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TBC1D24-Related Disorders

, MD, , MD, PhD, , MD, PhD, and , MD.

Author Information

Initial Posting: ; Last Update: December 7, 2017.

Summary

Clinical characteristics.

TBC1D24-related disorders comprise a continuum of features that were originally described as distinct, recognized phenotypes:

  • DOORS syndrome (deafness, onychodystrophy, osteodystrophy, mental retardation, and seizures). Profound sensorineural hearing loss, onychodystrophy, osteodystrophy, intellectual disability / developmental delay, and seizures
  • Familial infantile myoclonic epilepsy (FIME). Early-onset myoclonic seizures, focal epilepsy, dysarthria, and mild-to-moderate intellectual disability
  • Progressive myoclonus epilepsy (PME). Action myoclonus, tonic-clonic seizures, progressive neurologic decline, and ataxia
  • Early-infantile epileptic encephalopathy 16 (EIEE16). Epileptiform EEG abnormalities which themselves are believed to contribute to progressive disturbance in cerebral function
  • Autosomal recessive nonsyndromic hearing loss, DFNB86. Profound prelingual deafness
  • Autosomal dominant nonsyndromic hearing loss, DFNA65. Slowly progressive deafness with onset in the third decade, initially affecting the high frequencies

Diagnosis/testing.

The diagnosis of a TBC1D24-related disorder is established in an individual with biallelic TBC1D24 pathogenic variants when the mode of inheritance is autosomal recessive (i.e., DOORS syndrome, FIME, PME, EIEE16, and DFNB86), and in an individual with a heterozygous TBC1D24 pathogenic variant when the mode of inheritance is autosomal dominant (DFNA65).

Management.

Treatment of manifestations: Hearing aids or cochlear implants as needed for hearing loss; early educational intervention and physical, occupational, and speech therapy for developmental delay; symptomatic pharmacologic management for seizures; routine management of visual impairment and renal and cardiac anomalies.

Surveillance: Neurology evaluations with EEGs depending on seizure frequency and/or progression of clinical manifestations; yearly audiologic evaluation to assess for possible progression of hearing loss and/or the efficacy of hearing aids; yearly dental evaluation.

Agents/circumstances to avoid: Excessive ambient noise, which may exacerbate hearing loss in heterozygotes for a TBC1D24 pathogenic variant that causes DFNA65.

Evaluation of relatives at risk: Molecular genetic testing for the familial TBC1D24 pathogenic variant(s) in older and younger sibs of a proband in order to identify as early as possible those who would benefit from early treatment of seizures and/or hearing loss.

Genetic counseling.

Most TBC1D24-related disorders are inherited in an autosomal recessive manner (DOORS syndrome, FIME, PME, EIEE16, and DFNB86). For autosomal recessive inheritance: At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Carrier testing for at-risk relatives requires prior identification of the TBC1D24 pathogenic variants in the family. Prenatal testing is possible for pregnancies at increased risk if the TBC1D24 pathogenic variants have been identified in an affected family member.

GeneReview Scope

TBC1D24-Related Disorders: Included Phenotypes 1
  • DOORS syndrome
  • Familial infantile myoclonic epilepsy (FIME)
  • Progressive myoclonus epilepsy (PME)
  • Early-infantile epileptic encephalopathy 16 (EIEE16)
  • Autosomal recessive nonsyndromic hearing loss, DFNB86
  • Autosomal dominant nonsyndromic hearing loss, DFNA65

For synonyms and outdated names see Nomenclature.

1.

For other genetic causes of these phenotypes see Differential Diagnosis.

Diagnosis

TBC1D24-related disorders comprise a continuum of recognized phenotypes:

  • DOORS syndrome (deafness, onychodystrophy, osteodystrophy, mental retardation, and seizures)
  • Familial infantile myoclonic epilepsy (FIME)
  • Progressive myoclonus epilepsy (PME)
  • Early-infantile epileptic encephalopathy 16 (EIEE16)
  • Autosomal recessive nonsyndromic hearing loss (DFNB86)
  • Autosomal dominant nonsyndromic hearing loss (DFNA65)

No formal diagnostic criteria have been published for any of theTBC1D24-related disorders.

Suggestive Findings

A TBC1D24-related disorder should be suspected in individuals with the following features of the recognized phenotypes that comprise a phenotypic continuum. (Information on additional features appears in Clinical Characteristics).

DOORS syndrome

  • Deafness (profound sensorineural hearing loss)
  • Onychodystrophy (short/absent nails)
  • Osteodystrophy (short phalanges)
  • Intellectual disability / developmental delay (formerly known as mental retardation)
  • Seizures
  • 2-oxoglutaric aciduria

Familial infantile myoclonic epilepsy (FIME). Early-onset myoclonic seizures

Progressive myoclonus epilepsy (PME). Action myoclonus, tonic-clonic seizures, progressive neurologic decline, and ataxia

Early-infantile epileptic encephalopathy 16 (EIEE16)

  • Early-onset seizures (unresponsive to medication) that can include myoclonic seizures or malignant migrating partial seizures of infancy
  • Extrapyramidal signs (e.g., dystonia), hemiparesis, and/or autonomic signs
  • Neurologic deterioration and early death
  • Progressive diffuse cerebral atrophy

Autosomal recessive nonsyndromic hearing loss, DFNB86. Prelingual nonsyndromic sensorineural deafness (See Deafness and Hereditary Hearing Loss Overview.)

