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Action Myoclonus – Renal Failure Syndrome

, MD, , MD, FRCP(C), and , MDCM, PhD, FCCMG.

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Estimated reading time: 32 minutes

Summary

Clinical description.

Action myoclonus – renal failure (AMRF) syndrome typically comprises a continuum of two major (and ultimately fatal) manifestations: progressive myoclonic epilepsy (PME) and renal failure; however, in some instances, the kidneys are not involved. Neurologic manifestations can appear before, simultaneously, or after the renal manifestations. Disease manifestations are usually evident in the late teens or early twenties. In the rare instances in which renal manifestations precede neurologic findings, onset is usually in late childhood/early adolescence but can range to the fifth or sixth decade. Neurologic manifestations begin with tremor at rest (which is exacerbated by fine motor activities) and progress to involuntary, action-activated myoclonic jerks that involve bulbar, proximal, and distal limb muscles; involuntary spontaneous myoclonic jerks; and generalized clonic-tonic-clonic seizures. Sensorimotor peripheral neuropathy and sensorineural hearing loss can also be observed. Renal manifestations include proteinuria that can progress to nephrotic syndrome and end-stage renal disease.

Diagnosis/testing.

The diagnosis of AMRF syndrome is suspected in a previously healthy teenager or young adult with the characteristic neurologic and/or renal manifestations. The diagnosis is confirmed in individuals with biallelic (homozygous or compound heterozygous) loss-of-function pathogenic variants in SCARB2.

Management.

Treatment of manifestations: Symptomatic pharmacologic and psychosocial support is the mainstay of care for the neurologic manifestations. Response to treatment is variable and may worsen over time, necessitating rehabilitative management. Renal insufficiency requires dialysis but response to treatment is poor, and renal transplantation is often necessary.

Prevention of secondary complications: Standard measures for prevention of aspiration pneumonia and sudden unexpected death in epilepsy should be followed.

Surveillance: Lifelong follow up should include regular monitoring of antiepileptic drug treatment and renal function (including urinary protein excretion, creatinine clearance, and estimated glomerular filtration rate) and periodic assessment of hearing and peripheral nerves.

Agents/circumstances to avoid: Diphenylhydantoin, carbamazepine, oxcarbazepine, and possibly lamotrigine increase myoclonus and should be avoided in any individual with PME.

Pregnancy management: Because some antiepileptic drugs can lead to an increased risk of malformations, growth retardation, or neurodevelopmental disabilities in exposed fetuses, standard measures for prevention of fetopathy should be followed.

Genetic counseling.

AMRF syndrome is inherited in an autosomal recessive manner. 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 and prenatal testing for pregnancies at increased risk require prior identification of the SCARB2 pathogenic variants in the family.

Diagnosis

Suggestive Findings

Action myoclonus – renal failure (AMRF) syndrome is suspected in a previously healthy teenager or young adult with the following neurologic and renal manifestations [Andermann et al 1986, Badhwar et al 2004, Vadlamudi et al 2006, Andermann 2011]:

Neurologic manifestations

  • A fine tremor of the fingers and hands, present at rest, exacerbated by fine motor activities such as writing and relieved by alcohol or propranolol, is commonly the first finding. The tremor can later involve the head, trunk, lower extremities, and sometimes tongue and voice. In the later stages of the disease, it becomes masked by striking myoclonic jerks.
  • Involuntary, action-activated myoclonic jerks, also induced by attempted and executed speech, involve bulbar as well as proximal and distal limb muscles. The myoclonus is reflex-sensitive to touch on the distal extremities.
  • Involuntary spontaneous myoclonic jerks of the face (particularly perioral) as well as synchronous and asynchronous jerks of the trunk and limbs also occur at rest.
  • Generalized clonic-tonic-clonic seizures, diurnal and/or nocturnal, start with a generalized clonic phase with preserved consciousness and proceed to unconsciousness with tonic-clonic features and urinary incontinence.
  • Other findings can include sensorimotor peripheral neuropathy (most often predominantly demyelinating or more rarely axonal) and sensorineural hearing loss (SNHL).

