Benign hereditary chorea (BHC) (MIM 118700) is a rare childhood-onset movement disorder characterized predominantly by non-progressive chorea. The first familial description reported a 32-member, five-generation African-American family from Mississippi, USA, in which the probands, two teenage brothers, presented with childhood-onset chorea and delayed motor development.1
A number of families with a similar phenotype to these original cases and an autosomal dominant pattern of inheritance were reported over subsequent years, although their descriptions were frequently heterogenous with atypical features such as dystonia, tremor, and myoclonus described.2–4 This led some to question the validity of BHC as single diagnosis and specific disease entity.5 Furthermore, 11 families, originally considered to have a clinical diagnosis of BHC, were later confirmed to have different genetically confirmed diagnoses, including Huntington’s disease (HD), myoclonus dystonia (MD), and ataxia telangiectasia (AT). This study also demonstrated the difficulties in classifying movement disorders, with significant discrepancy between experts when reviewing videotaped examinations.5
In 2000, genetic analysis of a four-generation Dutch family in tandem with another of the originally described BHC families, established linkage to a disease locus on the long arm of chromosome 14.2,6,7 Recombination analysis of six further families reduced the critical region to 8.4cM (14q13.1-q21.1). Using chromosomal markers and two-color fluorescence in situ hybridization (FISH) a de novo 1.2-Mb deletion involving the NKX2.1 gene, also known as TITF1, TTF1, and TEBP, was identified.8 Breedveld et al.9 confirmed further NKX2.1 variants in four additional unrelated BHC families, in whom whole gene deletion (WGD), frameshift, and missense mutations were identified. At the same time, NKX2.1 mutations were reported in a cohort of five pediatric patients with hypothyroidism and ongoing symptoms, despite adequate thyroxine replacement.10 The NKX2.1 clinical spectrum appeared to therefore extend beyond a pure neurological phenotype.
In this review, we discuss the spectrum of clinical, genetic, and pathological features considered to be typical of BHC, while also considering more recent reports that expand the spectrum of the disorder (Figure 1). We also discuss the current evidence for the physiological role of NKX2.1 protein, putative pathogenic disease mechanisms, and the relative efficacy of current treatment strategies.
We performed a systematic literature search of the PUBMED database using the key words “benign hereditary chorea”, “brain–lung–thyroid syndrome”, “NKX2.1”, “TITF1”, “TTF1”, and “TEBP ”. There was no restriction on year of publication but only those published in English and in peer-reviewed journals were included.
A single epidemiological study of the Welsh population estimated the prevalence of BHC to be ∼1:500,000 in 1978, although this is widely considered to be an underestimate due to poor recognition of subtle choreiform phenotypes.11 Other studies have reported that, in addition to gender-dependent variation in penetrance (100% in males, 75% in females), there is significant inter- and intrafamilial variation of clinical phenotypes.12 Larger cohort studies have also suggested that females are more commonly affected than males (0.64:0.46), whether additional genetic or environmental factors contribute to this observed gender difference remains undetermined.13,14
Classical motor symptoms in BHC include mild to moderate hypotonia with delayed motor milestones in infancy. Onset of chorea is usually in early childhood (median age 2.5–3 years), although this is likely to be younger due to relatively late recognition of the movement disorder phenotype.13–15 The chorea is often generalized, affecting all body parts (face, limbs, trunk) and tends to worsen with stress or excitement.16 In the majority of cases the chorea will remain stable, or improve during adolescence and early adulthood, with other movement disorders such as myoclonus becoming more prevalent and disabling.12,13
Other types of movement disorder have also been reported in NKX2.1 mutation-positive cases (either in isolation or in conjunction with chorea) and include ataxia, upper limb intention tremor, limb dystonia, and motor and vocal tics.13,14,17,18 Facial apraxia and axial dystonia have also been described, although in both cases, genetic confirmation was not possible as the clinical description pre-dated NKX2.1 gene identification.19,20 More recently, recurrent drop attacks and frequent falls have been reported, often resulting in significant trauma. In one of these cases, electroencephalography during a typical episode demonstrated that these features were not associated with ictal epileptiform activity, suggesting that they were a consequence of the movement disorder phenotype, rather than an epileptic event.17,21
