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ADAPTOR-RELATED PROTEIN COMPLEX 4, EPSILON-1 SUBUNIT; AP4E1

ADAPTOR-RELATED PROTEIN COMPLEX 4, EPSILON-1 SUBUNIT; AP4E1

HGNC Approved Gene Symbol: AP4E1Cytogenetic location: 15q21.2 Genomic coordinates (GRCh38): 15:50,908,568-51,005,899 (from NCBI)▼ DescriptionThe heterotetram...

HGNC Approved Gene Symbol: AP4E1

Cytogenetic location: 15q21.2 Genomic coordinates (GRCh38): 15:50,908,568-51,005,899 (from NCBI)

▼ Description
The heterotetrameric adaptor protein (AP) complexes sort integral membrane proteins at various stages of the endocytic and secretory pathways. AP4 is composed of 2 large chains, beta-4 (AP4B1; 607245) and epsilon-4 (AP4E1), a medium chain, mu-4 (AP4M1; 602296), and a small chain, sigma-4 (AP4S1; 607243).

▼ Cloning and Expression
By genomic sequence analysis, Dell'Angelica et al. (1999) identified AP4E1 within an EST and found that the deduced protein shares 25 to 30% identity with the trunk region of the AP subunits alpha (see AP2A1; 601026), gamma (see AP1G1; 603533), and delta (AP3D1). Northern blot analysis revealed a transcript of 7 kb. By gel filtration and Western blot analysis of human fibroblast cytosol, Dell'Angelica et al. (1999) found that AP4E1 shows an apparent molecular mass of about 140 kD and is part of a 280-kD complex containing AP4B1, AP4S1, and AP4M1. By immunoprecipitation and reprecipitation of HeLa cell lysates, they confirmed interaction between AP4B1 and AP4E1. Immunolocalization of the AP4B1 subunit in HeLa cells revealed that the AP4 complex associates with the trans-Golgi network or an adjacent structure. The association was sensitive to brefeldin-A treatment, indicating that the membrane localization of AP4 is dependent upon the small GTP-binding protein ARF1 (103180).

Hirst et al. (1999) identified AP4E1 within an EST from human testis and cloned the full-length cDNA by screening a human heart cDNA library. The deduced protein has a calculated molecular mass of about 127 kD. Within the first 600 amino acids, AP4E1 has homology with the AP subunits gamma, alpha, and delta, including conserved stretches that include a KRIGYL motif and a WIIGEY motif. AP4E1 contains a conserved 600-amino acid N-terminal domain, a hinge domain, and a C-terminal 'ear' domain. Northern blot analysis revealed ubiquitous but weak expression of a 7.5-kb transcript. By coimmunoprecipitation of pig brain cytosol, Hirst et al. (1999) confirmed an interaction between AP4E1 and AP4B1. Using yeast 2-hybrid analysis, they found a strong and specific interaction between AP4E1 and AP4S1.

Abou Jamra et al. (2011) found ubiquitous AP4S1 expression in all fetal and adult brain structures examined.

▼ Molecular Genetics
Autosomal Recessive Spastic Paraplegia 51

In 2 sibs, born of consanguineous Palestinian Jordanian parents, with spastic paraplegia-51 (SPG51; 613744), Moreno-De-Luca et al. (2011) identified a homozygous 192-kb deletion on chromosome 15q21.2 (chr15: 48,835,480-49,028,171) that included the 5-prime end of the AP4E1 gene and the 5-prime end of the SPPL2A gene (608238). Noting that mutation in the AP4M1 gene (602296), which forms a complex with AP4E1, causes a similar phenotype (SPG50; 612936), Moreno-De-Luca et al. (2011) concluded that disruption of the AP4E1 gene was responsible for the phenotype in their family, although they could not exclude a possible role for disruption of the SPPL2A gene. The authors proposed the designation 'AP4 deficiency syndrome' to refer to disorders caused by disruption of any of the 4 subunits of the AP4 complex.

By linkage analysis followed by candidate gene sequencing in a consanguineous Syrian family with mental retardation and spasticity, Abou Jamra et al. (2011) identified a homozygous truncating mutation in the AP4E1 gene (607244.0002). The authors concluded that AP4-complex-mediated vesicular trafficking plays a crucial role in brain development and function.

By homozygosity mapping followed by exon enrichment and next-generation sequencing in 136 consanguineous families (over 90% Iranian; less than 10% Turkish or Arab) segregating syndromic or nonsyndromic forms of autosomal recessive intellectual disability, Najmabadi et al. (2011) identified homozygosity for a frameshift mutation in the AP4E1 gene (607244.0003) in 3 affected members of a consanguineous family segregating SPG51.

In a pair of monozygotic twin sisters, born of consanguineous Moroccan parents, with SPG51, Kong et al. (2013) identified a homozygous nonsense mutation in the AP4E1 gene (R1105X; 607244.0005). The mutation, which was found by whole-exome sequencing and confirmed by Sanger sequencing, segregated with the disorder in the family. It was not present in 1,050 healthy controls or in several control databases. Analysis of patient cells showed normal AP4E1 mRNA levels, but barely detectable protein levels, suggesting an unstable mutant protein. There was also a severe impairment of AP4 complex formation compared to controls.

