title: Disruption of AP3B1 by a chromosome 5 inversion: a new disease mechanism in Hermansky-Pudlak syndrome type 2.
abstract: BACKGROUND
  Hermansky-Pudlak syndrome 2 (HPS2; OMIM #608233) is a rare, autosomal recessive disorder caused by loss-of-function genetic variations affecting AP3B1, which encodes the β3A subunit of the adaptor-related protein complex 3 (AP3). Phenotypic characteristics include reduced pigmentation, absent platelet dense granule secretion, neutropenia and reduced cytotoxic T lymphocyte (CTL) and natural killer (NK) cell function. To date HPS2 has been associated with non-synonymous, stop-gain or deletion-insertion nucleotide variations within the coding region of AP3B1.
  
  
  METHODS
  We describe a consanguineous female infant with reduced pigmentation, neutropenia and recurrent infections. Platelets displayed reduced aggregation and absent ATP secretion in response to collagen and ADP, indicating a platelet dense granule defect. There was increased basal surface expression of CD107a (lysosome-associated membrane protein 1(LAMP-1)) on NK cells and CTLs from the study subject and a smaller increase in the percentage of CD107a positive cells after stimulation compared to most healthy controls. Immunoblotting of protein extracts from EBV-transformed lymphoblasts from the index case showed absent expression of full-length AP-3 β3A subunit protein, confirming a phenotypic diagnosis of HPS2.The index case displayed a homozygous pericentric inv(5)(p15.1q14.1), which was also detected as a heterozygous defect in both parents of the index case. No loss of genetic material was demonstrated by microarray comparative genome hybridisation at 60kb resolution. Fluorescence in-situ hybridisation using the 189.6kb probe RP11-422I12, which maps to 5q14.1, demonstrated dual hybridisation to both 5q14.1 and 5p15.1 regions of the inverted Chr5. The RP11-422I12 probe maps from intron 1 to intron 16 of AP3B1, thus localising the 5q inversion breakpoint to within AP3B1. The probe RP11-211K15, which corresponds to an intergenic region on 5p also showed dual hybridisation, enabling localisation of the 5p inversion breakpoint.
  
  
  CONCLUSIONS
  This case report extends the phenotypic description of the very rare disorder HPS2. Our demonstration of a homozygous Chr5 inversion predicted to disrupt AP3B1 gene provides a novel pathogenic mechanism for this disorder.
authors.affiliation: 1Bristol Heart Institute & School of Cellular and Molecular Medicine,University of Bristol, Bristol, UK.
authors.forename: Matthew L
authors.initials: ML
authors.lastname: Jones
authors.forename: Sherina L
authors.initials: SL
authors.lastname: Murden
authors.forename: Claire
authors.initials: C
authors.lastname: Brooks
authors.forename: Viv
authors.initials: V
authors.lastname: Maloney
authors.forename: Richard A
authors.initials: RA
authors.lastname: Manning
authors.forename: Kimberly C
authors.initials: KC
authors.lastname: Gilmour
authors.forename: Vandana
authors.initials: V
authors.lastname: Bharadwaj
authors.forename: Josu
authors.initials: J
authors.lastname: de la Fuente
authors.forename: Subarna
authors.initials: S
authors.lastname: Chakravorty
authors.forename: Andrew D
authors.initials: AD
authors.lastname: Mumford
mesh_terms: D033963:Adaptor Protein Complex 3 / Q000235:genetics* / Q000378:metabolism; D033966:Adaptor Protein Complex beta Subunits / Q000235:genetics* / Q000378:metabolism; D001792:Blood Platelets / Q000378:metabolism / Q000648:ultrastructure; D007446:Chromosome Inversion; D002895:Chromosomes, Human, Pair 5 / Q000235:genetics*; D005260:Female; D005796:Genes; D022861:Hermanski-Pudlak Syndrome / Q000235:genetics* / Q000473:pathology; D006720:Homozygote; D006801:Humans; D015151:Immunoblotting; D017404:In Situ Hybridization, Fluorescence; D007223:Infant; D007694:Killer Cells, Natural / Q000378:metabolism; D052118:Lysosomal-Associated Membrane Protein 1 / Q000235:genetics / Q000378:metabolism; D051907:Lysosomal Membrane Proteins / Q000235:genetics / Q000378:metabolism; D010641:Phenotype; D010858:Pigmentation / Q000235:genetics; D021122:Protein Subunits / Q000378:metabolism; D013602:T-Lymphocytes, Cytotoxic / Q000378:metabolism
chemical_list: C439973:AP3B1 protein, human; D033963:Adaptor Protein Complex 3; D033966:Adaptor Protein Complex beta Subunits; C489116:LAMP1 protein, human; D052118:Lysosomal-Associated Membrane Protein 1; D051907:Lysosomal Membrane Proteins; D021122:Protein Subunits
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