Speech disturbance has been consistently reported in BHC patients. Dysarthria is most frequently reported, evident in 40% of cases from a single case series.14,22 Stammer as well as slow, slurred, stuttering, and explosive speech have also each been described in single cases.18,23,24 Interestingly, in the largest longitudinal cohort studied to date, no dysarthria was described but three unrelated cases were described as having vocal tics.13
Cognitive status in those with BHC has historically been considered normal, and previously postulated to be an integral marker in differentiating this form of chorea from more neurodegenerative forms associated with cognitive impairment, e.g. Huntington’s disease.16 However, prior to NKX2.1 gene discovery, several reports documented reduced IQ in those with clinical BHC syndrome, with one study finding average IQ to be 10 points lower in motor affected individuals than unaffected family members.2,25,26 Subsequent studies have documented variable findings on cognitive assessment. Although some, using standardized scales (Mini Mental State Examination [MMSE] and Wechsler Adult Intelligence Scale - Revised [WAIS-R]), reported reduced levels of cognitive functioning, others have described children performing academically above that expected for their age.17,23 Two recent case series have also demonstrated conflicting results. We reported grossly normal levels of intellect, gauged by attendance in mainstream education, while Gras et al identified learning difficulties in 20 out of 28 reported cases.13,14 Of these 20 patients, 14 underwent formal assessment. Within this subgroup, three were considered to have mental retardation (IQ < 70), three had borderline IQ levels (70–80) and eight were found to have IQs within normal limits (IQ > 80). The other six cases were historically reported to have learning difficulties during the course of their education, though no formal cognitive assessment was performed. It therefore remains to be determined whether these deficits in cognitive function reflect an integral part of the BHC phenotype or a secondary consequence of social embarrassment and isolation resulting in reduced periods of education.
Depression, apathy, psychosis, and prolonged inpatient psychiatric admission were all described in early reports of patients with BHC clinical syndrome.2,27 Psychosis, schizophrenia, post-partum psychosis, obsessive–compulsive disorder (OCD), and attention deficit hyperactivity disorder (ADHD) have all been reported in patients with NKX2.1 mutations.14,18,28,29 The recent large, longitudinal French study found seven out of 28 mutation-positive cases to have a diagnosis of ADHD, of whom nearly all (six out of seven) had IQ levels within the normal range.13
The combination of brain, lung, and thyroid involvement was first described in a single case involving a contiguous NKX2.1 gene deletion in which the patient had neonatal respiratory failure, a high serum thyrotropin concentration, motor delay, hypotonia, and truncal ataxia.30 However, it wasn’t until description of a clinically similar case two decades later that the term brain–lung–thyroid syndrome was coined.31 Typical respiratory symptoms include neonatal or infant respiratory distress syndrome, frequently resulting in mechanical ventilation followed by recurrent pulmonary infections, development of obstructive airways disease, and chronic interstitial lung disease. Thyroid dysfunction usually takes the form of congenital hypothyroidism, which in a proportion of cases may be due to thyroid agenesis.16 Two larger case series have demonstrated 30–36% patients have symptoms involving all three organs, 32–40% with brain and thyroid involvement, and 20–21% with brain and lung disease alone.13,14 However, both cohorts only recruited participants from neurology/movement disorder outpatient clinics with likely ascertainment bias. In reality, the full triad of brain–lung–thyroid symptoms seems to be evident in up to 50% of cases with NKX2.1 mutations, while involvement of brain and thyroid appears to consistently involve 30% of all cases.15,32 It is postulated that brain–lung–thyroid syndrome and BHC may represent part of a continuous spectrum of disorders and that the associated lung and thyroid features were not recognized or investigated in early descriptions of the BHC clinical syndrome.12
Other clinical features have also been consistently reported, including developmental abnormalities of the urinary tract, such as hypospadias, vesico-urethral reflux, megabladder, duplex kidney, and recurrent urinary tract infections.3,14,28,33 Others have reported single cases manifesting hypo- or oligodontia,34 hypoparathyroidism,34 short stature, webbed neck,18 joint hypermobility,22 sensorineural hearing loss,35 seizures,36 optic nerve glioma,2 growth hormone deficiency, visual disturbance, pes cavus, and kyphosis.14 It is currently uncertain whether these features are disease related or incidental findings.
