Funded by the NIH • Developed at the University of Washington, Seattle
Funded by the NIH • Developed at the University of Washington, Seattle
[Includes: Cystic Fibrosis (CF, Mucoviscidosis) and Congenital Absence of the Vas Deferens (CAVD)]
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Authors:
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Samuel M Moskowitz, MD
James F Chmiel, MD Darci L Sternen, MS, CGC Edith Cheng, MS, MD Garry R Cutting, MD |
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Initial Posting:
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Last Update:
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Disease characteristics. CFTR-related disorders include cystic fibrosis (CF) and congenital absence of the vas deferens (CAVD). Cystic fibrosis affects epithelia of the respiratory tract, exocrine pancreas, intestine, male genital tract, hepatobiliary system, and exocrine sweat glands, resulting in complex multisystem disease. Pulmonary disease is the major cause of morbidity and mortality in CF. Affected individuals have lower airway inflammation and chronic endobronchial infection, progressing to end-stage lung disease characterized by extensive airway damage (bronchiectasis, cysts, and abscesses) and fibrosis of lung parenchyma. Meconium ileus occurs at birth in 15%-20% of newborns with CF. Pancreatic insufficiency with malabsorption occurs in the great majority of individuals with CF. More than 95% of males with CF are infertile as a result of azoospermia caused by absent, atrophic, or fibrotic Wolffian duct structures. CAVD occurs in men without pulmonary or gastrointestinal manifestations of CF. Affected men have azoospermia and are thus infertile.
Diagnosis/testing. Most commonly the diagnosis of cystic fibrosis (CF) is established in individuals with one or more characteristic phenotypic features of CF plus evidence of an abnormality in cystic fibrosis transmembrane conductance regulator (CFTR) function based on one of the following: presence of two disease-causing mutations in the CFTR gene or two abnormal quantitative pilocarpine iontophoresis sweat chloride values (>60 mEq/L) or transepithelial nasal potential difference (NPD) measurements characteristic of CF. The CFTR mutation detection rate varies by test method and ethnic background. In some symptomatic individuals, only one or neither disease-causing mutation is detectable; in some carriers, the disease-causing mutation is not detectable. The diagnosis of CFTR-related CAVD is established in males with azoospermia, low volume of ejaculated semen, absence of vas deferens on clinical or ultrasound examination, and at least one disease-causing mutation in CFTR.
Management. Treatment of manifestations: treatment/prevention of pulmonary complications using oral, inhaled, or IV antibiotics, bronchodilators, anti-inflammatory agents, mucolytic agents, and chest physiotherapy; lung or heart/lung transplantation in selected patients; topical steroids, antibiotics, and/or surgical intervention for nasal/sinus symptoms; special infant formulas to enhance weight gain; oral pancreatic enzyme replacement with meals in those who are pancreatic insufficient; supplemental feeding, often with an indwelling gastric feeding catheter to increase caloric intake; additional fat-soluble vitamins and zinc; oral ursodiol for biliary sludging/obstruction; management of CF-related diabetes mellitus (CFRD) by an endocrinologist; assisted reproductive technologies (ART) for male infertility. Prevention of secondary complications: airway clearance using chest physiotherapy (CPT) and a variety of airway clearance techniques (ACTs); antibiotics to eradicate initial airway infection and prevent chronic airway infection; immunizations. Surveillance: regularly scheduled visits to CF care providers to monitor for subtle changes in physical examination, pulmonary function studies, chest radiographs, specific blood and urine tests; cultures of respiratory tract secretions at least four times yearly in patients who are not yet chronically infected with P. aeruginosa; random serum glucose concentration measured annually to screen for CFRD; assessment of bone density starting in adolescence. Agents/circumstances to avoid: respiratory irritants (smoke, dust); respiratory infectious agents (especially viruses); dehydration. Testing of relatives at risk: sweat chloride testing of sibs and mothers of affected individuals to determine if they may have mild or not yet symptomatic forms of CF. Therapies under investigation: antibiotic agents/treatment schedules to delay chronic respiratory tract infection.