Autosomal dominant nonsyndromic hearing loss, DFNA65. Adult-onset nonsyndromic sensorineural deafness (See Deafness and Hereditary Hearing Loss Overview.)

Establishing the Diagnosis

The diagnosis of a TBC1D24-related disorder is established in a proband by identification of (Table 1):

Molecular genetic testing approaches can include single-gene testing, use of a multigene panel, and more comprehensive genomic testing:

  • Single-gene testing. Sequence analysis of TBC1D24 is performed first and followed by gene-targeted deletion/duplication analysis if only one or no pathogenic variant is found.
    Note: (1) The diagnostic yield appears to be highest in individuals with all five typical features of DOORS syndrome [Campeau et al 2014]. (2) The proportion of epilepsy caused by pathogenic variants in TBC1D24 appears to be small, and clinically distinctive signs and symptoms have not yet been identified.
  • A multigene panel that includes TBC1D24 and other genes of interest (see Differential Diagnosis) may be considered. Note: (1) The genes included in the panel and the diagnostic sensitivity of the testing used for each gene vary by laboratory and over time. (2) Some multigene panels may include genes not associated with the condition discussed in this GeneReview; thus, clinicians need to determine which multigene panel is most likely to identify the genetic cause of the condition at the most reasonable cost while limiting identification of variants of uncertain significance and pathogenic variants in genes that do not explain the underlying phenotype. (3) In some laboratories, panel options may include a custom laboratory-designed panel and/or custom phenotype-focused exome analysis that includes genes specified by the clinician. (4) Methods used in a panel may include sequence analysis, deletion/duplication analysis, and/or other non-sequencing-based tests.
    For an introduction to multigene panels click here. More detailed information for clinicians ordering genetic tests can be found here.
  • More comprehensive genomic testing (when available) including exome sequencing, mitochondrial sequencing, and genome sequencing may be considered. Such testing may provide or suggest a diagnosis not previously considered (e.g., mutation of a different gene or genes that results in a similar clinical presentation).
    For an introduction to comprehensive genomic testing click here. More detailed information for clinicians ordering genomic testing can be found here.

Table 1.

Molecular Genetic Testing Used in TBC1D24-Related Disorders

Gene 1Test MethodProportion of Probands with Pathogenic Variants 2 Detectable by This Method
TBC1D24Sequence analysis 3
  • DOORS syndrome: 9/18 families with all 5 major features 4
  • FIME: rare 5, 6
  • PME: rare 5, 7
  • EIEE16: rare 5, 8
  • DFNB86: rare 5, 9
  • DFNA65: rare 5, 10
Gene-targeted deletion/duplication analysis 11Unknown 12
Unknown 13

FIME = familial infantile myoclonic epilepsy

PME = progressive myoclonus epilepsy

EIEE = early-infantile epileptic encephalopathy

1.
2.

See Molecular Genetics for information on allelic variants detected in this gene.

3.

Sequence analysis detects variants that are benign, likely benign, of uncertain significance, likely pathogenic, or pathogenic. Pathogenic variants may include small intragenic deletions/insertions and missense, nonsense, and splice site variants; typically, exon or whole-gene deletions/duplications are not detected. For issues to consider in interpretation of sequence analysis results, click here.

4.
5.

Although a significant proportion of individuals with this phenotype have a genetic etiology, TBC1D24 is a rare cause.

6.
7.

One family reported [Muona et al 2015]

8.

Three families reported [Guven & Tolun 2013, Milh et al 2013, Lozano et al 2016]; one individual in a cohort of 359 individuals with epileptic encephalopathy [de Kovel et al 2016]

9.

Recessive deafness; five families reported [Rehman et al 2014, Bakhchane et al 2015]

10.

Dominant deafness; two families reported [Azaiez et al 2014, Zhang et al 2014]

11.

Gene-targeted deletion/duplication analysis detects intragenic deletions or duplications. Methods that may be used include: quantitative PCR, long-range PCR, multiplex ligation-dependent probe amplification (MLPA), and a gene-targeted microarray designed to detect single-exon deletions or duplications.

12.

No data on detection rate of gene-targeted deletion/duplication analysis are available.

13.

Genetic heterogeneity for DOORS syndrome is likely. Exome analysis in some of these families did not reveal a commonly mutated gene [Campeau et al 2014], but an extension of this study is ongoing.

Clinical Characteristics

Clinical Description

While initial reports suggested specific phenotypes associated with pathogenic variants in TBC1D24, several recent publications suggest that the features represent a phenotypic spectrum ranging from a mild form of familial infantile myoclonic epilepsy (FIME) to a combination of epilepsy with variable other features, to DOORS syndrome [Balestrini et al 2016]. Features seen in individuals with biallelic TBC1D24 pathogenic variants who do not have DOORS syndrome include parkinsonism [Banuelos et al 2017], ataxia, dysarthria, axial hypotonia, hearing loss, visual impairment, mild dysmorphic facial features, developmental delay or intellectual disability, and microcephaly [Balestrini et al 2016].