An increasing number of case reports describe malignancy in those with NKX2.1 mutations. Lung carcinoma is most frequently reported, often preceded by a history of pulmonary disease such as alveolar proteinosis, asthma, or recurrent respiratory tract infections.13,18,31,37 In addition, two terminal cases of leukemia and a single case of small cell bladder carcinoma have also been described.22,38 Interestingly in a mouse model of lung carcinoma, reduced TITF1 signaling contributes to tumor formation. Gene overexpression has also been recognized in a broad spectrum of malignancy subtypes including lung, thyroid, and prostate.39 More recently, copy number variant analysis in lung adenocarcinoma led to the suggestion of 14q13.3 amplification as a candidate proto-oncogene.40
NKX2.1 testing should be considered in all those with a history of neonatal hypotonia, motor developmental delay, and early-onset hyperkinetic movement disorder with predominant chorea and/or dystonia. Clearly, early evidence of comorbid lung or thyroid symptoms would serve to reinforce the importance of genetic testing. However, these neurological features (in the presence of normal cerebral magnetic resonance imaging) are common to a number of other genetically determined movement disorders that would need to be considered in the clinical differential diagnosis. Those to consider would include myoclonus dystonia (particularly those with SGCE mutations), given the difficulties in discriminating distal low amplitude myoclonus and fine choreiform movements.41 Allan–Herndon–Dudley syndrome, caused by SLC16A2 mutations, is a rare X-linked hyperkinetic movement disorder. Its presentation frequently mirrors that of BHC with initial hypotonia and subsequent development of involuntary limb movements, frequently with dystonia.42 Finally, autosomal dominant inherited adenylate cyclase-5 (ADCY5) mutations have recently been identified in a number of families presenting with early-onset chorea and dystonia, and will become an increasingly important differential diagnosis in those with infancy-onset hyperkinetic movement disorders.43
Although cerebral imaging is normal in the majority of NKX2.1 mutation-positive cases, a variety of structural and enhancement anomalies of uncertain significance have also been reported. These have included ventricular dilatation, empty sella, bilateral pallidal hyperintensity, and cystic masses in the pituitary and sella turcica.10,14,23,44 Volumetric analysis of two mutation-positive cases found bilateral reduction of striatal volumes, while magnetic resonance spectroscopy identified altered myoinositol, creatine, and N-acetyl-aspartate levels.45 Single-photon emission computed tomography scans of two pediatric cases found reduced technetium-99 uptake in the basal ganglia, while positron emission tomography (PET) studies of dopamine transmission demonstrated reduced post-synaptic D2 receptor function.37,46 Although neurodegeneration is not considered to occur in those with NKX2.1 mutations, fludeoxyglucose-PET findings have been inconsistent in a small number of cases demonstrating both normal and reduced cortico-striatal glucose metabolism, the latter often being a feature observed in neurodegenerative forms of chorea such as Huntington’s disease.28,38 Overall, these imaging findings are predominantly from single case reports or small case series, so their relevance to NKX2.1-related disease remain uncertain and may represent incidental findings. Future studies, involving larger cohorts, using standardized imaging techniques, are required before conclusions can be drawn regarding the specificity of radiological findings.