Genetic counseling. CFTR-related disorders are inherited in an autosomal recessive manner. Sibs of a proband with cystic fibrosis and brothers of a proband with CAVD have a 25% chance of being affected, a 50% chance of being asymptomatic carriers, and a 25% chance of being unaffected and not carriers. Molecular genetic testing for disease-causing mutation(s) in the CFTR gene is used for carrier detection in population screening programs. Prenatal testing is available for pregnancies at increased risk for CFTR-related disorders if the disease-causing mutations in the family are known.
Cystic fibrosis (CF). Phenotypic features of CF include but are not limited to the following:
Chronic sinopulmonary disease (chronic cough and sputum production, chronic wheeze and air trapping, obstructive lung disease on lung function tests, persistent colonization with pathogens commonly found in individuals with CF, chronic chest radiograph abnormalities, chronic pansinusitis, digital clubbing)
Gastrointestinal/nutritional abnormalities (meconium ileus, rectal prolapse, malabsorption/pancreatic insufficiency, steatorrhea, hypoproteinemia, fat-soluble vitamin deficiencies, failure to thrive, distal intestinal obstructive syndrome, recurrent pancreatitis, biliary sludging, elevation of transaminases and gamma-glutamyl transferase, direct hyperbilirubinemia, chronic hepatobiliary disease)
Obstructive azoospermia
Salt-loss syndromes (acute salt depletion, chronic metabolic alkalosis, hyponatremic hypochloremic dehydration)
The diagnosis of CF is established in individuals with the following:
One or more characteristic phenotypic features of CF
and
Evidence of an abnormality in cystic fibrosis transmembrane conductance regulator (CFTR) function based on one of the following:
Presence of two disease-causing mutations in the
CFTR gene
or
Two abnormal quantitative pilocarpine iontophoresis sweat chloride values (>60 mEq/L)
or
Transepithelial nasal potential difference (NPD) measurements characteristic of CF
The diagnosis of CF may be made in the absence of phenotypic features of CF in the following settings:
Diagnosis in a newborn screening program (based on the presence of two disease-causing mutations in the CFTR gene or abnormal sweat chloride value). In 2002, 12.8% of newly diagnosed individuals were identified through newborn screening [CFF Patient Registry 2003].
In utero diagnosis by presence of two disease-causing mutations in the CFTR gene. In 2002, 4.0% of newly diagnosed individuals were identified by prenatal diagnosis [CFF Patient Registry 2003].
Congenital absence of the vas deferens (CAVD). The diagnosis of CFTR-related CAVD is established in males with the following:
Azoospermia (absence of sperm in the semen)
Absence of the vas deferens on palpation (Rarely, a thin fibrous cord representing a rudimentary vas deferens may be present.)
An identifiable mutation in one or both CFTR alleles [Dork et al 1997]
Additional features that may be seen include the following:
A low volume of ejaculated semen (<2 mL; normal: 3-5 mL) with a specific chemical profile
Evidence of abnormalities of seminal vesicles or vas deferens on rectal ultrasound examination
Sweat chloride. The National Committee for Clinical Laboratory Standards has published guidelines for the appropriate performance of the quantitative pilocarpine iontophoresis procedure [Wayne 1994 , Legrys 1996]. Centers accredited by the Cystic Fibrosis Foundation are required to adhere to this protocol; alternative sweat test procedures are not acceptable. This test is positive in more than 90% of individuals with CF.
A minimum sweat weight of 75 mg must be collected during a 30-minute period to assure a sweat rate of 1 g/M 2/min.
A chloride concentration greater than 60 mEq/L in sweat on two separate occasions is diagnostic.
Note: Sweat chloride levels higher than 160 mEq/L are not physiologically possible and should be attributed to technical error. (2) False positive sweat chloride results may be associated with other conditions, most notably mucopolysaccharidosis type 1 (Hurler syndrome). (3) False negative sweat chloride results may occur in the setting of acute CF-related salt losses. (4) When CF is suspected in an individual with hyponatremia and hypochloremia, sweat testing should be deferred until electrolyte balance has been restored and fluid status stabilized.