Renal manifestations

  • Proteinuria, the first manifestation of renal disease, is initially mild and asymptomatic.
  • Renal disease can progress to nephrotic syndrome and end-stage renal disease.
  • In some families, renal manifestations (eventually requiring renal transplantation) appear first in late childhood or early teens and neurologic manifestations in the late twenties or early thirties [Badhwar et al 2004].
  • Histologic changes include interstitial fibrosis, atrophy, focal sclerosing glomerulonephritis sometimes with features of collapsing glomerulopathy (a severe variant of glomerulosclerosis), or membranous nephropathy. No storage was observed [Andermann et al 1986, Badhwar et al 2004, Berkovic et al 2008].
  • Note: Immunostaining can show IgM and complement present in the glomerular loops and in the mesangium [Andermann et al 1986]. In one patient, a C1q nephropathy has been shown by immunohistochemistry [Balreira et al 2008, Chaves et al 2011], indicating an immune complex-mediated glomerular disease. In this patient, extensive tubular abnormalities were also present, as well as granular material in cortical tubules. No Gaucher cells were observed.

A classification that takes into account the age at disease onset and the clinical manifestations has been proposed (Table 2).

Establishing the Diagnosis

The diagnosis of action myoclonus – renal failure (AMRF) syndrome is confirmed in individuals with biallelic (homozygous or compound heterozygous) loss-of-function pathogenic variants in SCARB2 [Balreira et al 2008, Berkovic et al 2008] (Table 1).

Note: Molecular diagnosis performed early in the disease course may eliminate the need for numerous invasive neurologic and renal investigations.

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

Single-gene testing

A multigene panel for progressive myoclonic epilepsy (PME) or epilepsy that includes SCARB2 and other genes of interest (see Differential Diagnosis) may also be used. Note: (1) The genes included in the panel and the diagnostic sensitivity of the testing used for each gene vary by laboratory and are likely to change 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 if serial single-gene testing (and/or use of a multigene panel that includes SCARB2) fails to confirm a diagnosis in an individual with features of action myoclonus – renal failure syndrome. Such testing may provide or suggest a diagnosis not previously considered (e.g., mutation of a different gene 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 Action Myoclonus – Renal Failure Syndrome

Gene 1Test MethodProportion of Probands with Pathogenic Variants 2 Detectable by This Method
SCARB2Sequence analysis 3Estimated from Table 4: 97.5% 4
Targeted analysis of pathogenic variantsFootnote 5
Gene-targeted deletion/duplication analysis 6None reported
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.

About 20 pathogenic variants have been reported to date (see Tables 3 and 4). About 2.5% of affected individuals are compound heterozygotes with an identified intragenic pathogenic variant in one allele and an unidentified variant in the other.

5.

Testing for the pathogenic variants c.862C>T (see also Table 4) (exon 7) and c.1187+3insT (see also Table 4) (intron 9) in persons of French-Canadian origin. In this population, the vast majority of probands are homozygous for c.862C>T, whereas a small proportion are compound heterozygotes for both pathogenic variants [Dibbens et al 2011].

6.

Gene-targeted deletion/duplication analysis detects intragenic deletions and duplications. Methods used may 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.

Clinical Characteristics

Clinical Description

Action myoclonus – renal failure (AMRF) syndrome typically comprises a continuum of two major (and ultimately fatal) manifestations: progressive myoclonic epilepsy (PME) and renal failure (Table 2); however, in some instances, renal failure is not observed. Thus, progressive myoclonus epilepsy without renal failure caused by biallelic SCARB2 pathogenic variants is considered to be one end of the spectrum of AMRF [Badhwar et al 2004, Dibbens et al 2009, Rubboli et al 2011, Guerrero-López et al 2012, Zeigler et al 2014].

The age of onset varies, even within the same family.

  • Neurologic manifestations can appear before (in 1/3 of the cases), simultaneously, or after the renal manifestations. In juvenile AMRF onset is usually in the late teens or early twenties [Andermann et al 1986, Badhwar et al 2004].
  • In some persons renal manifestations occur early (late childhood or early teens) and neurologic involvement much later (late 20s or early 30s) [Badhwar et al 2004, Hopfner et al 2011].
  • In three persons of Japanese heritage who did not develop renal failure, neurologic manifestations appeared in the fifth or sixth decade [Higashiyama et al 2013, Fu et al 2014].

The neurologic and renal manifestations progress independently. Of note, the neurologic manifestations are not the result of a metabolic encephalopathy due to renal failure and are not improved by dialysis or by renal transplantation [Andermann et al 1986, Badhwar et al 2004].

