essential telomerase genes, hTERT and hTR, are the causal genetic defect in up to one-sixth of pulmonary fibrosis families. The presence of telomerase mutations in this subset is significant for clinical decisions as affected individuals can develop extra-pulmonary complications related to telomere shortening such as bone marrow failure and cryptogenic liver cirrhosis. There is also evidence that IPF is an ancestral manifestation of autosomal dominant telomere syndromes where, with successive generations, the disease evolves from pulmonary fibrosis into a bone marrow failure-predominant disorder, defining a unique form of genetic anticipation. Here I review the significance of telomere defects for understanding the genetics, disease patterns and pathophysiology of IPF. The importance of this diagnosis for patient care decisions will also be discussed.
Idiopathic pulmonary fibrosis (IPF) is perhaps the most devastating of the idiopathic disorders in medicine. It is estimated to affect as many as 100,000 individuals in the United States. Characterized by an unrelenting progression of parenchymal lung scarring, its most famous attribute has long been its “idiopathic” or unknown etiology. Recently, genetic clues have opened possibilities for a new understanding of IPF. Here I review the significance of telomere biology to understanding the genetics, pathophysiology and disease patterns seen in IPF patients.
IPF has a well-characterized, progressive clinical course. Most individuals are diagnosed after presenting with worsening respiratory complaints. From the time of diagnosis, IPF patients live on average 3 years, although in some individuals, the natural history may be more protracted. Several clinical risk factors are known to be linked with IPF. Age is the biggest with the great majority of individuals diagnosed after the age of 60. IPF is also diagnosed more frequently in males with a nearly 2:1 ratio. Cigarette smoke is known to accelerate disease onset with those having a smoking history presenting as much as a decade earlier than never smokers. A positive family history is also a major risk factor with up to 20% of IPF patients reporting an affected family member. Understanding the genetics which underlie familial clustering of pulmonary fibrosis has held promise for understanding its etiology.
IPF has a recognizable constellation of clinical findings which are often sufficient to make the diagnosis without the need for lung biopsy. On imaging studies, IPF has a characteristic honeycombing pattern which preferentially affects the basilar and peripheral areas of the lung (Figure 1). When available, histology shows a destructive pattern of alveolar architecture, and is classically known as usual interstitial pneumonia. Usual interstitial pneumonia is a histological hallmark of end-stage lung disease, and the “usual” terminology is derived from the fact that IPF/usual interstitial pneumonia is the most common of the interstitial lung diseases.
The treatment for IPF is supportive, and there are currently no known therapies that alter its natural history. Patients with IPF do not respond to immunosuppressive drugs, in contrast to fibrotic lung disease that occurs in the setting of autoimmune disorders. In eligible patients, transplant is considered, and since 2007, IPF has become the most common indication for lung transplant in the United States. Transplantation is however limited by organ availability and procedure morbidity. Despite significant advances in transplant medicine, recipients have an average survival of 5 years. A better understanding of its etiology holds promise for advancing therapeutic paradigms in IPF.
Telomerase is a remarkable enzyme that maintains chromosome ends[7–9]. Mutations in the essential genes coding for the enzyme telomerase are the most commonly identified genetic risk factor in IPF. Telomerase has two essential components: hTERT, the telomerase reverse transcriptase, and hTR, a specialized RNA that contains a template for telomere repeat addition. hTERT uses the template within hTR to add new (TTAGGG)n repeats onto the 3’ end of chromosomes. In cells, telomerase forms a holoenzyme complex with other proteins. Its biogenesis and stability depend on an essential protein dyskerin which is coded by the X-chromosome DKC1 gene. Dyskerin binds to an RNA motif within hTR known as the box H/ACA, which is critical for hTR integrity([13, 14], reviewed in). Mutations in hTERT, hTR and DKC1 cause telomerase loss of function, and decrease telomerase activity[12, 16, 17] (Figure 2). The loss of function accelerates the telomere shortening which normally occurs with age. Syndromes associated with mutant telomerase genes are therefore considered premature aging syndromes.
The consequences of telomerase deficiency were studied in model organisms before telomerase mutations were discovered in the setting of human disease[18, 19]. In mouse models where telomerase deletion was engineered, it was initially recognized that telomerase itself is not essential[18, 19]. In first generation mice that were null for telomerase, no phenotypic defects could be identified. However, after breeding for successive generations, mice that have short telomeres developed degenerative disease that was most evident in tissues of high turnover[19, 20]. For example, the bone marrow has a significant requirement for intact regenerative potential to sustain the high turnover demands, and short telomeres limit the self-renewal capacity of hematopoietic stem cells causing ineffective hematopoiesis[19–21]. The phenotypes of telomerase null mice are identical whether TR or TERT is deleted, and both TR and TERT null mice develop phenotypes only when telomeres are short[22, 23]. These genetically defined models have established telomere shortening as a mechanism of genetic anticipation, whereby phenotypes worsen in successive generations due to the accumulation of dysfunctional telomeres[16, 18, 21]. The absence of telomerase alone is therefore not sufficient to mediate degenerative disease, but the short telomere length which is the primary genetic determinant of phenotype severity.