Transepithelial nasal potential difference (NPD). Respiratory epithelia regulate ion transport and alter content of the airway surface fluid by active transport mechanisms. The absence of functional CFTR at the apical surface with resultant alterations in chloride efflux and sodium transport produces an abnormal electrical potential difference across epithelial surfaces. The protocol for NPD measurements in individuals older than age six years is well described, standardized, and safely performed in many specialized CF centers worldwide [Schuler et al 2004 , Standaert et al 2004].
Individuals with CF have the following:
A raised (more negative) baseline NPD reflecting enhanced sodium absorption across a relatively chloride-impermeable membrane
A greater change in NPD during perfusion of the nasal mucosa with amiloride, an inhibitor of sodium channel activity
Minimal change in NPD in response to perfusion with amiloride/low chloride/beta-agonist, as a measure of cAMP-mediated chloride transport via CFTR
Newborn screening. Newborn screening using immunoreactive trypsinogen (IRT) assays performed on blood spots has been implemented in most of the United States [National Newborn Screening Status Report (pdf)]. Trypsinogen is synthesized in the pancreas; in CF, its release into the circulation appears to be enhanced by abnormal pancreatic duct secretions. Thus, IRT levels are elevated in cystic fibrosis. The benefits of newborn screening across various populations have been reviewed [Wagener et al 2004 , Grosse et al 2006].
In keeping with the use of IRT as a screening test, the definition of a positive result favors sensitivity over specificity, resulting in a decreased positive predictive value and a significant false positive rate when IRT is used by itself. Some states in the US utilize serial IRT testing to improve sensitivity and specificity [Sontag et al 2005].
Abnormal IRT results are therefore evaluated through sweat testing and/or molecular genetic testing of CFTR [Gregg et al 1997 , Pollitt 1998 , Wilcken & Wiley 2003].
Semen analysis. Additional findings of the semen of men with CAVD include the following [Holsclaw et al 1971]:
GeneReviews designates a molecular genetic test as clinically available only if the test is listed in the GeneTests Laboratory Directory by either a US CLIA-licensed laboratory or a non-US clinical laboratory. GeneTests does not verify laboratory-submitted information or warrant any aspect of a laboratory's licensure or performance. Clinicians must communicate directly with the laboratories to verify information. —ED.
Gene. CFTR is the only gene known to be associated with the CFTR-related disorders, CF and CAVD.
Clinical uses
Clinical testing
Mutation panels. Several panels are available. In 2004, the American College of Medical Genetics (ACMG) decreased the recommended mutation panel from 25 to 23 mutations [Watson et al 2004]. The mutation detection rate for the 23-mutation panel (Table 8) varies with ethnic background (Table 1) [Richards et al 2002].
5T/TG tract analysis
The poly T tract, a string of thymidine bases located in intron 8 of the CFTR gene, can be associated with CFTR-related disorders depending on its size. The three common variants of the poly T tract are 5T, 7T, and 9T. Both 7T and 9T are considered polymorphic variants and 5T is considered a variably penetrant mutation. The 5T variant is thought to decrease the efficiency of intron 8 splicing. Poly T testing is appropriate as a reflex test when an R117H mutation is detected or an adult male is being evaluated for CAVD.
The TG tract lies just 5' of the poly T tract. It consists of a short string of TG repeats that commonly number 11, 12, or 13. A longer TG tract (12 or 13) in conjunction with a shorter poly T tract (5T) has the strongest adverse effect on proper intron 8 splicing [Cuppens et al 1998 , Groman et al 2004].
Males with CAVD or suspected CAVD, individuals with non-classic CF, or adult carriers of 5T who wish to further refine their reproductive risks are all appropriate for 5T/TG tract typing.
Deletion analysis. MLPA (multiplex-ligation-dependent profbe amplification) can detect deletions not identified by sequence analysis. Mutation detection rate is not known.
Sequence analysis. Sequencing of all exons, intron/exon borders, promoter regions and specific intronic regions detects more than 98% of CFTR mutations [Strom et al 2003].
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1. The original 25-mutation panel recommended by the American College of Medical Genetics [Grody et al 2001] included the 23 mutations listed in Table 8
, 1078delT, and I148T. The 23-mutation panel recommended in 2004 is expected to have a similar mutation detection rate [Watson et al 2004]. Other panels may have significantly different mutation detection rates.