DOORS Syndrome

The five major features of DOORS syndrome are profound sensorineural hearing loss, onychodystrophy, osteodystrophy, intellectual disability / global developmental delay, and seizures [James et al 2007, Campeau et al 2014].

The sensorineural hearing loss is often profound and prelingual. Some have benefited from cochlear implants.

The onychoosteodystrophy affects the hands and feet equally. Small or absent nails (onychodystrophy) and hypoplastic terminal phalanges (osteodystrophy) are noted in most individuals. A triphalangeal thumb is present in one third of affected individuals.

The intellectual disability (previously referred to as mental retardation) can vary significantly in degree but is often severe [Balestrini et al 2016]. The motor and language skills were most delayed in two children where such details are available [Nomura et al 2009, Girish et al 2011]. One child had autism spectrum disorder [Nomura et al 2009].

Seizures, present in most individuals with DOORS syndrome, usually start in the first year of life. The seizures are more often generalized tonic-clonic, but myoclonic, partial, and absence seizures also occur. Occasionally their frequency or severity increases. In several instances, the seizures have been difficult to control even with multiple antiepileptic medications, and have led to status epilepticus and death.

On MRI hyperintense T2-weighted signal anomalies may be observed in the cerebellar hemispheres and the frontal region [Campeau et al 2014].

Nonspecific dysmorphic features. A wide nasal base and a bulbous nose are the most common facial dysmorphisms. Other findings in a minority of individuals include narrow forehead, narrow or high arched palate, broad alveolar ridge, short frenulum, and nevus simplex on the glabella and nose.

Other. In individuals with DOORS syndrome the additional anomalies noted are the following:

  • Microcephaly in one third of individuals
  • Other cranial anomalies (sagittal craniosynostosis in 1 individual; frontal bossing, trigonocephaly, or brachycephaly in several other affected individuals)
  • Dental anomalies (delayed eruption, wide spacing, and abnormal shape, size, and number)
  • Congenital heart defects (double outlet right ventricle)
  • Skeletal anomalies (e.g., calcaneal deformities)
  • Hypothyroidism
  • Renal and urinary tract anomalies (e.g., hydronephrosis, nephrocalcinosis) [Campeau et al 2014]
  • Elevated levels of urinary 2-oxoglutaric acid, which can fluctuate between normal and elevated over time [Patton et al 1987, van Bever et al 2007, Campeau et al 2014]
  • Visual impairment in six of 14 individuals [Balestrini et al 2016]
  • Peripheral neuropathy in one individual with confirmed TBC1D24 pathogenic variants [Balestrini et la 2016] and three individuals who either did not undergo genetic testing or in whom no TBC1D24 pathogenic variant was identified

Familial Infantile Myoclonic Epilepsy (FIME)

FIME is characterized by early-onset myoclonic seizures.

Findings include focal epilepsy, dysarthria, mild-to-moderate intellectual disability, and cortical thickening and cerebellar atrophy with high T2-weighted and FLAIR on MRI (in 4 sibs of an Israeli Arab family [Corbett et al 2010, Afawi et al 2013]).

Intellect may be normal: all seven members of an Italian family who had FIME and biallelic TBC1D24 pathogenic variants also had normal intelligence. Six had normal brain MRI and one had periventricular nodular heterotopia [Zara et al 2000, de Falco et al 2001, Falace et al 2010].

Progressive Myoclonus Epilepsy (PME)

PME is characterized by action myoclonus, tonic-clonic seizures, progressive neurologic decline, and ataxia.

For the child described with PME caused by biallelic pathogenic variants in TBC1D24, tonic seizures started at age 36 hours. Developmental delay with later regression was then noted. Myoclonus started at age eight months and tonic-clonic seizures at age 3.5 years. Ataxia, spasticity, supranuclear gaze palsy, and visual decline were also noted. Although the initial clinical diagnosis was of an epileptic encephalopathy, a florid PME pattern was apparent by age nine years [Muona et al 2015]. There were no digital anomalies or deafness [Sam Berkovic, MD, personal communication].

Early-Infantile Epileptic Encephalopathy 16 (EIEE16)

Epileptic encephalopathies are defined by the International League Against Epilepsy (ILAE) as conditions in which epileptiform EEG abnormalities themselves are believed to contribute to progressive disturbance in cerebral function [Engel 2001, Berg et al 2010].

Findings may include:

  • Myoclonic epilepsy with episodic dystonia, hemiparesis, autonomic signs, and lethargy evolving to chronic dystonia, progressive diffuse cerebral atrophy, and early death (in 5 Turkish families [Duru et al 2010, Guven & Tolun 2013]);
  • Malignant migrating partial seizures in infancy with progressive diffuse cerebral atrophy of the gray matter (sparing the posterior fossa) and early death (in 2 French sibs [Milh et al 2013]).

Autosomal Recessive Nonsyndromic Hearing Loss, DFNB86

Findings observed include profound prelingual deafness with hearing thresholds above 90 dB for all test frequencies (in 2 consanguineous Pakistani families; 1 affected family member and 1 individual with a heterozygous TBC1D24 pathogenic variant also had seizures [Rehman et al 2014]) (see Deafness and Hereditary Hearing Loss Overview).