In the setting of human disease, mutations in telomerase enzyme components were first identified in a rare syndrome dyskeratosis congenita (DC)[12, 24]. This disorder, as is delineated below, represents a more severe presentation of a spectrum of telomere syndromes where IPF represents an attenuated form. DC is classically defined based on a triad of mucocutaneous manifestations: reticular skin pigmentation, nail dystrophy and oral mucosal leukoplakia. In childhood, bone marrow failure is the most frequent complication of DC, while pulmonary fibrosis is a frequent cause of mortality in adults. In DC patients with bone marrow failure, pulmonary fibrosis can be precipitated by pulmonary toxic drugs in the setting of bone marrow transplant. For example, the alkylating agent busulfan, which is used in myeloablative conditioning regimens, causes fatal pulmonary fibrosis in DC patients. Attenuated bone marrow transplant regimens that avoid pulmonary toxic drugs and minimize ionizing radiation exposure can delay the onset of this complication[27–29]. Even without precipitating toxins, pulmonary fibrosis is a significant and under-estimated complication of DC. In some large DC kindreds, pulmonary fibrosis is the major cause of premature mortality in the absence of bone marrow failure. Therefore DC can manifest as familial pulmonary fibrosis.
DC is a disease of telomere maintenance. Mutations in the DKC1 gene cause an X-linked form; while mutations in hTERT and hTR cause autosomal dominant disease[17, 30]. In 40–50% of cases, the genes for DC remain uncharacterized, although short telomeres are a uniform feature[31, 32]. Mutations in the TINF2 gene, encoding the telomere binding protein TIN2, cause severe DC which generally manifests in early childhood as a result of de novo mutations, but can rarely be transmitted in an autosomal dominant manner[33, 34]. In autosomal dominant DC, there is progressive shortening of telomeres across generations which causes genetic anticipation, similar to what is seen in the telomerase knockout mouse[30, 35]. Therefore, syndromes of telomere shortening and trinucleotide repeat expansion are the two characterized molecular mechanisms of genetic anticipation in Mendelian disorders[10, 36].
The study of familial forms of IPF has established that, in at least a substantial subset, IPF is a disease of telomere maintenance. Mutations in hTERT and hTR are risk factors for pulmonary fibrosis underlying the inheritance in 8–15% of familial cases[37, 38]. In these families, IPF is inherited as an autosomal dominant trait with age-dependent penetrance. Mutations in the essential telomerase genes are also found in 1–3% of sporadic IPF cases[39, 40]. Although the frequency of hTERT mutations is higher than hTR mutations, the mutant gene cannot be distinguished based on clinical features alone[10, 41]. Moreover, the clinical patterns of lung disease in mutation carriers are identical to those in whom mutations cannot be identified indicating that short telomeres are sufficient to cause the common form of IPF. Importantly, even within a given mutation carrier or family, there are often heterogeneous patterns of lung disease indicating that short telomeres can cause a spectrum of idiopathic interstitial lung histologies. Among these, IPF/usual interstitial pneumonia is most common seen in approximately 65% of cases[37, 38, 40, 42, 43]. The histologic and clinical pulmonary manifestations heretofore identified in telomerase mutation carriers are summarized in Table 1.
Mutant hTERT and hTR genes cause telomere shortening in IPF because of loss of function and haploinsufficiency (Figure 2). The short telomere defect is germline, and short telomere length as measured in peripheral blood lymphocytes can distinguish telomerase mutation carriers from their non-carrier relatives with some specificity[37, 39]. Because IPF is common, compared to the prevalence of DC and aplastic anemia which have also been linked to mutant telomerase genes, lung disease is the most common manifestation of telomere-mediated disorders (Table 2)[2, 10, 37–39, 44–47]. Thus, although the classic mucocutaneous features of DC are specific for identifying individuals with telomere-mediated disease, they identify only a small subset, perhaps less than 5% of all cases (Table 2).
