AP-4-associated hereditary spastic paraplegia (AP-4-HSP) is a group of slowly-progressing neurodegenerative disorders that generally present with global developmental delay, moderate to severe intellectual disability, impaired/absent speech, small head size (microcephaly), seizures, and progressive motor symptoms. Low muscle tone (hypotonia) in infancy develops into high muscle tone (hypertonia), resulting in spasticity of the legs that leads to the inability to walk (non-ambulation) and wheelchair reliance. Spasticity may progress to the upper extremities, leading to the partial or total loss of use of all four limbs and torso (tetraplegia).
Causes
AP-4-associated HSP is inherited in an autosomal recessive manner. The four genes that encode subunits of the AP-4 complex are AP4B1, AP4E1, AP4M1, and AP4S1. Parents each carrying a mutated gene have a 25% chance of having an affected child, a 50% chance of having an unaffected carrier child, and a 25% chance of having a child who is unaffected and does not carry a mutated gene.
Diagnosis
The diagnosis of AP-4-associated HSP is based on clinical characteristics and testing that may include a brain MRI showing characteristic features such as a thin corpus callosum, wide lateral ventricles, and changes in the white matter. A definitive diagnosis is reached by genetic testing.
Spastic paraplegia 4 (SPG4; also known as SPAST-HSP) should be suspected in individuals with the following:
- Characteristic clinical symptoms of insidiously progressive bilateral leg stiffness affecting gait with or without spasticity at rest and mild proximal weakness, often accompanied by urinary urgency
- Neurologic examination demonstrated corticospinal tract deficits affecting both legs (spastic weakness, hyperreflexia, and extensor plantar responses). Mildly impaired vibration sensation in the ankles is present in the majority of individuals.
- Family history consistent with autosomal dominant inheritance, or exclusion of other causes of spastic paraplegia in simplex cases (i.e., a single occurrence in a family)
Note: The presence of other signs/symptoms suggestive of complicated hereditary spastic paraplegia does not exclude SPAST-HSP, although it reduces its probability.
Brain and spinal cord MRI
- Often normal in individuals with SPAST-HSP
- Spinal cord atrophy can occur in SPAST-HSP, but is less pronounced than in other genetic causes of HSP.
- Mild vermis atrophy, a thin corpus callosum, subtle white matter changes, and/or cerebellar atrophy have been reported [Duning et al 2010, da Graça et al 2019].
- Electromyography (EMG) with nerve conduction velocities (NCV) is used to exclude peripheral nervous system involvement, which could raise the possibility of an alternative diagnosis as severe polyneuropathy is not a frequent symptom of SPAST-HSP. Karle et al performed neurophysiologic examinations of 128 individuals with HSP, including 35 individuals with SPAST-HSP, and showed that massively elongated central motor conduction time argued against SPAST-HSP; however, reduced amplitudes and prolonged latencies were reported, in particular in individuals with a SPAST pathogenic missense variant [Karle et al 2013].
- Single-gene testing. Sequence analysis of SPAST detects missense, nonsense, and splice site variants, as well as small intragenic deletions/insertions. The combination of in silico predictive algorithms and information retrieved from population databases is essential to establish the pathogenic role of variants of unknown significance [Richards et al 2015]. If no pathogenic variant is found on sequence analysis, perform gene-targeted deletion/duplication analysis to detect intragenic deletions or duplications.
- A multigene panel that includes SPAST and other genes of interest is most likely to identify the genetic cause of the condition while limiting the identification of variants of uncertain significance and pathogenic variants in genes that do not explain the underlying phenotype. 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. (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 this disorder, a multigene panel that also includes deletion/duplication analysis is recommended
Treatment
No specific treatment is known that would prevent, slow, or reverse HSP. Available therapies mainly consist of symptomatic medical management and promoting physical and emotional well-being. Therapeutics offered to HSP patients include:
- Baclofen – a voluntary muscle relaxant to relax muscles and reduce tone. This can be administered orally or intrathecally.
- Tizanidine – to treat nocturnal or intermittent spasms (studies available )
- Diazepam and clonazepam – to decrease the intensity of spasms
- Oxybutynin chloride – an involuntary muscle relaxant and spasmolytic agent, used to reduce spasticity of the bladder in patients with bladder control problems
- Tolterodine tartrate – an involuntary muscle relaxant and spasmolytic agent, used to reduce spasticity of the bladder in patients with bladder control problems
- Cro System – to reduce muscle overactivity (existing studies for spasticity
- Botulinum toxin – to reduce muscle overactivity (existing studies for HSP patients
- Antidepressants – (such as selective serotonin reuptake inhibitors, tricyclic antidepressants, and monoamine oxidase inhibitors) – for patients experiencing clinical depression
- Physical therapy – to restore and maintain the ability to move; to reduce muscle tone; to maintain or improve range of motion and mobility; to increase strength and coordination; to prevent complications, such as frozen joints, contractures, or bedsores
Management of symptoms
- Ages 0-3 years. Referral to an early intervention program is recommended for access to occupational, physical, speech, and feeding therapy as well as mental health services, special educators, and sensory-impairment specialists.