Autosomal Dominant Nonsyndromic Hearing Loss, DFNA65

Findings include slowly progressive deafness with onset in the third decade, initially affecting the high frequencies (in 1 Chinese family [Zhang et al 2014] and in a family of European descent [Azaiez et al 2014]) (see Deafness and Hereditary Hearing Loss Overview).

Heterozygotes, in the context of autosomal recessive disease. Two unrelated individuals with generalized tonic-clonic seizures and biallelic pathogenic TBC1D24 variants both had a family history of hearing loss, but the relatives with hearing loss were not tested for a heterozygous TBC1D24 pathogenic variant. In one family, the affected individual's brother had hearing loss and in the other family the affected individual's maternal grandmother had hearing loss [Balestrini et al 2016].

Increasing evidence points to an elevated susceptibility to seizure disorders in apparently unaffected individuals who have a heterozygous TBC1D24 pathogenic variant (i.e., a "carrier") as compared to the population frequency estimated at seven per 1,000 individuals.

Genotype-Phenotype Correlations

The TBC1D24 pathogenic variants that cause DOORS syndrome, FIME, EIEE16, DFNB86, and DFNA65 are located throughout the gene; no pattern has emerged to date. Most pathogenic variants causing one phenotype have not been demonstrated to cause the others, either within the same family or in different families. However, a heterozygous frameshift variant (c.1008delT) coupled with another pathogenic variant affecting the other TBC1D24 allele was identified in four affected individuals, two with DOORS and one sib pair with EIEE16 and early death.

In general, loss-of-function variants (frameshift, nonsense, or splice site) are associated with a more severe epilepsy phenotype with drug resistance and early death, except when the loss-of-function variant is located in the last exon. Pathogenic missense variants in or before the TBC domain are also associated with a higher risk of lethality.

The diagnosis of DOORS does not allow for a prediction of the epilepsy type [Balestrini et al 2016]. The location of new pathogenic variants cannot yet be used to predict a phenotype. This may change as more pathogenic variants are identified.

Nomenclature

The acronym "DOOR syndrome" was coined in 1975 [Cantwell 1975]. Subsequently, Qazi & Nangia [1984] suggested adding an S (DOORS syndrome) because of the seizures present in most individuals. Other terms used for this condition include digito-reno-cerebral syndrome [Eronen et al 1985] and Eronen syndrome [Le Merrer et al 1992].

EIEE16 has also been referred to as malignant migrating partial seizures of infancy (MMPSI) [Milh et al 2013], which is also known as epilepsy of infancy with migrating focal seizures.

Prevalence

The prevalence of TBC1D24-related disorders is very low. Fewer than 50 families with DOORS syndrome are known, and fewer than five each for the other TBC1D24-related disorders. A targeted sequencing study of a cohort of 359 individuals with epileptic encephalopathy identified one person with biallelic TBC1D24 variants [de Kovel et al 2016].

Differential Diagnosis

DOORS Syndrome

Table 2.

Disorders to Consider in the Differential Diagnosis of DOORS Syndrome

DisorderGene(s)MOIFeatures of the Disorder
Overlapping w/DOORS syndromeDistinguishing from DOORS syndrome
Coffin-Siris syndromeARID1A
ARID1B
SMARCA4
SMARCB1
SMARCE1
SOX11
AD 1ID/DD, aplastic or hypoplastic nails & terminal phalanges, seizuresVariably seen: coarse face, generalized hypertrichosis, scoliosis (some), gingival overgrowth (some), & 5th finger hypoplasia
Dominant deafness-
onychodystrophy syndrome
(OMIM 124480)
ATP6V1B2ADCongenital sensorineural deafness, onychodystrophyDental anomalies (conical, hypoplastic teeth) (some); absence of ID/DD & seizures
Nicolaides-Baraitser syndromeSMARCA2AD 1Severe ID/DD, seizuresCoarse face, prominent finger joints & broad distal phalanges, scoliosis (some)
Temple-Baraitser syndrome
(OMIM 611816)
KCNH1AD 1Severe ID/DD, seizures, nail hypoplasia/aplasia limited to 1st rays (thumb, great toe)Broad and proximally implanted thumbs, long great toes
Zimmermann-Laband syndrome
(OMIM 135500 and 616455)
KCNH1
ATP6B1B2
ADVariable ID/DD, seizures in ZLS due to pathogenic variants in KCNH1, hypoplasia or aplasia of nails & terminal phalanges, hearing loss (some)Coarse face, hypertrichosis, gingival overgrowth, scoliosis; no seizures in individuals w/ZLS caused by pathogenic variants in ATP6B1B2
Mabry syndrome
(OMIM 239300)
PIGVARSevere ID/DD, seizures, short terminal phalanges, and nail hypoplasiaHyperphosphatasia; absence of 2-oxoglutaric aciduria & deafness
Kaufman oculocerebrofacial
syndrome
(OMIM 244450)
UBE3BARID/DD, hearing loss (some), microcephaly, nail dysplasiaBlepharophimosis; hypoplastic/absent terminal phalanges rarely seen
Fetal anticonvulsant syndromen/an/aID/DD, nail hypoplasiaDental abnormalities w/delayed eruption, talipes equinovarus, otitis media w/effusion; absence of hearing loss & seizures

ID/DD = intellectual disability / developmental delay

1.