Even in the same family, the number and order of appearance of the clinical manifestations can vary. Neurologic manifestations may occur first or in isolation in some family members and renal manifestations may be first or isolated in other family members [Badhwar et al 2004]. In some families, all affected individuals have neurologic manifestations and none develop renal failure [Dibbens et al 2009, Rubboli et al 2011, Guerrero-López et al 2012, Fu et al 2014, Perandones et al 2014, Zeigler et al 2014]; however, some affected family members have proteinuria [Dibbens et al 2009, Guerrero-López et al 2012] or reduced creatinine clearance [Zeigler et al 2014].

The disease progresses relentlessly with neurologic deterioration (especially increasing severity of myoclonus) and renal failure leading to death within seven to 15 years after onset.

Table 2.

Clinical Manifestations of AMRF

Disease OnsetMajor ManifestationsCNS InvolvementRenal DiseaseOther Possible Manifestations
JuvenilePME and RF
  • Tremor
  • Progressive action myoclonus
  • Myoclonus at rest
  • Ataxia, dysarthria
  • Severe epileptic photosensitivity
  • GTCS (rare at onset; relatively infrequent even in later stages)
  • Proteinuria (progressing from mild to severe)
  • Segmental GP and/or TP
  • Renal failure
  • PNP
  • SNHL
  • DCM
PME
  • Tremor
  • Progressive action myoclonus
  • Myoclonus at rest
  • Ataxia, dysarthria
  • Severe epileptic photosensitivity
  • GTCS (infrequent at onset)
  • Mild proteinuria
  • Mild decrease of creatinine clearance
  • PNP
  • Mild generalized muscle atrophy w/out fasciculations
RFNeurologic manifestations as listed above may develop late in the course of the disease
  • May begin in early childhood or early teens
  • Proteinuria, progressive
  • Segmental GP
  • TP
  • Renal failure
LatePME
  • Tremor
  • Ataxia, dysarthria
  • Progressive action myoclonus
  • GTCS may occur infrequently
  • Cognitive decline
None reported

DCM = dilated cardiomyopathy; GP = glomerulopathy; GTCS = generalized tonic-clonic seizures; PME = progressive myoclonus epilepsy; PNP = peripheral neuropathy; RF = renal failure; SNHL = sensorineural hearing loss; TP = tubulopathy

Neurologic Disease

Fine tremor. The disease begins with bilateral fine tremor of the fingers that is noted at rest and increased by delicate movement such as writing, by intention of movement, and by maintaining an attitude in horizontal extension [Andermann et al 1986, Badhwar et al 2004, Vadlamudi et al 2006]. The tremor can be relieved by alcohol [Andermann et al 1986, Andermann 2011, Guerrero-López et al 2012]. It becomes progressively worse until it is masked by myoclonic jerks [Badhwar et al 2004, Vadlamudi et al 2006].

Action myoclonus. The fine tremor is followed by jerking movements first of the upper and then the lower extremities. Referred to as action myoclonus, these jerking movements are typically triggered by movements or intended movements. They are asynchronous and of variable severity.

With time, myoclonic jerks involve the proximal limbs; their amplitude and number increases by movements of the limbs, typically by walking down stairs. Action myoclonus can also involve the trunk. Attempts at speaking and executed speech can induce myoclonus of the bulbar musculature, contributing to the dysarthria. There is no palatal myoclonus.

Action myoclonus, which is also reflex-sensitive to touch over the distal extremities, can be exacerbated by anxiety, excitement, stress, and fatigue [Badhwar et al 2004, Zeigler et al 2014] and by auditory stimuli [Perandones et al 2012]. Some patients exhibit occasional myoclonus in response to startle [Badhwar et al 2004]. Of note, myoclonic jerks were significantly less frequent during pregnancy in one patient [Amrom et al, In press].

Action myoclonus represents the most disabling manifestation: it prevents affected individuals from being able to feed themselves and, thus, they become malnourished unless they receive assistance with feeding or are fed by artificial means. In the final stages, they may become bedridden or wheelchair bound. Swallowing difficulties can lead to aspiration pneumonia and death [Andermann et al 1986, Badhwar et al 2004, Vadlamudi et al 2006].

Myoclonus at rest. Subtle myoclonic movements of the eyelids, jaws, and perioral musculature appear at rest and while speaking. Ocular dysmetria can occur later in the disease course.

Clonic-tonic-clonic seizures, which can be diurnal or nocturnal, begin with generalized clonic jerking with preserved consciousness and proceed to unconsciousness with tonic-clonic features. They occur infrequently, starting with one per annum initially [Badhwar et al 2004]. TV viewing or other light stimulation may trigger generalized myoclonic seizures or tonic-clonic seizures [Rubboli et al 2011]. Photosensitivity can become so severe that affected individuals choose to live in almost complete darkness [Rubboli et al 2011].