Although telomerase mutations underlie inheritance in one-sixth of families, short telomere length is a common finding in other IPF patients even when the telomerase genes appear intact. Sporadic IPF cases (those who report no family history) have significantly shorter telomeres than age-matched controls in cross-sectional studies[39, 40]. The telomere shortening can be detected in lymphocytes, granulocytes as well as alveolar epithelial cells, implicating a global telomere defect in these individuals. Since telomere length is in part genetically determined[16, 48], this observation indicates that IPF may be more likely to develop in individuals with the shortest telomere lengths in the population. There is also evidence that in these patients, short telomere length may be a risk factor for disease outside the lung. A subset of sporadic IPF that lack an apparent telomerase mutation also develops cryptogenic liver cirrhosis (3% in one study). Because liver disease is known to occur in DC[30, 49], its increased incidence in sporadic IPF suggests that it may represent a complication of telomere-mediated disease outside the lung in these patients even when they have no identifiable telomerase mutations . Moreover, several studies have shown that IPF patients have an increased incidence of diabetes, as much as 3-fold higher than age-matched controls[50, 51]. The incidence of insulin-requiring diabetes is also higher in IPF patients than controls. Short telomeres are sufficient to cause insulin secretion defects and glucose intolerance in telomerase deficient mice. The short telomere defect may therefore concurrently be a risk factor for the development of diabetes alongside IPF. Therefore the short telomere defect in sporadic IPF cases may be a risk factor for telomere-related disease outside the lung.
Since the telomere shortening in IPF patients is systemic, IPF patients and their relatives who carry telomerase mutations can have extra-pulmonary manifestations of telomere-mediated disease. This may appear as subtle to overt signs of bone marrow failure including macrocytosis of red blood cells, single lineage cytopenias, or aplastic anemia[30, 37, 43]. In addition, myelodysplastic syndromes (MDS), and acute myeloid leukemia (AML) occur more frequently in IPF families[42, 43, 53]. IPF and bone marrow failure have not been considered related conditions outside the setting of DC, but their co-occurrence in the absence of DC features can clinically identify families that carry telomerase mutations. A recent study found that the co-occurrence of these disorders within a single family is highly specific for the presence of a germline defect in telomerase. When present in consecutive generations, the syndrome complex of IPF and bone marrow failure predicted the presence of a mutant hTERT or hTR gene in 10 of 10 families (100%). A personal and family history for cytopenias in IPF patients should therefore increase the index of suspicion for a telomerase-associated disorder and prompt consideration for genetic counseling and work-up.
Although they are specific, the syndrome complex of IPF and bone marrow failure is not sensitive as the majority of IPF families who carry mutant telomerase genes do not have extra-pulmonary phenotypes at the time of assessment and cannot be readily distinguished clinically. A high index of suspicion for hematologic abnormalities in IPF patients is nonetheless significant even in the absence of overt signs as the limited bone marrow reserves in telomerase mutation carriers may make them exquisitely sensitive to develop toxicities when receiving myelosuppressive drugs. IPF patients may be exposed to bone marrow suppressive drugs in several settings including cytotoxic chemotherapy for the treatment of cancer or immunosuppressive drugs in the post-transplant setting. A careful examination of complete blood counts in IPF patients is thus particularly relevant to treatment decisions. The hematologic manifestations of telomere-mediated disease reported in IPF families are summarized in Table 1.
In addition to bone marrow failure, IPF patients with telomerase mutations may be at risk for developing other complications of telomere-mediated disease. For example, in some cases, liver cirrhosis may co-occur with IPF in the same individual or in other affected family members. Thus, although IPF may be a highly penetrant phenotype in telomerase mutation carriers, affected individuals are at risk for developing extra-pulmonary disease. Vigilance for the entire telomere syndrome complex of lung, bone marrow and liver disease has implications for surveillance and treatment decisions in both IPF patients and their at-risk family members (Table 1).
In autosomal dominant telomere syndromes, progressive telomere shortening due to telomerase haploinsufficiency causes an earlier onset of disease in successive generations. The extent of genetic anticipation ascertained at any given time by a carefully obtained family history depends in part on the degree of functional impairment of the mutant telomerase[10, 42]. Functionally null mutations may cause more obvious genetic anticipation than hypomorphic mutations that minimally compromise telomerase function[30, 42]. Recent observations indicate that, unique to telomere syndromes, genetic anticipation causes not only an earlier onset of disease, but an evolving pattern of disease in successive generations. Within a single family, a mutant hTERT and hTR gene can manifest as IPF, bone marrow failure, or a combined phenotype. There appears to be a several decade difference in onset, with IPF manifesting in older individuals, and bone marrow failure in younger ones (mean age 51 vs. 14). In families, this pattern can be appreciated as a generation effect whereby IPF is an ancestral manifestation occurring in older generations, and bone marrow failure manifesting in children (Figure 3). This evolving pattern of disease which correlates with successive telomere shortening highlights several important points. First, bone marrow failure is a more severe manifestation of telomere-mediated disease and thus generally appears early in life, reflecting the exquisite sensitivity of the hematopoietic system to telomere reserves. Pulmonary fibrosis, on the other hand, appears to be an attenuated form of disease whereby cumulative age-related changes are required to provoke the fibrosis process. The predictable and evolving pattern of disease in autosomal dominant telomere syndromes is, to our knowledge, a unique and distinct feature of autosomal dominant disorders which display anticipation. Awareness of the changing pattern of disease has implications for genetic counseling and for surveillance in children and grandchildren of IPF patients who may carry mutations and who may be at risk for developing bone marrow failure, or possibly, the overt DC phenotype (Figure 3).
