Alternative titles; symbolsDJC6AUXILIN, BOVINE, HOMOLOG OFKIAA0473HGNC Approved Gene Symbol: DNAJC6Cytogenetic location: 1p31.3 Genomic coordinates (GRCh38):...
Alternative titles; symbols
HGNC Approved Gene Symbol: DNAJC6
Cytogenetic location: 1p31.3 Genomic coordinates (GRCh38): 1:65,264,748-65,415,870 (from NCBI)
The DNAJC6 gene encodes auxilin, a neuronal protein that functions specifically in the pathway of clathrin-mediated endocytosis. It shares homology with the ubiquitously expressed GAK (602052), and both proteins act as cochaperones to support the HSC70 (600816)-dependent clathrin uncoating of clathrin-coated vesicles (summary by Yim et al., 2010).
DNAJC6 belongs to the evolutionarily conserved DNAJ/HSP40 family of proteins, which regulate molecular chaperone activity by stimulating ATPase activity. DNAJ proteins may have up to 3 distinct domains: a conserved 70-amino acid J domain, usually at the N terminus, a glycine/phenylalanine (G/F)-rich region, and a cysteine-rich domain containing 4 motifs resembling a zinc finger domain (Ohtsuka and Hata, 2000).
▼ Cloning and Expression
By sequencing clones obtained from a size-fractionated brain cDNA library, Seki et al. (1997) cloned DNAJC6, which they designated KIAA0473. The predicted protein contains 913 amino acids and shares 95% amino acid identity with bovine auxilin (Ishikawa et al., 1997). Using RT-PCR, Ishikawa et al. (1997) detected expression of DNAJC6 in all tissues tested, with predominant expression in brain.
Ohtsuka and Hata (2000) stated that the 910-amino acid bovine auxilin protein, which is homologous to DNAJC6, has a J domain at residues 841 to 910, but it does not have G/F-rich or cys-rich domains. PSORT analysis suggested both a nuclear and cytoplasmic localization.
Koroglu et al. (2013) found abundant expression of the DNAJC6 gene in various human brain regions, including cerebellum, corpus callosum, cortex, striatum, brainstem, pons, putamen, spinal cord, and substantia nigra. There was very low expression in nonneural tissues, such as leukocytes, liver, adipose tissue, skeletal muscle, and bone marrow.
▼ Biochemical Features
Fotin et al. (2004) used electron cryomicroscopy to determine the 12-angstrom resolution structures of in vitro-assembled clathrin (see 118955) coats in association with a C-terminal fragment of auxilin that contains both the clathrin-binding region and the J domain. They located the auxilin fragment by computing differences between these structures and those lacking auxilin. Auxilin binds within the clathrin lattice near contacts between inward-projecting C-terminal helical tripod and the crossing of 2 'ankle' segments; it also contacts the terminal domain of yet another clathrin 'leg.' Auxilin therefore recruits HSC70 (600816) to the neighborhood of a set of critical interactions. Auxilin binding produces a local change in heavy-chain contacts, creating a detectable global distortion of the clathrin coat. Fotin et al. (2004) proposed a mechanism by which local destabilization of the lattice promotes general uncoating.
By radiation hybrid analysis, Ishikawa et al. (1997) mapped the DNAJC6 gene to chromosome 1.
▼ Gene Function
Auxilin is selectively expressed in neurons and is enriched in nerve terminals, where it plays a role in clathrin-mediated endocytosis (summary by Edvardson et al., 2012).
▼ Molecular Genetics
In 2 brothers, born of consanguineous Palestinian parents, with juvenile-onset Parkinson disease-19A (PARK19A; 615528), Edvardson et al. (2012) identified a homozygous loss-of-function mutation in the DNAJC6 gene (608375.0001). The mutation was found by homozygosity mapping combined with whole-exome sequencing. Because the DNAJC6 gene plays a role in clathrin-mediated endocytosis, the findings suggested that a defect in the neuronal endocytic/lysosomal pathway contributes to the pathogenesis of Parkinson disease. Edvardson et al. (2012) noted that other significant genes mutated in Parkinson disease (PD), including SNCA (163890) and LRRK2 (609007), participate in synaptic vesicle recycling, underscoring the role of the endosomal/lysosomal pathway in PD. Mutations in the DNAJC6 gene were not found in 15 additional patients with early-onset Parkinson disease.
Koroglu et al. (2013) identified a homozygous loss-of-function mutation in the DNAJC6 gene (608375.0002) in affected members of a consanguineous Turkish family with severe juvenile-onset PARK19A and mental retardation. The mutation was found by homozygosity mapping and whole-exome sequencing.
In affected members of 2 unrelated families with early-onset PARK19B (see 615528), Olgiati et al. (2016) identified 2 different homozygous mutations in the DNAJC6 gene: a missense mutation (R927G; 608375.0003) and a putative splice site mutation (c.2223A-T; 608375.0004). Patient fibroblasts from both families showed significantly decreased, but detectable, levels of DNAJC6 compared to controls, suggesting a loss of function effect. The patients had onset of symptoms between the third and fifth decades of life, features of classic PD, slow disease progression, and good response to dopaminergic medication. Olgiati et al. (2016) noted that the phenotype in these patients was not as severe as that observed in patients with truncating mutations, suggesting that some residual activity may mitigate the phenotype. The families accounted for 2 (2.2%) of 92 probands with autosomal recessive PD who underwent sequencing of the DNAJC6 gene.