Ataxia and dysarthria, common findings, can be distinguished from myoclonic jerks by the presence of the cerebellar abnormalities of pendular reflexes, abnormal rebound, and hypermetric ocular saccades.

Progressive myoclonus ataxia. Some patients develop significant progressive ataxia before or after the appearance of myoclonus and, thus, have been reported to have "progressive myoclonus ataxia."

A woman age 29 years had a history of clumsiness in the lower limbs, mild gait instability, and difficulties in riding a bicycle beginning at age 21 years, two years before the onset of progressive myoclonus at age 23 years. Renal failure was evident at age 25 years, and bilateral severe SNHL was diagnosed at age 27 years [Perandones et al 2012, Perandones et al 2014].

A woman age 22 years developed postural hand tremor exacerbated by fine voluntary movements and stress. The tremor slowly worsened with multifocal spontaneous and stimulus-sensitive myoclonic jerks. No other seizures were reported, and the EEG did not show any epileptic activity (of note, the patient was taking several antiepileptic medications). Ataxia was first reported when she was wheel-chair bound at age 27 years. The co-occurrence of tremor, myoclonus, and ataxia in the same patient – without generalized tonic-clonic seizures – increases the complexity of the clinical picture of this disorder [Guerrero-López et al 2012].

Peripheral neuropathy. In some families, a sensorimotor peripheral neuropathy (most often predominantly demyelinating or more rarely axonal) may be present.

Some affected individuals may be diagnosed with a predominantly demyelinating peripheral neuropathy before the onset of renal failure [Badhwar et al 2004, Costello et al 2009, Dibbens et al 2011, Hopfner et al 2011]. In the German Family I reported by Badhwar et al [2004], one family member with AMRF who was asymptomatic for polyneuropathy was initially found to have a predominantly axonal neuropathy by nerve conduction studies; several years later, he and his two affected sibs were reported to have predominantly demyelinating polyneuropathy [Hopfner et al 2011].

Mild generalized muscle atrophy was observed in one individual who exhibited mild generalized reduced tone and no fasciculations [Zeigler et al 2014].

Renal Disease

Mild proteinuria may progress to nephrotic syndrome and ultimately to renal failure (Table 2).

Dialysis and renal transplantation can prolong survival, but do not improve the neurologic features.

Mental Status

Cognitive function. Unlike individuals with most other types of progressive myoclonus epilepsy, the majority of individuals with AMRF syndrome remain mentally alert. However, dementia has been documented in two unrelated individuals of Japanese ancestry, one with the juvenile-onset form, and the other with the late-onset form, who have different SCARB2 pathogenic variants [Fu et al 2014].

Psychological complications. Individuals with AMRF may exhibit somatic concerns or depressed mood, or may in exceptional cases commit suicide [Amrom et al, In press].

Other Findings

Cardiac disease. In a German family, echocardiography revealed dilated cardiomyopathy in two of three affected sibs at ages 14 and 21 years [Hopfner et al 2011]. In addition, these sibs had sensorimotor peripheral neuropathy.

Hearing loss / deafness. Frank or subclinical sensorineural hearing loss (SNHL) can be part of the spectrum of AMRF syndrome. Three individuals with AMRF had adult-onset SNHL which was mild in one individual from one family [Rubboli et al 2011], and severe and asymmetric in an individual from a second family [Perandones et al 2012], in which a sister had preclinical hearing loss [Perandones et al 2014].

Thus in the same family, the SNHL can be severe or subclinical or absent [Badhwar et al 2004, Perandones et al 2012, Perandones et al 2014].

Common causes of death in AMRF. Sudden death may occur during or after a generalized epileptic seizure due to aspiration, severe myoclonus and unmanageable saliva, or an undetermined cause. Death can also occur due to aspiration pneumonia, renal failure, or rejection of a renal transplant [Andermann et al 1986, Badhwar et al 2004, Vadlamudi et al 2006, Rubboli et al 2011].

Late-Onset AMRF

Disease onset in the fifth or sixth decade has been reported in two Japanese families.

Higashiyama et al [2013] reported one family with two sibs with AMRF without renal failure. The sister presented with myoclonic jerks at age 43 years; her older brother presented with gait difficulties at age 52 years. Both were homozygous for the SCARB2 pathogenic variant c.1385_1390del6insATGCATGCACC.

