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Hermansky–Pudlak syndrome (HPS) is a rare autosomal recessive disorder that results in a syndrome of oculocutaneous albinism, a bleeding tendency, and lipofuscinosis.1 HPS is, however, the commonest genetic disorder in Puerto Rico, affecting 1 in 18002; clusters of kindreds from the Swiss Valais,3 in southern Holland, and Japan, as well as sporadic cases have also been reported.
In HPS patients from Puerto Rico a specific genetic abnormality is almost ubiquitous, that is a 16 base pair frameshift duplication affecting codons 491–496 in exon 15 of a gene (HPS 1) at chromosome segment 10q23.4 A number of other mutations in HPS-1 have also been described in HPS patients from Europe and Japan.5 There is an apparent frameshift hot spot at codons 321–322; however, fewer than half such non-Puerto Rican HPS patients have an identified mutation.5 More recently two brothers with HPS were reported to have mutations in the β3A subunit of adaptor complex 3, which is thought to be responsible for the formation of pigment forming vesicles (melanosomes) and platelet storage vesicles (dense bodies).6 These brothers did, however, have the additional finding of neutropenia, which on examination of the bone marrow was associated with granulocytic hypoplasia.
The Puerto Rican patients have an interesting phenotype with a significant risk of developing pulmonary fibrosis and granulomatous colitis. Witkop2 reported death resulting from restrictive lung disease (68%), haemorrhage (17%), or granulomatous colitis (15%) in HPS patients aged 30–50 years. Hitherto there have only been isolated case reports of granulomatous colitis occurring in non-Puerto Rican patients.7 Importantly therefore it does not as yet appear that HPS patients originating from outside Puerto Rico have a similar long term risk of these complications. This difference in phenotype may relate to different genetic defects, modifying effects of as yet undefined genetic loci, or for those patients with HPS-1 mutations the possibility that transcripts of different lengths generated by the different frameshifts may have differing functional capabilities.
HPS thus appears to be both a clinically and a biologically diverse disorder, a view that is supported by the existence of a number of mouse models for this condition. For example, the pale ear mouse is the murine analogue of the HPS-1 mutations,8,9 and the pearl10 and mocha11 mice have defects in the AP-3 adaptor complex. There are also a number of mouse models for HPS which thus far lack a human homologue, including, for example, the cappuccino mouse recently reported by Gwynn and colleagues.12 The existence of several patient clusters as well as multiple animal models implies that there are additional mechanisms responsible for generating the phenotype of HPS in man. Both the HPS-1 protein and the AP-3 adaptor complex appear to have roles in formation of granular organelles and their tracking within the cell (reviewed by Spritz13). This is important as discovery of further abnormalities is likely to yield significant insight into the mechanisms of cytoplasmic organelle trafficking and also have implications for our understanding of platelet function.
Great Ormond Street Hospital for Children NHS Trust (GOSH) has both a Comprehensive Care Centre for Haemophilia and other bleeding disorders and a large ophthalmology department with a recognised interest in albinism. We have identified 23 children with HPS treated at GOSH. In this report we review the ethnic origin, haematological findings, and clinical haemorrhagic tendency in these children.
PATIENTS AND METHODS
Patients with HPS were identified from the database recording all children reviewed in the Haemophilia Comprehensive Care Centre. These children were either referred for evaluation of a possible bleeding tendency or referred from the specialist ophthalmology clinic for investigation of potential bleeding diathesis. All children were seen by both the ophthalmological and haematological services. The notes of the patients were reviewed, as were the results of haematology laboratory evaluation. The patients were contacted and details of ethnic origin, family, and medical history, in particular haemorrhagic tendency confirmed.