2. Grody et al 2001 3. Palomaki et al 2002 4. Using an assay to sequence all the coding sequences, splice donor and acceptance sites, the promotor region, and two intronic sequences [Strom et al 2003] |
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Interpretation of test results used in diagnosis of individuals suspected of having CF
For issues to consider in interpretation of sequence analysis results, click here .
The number of abnormal alleles detected (two, one, or none) (Table 2) depends on the mutation detection rate (Table 1).
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Calculated using Hardy-Weinberg Rule
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Interpretation of test results used in diagnosis of individuals suspected of having CAVD. The percentage of mutant CFTR alleles and 5T variant alleles detected in males with CAVD is summarized in Table 3 .
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Interpretation of results of CF carrier testing. It is recommended that in population screening programs cystic fibrosis carrier testing be performed using the ACMG 23-mutation panel (Table 8).
5T and TG tract typing should not be included in a routine carrier screen.
If an individual has the R117H mutation, reflex testing for the variants 5T/7T/9T is recommended.
If the individual has the 5T allele, family studies are recommended to determine if the 5T allele is in cis configuration or trans configuration with the R117H allele.
The 5T/TG tract analysis is not able to provide a specific risk figure for developing symptoms or having a child who develops symptoms of non-classic CF or CAVD; it is able to assign risk as "increased" or "decreased."
A person with ΔF508 (Phe508del) and 5T/11TG is less likely to develop non-classic CF, but it is still possible.
An individual with ΔF508 and 5T/12TG or ΔF508 and 5T/13TG is more likely, though not certain, to develop CAVD or non-classic CF [Groman et al 2004].
Cystic fibrosis. In most circumstances, the following strategy is indicated:
Quantitative pilocarpine iontophoresis for sweat chloride concentrations (remains the primary test for the diagnosis of CF, accurately diagnosing >90% of cases)
Molecular genetic testing of CFTR for diagnostic purposes if sweat chloride testing is unavailable or uninformative (CFTR molecular genetic testing for prognostic and epidemiologic purposes is appropriate for individuals diagnosed with CF based on sweat chloride testing.)
Transepithelial nasal potential difference (NPD) measurements to confirm the diagnosis of CF in symptomatic individuals with borderline or nondiagnostic sweat tests in whom only one or no CFTR disease-causing mutation has been detected
In the following special circumstances, CFTR molecular genetic testing is the initial diagnostic test:
Prenatal testing in a high-risk fetus
Prenatal diagnosis in low-risk fetus with echogenic bowel
Symptomatic infants (i.e., those with meconium ileus) who are too young to produce adequate volumes of sweat
Testing of symptomatic sibs of an affected individual in whom both CFTR mutations have been identified
CAVD. The diagnosis of CAVD is generally made in three steps:
Evaluation of infertility. A male with severe oligospermia (<5 million), azoospermia, or very low volume of semen (<2 mL) proceeds to Step 2.
Clinical evaluation by a urologist. If absence of the vas deferens is diagnosed by palpation, the workup proceeds to Step 3. (Imaging may be used but is not usually necessary if the clinical examination is consistent.)
Molecular genetic testing for CFTR mutations
An increased prevalence of CFTR mutations has been noted in individuals with idiopathic pancreatitis, bronchiectasis, allergic bronchopulmonary aspergillosis, and chronic rhinosinusitis. The reader is referred to the following references for further information: Cohn et al 1998 , Luisetti et al 1998 , Mickle & Cutting 1998 , Wang et al 2000 , Cohn et al 2004 , Cohn et al 2005 , Nick & Rodman 2005 , Weiss et al 2005 .
At present, DNA testing is of unknown and unclear utility for these conditions.