Pathogenic variants are typically (or always) de novo.

Table 3.

Other Conditions with 2-Oxoglutaric Aciduria

DisorderGeneMOIFeatures of the Disorder
Overlapping w/TBC1D24-related disordersDistinguishing from TBC1D24-related disorders
Combined D-2- and
L-2-hydroxyglutaric aciduria
(OMIM 615182)
SLC25A1ARSevere DD, seizuresSevere neonatal encephalopathy w/early death, no skeletal manifestations, ↑ D-2- & L-2-hydroxyglutaric acid
3-methylcrotonyl-CoA
carboxylase 1 deficiency
(OMIM 210200)
MCCC1AR2-oxoglutaric aciduria in 1 individual; ID, DD, seizuresUrinary excretion of 3-hydroxyisovalerate & 3-methylcrotonylglycine, metabolic decompensation
2-ketoglutarate
dehydrogenase deficiency
(OMIM 203740)
UnknownARElevated 2-oxoglutaric acidProgressive neurodegenerative disorder; development initially normal

ID = intellectual disability

DD = developmental delay

Familial Infantile Myoclonic Epilepsy (FIME) and Progressive Myoclonus Epilepsy (PME)

Table 4.

Disorders to Consider in the Differential Diagnosis of TBC1D24-Related FIME and TBC1D24-Related PME

DisorderGene(s)MOIFeatures of the Disorder
Overlapping w/TBC1D24-Related FIME/PMEDistinguishing from TBC1D24-Related FIME/PME
Juvenile myoclonic epilepsy
(OMIM 254770)
EFHC1ADMyoclonic seizures, generalized tonic-clonic seizuresTypical 3-Hz polyspike EEG, normal intelligence, later mean age of onset (~10 yrs), myoclonic seizures (jerks) typically in the morning
Early-infantile myoclonic encephalopathy 3
(OMIM 609304)
SLC25A22ARMyoclonic refractory seizures, early onset ageBurst suppression on EEG, abnormal visual evoked potentials, spasticity
Unverricht-Lundborg diseaseCSTBARGeneralized tonic-clonic seizures, myoclonic seizuresNo or mild decline in intellectual performance, EEG always abnormal, later onset age
Progressive myoclonus epilepsy, Lafora typeEPM2A
NHLRC1
ARGeneralized myoclonus and/or generalized tonic-clonic seizuresProgressive neurologic degeneration in previously healthy adolescents; Lafora bodies
Neuronal ceroid-lipofuscinosesATP13A2
CLN3
CLN5
CLN6
CLN8
CTSD
CTSF
DNAJC5
GRN
KCTD7
MFSD8
PPT1
TPP1
AR 1Myoclonus, seizuresProgressive intellectual & motor deterioration w/vision loss
MERRFMT-TF
MT-TI
MT-TK
MT-TL1
MT-TP
MaternalMyoclonus, generalized epilepsy, hearing loss, ataxiaNormal early development, ragged-red fibers on muscle biopsy, lactic acidosis; cardiomyopathy in some
POLG-related disordersPOLGAR
AD
Myoclonus, seizures, ataxiaVariable phenotype that may include ophthalmoplegia, neuropathy, liver dysfunction
SCN1A-related seizure disordersSCN1AADMyoclonic epilepsy, generalized tonic-clonic/ hemiclonic & focal seizuresNot associated w/hearing loss
Action myoclonus-renal failure syndrome
(OMIM 254900)
SCARB2ARProgressive myoclonic epilepsyOnset in late teens or early 20s w/tremors, proteinuria, & development of renal failure possible
PRICKLE1-related progressive myoclonus epilepsy (PME) with ataxia
(OMIM 612437)
PRICKLE1ARMyoclonic seizures, generalized convulsive seizures, ataxiaNormal intellect
1.

Neuronal ceroid-lipofuscinosis (NCL) is inherited in an autosomal recessive manner with the exception of adult NCL, which can be inherited in either an autosomal recessive or an autosomal dominant manner.

Early-Infantile Epileptic Encephalopathy (EIEE)

EIEE is a rare form of epilepsy in which affected children develop intractable seizures in the first weeks or months of life resulting in severe developmental disabilities or death in infancy.

EIEE is genetically heterogeneous. Causes can include the following:

See Epileptic encephalopathy, early-infantile – OMIM Phenotypic Series to view genes associated with this phenotype in OMIM.

Hereditary Hearing Loss and Deafness

For the differential diagnosis of hereditary hearing loss and deafness, see Hereditary Deafness and Hearing Loss Overview.

