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BLOC1S3
biogenesis of lysosomal organelles complex 1 subunit 3
BLOC1S3 is located on the long (q) arm of chromosome 19, at band 19q13.32. Arm ratio per GRCh38 - banding schematic.
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Overview
BLOC1S3 is located on chromosome 19 and encodes biogenesis of lysosomal organelles complex 1 subunit 3, a 202-amino acid protein that forms part of the eight-subunit BLOC-1 complex. This complex operates as a coordinator in the biogenesis pathway for lysosome-related organelles, specialised membrane-bound compartments that perform tissue-specific functions distinct from conventional lysosomes.
Variants in BLOC1S3 follow an autosomal recessive inheritance pattern, meaning two altered copies - one inherited from each parent - are required to cause disease. Pathogenic changes in this gene result in Hermansky-Pudlak syndrome type 8, a disorder characterised by oculocutaneous albinism, prolonged bleeding due to platelet dysfunction, and in some cases additional complications affecting the lungs or intestines.
What the gene does
The BLOC1S3 protein functions within the BLOC-1 complex to facilitate the proper formation and trafficking of lysosome-related organelles. These specialised compartments include melanosomes in melanocytes, which synthesise and distribute melanin pigment, and dense granules in platelets, which store molecules essential for blood clot formation.
BLOC-1 operates primarily during the early stages of organelle maturation, coordinating the delivery of cargo proteins and the organisation of membrane structures. The complex interacts with molecular machinery that sorts proteins destined for these organelles and ensures they reach the correct intracellular location. Without functional BLOC1S3, the entire BLOC-1 complex becomes unstable or non-functional, disrupting the biogenesis pathway.
In melanocytes, this disruption prevents normal melanosome development, reducing melanin production and resulting in lighter skin, hair, and eye colour than would otherwise occur based on genetic background. In platelets, impaired dense granule formation compromises the cell's ability to release clotting factors during vascular injury, leading to prolonged bleeding times and easy bruising.
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Chromosome location
BLOC1S3 is located on the long arm of chromosome 19 at position 19q13.32. This chromosomal region contains multiple genes involved in diverse cellular processes. The precise genomic coordinates and exon structure contribute to the gene's regulation and the potential impact of structural variants affecting this locus.
Protein structure
The BLOC1S3 protein comprises 202 amino acids with a largely disordered N-terminal region spanning amino acids 1-82. This extended disordered region may provide structural flexibility important for protein-protein interactions within the BLOC-1 complex or allow the protein to adopt different conformations during organelle biogenesis. The remainder of the protein likely contains structured elements that mediate binding to other BLOC-1 subunits, though detailed structural characterisation of the full complex remains an active area of research.
Key variants
Pathogenic variants in BLOC1S3 are typically loss-of-function changes that prevent production of a functional protein or severely impair its ability to participate in the BLOC-1 complex. These include nonsense variants that introduce premature stop codons, frameshift variants caused by small insertions or deletions, and variants affecting splice sites that disrupt normal gene processing. The variant spectrum reflects the autosomal recessive nature of associated disease, with affected individuals carrying two pathogenic variants.
Sample of pathogenic variants
5 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.131C>A | p.Ser44Ter | Pathogenic | - | Hermansky-Pudlak syndrome 8 |
c.338_341del | p.Leu113fs | Pathogenic | - | Hermansky-Pudlak syndrome 8 |
c.385_403del | p.Ser129fs | Pathogenic | - | Hermansky-Pudlak syndrome 8 |
c.444_467del | p.Gln150_Ala157del | Pathogenic | - | Hermansky-Pudlak syndrome 8 |
c.448del | p.Gln150fs | Pathogenic | - | Hermansky-Pudlak syndrome 8 |
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 BLOC1S3 cause Hermansky-Pudlak syndrome type 8, a form of Hermansky-Pudlak syndrome characterised primarily by oculocutaneous albinism and platelet storage pool deficiency. Individuals with this condition typically present with reduced pigmentation of the skin, hair, and eyes compared to unaffected family members, along with vision problems including nystagmus and reduced visual acuity. Bleeding manifestations range from easy bruising to prolonged bleeding after injury or surgery. Hermansky-Pudlak syndrome type 8 is generally considered a milder form compared to some other genetic subtypes, with pulmonary fibrosis and granulomatous colitis being less commonly reported.
No disease links recorded for this gene in our reference set.
Inheritance pattern
Conditions caused by pathogenic BLOC1S3 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
BLOC1S3 is included on multiple NHS Genomic Medicine Service gene panels. It appears on the Albinism or congenital nystagmus panel (green classification, version R39), reflecting its role in pigmentation disorders. The gene is also listed on the Bleeding and platelet disorders panel and the Inherited bleeding disorders panel, both with green classifications, recognising the platelet dysfunction associated with BLOC1S3-related conditions. Green classification indicates strong evidence supporting the gene-disease relationship and its inclusion in clinical diagnostic testing pathways.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
What does it mean to be a carrier of a BLOC1S3 variant?
Carriers have one pathogenic variant and one typical copy of BLOC1S3, which is sufficient for normal BLOC-1 complex function. Carriers do not develop Hermansky-Pudlak syndrome and typically have normal pigmentation and blood clotting. However, if both parents are carriers, each pregnancy has a 25% chance of the child inheriting two pathogenic variants and being affected.
How is Hermansky-Pudlak syndrome type 8 diagnosed?
Diagnosis typically involves clinical assessment of pigmentation and bleeding history, eye examination documenting features of albinism, and platelet function studies showing absent or reduced dense granules. Genetic testing confirming biallelic pathogenic variants in BLOC1S3 provides definitive diagnosis and enables carrier testing for at-risk family members.
Is BLOC1S3 tested on the NHS?
Yes, BLOC1S3 is included on NHS Genomic Medicine Service panels for albinism, congenital nystagmus, and inherited bleeding disorders. Testing may be arranged through clinical genetics services when an individual's symptoms suggest Hermansky-Pudlak syndrome or a related condition affecting pigmentation or platelet function.