Familial Persistent Stuttering 1

In affected members of a large family of Cameroon descent (CAMST01) with autosomal dominant familial persistent stuttering-1 (STUT1; 184450), originally reported by Raza et al. (2013), Raza et al. (2015) identified heterozygosity for 2 missense variants in the AP4E1 gene that occurred in cis (V517I and E801K; 607244.0004). The mutations were found by whole-exome sequencing and segregated with the disorder in the family. The same mutations on the same haplotype were subsequently found in 2 of 96 additional individuals from Cameroon with stuttering. Sequencing of the AP4E1 gene in unrelated affected individuals, including 93 from Cameroon, 132 from Pakistan, and 711 from North America, revealed 23 other rare variants in this gene, including small (1- to 3-bp) deletions, insertions, duplications, frameshifts, and stop codons; no truncating mutations were found in 558 ethnically matched control individuals. Investigation of large population databases, including the 1000 Genomes Project and Exome Sequencing Project, which are not phenotyped for speech fluency, found 3 loss-of-function AP4E1 variants among about 19,000 chromosomes, as well as several rare missense variants; all of these occurred at a significantly lower frequency compared to the variants observed in stuttering individuals. Studies in a yeast 2-hybrid system supported a direct interaction between the AP4 complex and NAGPA (607985), variations in which have also been implicated in stuttering (STUT2; 609261). The findings suggested that defects in intracellular trafficking play a role in persistent stuttering.

▼ Animal Model
De Pace et al. (2018) found that Ap4e1-knockout mice exhibited a range of neurologic phenotypes, including hindlimb clasping, decreased motor coordination, and weak grip strength. In addition, the knockout mice had thin corpus callosum and axonal swellings in various regions of brain and spinal cord. Knockout mice showed accumulation of Atg9a (612204) at the trans-Golgi network (TGN) in various neuronal types. Similar accumulation of ATG9A was observed in TGN of skin fibroblasts from human patients with AP4M1 mutations. The results indicated that AP4 deficiency impaired delivery of ATG9A from the Golgi complex to the cell periphery. This defect was associated with increased tendency to accumulate mutant huntingtin aggregates in axons of Ap4e1-knockout neurons.

▼ ALLELIC VARIANTS ( 5 Selected Examples):

.0001 SPASTIC PARAPLEGIA 51, AUTOSOMAL RECESSIVE
AP4E1, 192-KB DEL
In 2 sibs, born of consanguineous Palestinian Jordanian parents, with spastic paraplegia-51 (SPG51; 613744), Moreno-De-Luca et al. (2011) identified a homozygous 192-kb deletion on chromosome 15q21.2 (chr15: 48,835,480-49,028,171) that included the 5-prime end of the AP4E1 gene and the 5-prime end of the SPPL2A gene (608238). The unaffected mother was heterozygous for the deletion, but no DNA was available from the father, who was inferred to be heterozygous for the deletion. Moreno-De-Luca et al. (2011) concluded that disruption of the AP4E1 gene was responsible for the phenotype, although they could not exclude a possible role for disruption of the SPPL2A gene.

.0002 SPASTIC PARAPLEGIA 51, AUTOSOMAL RECESSIVE
AP4E1, 4-BP DEL, IVS5DS, 542+1GTAA
In 2 affected members of a consanguineous Syrian family with mental retardation and spasticity (SPG51; 613744), Abou Jamra et al. (2011) identified a homozygous 4-bp deletion (542+1delGTAA) in the donor splice site of intron 5 of the AP4E1 gene, resulting in the skipping of exon 5, a frameshift, and premature termination of the protein in exon 6. The mutation was not found in 740 control chromosomes.

.0003 SPASTIC PARAPLEGIA 51, AUTOSOMAL RECESSIVE
AP4E1, VAL454FS
In family M254, in which 3 of 4 children of first-cousin parents had mental retardation, microcephaly, and spastic paraplegia (SPG51; 613744), Najmabadi et al. (2011) identified a causative homozygous 2-bp insertion at genomic coordinate chr15:49029357 (NCBI36), resulting in a frameshift at codon 454.

.0004 STUTTERING, FAMILIAL PERSISTENT, 1
AP4E1, VAL517ILE AND GLU801LYS
In affected members of a large family of Cameroon descent (CAMST01) with autosomal dominant familial persistent stuttering-1 (STUT1; 184450), originally reported by Raza et al. (2013), Raza et al. (2015) identified heterozygosity for 2 variants in the AP4E1 gene that occurred in cis: a c.1549G-A transition (c.1549G-A, NM_007347.4) in exon 14, resulting in a val517-to-ile (V517I) substitution in the trunk domain, and a c.2401G-A transition (c.2401G-A, NM_00737.4) in exon 18, resulting in a glu801-to-lys (E801K) substitution in the hinge domain. The mutations, which were found by whole-exome sequencing, segregated with the disorder in the family. Both mutations occurred at highly conserved residues and were not found in the 1000 Genomes Project or Exome Sequencing Project databases or in 558 controls. Sequencing of 96 unrelated affected Cameroonians identified the same 2 variants in 2 individuals; haplotype analysis indicated a founder effect between these individuals and the family. In vitro functional expression studies in HEK293 cells showed that the mutations resulted in slightly decreased assembly of the AP4 complex (about 80% of wildtype).

.0005 SPASTIC PARAPLEGIA 51, AUTOSOMAL RECESSIVE
AP4E1, ARG1105TER
In a pair of monozygotic twin sisters, born of consanguineous Moroccan parents, with spastic paraplegia-51 (SPG51; 613744), Kong et al. (2013) identified a homozygous C-to-T transition in the AP4E1 gene, resulting in an arg1105-to-ter (R1105X) substitution. The mutation, which was found by whole-exome sequencing and confirmed by Sanger sequencing, segregated with the disorder in the family. It was not present in 1,050 healthy controls or in several control databases. Analysis of patient cells showed normal AP4E1 mRNA levels, but barely detectable protein levels, suggesting an unstable mutant protein. There was also a severe impairment of AP4 complex formation compared to controls. In addition to SPG51, the patients had IMD86 (619549) caused by a homozygous mutation in the SPPL2A gene (608238.0001).

Tags: 15q21.2