BackgroundHermansky-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.Case presentationWe 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.ConclusionThis 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.

No abstract

4919 Hermansky Pudlak Syndrome (HPS) is a genetically heterogeneous group of rare disorders that are classified as ‘lysosome biogenesis disorders’ and are characterized by variegated pigmentation, variable immune deficiencies, and platelet dysfunction. There are 8 known variants of HPS in humans. Lysosomes are membrane-bound organelles that contain hydrolytic enzymes necessary for intracellular degradation and represent the main cellular degradation compartment within the vacuolar system of the eukaryotic cell 1. Biogenesis of lysosomes partly depends on the function of the biogenesis of lysosome-related organelles complex-1 (BLOC-1), −2, and −3 proteins2. The mutated proteins known to give rise to the HPS variants occur in one or more of the cytosolic BLOC complexes, each being composed of different proteins, including the Adaptor Proteins 1–4 (AP1-4) 3. Also, since mutations in the AP3B1 gene encoding the b subunit of the AP3 complex cause the only HPS subtype which is associated with an immune disorder along with other lysosomal abnormalities 4, this indicates that AP3 is important for lysosome function within the immune system. In the absence of AP3, cytotoxic T lymphocyte (CTL) mediated killing is severely impaired and cytotoxic lysosomal granules are unable to move along microtubules when the CTL recognizes target cells 5. We report a case of Hermansky Pudlak Syndrome Type 2 in a consanguineous female infant of Arabic origin who presented with severe sepsis, occulocutaneous albinism, platelet nucleotide deficiency, neutropaenia and a homozygous balanced inversion of chromosome 5; inv(5)(p15q13). Further molecular genetic analysis revealed a breakpoint in the AP3B1 gene, thereby confirming a genetic diagnosis of HPS2. Case Report: A five month old female infant presented with respiratory sepsis requiring ventilation. Clinical examination revealed occulocutaneous albinism (Figure 1) and hepatosplenomegaly. Laboratory data revealed severe neutropenia (0.2×109/l), CD4 lymphopaenia (446/microlitre), normal immunoglobulins, marked reduction of platelet nucleotides and abnormal platelet aggregation studies. There was no evidence of haemophagocytic syndrome at presentation. Neutrophil activity was normal and there was no evidence of B-cell dysfunction. Unexpectedly, her CTLs demonstrated normal lysosomal granule release on activation. Karyotyping showed pericentric balanced inversion in chromosome 5, inv (5)(p15q13). Parental karyotyping revealed heterozygous inversion of chromosome 5 at the same breakpoint. Array comparative genomic hybridization confirmed absence of copy number gain or loss in the inversion. Bacterial artificial chromosome cloning using Fluorescent in situ hybridization confirmed the breakpoint to be within the APB3B1 gene. She made a good recovery from her initial sepsis and continues to remain neutropaenic and CD4 lymphopaenic on follow up for 12 months, and has required a number of admissions with viral sepsis (adenovirus and respiratory syncitial virus). In summary, this is a first report of homozygous pericentric balanced inversion of chromosome 5 inv(5)(p15q13), leading to disruption of the AP3B1 gene, giving rise to a clinical phenotype of Hermansky Pudlak Syndrome Type 2. The normal cytotoxic lymphocyte function remains unexplained. Further molecular genetic work is in progress to map the breakpoint in the AP3B1 gene and the corresponding gene(s) involved in the inversion. Disclosures: No relevant conflicts of interest to declare.

A three and a half-year-old girl of Indian ethnic origin presented with a 2-day history of fever and sore throat. She had also been passing red urine for the previous 24 hours. Her blood count showed: white cell count 27.5 x 10 9 /l, red cell count 2.76 x 10 12 /l, hemoglobin concentration 81 g/l, hematocrit 0.22 l/l, mean cell volume 80 fl, mean cell hemoglobin 29.6 pg, mean cell hemoglobin concentration (MCHC) 370 g/l. The increased MCHC suggested the presence of spherocytes, irregularly contracted cells or some other hyper-dense cells. A blood film confirmed the presence of numerous spherocytes and in addition was leucoerythroblastic and showed red cell agglutination and erythrophagocytosis by neutrophils (Figures). Other neutrophils had large vacuoles, similar in size to an erythrocyte, containing amorphous debris. There were also atypical lymphocytes, which appeared reactive. Lactate dehydrogenase and bilirubin were increased. The reticulocyte count was initially normal (46 x 10 9 /l) but subsequently rose. The combination of marked spherocytosis, red cell agglutinates and erythrophagocytosis was considered strongly suggestive of paroxysmal cold haemoglobinuria (PCH) and confirmatory tests were performed. A direct antiglobulin test was positive for complement (111) and negative for immunoglobulin (Ig) G. A DonathLandsteiner test was positive. Red cell transfusion was required and full recovery had occurred by 3 weeks.Although the Donath-Landsteiner antibody is an IgG antibody it is usual for the direct antiglobulin test in PCH to show complement only since the antibody that binds to the cell and fixes complement in the cold detaches from the red cell membrane on warming. Confirmation of the diagnosis is by demonstration of the presence of an anti-P antibody and biphasic haemolysis. However the blood film appearances are highly characteristic and permit a rapid presumptive diagnosis.

No abstract