Cystic fibrosis (CF) affects the epithelia in several organs resulting in a complex, multisystem disease that includes the exocrine pancreas, intestine, respiratory tract, male genital tract, hepatobiliary system, and exocrine sweat glands. Disease expression varies by severity of CFTR mutations [De Braekeleer et al 1997], genetic modifiers [Drumm et al 2005 , Blackman et al 2006 , Vanscoy et al 2007], and environmental factors [Goss et al 2004]. The range extends from early childhood death as a result of progressive obstructive lung disease with bronchiectasis, to pancreatic insufficiency with gradually progressive obstructive lung disease during adolescence and increasing frequency of hospitalization for pulmonary disease in early adulthood, to recurrent sinusitis and bronchitis or male infertility in young adulthood.
The great majority of individuals with CF are pancreatic insufficient. Individuals with CF and pancreatic sufficiency (<10%) have a milder clinical course with greater median survival (i.e., 56 years [1995 CFF Patient Registry]) than those with pancreatic insufficiency.
The overall median survival is 36.5 years (95% confidence intervals, 33.7-40.0 years) [CFF Patient Registry 2006]. A gender difference is present in CF, with greater median survival in males than in females [CFF Patient Registry 1999].
Respiratory. Pulmonary disease remains the major cause of morbidity and mortality in CF [CFF Patient Registry 2006]. Affected individuals have lower airway inflammation and chronic endobronchial infection. Failure of lung defenses leads to bacterial endobronchitis (most commonly Staphylococcus aureus and Pseudomonas aeruginosa) with resulting airway obstruction and intense neutrophilic inflammation.
Early manifestations are chronic cough, intermittent sputum production, and exertional dyspnea. As the lung disease progresses as a result of chronic endobronchitis, structural injury to the airways occurs with resulting bronchiectasis. End-stage lung disease is characterized by extensive damage to the airways (cysts/abscesses) and accompanying fibrosis of lung parenchyma adjacent to airways.
Gastrointestinal. Meconium ileus occurs in 15%-20% of newborns diagnosed with CF.
Pancreatic insufficiency with malabsorption occurs in the great majority of individuals with CF. Exocrine pancreatic insufficiency is caused by inspissation of secretions within the pancreatic ducts and ultimately interstitial fibrosis. The clinical manifestations are steatorrhea and poor growth related to fat malabsorption and hemolytic anemia, defective coagulation, or skin rashes related to deficiencies of fat-soluble vitamins and zinc. Acute or chronic recurrent pancreatitis can be a presenting manifestation of CF, and is much more common among those with pancreatic sufficiency (10% prevalence) than those with pancreatic insufficiency (0.5% prevalence) [De Boeck et al 2005].
Cystic fibrosis-related diabetes mellitus (CFRDM) may present in adolescence. It is diagnosed in 7% of those age 11 to 17 years [CFF Patient Registry 2006]. The prevalence increases in adulthood. The etiology is a combination of reduced insulin secretion (secondary to fibrosis of the pancreas and reduced number of islet cells) and peripheral insulin resistance [Lanng 1996 , Hardin et al 1997].
Hepatobiliary disease, with elevation of serum concentration of liver enzymes in school-age children, infrequently progresses to biliary cirrhosis in adolescents and adults. Prevalence of liver disease varies based on definition, with the overall rate reported as 6.1% in the 2003 CFF Patient Registry . As liver disease progresses, portal hypertension and varices develop. Liver disease is second to pulmonary disease (plus organ transplantation complications) as a cause of mortality in CF (1.7% of deaths) [CFF Patient Registry 2003].
Fertility. More than 95% of males with CF are infertile as a result of azoospermia caused by altered vas deferens, which may be absent, atrophic, or fibrotic. The body and tail of the epididymis and seminal vesicles may be abnormally dilated or absent.
Women with CF are fertile, although a few females have abnormal cervical mucus that may contribute to infertility. The rate of live births among females with CF age 13-45 years is 1.9 per 100 [CFF Patient Registry 2003].
Pregnancy. The survival of individuals with CF has improved considerably over the past few decades. Currently, the average median survival is approximately 37 years and pregnancy in women with CF has become an important issue.
Early reports of such pregnancies were discouraging. Historically, the predictors of poor pregnancy outcome for mother and/or fetus have been a forced vital capacity (FVC) of less than 50% of the predicted value and poor nutritional status. In fact, an FVC of less than 50% of the predicted value was an absolute contraindication to pregnancy.