Management

Evaluations Following Initial Diagnosis

To establish the extent of disease and needs in an individual diagnosed with a TBC1D24-related disorder, the following evaluations are recommended if they have not already been completed:

DOORS syndrome

  • Audiology evaluation
  • Ophthalmology evaluation
  • Dental examination
  • Otolaryngology consultation
  • Echocardiogram
  • Renal ultrasound examination
  • Neurology consultation
  • Consultation with a clinical geneticist and/or genetic counselor

FIME, PME, EIEE16, and TBC1D24-related nonsyndromic deafness syndromes (DFNA65 and DFNB86). One should consider some of the above evaluations depending on the signs and symptoms, since clinical overlap between all TBC1D24-related conditions exists.

Treatment of Manifestations

Deafness. Consider hearing aids or cochlear implants as needed for hearing loss (see Hereditary Hearing Loss and Deafness Overview). Cochlear implants at age one have been beneficial in one individual with DOORS syndrome [Campeau et al 2014].

Visual impairment should be managed in the standard fashion.

Heart defects should be managed according to the anomalies detected.

Renal anomalies. Refer to an urologist or nephrologist, as indicated.

Seizures. Symptomatic pharmacologic management is warranted, as no controlled studies have compared the efficacy of different antiepileptic drugs in TBC1D24-related disorders. A variety of different antiepileptic agents have been used to achieve seizure control [Balestrini et al 2016].

The management of epilepsy in many genetic epilepsies is complex; general recommendations from the UK National Institute for Health and Care Excellence are available online.

Global Developmental Delay / Intellectual Disability Educational Issues

The following information represents typical management recommendations for individuals with developmental delay / intellectual disability in the United States; standard recommendations may vary from country to country.

Ages 0-3 years. Referral to an early intervention program is recommended for access to occupational, physical, and speech therapy. In the United States, early intervention is a federally funded program available in all states.

Ages 3-5 years. In the United States, developmental preschool through the local public school district is recommended. Before placement, an evaluation is made to determine needed services and therapies and an individualized education plan (IEP) is developed.

Ages 5-21 years

  • In the United States, an individualized education program (IEP) based on the individual's level of function should be developed by the local public school district. Affected children are permitted to remain in the public school district until age 21.
  • Discussion about transition plans including financial, vocation/employment, and medical arrangements should begin at age 12 years. Developmental pediatricians can provide assistance with transition to adulthood.

All ages. Consultation with a developmental pediatrician is recommended to ensure the involvement of appropriate community, state, and educational agencies and to support parents in maximizing quality of life. Some issues to consider:

  • Private supportive therapies based on the affected individual's needs may be considered. Specific recommendations regarding type of therapy can be made by a developmental pediatrician.
  • In the United States:
    • Enrollment in Developmental Disabilities Administration (DDA) is recommended. DDA is a public agency that provides services and support to qualified individuals. Eligibility differs by state but is typically determined by diagnosis and/or associated cognitive/adaptive disabilities.
    • Families with limited income and resources may also qualify for supplemental security income (SSI) for their child with a disability.

Motor Dysfunction

Gross motor dysfunction

  • Physical therapy is recommended to maximize mobility and to reduce the risk for later-onset orthopedic complications (e.g., contractures, scoliosis, hip dislocation).
  • Consider use of durable medical equipment as needed (e.g., wheelchairs, walkers, bath chairs, orthotics, adaptive strollers).

Fine motor dysfunction. Occupational therapy is recommended for difficulty with fine motor skills that affect adaptive function such as feeding, grooming, dressing, and writing.

Communication issues. Consider evaluation for alternative means of communication (e.g., Augmentative and Alternative Communication [AAC]) for individuals who have expressive language difficulties.

Social/Behavioral Issues

Children may qualify for and benefit from interventions used in treatment of autism spectrum disorder, including applied behavior analysis (ABA). ABA therapy is individualized therapy targeted to each child's behavioral, social, and adaptive strengths and weaknesses and is typically performed one-on-one with a board-certified behavior analyst.

Consultation with a developmental pediatrician may be helpful in guiding parents through appropriate behavioral management strategies or providing prescription medications when necessary.

Surveillance

Neurology evaluations with EEGs are appropriate, depending on seizure frequency and/or progression of clinical manifestations.

  • Individuals with epilepsy, irrespective of cause, should have periodic ECGs as interictal and ictal abnormalities may predispose to sudden unexplained death in epilepsy (SUDEP).

Perform yearly audiologic evaluation to assess for possible progression of hearing loss and/or the efficacy of hearing aids.

Yearly dental evaluation is appropriate.

Agents/Circumstances to Avoid

Heterozygotes for a TBC1D24 pathogenic variant causing autosomal dominant deafness, DFNA65 should avoid excessive ambient noise as it may exacerbate hearing loss.

Evaluation of Relatives at Risk

Molecular genetic testing for the familial TBC1D24 pathogenic variant(s) in older and younger sibs of a proband is appropriate in order to identify as early as possible those who would benefit from early treatment of seizures and/or deafness.

See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.

Pregnancy Management

In general, no information on specific prenatal presentations is available.

Polyhydramnios is often noted when a fetus has DOORS syndrome [James et al 2007]. A subsequent affected pregnancy in one family with DOORS syndrome was terminated due to an elevated nuchal translucency of 5.1 mm at 12 weeks' estimated gestational age [Balestrini et al 2016].