Evaluation of platelet function
A platelet count and standard coagulation screen (activated partial thromboplastin time, prothrombin time, and thrombin time) were obtained, and in each case the blood film was reviewed for abnormalities by a haematologist. Platelet aggregation was performed using standard laboratory techniques and a light aggregometer (Biodata machine). Platelet nucleotides were quantitated using the luciferin–luciferase reaction and a luminometer (LKB Wallac LabSystems). Coagulation factor assays were performed if the coagulation screen was abnormal, if there was significant clinical history of haemorrhagic tendency, or a family history of either. In addition whole blood samples taken into citrate were used to perform an in vitro bleeding time assay using the PFA-100 analyser (Sysmex) as an extra test of platelet function. The PFA-100 is an in vitro assessment of the bleeding time, it determines time to closure of an aperture coated with collagen and either adrenaline or ADP. There is only limited experience and validation of this technique in paediatrics, as most validation work has been performed on the adult patient population.
Twenty three patients were identified with well defined HPS in that they had classical features of occulocutaneous tyrosinase positive albinism in conjunction with clear laboratory evidence of defective platelet function.
Ethnic origin and family history
None of the 23 children were of Puerto Rican ancestry. Thirteen were of Turkish extraction: 12 came from four different families with at least one other affected member, and one Turkish child is the sole affected member of a fifth family. Figure 1 shows the family trees of the four Turkish kindreds. Two of the kindreds originate from the Turkish/Kurdish region (Sivas and Trazbon respectively). There is a relatively high frequency of intermarriage and some consanguinity in each of the Turkish families; many have additional relatives with fair skin and eyes and visual problems.
Four children who originate from Pakistan are also reported. Three are siblings whose parents are first cousins and who have distant relatives with features suggestive of a diagnosis of HPS. The remaining one is an index case. Both these Pakistani kindreds have consanguineous relationships. One of the three affected siblings developed complete heart block and required insertion of a permanent pacemaker at the age of 8 years. This cardiac conduction defect was thought to be independent of the diagnosis of HPS, as cardiac structure was normal. No further manifestations have occurred in either this child or the other three children. The six remaining children, patients 18–23, are of neither Turkish nor Pakistani origin. Five are of Northern European extraction and one originates from Peru.
Clinical history of haemorrhagic tendency
A bleeding history was elicited from the patients and their families by staff at the Haemophilia Centre, both at the time of initial referral of the patients and during follow up visits. Particular attention was paid to bleeding following circumcision, dental eruptions, and extraction, bleeding following: other operations (especially relevant as many of these children had surgery to correct a squint), and apparent spontaneous bruising and bleeding.
Overall 9/23 (39%) of the children had no history of excessive bleeding. Twelve (52%) of the patients experienced relatively trivial bleeding symptoms; four had recurrent nosebleeds requiring cautery in two cases. Only one (patient 23) of the patients who underwent ocular surgery experienced excessive bleeding (which was prior to a diagnosis of HPS); the remaining patients received prophylactic treatment which successfully prevented haemorrhage following these procedures. One female had significant menorrhagia following the menachre, which was poorly controlled with tranexamic acid and eventually required ongoing iron replacement and oestrogen therapy. The second major bleeding event occurred in patient 21. This 7 week old male infant presented with a suppressed level of consciousness; he was found to have both subdural and retinal haemorrhages. In this case, however, the infant was exclusively breast fed and had received inadequate vitamin K prophylaxis (only one oral dose) to prevent haemorrhagic disease of the newborn. This case has previously been reported,14 and it seems most likely that haemorrhage was caused by a combination of vitamin K deficiency and the platelet function defect of HPS. Importantly this infant was treated with desmopressin (DDAVP) and subsequently suffered hyponatraemia induced fitting. DDAVP should only be used with extreme caution, if at all, in infants below the age of 1 year, and its use requires meticulous attention to fluid balance.
Evaluation of platelet function
Platelet function was evaluated by a number of different means, including a clinical history, platelet aggregation, and platelet nucleotides. A template bleeding time was performed on the first 11 patients but this technique has now largely been abandoned, and in part, replaced by the so called in vitro bleeding time test using the PFA-100 machine. The PFA-100 is an in vitro assessment of bleeding time; it determines time to closure of an aperture coated with collagen and either adrenaline or ADP. There is only limited experience and validation of this technique in paediatrics, as most validation work has been performed on the adult patient population. Table 1 shows the results of these assays. Patient 18 had electron microscopy studies performed on their platelets showing reduced numbers of dense granules; a typical result is shown (fig 2).
