A deleterious mutation in DNAJC6 encoding the neuronal-specific clathrin-uncoating co-chaperone auxilin, is associated with juvenile parkinsonism

Abstract

Parkinson disease is caused by neuronal loss in the substantia nigra which manifests by abnormality of movement, muscle tone, and postural stability. Several genes have been implicated in the pathogenesis of Parkinson disease, but the underlying molecular basis is still unknown for ∼70% of the patients. Using homozygosity mapping and whole exome sequencing we identified a deleterious mutation in DNAJC6 in two patients with juvenile parkinsonism. The mutation was associated with abnormal transcripts and marked reduced DNAJC6 mRNA level. DNAJC6 encodes the HSP40 Auxilin, a protein which is selectively expressed in neurons and confers specificity to the ATPase activity of its partner Hcs70 in clathrin uncoating. In Auxilin null mice it was previously shown that the abnormally increased retention of assembled clathrin on vesicles and in empty cages leads to impaired synaptic vesicle recycling and perturbed clathrin mediated endocytosis. Endocytosis function, studied by transferring uptake, was normal in fibroblasts from our patients, likely because of the presence of another J-domain containing partner which co-chaperones Hsc70-mediated uncoating activity in non-neuronal cells. The present report underscores the importance of the endocytic/lysosomal pathway in the pathogenesis of Parkinson disease and other forms of parkinsonism.

Figure 1. The c.801 −2 A−>G mutation in the DNAJC6 gene.

The green arrow points at the first nucleotide of exon 7 and the mutation affects the preceding AG splice acceptor site of intron 6 which is changed to GG in the patient (A). The sequence of an obligate heterozygote is shown in (B) and that of a control in (C). Schematic representation of the mutation site at the genomic level (D) and its impact on the cDNA (E). Chromatogram of cDNA from a patient encompassing the 3′ junction of exon 6 (F) and demonstrating a transcript lacking exon 7 and another transcript where nNext to the last base of exon 6 (blue arrow) overlapping exon 8 sequence is the intronic sequences from intron 6 (c.801 −91). The normal exon 6/exon 7 spliced form is undetectable.

Figure 2. Schematic representation of clathrin-mediated endocytosis.

Plasma membrane molecules (in this case the dopamine receptor) associate with nascent clathrin-coated pits which then mature invaginate and finally pinch off to form clathrin-coated vesicles. The shedding of the coat takes place after the vesicle buds from the plasma membrane. This process is driven by Hsc70 ATP hydrolysis activity which is recruited to clathrin coats by Auxilin. The uncoated vesicle fuses with the membrane of a target compartment and delivers its cargo. Clathrin molecules are directed to the plasma membrane for re-use.

Figure 3. Family pedigree.

The patients are represented by filled symbols.