Therapies Under Investigation

Search ClinicalTrials.gov in the US and www.ClinicalTrialsRegister.eu in Europe for access to information on clinical studies for a wide range of diseases and conditions. Note: There may not be clinical trials for this disorder.

Genetic Counseling

Genetic counseling is the process of providing individuals and families with information on the nature, inheritance, and implications of genetic disorders to help them make informed medical and personal decisions. The following section deals with genetic risk assessment and the use of family history and genetic testing to clarify genetic status for family members. This section is not meant to address all personal, cultural, or ethical issues that individuals may face or to substitute for consultation with a genetics professional. —ED.

Mode of Inheritance

Most TBC1D24-related disorders (DOORS syndrome, FIME, PME, EIEE16, and DFNB86) are inherited in an autosomal recessive manner.

Risk to Family Members – Autosomal Recessive Inheritance

Parents of a proband

  • The parents of an affected child are obligate heterozygotes (i.e., carriers of one TBC1D24 pathogenic variant).
  • Heterozygotes (carriers) are typically asymptomatic and are not at risk of developing an autosomal recessive TBC1D24-related disorder. It is possible that some heterozygotes (carriers) could have an elevated susceptibility to seizure disorders related to certain TBC1D24 pathogenic variants, but genotype-phenotype correlation is lacking and no risk estimates are available.

Sibs of a proband

  • At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier.
  • Heterozygotes (carriers) are asymptomatic and are not at risk of developing the disorder.

Offspring of a proband

  • Individuals with TBC1D24-related DOORS syndrome, TBC1D24-related PME, and EIEE16 have not reproduced.
  • The offspring of an individual with a FIME or DFNB86 are obligate heterozygotes (carriers) for a pathogenic variant in TBC1D24.

Other family members. Each sib of the proband's parents is at a 50% risk of being a carrier of a TBC1D24 pathogenic variant.

Carrier (Heterozygote) Detection

Carrier testing for at-risk relatives requires prior identification of the TBC1D24 pathogenic variants in the family.

Related Genetic Counseling Issues

See Management, Evaluation of Relatives at Risk for information on evaluating at-risk relatives for the purpose of early diagnosis and treatment.

The following points are noteworthy:

  • Communication with individuals who are members of the Deaf community and who sign requires the services of a skilled interpreter.
  • Members of the Deaf community may view deafness as a distinguishing characteristic and not as a handicap, impairment, or medical condition requiring a "treatment" or "cure," or to be "prevented."
  • Many deaf people are interested in obtaining information about the cause of their own deafness, including information on medical, educational, and social services, rather than information about prevention, reproduction, or family planning. It is, therefore, important to ascertain and address the questions and concerns of the family/individual.
  • The use of certain terms is preferred: probability or chance vs risk; deaf and hard-of-hearing vs hearing impaired. Terms such as "abnormal" should be avoided.

Family planning

  • The optimal time for determination of genetic risk, clarification of carrier status, and discussion of the availability of prenatal testing is before pregnancy.
  • It is appropriate to offer genetic counseling (including discussion of potential risks to offspring and reproductive options) to young adults who are affected, are carriers, or are at risk of being carriers.

DNA banking is the storage of DNA (typically extracted from white blood cells) for possible future use. Because it is likely that testing methodology and our understanding of genes, allelic variants, and diseases will improve in the future, consideration should be given to banking DNA of affected individuals.

Prenatal Testing and Preimplantation Genetic Diagnosis

Once the TBC1D24 pathogenic variants have been identified in an affected family member, prenatal testing for a pregnancy at increased risk and preimplantation genetic diagnosis are possible.

Differences in perspective may exist among medical professionals and within families regarding the use of prenatal testing, particularly if the testing is being considered for the purpose of pregnancy termination rather than early diagnosis. While most centers would consider decisions regarding prenatal testing to be the choice of the parents, discussion of these issues is appropriate.

Resources

GeneReviews staff has selected the following disease-specific and/or umbrella support organizations and/or registries for the benefit of individuals with this disorder and their families. GeneReviews is not responsible for the information provided by other organizations. For information on selection criteria, click here.

  • American Epilepsy Society (AES)
  • American Society for Deaf Children (ASDC)
    800 Florida Avenue Northeast
    Suite 2047
    Washington DC 20002-3695
    Phone: 800-942-2732 (Toll-free Parent Hotline); 866-895-4206 (toll free voice/TTY)
    Fax: 410-795-0965
    Email: info@deafchildren.org; asdc@deafchildren.org
  • Canadian Epilepsy Alliance
    Canada
    Phone: 1-866-EPILEPSY (1-866-374-5377)
  • Epilepsy Foundation
    8301 Professional Place East
    Suite 200
    Landover MD 20785-7223
    Phone: 800-332-1000 (toll-free)
    Email: ContactUs@efa.org
  • National Association of the Deaf (NAD)
    8630 Fenton Street
    Suite 820
    Silver Spring MD 20910
    Phone: 301-587-1788; 301-587-1789 (TTY)
    Fax: 301-587-1791
    Email: nad.info@nad.org
  • National Institute of Neurological Disorders and Stroke (NINDS)
    PO Box 5801
    Bethesda MD 20824
    Phone: 800-352-9424 (toll-free); 301-496-5751; 301-468-5981 (TTY)

Molecular Genetics

Information in the Molecular Genetics and OMIM tables may differ from that elsewhere in the GeneReview: tables may contain more recent information. —ED.