There are currently no consensus nor formal guidelines for the treatment of chorea in patients with BHC. Multiple reports have described single cases treated with a variety of agents including trihexyphenidyl, corticosteroids, sodium valproate, propranolol, ropinirole, and sulpiride, each with varying results.14,32,46 Levodopa has been the most consistent in providing symptomatic improvement (relatively high doses of 7–9 mg/kg/day), predominantly when given during childhood, and in conjunction with physiotherapy.21,22,47 Cases in which there has been little response to L-dopa have tended to use lower levodopa doses, principally limited due to tolerability.13,14 Tetrabenazine also led to motor improvements in five cases at doses of 0.5 mg/kg/day in children and 37.5 mg/day in adults. Cessation of a 75 mg/day dosage in one case led to worsening of chorea and disturbances to sleep and behavior.13,28 Olanzapine, for treatment of psychosis, improved chorea in a single NKX2.1 mutation-positive case, while in another the motor symptoms were left unchanged.18 Trihexyphenidyl and clonazepam have also been considered effective, each in single cases, although neither led to complete resolution of motor symptoms.14
NKX2.1 belongs to the natural killer gene family of highly conserved homeodomain-containing transcription factors. The gene comprises three coding exons giving rise to five transcripts. The two most common isoforms are 371 and 401 amino acids in length (ENST00000498187 and ENST00000354822) differing only in their N-terminal sequences.48 The majority of pathogenic mutations lead to protein truncation, either before or within the DNA-binding domain, preventing DNA binding, and lead to loss-of-function.9In vitro studies have demonstrated that the mutant protein has a dominant-negative effect on transcriptional activity of the wild-type protein, suggesting that the clinical phenotype is due to NKX2.1 haploinsufficiency.15,29
A broad spectrum of NKX2.1 mutations and diverse clinical phenotypes has been reported to date (Table 1, Figure 2). A correlation between phenotype severity and type of mutation has been postulated. Larger contiguous gene deletions have been associated with a more severe spectrum of the brain–lung–thyroid triad (often with additional clinical characteristics), while point nonsense mutations, affecting the terminal regions of the protein, are reported in those with a milder clinical phenotype.16 However, this genotype–phenotype association is not seen in all cases, suggesting that there may be other contributory environmental and/or epigenetic factors.13,14NKX2.1 mutations also demonstrate considerable intrafamilial variability, with severity of the movement disorder ranging from significant to those with minimal functional impairment. There may also be individual heterogeneity in the expression of other motor and non-motor features between members of the same kindred.49 A full summary of all known mutations and clinical features of cases with contiguous gene deletions involving NKX2.1 is shown in Table 1 and Figure 2.
|Authors||Year of Publication||Deletion size and Chromosomal Location*||Genes Involved1||Organ Involvement||Additional Clinical Characteristics|
|Devriendt et al.30||1998||<13cM deletion involving 14q13-q21||NKX2.1, PAX9||Hypotonia, truncal ataxia, motor DD||RDS, mechanical ventilation||Congenital hypothyroidism||Nil observed|
|Iwatani et al. (1)62||2000||14q12-q13.3||NKX2.1, PAX9||Motor DD||Bronchiolitis, recurrent RTI, RDS||Congenital hypothyroidism||Feeding difficulties, failure to thrive, microcephaly, hearing loss, cognitive impairment, dysmorphic facial features (e.g. hypertelorism, high arched palate), lower limb contractures|
|Iwatani et al. (2)62||2000||14q12-q13.3||NKX2.1, PAX9||Motor DD||Died of respiratory failure aged 3 years||Nil observed||Feeding difficulties, failure to thrive, microcephaly, hearing loss, cognitive impairment|
|Breedveld et al.9||2002||1.2Mb||NKX2.1, MBIP, NKX2.8, PAX9, SLC25A21||Chorea||Nil observed||Nil observed||Nil observed|
|Krude et al.10||2002||14q11.2-q13.3||NKX2.1||Severe choreoathetosis||RDS, recurrent RTI||Thyroid hypoplasia||Nil observed|
|Devos et al. (1)34||2006||0.9Mb||NKX2.1, MBIP, NKX2.8, PAX9, SLC25A21, MIPOL1||Chorea, hypotonia||Nil observed||Hypothyroidism||Diarrhea|
|Devos et al. (2)34||2006||0.9Mb||NKX2.1, MBIP, NKX2.8, PAX9, SLC25A21, MIPOL1||Chorea, motor DD, hypotonia, cerebellar signs||Interstitial pneumonia||Hypothyroidism||Diarrhea, malabsorption, osteoporosis, hypoparathyroidism, dry eyes|