- 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 for those who qualify based on the established motor, language, social, and/or cognitive delay. The early intervention program typically assists with this transition.
- Ages 5-21 years. In the United States, an IEP based on the individual’s level of function should be developed by the local public school district and will dictate specially designed instruction/related services. Discussion about transition plans including financial and medical arrangements should begin at the age of 12 years. Developmental pediatricians can assist with the transition to adulthood.
Motor Dysfunction
- Gross motor dysfunction – Physical therapy is recommended to maximize mobility.Consider the use of durable medical equipment and positioning devices as needed (e.g., wheelchairs, walkers, bath chairs, orthotics, and adaptive strollers).
- Fine motor dysfunction – Occupational therapy is recommended for difficulty with fine motor skills that affect adaptive functions such as feeding, grooming, dressing, and writing.
- Oral-motor dysfunction – Oral-motor dysfunction should be reassessed at regular intervals and clinical feeding evaluations and/or radiographic swallowing studies should be obtained for choking/gagging during feeds, poor weight gain, frequent respiratory illnesses, or feeding refusal that is not otherwise explained.
- Communication issues – Speech therapy is recommended. Consider evaluation for alternative means of communication for individuals who have expressive language difficulties.
Investigational Therapies
Dr. Darius Ebrahimi-Fakhari of Boston Children’s Hospital is pursuing a drug screening experiment on cells derived from AP-4-associated HSP patients. The process begins with obtaining skin cells from the patient and the same gender parent. IPSC stem cells are developed from these skin cells and then differentiated into neurons, which can be maintained and studied in a lab. The goal of the project is to test whether various compounds which are FDA-approved for other disorders might offer some benefit to the cells affected by AP-4-associated HSPs. Potentially, the drug screening research may help identify a treatment that benefits all four of these diseases.
Molecular Pathogenesis
AP-4-associated hereditary spastic paraplegia (HSP) is caused by biallelic pathogenic variants in one of four genes (AP4B1, AP4E1, AP4M1, AP4S1) that encode subunits of the AP4 complex (β4, ε, μ4, σ4, respectively). Loss of any one subunit renders the entire heterotetrameric complex nonfunctional; hence, loss-of-function variants in any one of the four genes cause the same cellular outcome – loss of AP-4 complex function [Hirst et al 2013, Frazier et al 2016]. Reduction or loss of the mutated subunit causes a reduction in the whole-cell level of the other subunits as they are no longer able to be incorporated into a stable complex, and so are degraded by the cell. Evolutionary studies also support the obligate nature of the AP-4 complex because organisms either have all four AP-4 HSP-related genes or none at all [Hirst et al 2013].
The AP-4 complex is ubiquitously expressed in human tissues, including in the central nervous system [Hirst et al 2013]. At a steady-state, AP-4 localizes at the subcellular level to the trans-Golgi network (TGN), where it functions in the sorting of transmembrane cargo proteins into transport vesicles for TGN export. In AP-4-deficient cells these cargo proteins will be missorted and so will become mislocalized in the cell, likely affecting their function. A number of proteins have been suggested to be AP-4 cargo proteins. Evidence has emerged supporting a role for AP-4 in the post-Golgi trafficking of the autophagy protein ATG9A (see BioRxiv) [Mattera et al 2017, Davies et al 2018, De Pace et al 2018].
AP4B1
Gene structure. The predominant AP4B1 transcript NM_006594.4 consists of 11 exons. Alternatively, spliced transcript variants that encode different isoforms are known.
Normal gene product. The NM_006594.4 transcript encodes the AP-4 complex subunit beta-1 (also known as β4) predicted to be 739 amino acids in length (NP_006585.2) [Dell’Angelica et al 1999, Hirst et al 1999]. The sequence of β4 is strongly conserved through evolution with orthologs in mammals and other vertebrates. β4 assembles into protein complex AP-4 (see Molecular Pathogenesis). β4 domains include one involved in the assembly of the AP-4 complex and a second that binds an accessory protein for AP-4 known as trypsin [Frazier et al 2016].
Abnormal gene product. Most AP4B1 pathogenic variants identified to date predict truncation and/or destabilization of the protein, suggesting that pathogenicity results from the loss of function of the β4 protein [Ebrahimi-Fakhari et al 2018].
AP4E1
Gene structure. The AP4E1 transcript NM_007347.4 consists of 21 exons. Alternatively spliced transcript variants that encode different isoforms are known.
Normal gene product. The NM_007347.4 transcript encodes the AP-4 complex subunit epsilon-1 (also known as ε) predicted to be 1,137-amino acids in length (NP_031373.2) [Dell’Angelica et al 1999, Hirst et al 1999]. The sequence of ε is strongly conserved through evolution with orthologs in mammals and other vertebrates. ε assembles into protein complex AP-4 (see Molecular Pathogenesis). ε domains include one involved in the assembly of the AP-4 complex, and a second shown to bind an AP-4 accessory protein known as tepsin [Mattera et al 2015].