Fu et al [2014] reported single affected individuals from two other Japanese families, one of whom (Patient 1) had late-onset disease. Patient 1 presented with onset of difficulties going up and down the stairs at age 45 years. Of note, he was homozygous for the same pathogenic variant as the two sibs reported by Higashiyama et al [2013]. Although the two families with the late-onset form are likely related as they originate from the same rural area, the precise relationship is unknown [Hiroshi Doi, 2014, personal communication; Hitoshi Takahashi, June 2014, personal communication]. Of note, Patient 2 reported by Fu et al [2014] presented at age 20 years, was unrelated to Patient 1, and was homozygous for a different SCARB2 pathogenic variant.

Specialized Studies

EEG findings [Andermann et al 1986, Badhwar et al 2004]. Background activity may be normal in some patients or show diffuse slowing at 6.5 to 7.5 Hz. Relatively low-voltage spike and spike-wave discharges, rather infrequent, bilaterally synchronous and generalized or confined to the central vertex or both occipital regions, increased by hyperventilation and intermittent photic stimulation, may be present.

Some of the brief spike potentials are difficult to distinguish from muscle potentials except that they are seen at the vertex where there is no muscle artefact. The electromyogram myoclonic potentials are sometimes associated with cerebral potentials and at other times occur independently, suggesting a subcortical origin with a secondary corticoreticular generalization.

Intermittent photic stimulation may produce whole-body myoclonus with multiple spikes in the EEG record associated with slow waves [Andermann et al 1986]. These generalized spike-polyspike-wave bursts can outlast the duration of light stimulation [Rubboli et al 2011].

Myoclonic seizures can be triggered by eye closure and resolve by eye opening [Rubboli et al 2011].

Overnight sleep recording can show fast spikes over the vertex spreading to bilateral frontocentral regions during rapid eye movement (REM) sleep.

Follow up over the course of the disease shows a preserved alpha background activity at disease onset, with rare generalized or focal epileptiform discharges. Over the years, irregular slower theta and delta waves progressively intermix with the alpha waves, and the epileptic activity becomes more frequent [Rubboli et al 2011].

MRI findings. Brain MRI may be normal or show mild diffuse cerebral and cerebellar atrophy [Andermann et al 1986, Badhwar et al 2004, Perandones et al 2012].

Electromyography findings. Nerve conduction analysis can show slowed nerve conduction velocities and prolonged F-waves, consistent with a mixed, mainly demyelinating polyneuropathy [Dibbens et al 2011, Hopfner et al 2011, Rubboli et al 2011].

In one patient, concentric needle electromyography (EMG) was suggestive of chronic anterior horn involvement [Zeigler et al 2014].

Brain histologic findings. A constant and pathognomonic finding is the presence of small and large autofluorescent pigment granules up to 10 µm in size in astrocytes and in certain cells in the meninges. The pigment granules are more prominent in laminae I and II of the cerebral cortex, the globus pallidus and putamen, and the Bergmann astrocytes in the cerebellar cortex; they are not seen in the thalamus, brain stem nuclei, dentate nuclei of the cerebellum, or spinal cord gray matter. The granules are both separate from as well as adjacent to glial cell nuclei, suggesting that at least some were within astrocytes [Andermann et al 1986, Badhwar et al 2004, Berkovic et al 2008].

Neurons contain normal amounts of lipofuscin and no pigment granules [Andermann et al 1986, Badhwar et al 2004]. In two affected individuals of Japanese ancestry extraneuronal brown pigment deposition, exclusively in astrocytic cytoplasm and surrounded by a membrane, was widely scattered throughout the brain [Fu et al 2014].

Click here for information about specialized studies for biologic and histologic findings.

Genotype-Phenotype Correlations

No clear genotype-phenotype correlation is evident.

Disease severity may vary among affected individuals within a family who have the same pathogenic variants. In some affected individuals, phenotypic variability may be explained by the presence of an additional pathogenic variant in another epilepsy-related gene [He et al 2014].

Nomenclature

Action myoclonus – renal failure (AMRF) syndrome has been referred to as:

  • Familial myoclonus with renal failure
  • Progressive myoclonus epilepsy with renal failure
  • Epilepsy, progressive myoclonic 4, with or without renal failure, EPM4. However, the presence or absence of renal failure represents only part of the clinical spectrum of AMRF.

The term progressive myoclonus epilepsy (PME) covers a large group of diseases characterized by myoclonus, epilepsy, and progressive neurologic deterioration.