However, with increasingly improved pulmonary treatment, aggressive management of infections with a greater variety of antibiotics, and improved nutrition, pregnancies today are well tolerated, especially in women with mild to moderate disease [Edenborough et al 2000 , Gilljam et al 2000 , Gillet et al 2002 , Cheng et al 2006]. In these women, the risk factors for deteriorating health and early death after pregnancy are the same as for the non-pregnant adult population. In a recent study, Goss et al (2003) adjusted for FEV(1) percent predicted, weight, height, and pulmonary exacerbation rate per year and found that pregnancy was not associated with an increased risk of death. In fact, pregnancy did not appear to be harmful even in a subset of women with diabetes mellitus or with FEV(1) less than 40% of predicted. Important predictors of pregnancy outcome for the fetus are the severity of maternal pulmonary impairment and nutritional status, in that deterioration during pregnancy may precipitate preterm delivery.
The risk for congenital anomalies in the fetus is not increased over background.
Breastfeeding is possible.
Men without clinically apparent pulmonary or gastrointestinal manifestations of CF may have CAVD. Hypoplasia or aplasia of the vas deferens and seminal vesicles may occur either bilaterally or unilaterally. CAVD does not pose a health risk per se to the affected male. Testicular development and function and spermatogenesis are usually normal.
CAVD is generally identified during evaluation of infertility or as an incidental finding at the time of a surgical procedure, such as orchidopexy.
Cystic fibrosis. The best correlation between genotype and phenotype is seen in the context of pancreatic function. The most common mutations have been classified as pancreatic sufficient or pancreatic insufficient. Individuals with pancreatic sufficiency usually have either one or two PS alleles, indicating that PS alleles are dominant with respect to pancreatic phenotype.
In contrast, genotype-phenotype correlation is generally poor for pulmonary disease in CF. Pulmonary disease among individuals with identical genotypes varies widely, a finding that may be accounted for in part by genetic modifiers or environmental factors (see Cutting 2005 , Drumm et al 2005 , Braun et al 2006).
Compound heterozygotes with the ΔF508/A455E mutations have better pulmonary function than individuals who are homozygous for ΔF508 [De Braekeleer et al 1997].
The severity of lung disease in individuals with one or two R117H mutations depends on the presence of a variation in the poly T tract of intron 8 [Massie et al 2001]. Individuals with a CFTR disease-causing mutation plus the 5T variant in cis configuration with the R117H mutation usually develop the lung disease of CF, but those individuals with R117H and the 7T variant or the 9T variant have a highly variable phenotype that can range from no symptoms to mild lung disease [Kiesewetter et al 1993 , Chmiel et al 1999].
Because A455E and R117H mutations are associated with pancreatic sufficiency, the less severe lung disease seen in individuals with these mutations could be the consequence of better nutritional status.
CAVD. CAVD usually results from the combination of one severe CF mutation on one chromosome with either a mild CF mutation or the 5T allele on the other chromosome (Table 3). However, some overlap exists between the CAVD phenotype and a very mild CF phenotype, with some fraction of individuals with CAVD also reporting respiratory or pancreatic problems [Dork et al 1997 , Gilljam et al 2004]. Moreover, the 5T allele may be associated with lung disease in adult females with CF-like symptoms [Noone et al 2000]. Thus, caution must be exercised in attempting to use genotype to predict the future course of individuals initially diagnosed with CAVD only.
At-risk individuals. Genotype-phenotype correlations are most relevant for genetic counseling of two carriers who have not had an affected child but who have been detected either through evaluation of at-risk family members or screening programs. The considerations in predicting the phenotype of potential offspring are the same as described above for probands with CF or CAVD.
In general, prediction of severity of pancreatic disease on the basis of genotype is most reliable, while prediction of the severity of respiratory disease is less reliable.
Prediction of the risk of CAVD from genotype is reasonably reliable, but couples should be aware that mild respiratory and/or pancreatic disease can also occur in individuals with genotypes usually associated with CAVD. The mechanism of partial penetrance of the 5T allele for CAVD appears to be variation in the length of the adjacent TG tract (estimated at 60% in one study) [Groman et al 2004].
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