Abnormal gene product. Most AP4E1 pathogenic variants identified to date predict truncation and/or destabilization of the protein, suggesting that pathogenicity results from loss of function of the ε protein [Abou Jamra et al 2011].
AP4M1
Gene structure. The AP4M1 transcript NM_004722.3 has 15 exons. There are multiple splice variants.
Normal gene product. The transcript NM_004722.3 encodes AP-4 complex subunit mu-1 (also known as µ4) predicted to be 453 amino acids in length (NP_004713.2) [Dell’Angelica et al 1999, Hirst et al 1999]. The sequence of µ4 is strongly conserved through evolution with orthologs in mammals and other vertebrates. µ4 assembles into protein complex AP-4 (see Molecular Pathogenesis). The µ4 domains include one that is involved in the assembly of the AP-4 complex, and a protein-protein interaction module known as a Mu homology domain that in adaptor protein complexes binds to linear sorting motifs in transmembrane cargo proteins.
Abnormal gene product. Most AP4M1 pathogenic variants identified to date predict truncation and/or destabilization of the protein, suggesting that pathogenicity results from loss of function of the µ4 protein [Verkerk et al 2009].
AP4S1
Gene structure. AP4S1 consists of six exons. There are multiple alternative splice variants encoding different protein isoforms.
Normal gene product. The NM_007077.4 transcript encodes AP-4 complex subunit sigma-1 (also known as σ4) predicted to be 159 amino acids in length (NP_009008.2) [Dell’Angelica et al 1999, Hirst et al 1999]. The sequence of σ4 is strongly conserved through evolution with orthologs in mammalians and other vertebrates. σ4 assembles into a protein complex, named AP-4 (see Molecular Pathogenesis). σ4 has a 1-142 amino acid region that is involved in the assembly of the AP-4 complex.
Abnormal gene product. Most AP4S1 pathogenic variants identified to date predict truncation and/or destabilization of the protein, suggesting that pathogenicity results from loss of function of the σ4 protein [Abou Jamra et al 2011, Hardies et al 2015].
The differential diagnosis includes the following:
- Structural abnormalities involving the brain or spinal cord (e.g., tethered cord syndrome and spinal cord compression)
- Vascular abnormalities including arteriovenous abnormalities or fibrocartilaginous embolism
- Demyelinating disorders including primary progressive multiple sclerosis or Devi’s disease (neuromyelitis optica)
- Paraneoplastic myelopathies including those associated with anti-GAD65 antibodies
- Other motor neuron disorders such as slowly progressive amyotrophic lateral sclerosis (ALS) or primary lateral sclerosis (PLS)
- Leukodystrophies such as steadily progressive multiple sclerosis, B12 deficiency, Krabbe disease, metachromatic leukodystrophy, and adrenomyeloneuropathy
- Spinocerebellar ataxias (SCAs) including Machado-Joseph disease (SCA 3), Friedreich ataxia, spastic ataxia of Charlevoix-Saguenay, or other spastic ataxias. See Hereditary Ataxias.
- Diplegic form of cerebral palsy (CP) with corresponding MR imaging abnormalities. However, several individuals with presumed CP have had pathogenic variants in genes associated with HSP identified on molecular genetic testing associated with either an autosomal dominant or autosomal recessive inheritance [Rainier et al 2006, Hedera 2013].
- Infections including human immunodeficiency virus (HIV/AIDs), tropical spastic paraplegia (also known as human T-cell leukemia virus 1 [HTLV1] -associated myelopathy), and neurosyphilis
- Metabolic disorders (reviewed in Hedera [2016]) include homocysteine remethylation defects (due to methylene tetrahydrofolate reductase [MTHFR] deficiency), cobalamin C disease, urea cycle defects, biotinidase deficiency, phenylketonuria, glycine encephalopathy, cerebral folate deficiency, homocarnosinosis, cerebrotendinous xanthomatosis, adult polyglucosan body disease, and nucleoside phosphorylase deficiency
- Nutritional disorders (reviewed in Hedera [2016]) include copper deficiency, vitamin B12 and E deficiencies
- Early-onset dementias including frontotemporal dementia-ALS (see Amyotrophic Lateral Sclerosis Overview) and familial Alzheimer’s disease caused by pathogenic variants in PSEN1 (encoding presenilin-1), PSEN2 (encoding presenilin-2), or APP (encoding amyloid precursor protein) (see Alzheimer Disease)
- Dopa-responsive dystonia. It is important to consider dopa-responsive dystonia in all individuals – particularly children – with progressive gait disturbance. See GTP Cyclohydrolase 1-Deficient Dopa-Responsive Dystonia, Tyrosine Hydroxylase Deficiency.
References
Antley-Bixler Syndrome