Prevalence

Exact prevalence figures are not available. To the authors' knowledge, 38 affected individuals from 26 families have been reported to date.

AMRF was first reported in several French-Canadian families [Andermann et al 1986]; it occurs worldwide, including Europe; North, Central and South America; Australia; Asia; and the Middle-East.

Families without renal failure have been reported in Italy [Dibbens et al 2009], Spain [Guerrero-López et al 2012], Asia [Higashiyama et al 2013, Fu et al 2014, He et al 2014], and the Middle-East [Zeigler et al 2014].

Differential Diagnosis

At the onset of the disease, three non-progressive conditions should be considered in the differential diagnosis:

In individuals with a PME phenotype who do not have biallelic SCARB2 pathogenic variants, the following disorders should be considered:

See Epilepsy, progressive myoclonic: OMIM Phenotypic Series to view genes associated with this phenotype in OMIM.

Charcot-Marie-Tooth neuropathy with focal segmental glomerulonephritis (OMIM 614455) should be considered in persons with peripheral neuropathy and glomerulonephritis [Boyer et al 2011]. Inheritance is autosomal dominant; heterozygous pathogenic variants in INF2 are causative.

Management

Evaluations Following Initial Diagnosis

To establish the extent of disease and needs in an individual diagnosed with action myoclonus – renal failure (AMRF) syndrome, the following evaluations are recommended:

  • Clinical evaluation including cognitive function and school performance, emotional features, eye movements, coordination, handwriting, walking
  • Examination of myoclonus including evaluation at rest, with action, and in response to stimuli
  • EEG evaluation including photosensitivity before therapy is initiated, as it is most characteristic before the use of anticonvulsant medication
  • Renal function
  • Audiogram and brain stem auditory evoked potentials (BAEPs) to assess the possibility of clinical or subclinical sensorineural hearing loss [Rubboli et al 2011, Perandones et al 2012, Perandones et al 2014]
  • Nerve conduction velocities (NCV) and needle electromyography (EMG)
  • Consultation with a clinical geneticist and/or genetic counselor

Treatment of Manifestations

Neurologic manifestations. Symptomatic pharmacologic and psychosocial support is the mainstay of care for the neurologic manifestations. Response to treatment is variable and may deteriorate over time, necessitating rehabilitative management.

  • Valproic acid, the first drug of choice, diminishes myoclonus and the frequency of generalized seizures.
  • Clonazepam may be used as add-on therapy for the treatment of myoclonic seizures.
  • Levetiracetam appearss to be effective for both myoclonus and generalized seizures, and is recommended in women of child bearing age.
    Note: Lamotrigine is not effective in controlling the myoclonus, and may aggravate myoclonus in some patients [Guerrini et al 1998].

Renal manifestations. Renal insufficiency requires dialysis but response to treatment is poor, and renal transplantation is often necessary.

Prevention of Secondary Complications

Standard measures for prevention of aspiration pneumonia and sudden unexpected death in epilepsy (SUDEP) should be followed; offering psychosocial support may be helpful.

Surveillance

Lifelong clinical follow up should include the following:

  • Neurologic
    • Monitoring of antiepileptic drug treatment (drug levels and clinical assessment of biological effects);
    • Periodic assessment of hearing by audiograms and BAEPs and of the peripheral nerves by NCV and needle EMG.
  • Renal function monitoring by measurement of: blood pressure; body weight; serum concentrations of creatinine, albumin, and cholesterol; 24-hour urinary protein; and creatinine clearance

Agents/Circumstances to avoid

Diphenylhydantoin, carbamazepine, oxcarbazepine, and possibly lamotrigine increase myoclonus and should be avoided in any individual with progressive myoclonic epilepsy (PME), including AMRF.

Evaluation of relatives at risk

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

Pregnancy Management

Some antiepileptic drugs can lead to an increased risk of malformations, growth retardation, or neurodevelopmental disabilities in exposed fetuses. However, when pregnant women experience prolonged seizures during pregnancy, the risk of adverse fetal outcomes is increased. Therefore, it is recommended that women with a known seizure disorder continue to take antiepileptic drugs during pregnancy. Standard measures for prevention of fetopathy should be followed. These include:

  • Possible changes of medication prior to pregnancy;
  • Spacing of the antiepileptic drugs into four doses a day or taking extended release medications, so that the drug levels do not have significant peaks or troughs;
  • Monitoring the dosages and drug levels of antiepileptic drugs during pregnancy and after the delivery.