▼ Animal Model
Yim et al. (2010) found that Dnajc6-null mice had a high rate of early postnatal mortality, although surviving pups had a normal life span despite decreased body weight. Knockout mice had impaired synaptic vesicle recycling, with an increased number of clathrin-coated vesicles, and impaired clathrin-mediated endocytosis of synaptic vesicles in neuronal culture. There was an upregulation of Gak, but this did not fully compensate for the lack of Dnajc6.
In brains of Dnajc6-null mice, Edvardson et al. (2012) did not find alteration in substantia nigra morphology or dopamine transporter abundance or distribution, in agreement with the lack of gait or movement abnormalities in the mutant mice.
▼ ALLELIC VARIANTS ( 5 Selected Examples):
.0001 PARKINSON DISEASE 19A, JUVENILE-ONSET
DNAJC6, IVS6AS, A-G, -2
In 2 brothers, born of consanguineous Arab-Muslim parents of Palestinian origin, with juvenile-onset Parkinson disease-19A (PARK19A; 615528), Edvardson et al. (2012) identified a homozygous c.801-2A-G transition in intron 6 of the DNAJC6 gene. Analysis of patient cells showed that the mutation resulted in the generation of 2 misspliced transcripts: an in-frame exon 7-skipped transcript lacking residues 268-328, and an out-of-frame transcript with an insertion of the last 91 nucleotides of intron 6, resulting in the addition of 8 residues before a termination codon. The normally spliced transcript was undetectable, and the mutant transcripts were unstable, consistent with a loss of function. The mutation was found by homozygosity mapping combined with whole-exome sequencing. The mutation was confirmed by Sanger sequencing and segregated with the disorder in the family. It was not found in the 1000 Genomes Project or Exome Sequencing Project databases, or in 208 ethnically matched controls. Heterozygous family members had no neurologic deficits.
Jesus et al. (2014) did not find the c.801-2A-G mutation in 356 patients with Parkinson disease from southern Spain, including 21 patients with juvenile-onset PD (before age 35 years) and 36 patients with early-onset PD (between 35 and 45 years).
.0002 PARKINSON DISEASE 19A, JUVENILE-ONSET
In 4 affected individuals from a large, highly consanguineous Turkish family with juvenile-onset Parkinson disease-19A (PARK19A; 615528) and mental retardation, Koroglu et al. (2013) identified a homozygous c.2200C-T transition in exon 16 of the DNAJC6 gene, resulting in a gln734-to-ter (Q734X) substitution, truncation of the protein by about 20%, and a null allele. The mutation was found by homozygosity mapping combined with whole-exome sequencing.
.0003 PARKINSON DISEASE 19B, EARLY-ONSET
In 2 sibs, born of Dutch parents (family GPS-0313), with Parkinson disease-19B (PARK19B; see 615528), Olgiati et al. (2016) identified a homozygous c.2779A-G transition (c.2779A-G, NM_001256864.1) in the DNAJC6 gene, resulting in an arg927-to-gly (R927G) substitution at a highly conserved residue in the J domain, which is a known crucial functional domain. The mutation, which was found by Sanger sequencing of the DNAJC6 gene, linkage analysis, and exome sequencing, segregated with the disorder in the family and was not found in the dbSNP (build 141), 1000 Genomes Project, Exome Variant Server (ESP6500S1-V2), or ExAC databases. The patients developed Parkinson disease in the third decade of life. Patient fibroblasts showed significantly decreased, but detectable, levels of DNAJC6 compared to controls, suggesting a loss-of-function effect.
.0004 PARKINSON DISEASE 19B, EARLY-ONSET
In 2 sibs, born of parents (family PAL-50) from southern Brazil, with Parkinson disease-19B (PARK19B; see 615528), Olgiati et al. (2016) identified a homozygous A-to-T transversion in the DNAJC6 gene located 5 bases before the end of exon 15 (c.2223A-T, NM_001256864.1) and predicted to result in a splicing defect. The mutation, which was found by Sanger sequencing of the DNAJC6 gene, linkage analysis, and exome sequencing, segregated with the disorder in the family and was not found in the dbSNP (build 141), 1000 Genomes Project, Exome Variant Server (ESP6500S1-V2), or ExAC databases. If translated, the mutation would result in a synonymous change, thr741-to-thr (T741T). Both patients also carried a heterozygous missense variant (L363P) in the GBA gene (606463), variants in which are known to be risk factors for Parkinson disease. The patients developed Parkinson disease at 42 and 31 years of age. Patient fibroblasts showed significantly decreased, but detectable, levels of DNAJC6 compared to controls, suggesting a loss-of-function effect.
.0005 PARKINSON DISEASE 19A, JUVENILE-ONSET
In a girl, born of consanguineous parents of Yemeni origin, with onset of Parkinson disease-19A (PARK19A; 615528) at age 10.5 years, Elsayed et al. (2016) identified a homozygous c.2365C-T transition in exon 16 of the DNAJC6 gene, resulting in a gln789-to-ter (Q789X) substitution. The mutation, which was found by targeted sequencing of a PD panel of genes and confirmed by Sanger sequencing, segregated with the disorder in the family. Functional studies of the variant and studies of patient cells were not performed, but the mutation was predicted to result in nonsense-mediated mRNA decay and a complete loss of protein function.