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AP3D1
adaptor related protein complex 3 subunit delta 1
AP3D1 is located on the short (p) arm of chromosome 19, at band 19p13.3. Arm ratio per GRCh38 - banding schematic.
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Overview
AP3D1 is located on chromosome 19 at band p13.3 and encodes a 1,153 amino acid protein that functions as the delta subunit of adaptor protein complex 3. This complex plays a critical role in the formation and trafficking of lysosome-related organelles, specialised compartments found in many cell types. Melanocytes require functional AP-3 to build melanosomes, the pigment-containing structures responsible for skin and hair colour, whilst platelets depend on it to form dense granules that store molecules essential for blood clotting. When both copies of AP3D1 carry pathogenic variants, the resulting loss of AP-3 function impairs these processes, producing the clinical features of Hermansky-Pudlak syndrome type 10.
What the gene does
The AP3D1 protein serves as the large delta subunit of the heterotetrameric AP-3 complex, which also comprises beta, mu, and sigma subunits. This complex recognises specific sorting signals on cargo proteins and facilitates their packaging into vesicles destined for lysosome-related organelles. The delta subunit contributes to the structural scaffold that enables AP-3 to bind both cargo proteins and the lipid membranes of donor compartments, coordinating the assembly of transport vesicles. In melanocytes, AP-3 directs enzymes involved in melanin synthesis to developing melanosomes, ensuring proper pigment production. In platelets, the complex is essential for delivering components to dense granules, which release adenosine diphosphate and serotonin during clot formation. Beyond these well-characterised roles, AP-3 participates in trafficking to other specialised compartments, including lytic granules in immune cells, highlighting its broad importance in cellular organisation.
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Chromosome location
AP3D1 is situated on the short arm of chromosome 19 at cytogenetic band p13.3. This region lies near the telomere of chromosome 19p and contains numerous genes involved in diverse cellular processes. The precise chromosomal position places AP3D1 within a gene-dense interval that has been well characterised through genome sequencing projects.
Protein structure
The AP3D1 protein comprises 1,153 amino acids and features an architecture dominated by HEAT repeats, a structural motif consisting of paired alpha helices that stack to form elongated, flexible scaffolds. The protein contains eleven consecutive HEAT repeats spanning amino acids 34 through 585 (HEAT 1 through HEAT 11), which together create the characteristic solenoid structure required for protein-protein interactions within the AP-3 complex. These repeats enable the delta subunit to interact with other AP-3 components and assist in recognising cargo molecules. The C-terminal half of the protein includes two coiled-coil regions at amino acids 659-679 and 725-756, which likely contribute to oligomerisation or interactions with trafficking machinery. Extensive disordered regions are present at amino acids 629-696 and 726-920, segments that may provide conformational flexibility during vesicle assembly and cargo recognition.
Key variants
Pathogenic variants in AP3D1 are inherited in an autosomal recessive pattern, meaning that affected individuals carry alterations in both gene copies. Loss-of-function variants, including nonsense and frameshift changes, are commonly observed and result in absent or severely truncated protein products. Missense variants affecting the HEAT repeat domain can disrupt the structural integrity of the delta subunit, impairing AP-3 complex assembly or cargo recognition. The clinical severity of Hermansky-Pudlak syndrome type 10 generally reflects the degree to which AP-3 function is compromised.
Sample of pathogenic variants
1 pathogenic / likely-pathogenic variants from ClinVar, ranked by review status (expert-panel-reviewed first). This is a sample; recurrent founder variants in a specific population may not appear here - see the full ClinVar listing via the link above.
| Variant (HGVS) | Protein change | Classification | Evidence | Associated condition |
|---|---|---|---|---|
c.3565_3566del | p.Val1189fs | Pathogenic | - | Hermansky-Pudlak syndrome 10 |
Evidence stars indicate ClinVar review status. Individual variant interpretation should always be performed by a qualified clinical laboratory - many variants remain classified as Variants of Uncertain Significance (VUS) pending more research.
Associated conditions
Biallelic pathogenic variants in AP3D1 cause Hermansky-Pudlak syndrome type 10, a rare autosomal recessive condition characterised primarily by oculocutaneous albinism and a bleeding diathesis. Individuals typically present with reduced pigmentation of the skin, hair, and eyes due to defective melanosome formation, alongside prolonged bleeding times resulting from platelet dense granule deficiency. Unlike some other Hermansky-Pudlak syndrome subtypes, type 10 is generally not associated with pulmonary fibrosis or granulomatous colitis, though the full phenotypic spectrum continues to be defined as additional cases are reported.
No disease links recorded for this gene in our reference set.
Inheritance pattern
Conditions caused by pathogenic AP3D1 variants typically follow autosomal recessive inheritance.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
UK clinical status
AP3D1 is included on several NHS Genomic Medicine Service gene panels, reflecting its clinical relevance in diagnosing inherited bleeding and platelet disorders. The gene appears on the Bleeding and platelet disorders panel (rated green, version R90) and the Inherited bleeding disorders panel (rated green), indicating strong evidence supporting its role in these conditions. Green classification denotes high confidence that variants in this gene are causative for the associated phenotypes, guiding diagnostic testing in the NHS.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
What is the inheritance pattern for AP3D1-related conditions?
AP3D1-related Hermansky-Pudlak syndrome type 10 follows an autosomal recessive inheritance pattern. This means an individual must inherit a pathogenic variant from both parents to develop the condition, whilst carriers with one variant typically remain unaffected.
How does AP3D1 affect pigmentation?
The AP3D1 protein is essential for trafficking enzymes and structural proteins to melanosomes, the organelles where melanin pigment is synthesised and stored. When AP3D1 function is lost, melanosomes fail to mature properly, resulting in reduced pigmentation of the skin, hair, and eyes.
Why do AP3D1 variants cause bleeding problems?
Platelets require functional AP-3 complex to form dense granules, specialised storage compartments that release clotting factors when a blood vessel is injured. Pathogenic AP3D1 variants prevent normal dense granule formation, reducing the platelet's ability to contribute to effective clot formation and leading to prolonged bleeding.