In addition, folic acid should be prescribed at 1 mg/day for all women of childbearing age and increased to 5 mg/day when planning a pregnancy (ideally 3 months prior to conception) and during the pregnancy, in order to prevent possible neural tube defects and other congenital malformations that can be associated with fetal exposure to antiepileptic drugs.

Therapies Under Investigation

A trial of enzyme replacement therapy (ERP) with imiglucerase (60 U/kg every two weeks) in two sibs with AMRF for a period of one year did not improve the clinical status. Substrate reduction therapy (SRT) with miglustat (600 mg daily) administered to one of the two sibs resulted in a significant reduction of myoclonus [Chaves et al 2011].

Search Clinical Trials.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.

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

Action myoclonus - renal failure (AMRF) syndrome is inherited in an autosomal recessive manner.

Risk to Family Members

Parents of a proband

  • The parents of an affected individual are obligate heterozygotes (i.e., carriers of one SCARB2 pathogenic variant).
  • Heterozygotes (carriers) are asymptomatic.

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.

Offspring of a proband

  • The offspring of an individual with action myoclonus-renal failure syndrome are obligate heterozygotes (carriers) for a SCARB2 pathogenic variant.
  • If the reproductive partner of an affected individual is a carrier of an SCARB2 pathogenic variant, the offspring have a 50% chance of being affected and a 50% chance of being a carrier.

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

Carrier (Heterozygote) Detection

Carrier testing for at-risk relatives requires prior identification of the SCARB2 pathogenic variants in the family.

Related Genetic Counseling Issues

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 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 SCARB2 pathogenic variants have been identified in an affected family member, prenatal testing for a pregnancy at increased risk and preimplantation genetic diagnosis for AMRF are possible.

Fetal ultrasound examination. Prenatal testing by level 2 ultrasound examination does NOT detect AMRF, as disease onset is usually in the late teens or early twenties.

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 Kidney Fund
    11921 Rockville Pike
    Suite 300
    Rockville MD 20852
    Phone: 866-300-2900
    Email: helpline@kidneyfund.org
  • Canadian Epilepsy Alliance
    Canada
    Phone: 1-866-EPILEPSY (1-866-374-5377)
  • Citizens United for Research in Epilepsy (CURE)
  • Epilepsy Foundation
    8301 Professional Place East
    Suite 200
    Landover MD 20785-7223
    Phone: 800-332-1000 (toll-free)
    Email: ContactUs@efa.org
  • Kidney Foundation of Canada
    310-5160 Decarie Blvd.
    Montreal Ontario H3X 2H9
    Canada
    Phone: 800-361-7494 (toll-free); 514-369-4806
    Fax: 514-369-2472
    Email: info@kidney.ca
  • NephCure Kidney International
    15 Waterloo Avenue
    Berwyn PA 19312
    Phone: 866-NephCure; 866-637-4287
    Email: info@nephcure.org

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.

Action Myoclonus - Renal Failure Syndrome: Genes and Databases

GeneChromosome LocusProteinLocus-Specific DatabasesHGMDClinVar
SCARB24q21​.1Lysosome membrane protein 2SCARB2 databaseSCARB2SCARB2

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 Action Myoclonus - Renal Failure Syndrome (View All in OMIM)

254900EPILEPSY, PROGRESSIVE MYOCLONIC, 4, WITH OR WITHOUT RENAL FAILURE; EPM4
602257SCAVENGER RECEPTOR CLASS B, MEMBER 2; SCARB2

Gene structure. SCARB2 contains 5.5 kb with 12 exons and codes for two transcript variants; the longer variant is NM_005506.3. For a detailed summary of gene and protein information, see Table A, Gene.

Benign variants. A few sequence alterations that are not believed to have a primary role in disease causation have been found in unaffected individuals [Dibbens et al, unpublished data]. The extent to which these variants may influence the phenotype or clinical disease expression remains to be established.

Pathogenic variants. Known pathogenic variants include nonsense, missense, deletion, frameshift, and splice site variants, leading to retention of SCARB2 protein (previously known as LIMP-2) in the endoplasmic reticulum, but differentially affecting the binding to β-glucocerebrosidase (β-GC) [Blanz et al 2010]. To the authors' knowledge, all pathogenic variants identified to date are loss-of-function variants.

See Table 3 and Table 4 (pdf).

Table 3.