|Devos et al. (3)34||2006||0.9Mb||NKX2.1, MBIP, NKX2.8, PAX9, SLC25A21, MIPOL1||Chorea, motor DD, hypotonia, cerebellar signs||Interstitial pneumonia||Hypothyroidism||Diarrhea, malabsorption, osteoporosis, dry eyes|
|Carre et al.15||2009||14q13||NKX2.1||Ataxia, motor DD||RDS, mechanical ventilation, Recurrent RTI||Congenital hypothyroidism||Mental retardation, impaired saccadic eye movements, absent corpus callosum|
|Accornero et al.63||2010||1.2Mb||NKX2.1, MBIP, NKX2.8, PAX9, SLC25A1||Chorea||Nil observed||Subclinical hypothyroidism||Duplication of pituitary stalk|
|Uematsu et al.64||2012||2.6Mb deletion involving 14q12-q13||NKX2.1||Motor DD, choreoathetosis||Recurrent RTI||Nil observed||Bilateral reduced cerebral blood flow in basal ganglia (especially caudate nucleus) on ECD-SPECT imaging|
|Gras et al. (1)13||2012||13.8Mb deletion involving 14q13.2-q22.1||NKX2.1, MBIP, PAX9, SLC25A21, MPOL1, FOXA1, SEC23A, GEMIN2, TRAPPC6B, CTAGE5, FBXO33, LRFN5, FSCB, C14orf28, KLHL28, FAM179B, PRPF39, FKBP3, FANCM, MDGA2||Chorea, hypotonia||Nil observed||Nil observed||Nil observed|
|Gras et al. (2)13||2012||6.2-Mb deletion involving 14q13.2-q21.2||NKX2.1, MBIP, PAX9, SLC25A21, MPOL1, FOXA1, SEC23A, GEMIN2, TRAPPC6B, CTAGE5, FBXO33||Chorea, hypotonia||Nil observed||Hypothyroidism||Learning difficulties, ADHD|
|Gras et al. (3)13||2012||0.3-Mb deletion involving 14q13.3||NKX2.1, MBIP, PAX9||Chorea||Nil observed||Hypothyroidism||Learning difficulties, ADHD|
|Dale et al.65||2012||3.27-Mb deletion(Chr 14:35,327,739-38,602,335)||NKX2.1, PAX9, MIPOL1, SEC23A||Motor DD, gait disturbance||Nil observed||Nil observed||Mild intellectual disability, ligament laxity, oligodontia|
|Teissier et al.66||2012||14q13.3||NKX2.1||Chorea, dystonia, gait disturbance||COPD||Congenital hypothyroidism||Nil observed|
|Teissier et al.66||2012||14q13.3||NKX2.1||Chorea, hypotonia, ataxia||Nil observed||Congenital hypothyroidism||Dysmorphic features: hexadactyly, arched palate|
|Hamvas et al. (1)67||2013||14q13.1-q21.1||NKX2.1||Hypotonia, motor DD, ataxia||Hypoxia, recurrent RTI, pneumathoraces||Hypothyroidism||Absent corpus callosum|
|Hamvas et al. (2)67||2013||14q13.3||NKX2.1||Ataxia, motor DD||Recurrent infections||Hypothyroidism||Behavioral difficulties|
|Hamvas et al. (3)67||2013||14q13.3-q21.1||NKX2.1||Hypotonia, DD, ataxia||RDS, PH, RTI, oxygen therapy||Congenital hypothyroidism||Language delay|
|Hamvas et al. (4)67||2013||14q13.3-q21.1||NKX2.1||Hypotonia, developmental delay||RDS, PH||Congenital hypothyroidism||Microcephaly, language delay|
|Hamvas et al. (5)67||2013||Deletion of exons 1 & 2||NKX2.1||Hypotonia, motor DD, ataxia||Severe RDS||Congenital hypothyroidism||Language delay|
|Peall et al. (1)14||2014||0.36Mb deletion(Chr14:36,924,171-37,283,221)||SFTA3, NKX2.1, BX161496, NKX2.8, PAX9, SLC25A21||Hypotonia, dystonia, motor DD||Nil observed||Nil observed||Nil observed|
|Peall et al. (2)14||2014||4.7Mb deletion(Chr14:35,581,654-40,301,792)||RALGAPAI, BRMS1L, MBIP, SFTA3, NKX2.1, NKX2.8, PAX9, SLC25A21, MIPOL1, TTC6, SSTR1, CLEC14A, SEC23A, GEMIN2, TRAPPC6B, MIA2, CTAGE5, FBX033||Hypotonia, motor DD||Recurrent RTI||Congenital hypothyroidism||Growth hormone deficiency, visual impairment|
It is likely that BCH is genetically heterogeneous. In four families described in the original paper by Breedveld et al.,8 neither a NKX2.1 mutation nor linkage to chromosome 14 was identified. Genetic linkage analysis of two unrelated Japanese families, each demonstrating an autosomal dominant pattern of inherited, slowly progressive chorea, has suggested an alternative genetic locus on chromosome 8 (8q21.3-q23.3), and named benign hereditary chorea 2 (BHC2).50 Interestingly, the motor phenotype in both families differed somewhat from classical BHC, namely with onset of disease in adulthood and slowly progressive chorea. Furthermore, brain post-mortem analysis in one affected individual, who had died from a respiratory tract infection, revealed degeneration of the striatum and cerebral white matter together with evidence of a tauopathy similar to that seen in progressive supranuclear palsy.51
Mouse expression studies reveal complex NKX2.1 expression during embryonic development, with transcription of the gene evident in the forebrain, thyroid, and developing lungs.52NKX2.1 expression occurs early in brain development, in the progenitor and post-mitotic cells of the rostrobasal telencephalon, by the 11th somite stage.53 A NKX2.1 null knockout (KO) mouse model has no abnormal motor phenotype in its heterozygous form but NKX2.1–/– homozygous mice died at birth with absent lungs, pituitary, thyroid gland, and multiple abnormalities of the ventral forebrain.54