Table A.

TBC1D24-Related Disorders: Genes and Databases

GeneChromosome LocusProteinLocus-Specific DatabasesHGMDClinVar
TBC1D2416p13​.3TBC1 domain family member 24TBC1D24 @ LOVDTBC1D24TBC1D24

Data are compiled from the following standard references: gene from HGNC; chromosome locus from OMIM; protein from UniProt. For a description of databases (Locus Specific, HGMD, ClinVar) to which links are provided, click here.

Table B.

OMIM Entries for TBC1D24-Related Disorders (View All in OMIM)

220500DEAFNESS, ONYCHODYSTROPHY, OSTEODYSTROPHY, MENTAL RETARDATION, AND SEIZURES SYNDROME; DOORS
605021MYOCLONIC EPILEPSY, FAMILIAL INFANTILE; FIME
613577TBC1 DOMAIN FAMILY, MEMBER 24; TBC1D24
614617DEAFNESS, AUTOSOMAL RECESSIVE 86; DFNB86
615338EPILEPTIC ENCEPHALOPATHY, EARLY INFANTILE, 16; EIEE16
616044DEAFNESS, AUTOSOMAL DOMINANT 65; DFNA65

Gene structure. Transcript variant 1 (NM_001199107.1) encodes the longest isoform, and is composed of eight exons, with exon 1 being noncoding. Exon 3 is spliced out in isoform 2 (encoded by transcript NM_020705.2), which is expressed predominantly in non-neural tissues [Guven & Tolun 2013]. For a detailed summary of gene and protein information, see Table A, Gene.

Pathogenic variants

Table 5.

TBC1D24 Pathogenic Variants Discussed in This GeneReview

DNA Nucleotide ChangePredicted Protein ChangeReference Sequences
c.119G>Tp.Arg40LeuNM_001199107​.1
NP_001186036​.1
c.208G>Tp.Asp70Tyr
c.404C>Tp.Pro135Leu
c.533C>Tp.Ser178Leu
c.724C>Tp.Arg242Cys
c.1008delTp.His336GlnfsTer12

Note on variant classification: Variants listed in the table have been provided by the authors. GeneReviews staff have not independently verified the classification of variants.

Note on nomenclature: GeneReviews follows the standard naming conventions of the Human Genome Variation Society (varnomen​.hgvs.org). See Quick Reference for an explanation of nomenclature.

Normal gene product. TBC1D24 is a Tre2–Bub2–Cdc16 (TBC) domain-containing RAB GTPase-activating protein, which catalyzes the hydrolysis of GTP by small GTPases, thus regulating the proper transport of intracellular vesicles. TBC1D24 is the only TBC/RabGAP protein with a TLDc domain (TBC, LysM, Domain catalytic) of unknown function but thought to be involved in oxidative stress resistance and perhaps to have some enzymatic activity.

TBC1D24 has been demonstrated to interact with ARF6 when both proteins are overexpressed in cell culture [Falace et al 2010, Falace et al 2014]. In C. elegans, C31H2.1 (a TBC1D24 ortholog) was implicated in synaptic function by an RNAi screen [Sieburth et al 2005]. In Drosophila, the ortholog Skywalker (Sky) facilitates endosomal trafficking in synaptic vesicles by facilitating GTP hydrolysis by Rab35, thus controlling synaptic vesicle rejuvenation and neurotransmitter release [Uytterhoeven et al 2011]. Analysis of the crystal structure of Sky identified a cationic pocket that is preserved in human TBC1D24. This pocket is necessary for binding to the lipid membrane via phosphoinositides phosphorylated at the 4 and 5 positions [Fischer et al 2016]. Whether human TBC1D24 is able to also facilitate Rab protein-mediated GTP hydrolysis remains to be determined.

Abnormal gene product. TBC1D24-related disorders that are inherited in an autosomal recessive manner are thought to be the result of reduced function or loss of function of TBC1D24. Abrogation of the cationic pocket by introducing two human pathogenic variants p.Arg40 and p.Arg242 led to impaired synaptic vesicle trafficking and seizures in Drosophila [Fischer et al 2016]. Functional studies of other causative variants are limited and to date, no studies have been conducted examining the variant causing autosomal dominant hearing loss, p.Ser178Leu.

References

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Chapter Notes

Author Notes

Philippe M Campeau is Clinical Assistant Professor in Pediatrics at the Sainte-Justine Hospital of the University of Montreal. He is a clinical geneticist with interest in skeletal dysplasias and inborn errors of metabolism, and a principal investigator whose laboratory focuses on understanding the pathophysiology of newly identified skeletal dysplasia genes.

Acknowledgments

The authors would like to thank the families who participated in our study on DOORS syndrome and made this work possible.

Revision History

  • 7 December 2017 (ma) Comprehensive update posted live
  • 26 February 2015 (me) Review posted live
  • 31 July 2014 (pmc) Original submission
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