SCARB2 Pathogenic Variants Discussed in This GeneReview

DNA Nucleotide ChangePredicted Protein ChangeReference Sequences
c.862C>Tp.Gln288TerNM_005506​.3
NP_005497​.1
c.1187+3insT

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.

Variants included are found in persons of French-Canadian origin; see Table 1, footnote 5.

Normal gene product. The protein encoded by SCARB2 (scavenger receptor class B, member 2) is a glycosylated 478-residue type III transmembrane protein that belongs to the CD36 family of scavenger receptors and is found in all cell types.

SCARB2 (previously known as LIMP-2) has two known physiologic functions related to lysosome biology:

  • First, it is a major lysosomal integral membrane protein playing an important role in the biogenesis and maintenance of endosomes/lysosomes and interaction with the vesicle fission/fusion machinery [Eskelinen et al 2003].
  • Second, it is a specific trafficking receptor of β-glucocerebrosidase (β-GC), the enzyme deficient in Gaucher disease [Reczek et al 2007]. It binds β-GC for transfer from the endoplasmic reticulum to the lysosome in a mannose-6-phosphate independent manner. The released β-GC remains in the endosome/lysosome whereas SCARB-2 then integrates in the limiting membrane of lysosomes, and is therefore also known as lysosomal integral membrane protein type 2 or LIMP-2 [Reczek et al 2007, Braulke & Bonifacino 2009, Blanz et al 2010].
  • Third, LIMP-2 has recently been found to represent a substrate for cathepsin-F, the enzyme involved in ANCL (Kufs disease) type B, suggesting that there may be links between the pathogenesis of different forms of PME.

In addition, an extralysosomal function has been discovered in mice, human hearts, and myocytes in vivo, in which LIMP-2 has been identified as an important component of the structural organization of the cardiac myocyte intercalated disc, in particular adherens junctions where it interacts with N-cadherin [Schroen et al 2007].

SCARB2 protein also functions as a receptor for various viruses.

Abnormal gene product. Action myoclonus – renal failure syndrome (AMRF) is caused by deficiency of the SCARB2 protein.

In AMRF, the beta-glucocerebrosidase (β-GC) structure itself is not affected, but its receptor in the trans-Golgi network (TGN) is abnormal, therefore altering its trafficking along its biosynthetic pathway. Biochemical analysis reveals a severe deficiency of β-GC in skin fibroblasts despite normal enzyme activity on standard leukocyte screening assays (Family D in Badhwar et al [2004]; unpublished data) [Balreira et al 2008, Zeigler et al 2014].

This selective tissue involvement could be due to variable expression of the SCARB2 protein or to a variable pathway for the transport of β-GC, specific to tissue. Another hypothesis could be that SCARB2 is also a transporter of other as-yet unidentified proteins that also accumulate and/or are secreted in excess [Amrom et al, in press].

AMRF is remarkable for the preservation of normal intelligence in the majority of patients. This could be explained by the observation of extraneuronal but no intraneuronal storage of autofluorescent pigmented material [Andermann et al 1986, Badhwar et al 2004]. However, two Japanese patients with AMRF developed dementia; it is unclear whether this is linked to AMRF or whether these patients had a second disease [Fu et al 2014]. Autofluorescent pigmented granules were observed in the cytoplasm of the astrocytes in these patients. Neuropathologic studies are needed in order to identify the nature of the storage material and to better localize the cellular organelles at the site of storage.

Animal models. The limp-2/scarb2-/- knockout mice showed a triad encompassing pelvic junction obstruction, deafness, and peripheral demyelinating neuropathy [Gamp et al 2003].

Fluorescence immunohistochemical studies in these mice showed that deafness was due to an early loss of potassium channel KCNQ1/KCNE1 surface expression in marginal cells of the stria vascularis in the cochlea [Knipper et al 2006]. In vivo studies in mice lacking limp-2 have shown that β-GC is no longer sorted to lysosomes but is secreted into the extracellular environment [Reczek et al 2007].

In the limp-2 null mice, embryologic cardiac development is normal; however, these mice fail to mount a hypertrophic cardiac response to AngII-induced hypertension, and develop dilated cardiomyopathy. limp-2 protein was thus shown to not only be an important lysosomal protein but also to be an important part of the intercalated disc of the heart and crucial for the hypertrophic response to cardiac loading [Schroen et al 2007].

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Chapter Notes

Revision History

  • 17 December 2015 (me) Review posted live
  • 25 February 2014 (da) Original submission
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