The identification of short telomeres as a prevalent defect in IPF has opened new possibilities for studying the biology of age-related lung disease in genetically relevant models, such as the telomerase knockout mouse. When lung phenotypes were examined in mice with short telomeres, no obvious gross phenotypes were found. However, when mice with short telomeres were chronically challenged with cigarette smoke exposure, they developed air space destruction that is characteristic of emphysema. The emphysema defect was intrinsic to the lung parenchyma and was associated with cumulative DNA damage in alveolar epithelial cells. Emphysema is characterized by alveolar destruction and, like IPF, age and cigarette smoke are the primary risk factors. Interestingly, observations in families with telomerase mutations indicate that emphysema may be a first manifestation of telomere-related lung disease in some telomerase mutation carriers. The emphysema in this setting can manifest alone, or concurrently with pulmonary fibrosis as a mixed pulmonary defect. In one study examining radiographs of telomerase mutation carriers with IPF, there was evidence of superimposed emphysema in 20% of cases. Therefore, short telomeres manifest as a heterogeneous group of age-related lung disorders, among them IPF is most common, but the spectrum may encompass some forms of emphysema which may manifest alone or as a combined interstitial lung disease-emphysema syndrome (Table 1).
Emphysema is the fourth most common cause of death in the United States. The observation that short telomere length is a susceptibility factor in emphysema is significant as its genetics are not completely understood. Although traditionally considered distinct entities, emphysematous changes are known to co-occur with pulmonary fibrosis in 10% of individuals who smoke cigarettes[56, 57]. In these individuals, emphysematous changes appear in the lung apex, while IPF in the basilar portions. The recent studies in animals with telomere dysfunction, along with clinical observations in telomerase mutation carriers, indicate that IPF and emphysema, in at least a subset, may represent regional responses to cumulative injury superimposed on a short telomere genetic background. The study of regional effectors that determine whether emphysema or fibrosis is the predominant lung phenotype have the potential to yield a better understanding of the biology of these age-related disorders. There is therefore an intimate connection between telomere dysfunction and the genetics of several common, age-related lung disorders.
The studies in mice with short telomeres suggest that the short telomere defect alone is not sufficient to cause lung disease. Instead, short telomeres lower the threshold to cumulative damage that occurs with age. Similar to the multi-step progression for carcinogenesis, age-related lung disease may be the end result of multiple “hits” that occur with aging. In such a model, short telomeres, as genetically determined, would represent a first hit. With age, additional injuries accumulate and ultimately provoke what appears to be irreversible lung damage. The requirement for additional hits may explain why mutant telomerase genes in IPF patients manifest late in life where the average age of onset is in the sixth decade[26, 42, 43]. Indeed, symptomatic pulmonary disease in the setting of telomerase mutations is rare prior to the third decade, except when precipitated by toxic drugs in the setting of bone marrow transplant. Cigarette smoke exposure is a highly relevant example of an environmental second hit. Based on the fact that telomere dysfunction causes progressive irreversible stem cell failure in the hematopoietic system, we have previously proposed that telomere dysfunction may cause an irreversible stem cell failure in alveolar epithelial cells, a putative site of injury in IPF and emphysema. A better understanding of the regenerative mechanisms of alveolar epithelial cells can advance current paradigms for understanding the biology of telomere-mediated lung disease.
In summary, the intimate relationship between telomere biology and pulmonary fibrosis has opened new possibilities for a deeper understanding of a devastating, common lung disorder. The genetic clues in connection to telomerase suggest that IPF may be the most common, but not sole manifestation of telomere dysfunction in the lung. Careful clinical observations in affected families have established that IPF occurs in the setting of a syndrome complex which has extra-pulmonary manifestations. An appreciation for the discrete clinical manifestations of this telomere syndrome is significant for diagnostic decisions, genetic counseling and drug dosing for affected patients and their families. In addition to the immediately apparent impact on patient care, the connection between IPF and telomere biology opens the possibility of considering new paradigms that have the potential to improve treatment approaches in the future.
I am grateful for critical comments from lab members, and help with the illustrations from Jennifer Fairman. Work in my lab is supported by funds from the United States National Institutes of Health, the Maryland Stem Cell Research Foundation and the Flight Attendants Medical Research Institute.
Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.