The rostrobasal telencephalon later develops into several structures including the medial ganglionic eminence (MGE), a precursor of the globus pallidus. NKX2.1 has been found to facilitate migration of striatal interneurons from the MGE to the lateral ganglionic eminence, which later forms the striatum. These neurons then migrate to the cortex and form predominantly gamma-aminobutyric acid and calbindin expressing neurons. Failure of development of the MGE prevents migration of cholinergic neurons to the pallidum and ultimately to the striatum.55 Butt et al.56 investigated the role of TTF1 during embryogenesis by using a conditional loss of function approach. They found that TTF1 function was necessary for differentiation of specific subtypes of interneurons with early loss resulting in ectopic production of medium spiny projection neurons while later loss resulted in near normal levels of cortical interneurons.56
During lung and thyroid development, NKX2.1 binds to the transcriptional regulatory elements of several proteins, e.g. secretoglobulin 1A and surfactant proteins in the lung and thyroglobulin and thyroperoxidase in thyroid tissue.57–59 Lung pathology results from disrupted branching of the bronchial tree during development, resulting in fewer alveoli, interstitial fibrosis, and cyst-like air spaces. Thyroid development is frequently interrupted resulting in incomplete development or agenesis.60
Two patients with confirmed NKX2.1 mutations have undergone post-mortem examination, both having died from leukemia. In both, the gross macroscopic and microscopic appearances were considered normal.22,38 The original case was reviewed using immunohistochemical staining of striatal tissue, aimed at identifying the neurotransmitters of interneurons whose tangential migration is mediated by TITF1. This found reduced numbers of striatal interneurons and a reduced density of calbindin, met-enkephalin, and substance P immunoreactive fibers, corroborating a number of the findings from the NKX2.1 KO mouse model.61 These findings also support the idea that NKX2.1 mutation-positive BHC is a developmental disorder, manifesting during childhood with no further deterioration in adult life. This may also explain the few available reports of successful treatment are during childhood and adolescence with minimal change observed when treating adults.
The clinical features of NKX2.1 BHC have become more clearly delineated with increased reporting of mutation-positive cases, highlighting hypotonia, motor developmental delay, chorea, hypothyroidism, respiratory distress syndrome, and recurrent respiratory tract infections as the core features of brain–lung–thyroid syndrome. Our review illustrates that the full triad appears only to be present in approximately 30% of all mutation-positive cases, and therefore the absence of one or more disease-associated symptoms should not discourage genetic testing.
In this review, we have discussed the expanding phenotypic spectrum associated with NKX2.1 mutations, with some significant variation from original clinical descriptions. Long considered to be a disorder in which cognition was preserved, several standardized studies have now demonstrated that at least a proportion of cases are found to have reduced IQ. Furthermore, the reported developmental abnormalities of other organ systems (e.g. lung, thyroid, and possibly urinary tract and musculoskeletal system) provide evidence of the important role of NKX2.1 in normal development.
Future development of treatment strategies and novel therapies may also need to consider the role of NKX2.1 in early development. Symptomatic improvement with levodopa therapy in some cases may indicate a window of opportunity for treatment in underdeveloped neuronal networks. Improved understanding of the clinical spectrum and pathogenic mechanisms governing this condition will require future multicenter, longitudinal collaborative studies, new laboratory disease models, development of novel therapeutic approaches, and large-scale clinical trials.
1 Funding: None.
2 Financial Disclosures: None.
3 Conflict of Interest: The authors report no conflict of interest.
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