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AP3B1
adaptor related protein complex 3 subunit beta 1
AP3B1 is located on the long (q) arm of chromosome 5, at band 5q14.1. Arm ratio per GRCh38 - banding schematic.
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Overview
The AP3B1 gene is located on chromosome 5 and encodes the beta-3A subunit of adaptor protein complex-3 (AP-3), a heterotetrameric complex that plays a central role in intracellular membrane trafficking. AP-3 recognises specific sorting signals on cargo proteins and packages them into clathrin-coated vesicles for delivery to lysosomes and lysosome-related organelles. These organelles include melanosomes in pigment cells, dense granules in platelets, and lytic granules in immune cells.
Pathogenic variants in AP3B1 are inherited in an autosomal recessive pattern and cause Hermansky-Pudlak syndrome type 2, a rare condition affecting pigmentation, blood clotting, and immune function. The gene is included on multiple NHS Genomic Medicine Service panels, reflecting its relevance to disorders of pigmentation, bleeding, and immunity.
What the gene does
The AP3B1 protein forms part of the core scaffold of the AP-3 complex, working alongside three other subunits to identify cargo destined for lysosome-related organelles. The complex binds to tyrosine-based and dileucine-based sorting motifs on transmembrane proteins, recruiting them into budding vesicles at the trans-Golgi network and early endosomes. This sorting process is essential for the proper assembly and function of organelles such as melanosomes, which store melanin pigment, and platelet dense granules, which release molecules required for blood clotting.
Without functional AP3B1, cargo proteins fail to reach their intended destinations, resulting in defective organelle formation. Melanocytes produce smaller, irregularly shaped melanosomes with reduced melanin content, leading to lighter skin and hair colour. Platelets lack normal dense granules, impairing their ability to aggregate at sites of vascular injury. Immune cells exhibit reduced cytotoxic activity due to abnormal lytic granule formation, increasing susceptibility to infection.
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Chromosome location
AP3B1 is located on the long arm of chromosome 5 at position 14.1 (5q14.1). The gene spans approximately 190 kilobases of genomic DNA and contains 28 exons. The encoded protein comprises 1,094 amino acids and has a predicted molecular weight of approximately 121 kilodaltons.
Protein structure
The AP3B1 protein contains three disordered regions, which likely provide structural flexibility for interaction with other adaptor complex subunits and membrane-associated cargo. The first disordered region spans amino acids 1 to 31 at the N-terminus, the second extends from amino acids 268 to 292, and the third encompasses a larger segment from amino acids 662 to 811. These intrinsically disordered regions may facilitate conformational changes required for cargo recognition and vesicle coat assembly.
Key variants
Pathogenic variants in AP3B1 typically result in loss of protein function through nonsense mutations, frameshift deletions, or splice-site alterations that lead to premature termination. Missense variants affecting critical residues involved in subunit assembly or cargo binding have also been reported. Because the condition follows autosomal recessive inheritance, affected individuals carry biallelic pathogenic variants, whilst carriers with a single variant typically show no clinical features.
Sample of pathogenic variants
10 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.177del | p.Lys59fs | Pathogenic | ★★☆☆ | Autoinflammatory syndrome |
c.1945C>T | p.Arg649Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Hermansky-Pudlak syndrome 2 |
c.2640del | p.Gly881fs | Pathogenic/Likely pathogenic | ★★☆☆ | Hermansky-Pudlak syndrome 2 |
c.3015_3016dup | p.Thr1006fs | Pathogenic/Likely pathogenic | ★★☆☆ | Hermansky-Pudlak syndrome 2 |
c.310C>T | p.Arg104Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Hermansky-Pudlak syndrome 2 |
c.1219C>T | p.Arg407Ter | Pathogenic | ★☆☆☆ | Hermansky-Pudlak syndrome 2 |
c.1344_1347dup | p.Leu450fs | Pathogenic | ★☆☆☆ | Hermansky-Pudlak syndrome 2 |
c.1979G>A | p.Trp660Ter | Pathogenic | ★☆☆☆ | Hermansky-Pudlak syndrome 2 |
c.2470+1G>A | - | Pathogenic | ★☆☆☆ | Hermansky-Pudlak syndrome 2 |
c.2769del | p.Lys923fs | Pathogenic | ★☆☆☆ | Hermansky-Pudlak syndrome 2 |
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 AP3B1 cause Hermansky-Pudlak syndrome type 2, a multisystem disorder characterised by oculocutaneous albinism, prolonged bleeding due to platelet dysfunction, and immunodeficiency. Individuals present with reduced skin and hair pigmentation, nystagmus, and increased bruising or bleeding tendency. Some affected individuals experience recurrent infections due to impaired natural killer cell and T-lymphocyte function. Unlike some other forms of Hermansky-Pudlak syndrome, pulmonary fibrosis and granulomatous colitis are not prominent features of the type 2 variant, though variable expression has been observed.
No disease links recorded for this gene in our reference set.
Inheritance pattern
Conditions caused by pathogenic AP3B1 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
AP3B1 is included on several green-rated NHS Genomic Medicine Service gene panels, reflecting strong evidence for its clinical validity. These include panels for albinism or congenital nystagmus, bleeding and platelet disorders, inherited bleeding disorders, intellectual disability, pigmentary skin disorders, and primary immunodeficiency or monogenic inflammatory bowel disease. The gene also appears on panels for familial pulmonary fibrosis and autophagy disorders, though its association with these phenotypes is less well-established. Green-rated status indicates that AP3B1 variants are considered reportable findings in diagnostic genomic testing for these conditions within the NHS.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
What is the difference between being a carrier and being affected by an AP3B1-related condition?
Carriers have one pathogenic variant in AP3B1 and typically show no symptoms, whilst affected individuals inherit pathogenic variants from both parents and develop Hermansky-Pudlak syndrome type 2. Carrier screening can identify individuals at risk of having affected children.
Why does AP3B1 dysfunction affect both pigmentation and blood clotting?
The AP-3 complex delivers essential cargo proteins to multiple types of lysosome-related organelles, including melanosomes in pigment cells and dense granules in platelets. When AP3B1 is non-functional, both organelle types fail to form properly, causing reduced pigmentation and impaired platelet aggregation.
Is AP3B1 testing available on the NHS?
AP3B1 is included on multiple NHS Genomic Medicine Service panels for conditions such as albinism, bleeding disorders, and immunodeficiency. Testing is typically arranged through clinical genetics services when an individual's symptoms suggest a diagnosis within these categories.