Platelet aggregation was performed in 22/23 patients and was abnormal in 18, with a variety of defects ranging from an abnormal response to all agonists to more subtle changes; 7/18 had no clinical haemorrhagic tendency. Four of the children had normal platelet function as assessed by aggregation. The first two belong to Turkish kindreds: one only had an abnormal PFA result and had no bleeding history; the second child had abnormal results for all the other assessments of platelet function and suffered severe menorrhagia. The third child was from the Pakistani kindred and had no history of excessive bleeding, but had evidence of platelet dysfunction using all the other laboratory tests. The final child with normal platelet aggregation had no bleeding history; he did not have the PFA test performed but bleeding time and platelet nucleotides were abnormal.
The template bleeding time was abnormal in all 11 of the children on whom it was performed; 4/11 had no clinical history of excessive bleeding. Platelet nucleotide testing was available for 22 children; it was abnormal, showing a defect consistent with a storage pool abnormality (reduced total and dramatically reduced release nucleotides) in 21/22. Samples were analysed using the PFA-100 machine in 19 children; results were abnormal in 13. There was no difference in bleeding history for those patients with a normal or an abnormal PFA-100 result. Platelet nucleotide defects were thus the most specifically abnormal platelet function test. This is to be expected, however, as platelet nucleotide quantitation was used as the principal diagnostic test and appeared to display less variability than the other tests of platelet function which are routinely used.
Correlation between abnormal platelet function tests and haemorrhagic tendency
We examined whether the number of platelet function tests which were abnormal could predict an in vivo haemorrhagic tendency in the HPS patients. We considered only the results of platelet aggregation, platelet nucleotides, and the PFA-100 device as far fewer patients had a bleeding time performed. Overall all three of these tests were performed in 18/23 patients. Ten patients had abnormal platelet function recorded with each test; however, 8/10 had no history of increased bleeding. Seven patients had abnormal platelet function shown in two of the three assays. Five of these seven had a normal PFA-100 result and two of these five had a history of bleeding events (nosebleeds and excessive bleeding post-circumcision). Two of the children had normal platelet aggregation but the remaining assays were abnormal; one of these had severe menorrhagia. One child without a history of increased bleeding had an isolated abnormal PFA-100 result. Therefore there was no clear correlation between the number of abnormal assays and a bleeding tendency, nor was a particular assay more predictive than the others.
HPS is a rare autosomal recessive inherited disorder that is relatively common in parts of Puerto Rico and the Swiss Alps. Puerto Rican patients with HPS have a relatively well defined genetic defect and longer term consequences of this syndrome; the occurrence of both pulmonary function defects and granulomatous colitis are described.2,4 Little is known of the genetic basis or likely long term prognosis of this disease for non-Puerto Rican patients; although limited reports suggest that pulmonary complications are particularly associated with HPS-1 mutations in Puerto Rican patients and are not found in other patients.15
In this study we report for the first time clustering of HPS in families originating from Turkey, especially the Turkish/Kurdish border, and from Pakistan. Twelve children in this series were members of four Turkish kindreds in which there was intermarriage and consanguinity. A further child with unrelated parents also originated from this area. Two kindreds of Pakistani origin with a total of four affected children also have HPS; the parents of these children are also consanguineous. One child developed complete heart block and required insertion of a permanent pacemaker at the age of 8 years. This cardiac conduction defect was thought to be independent of the diagnosis of HPS, as cardiac structure was normal. No further manifestations have occurred in either this child or the other three children.
All these children had classical oculocutaneous albinism, although there was variation of the ocular and sometimes cutaneous features within each kindred (data not shown). This suggests either incomplete penetrance of the HPS gene(s) and/or the presence of additional factors, most likely genetic, that have the effect of modifying the phenotype. The platelet storage pool disorder was identified by platelet nucleotide analysis, defects of platelet aggregation, template bleeding time, and in the in vitro bleeding time, as evaluated by the PFA-100 analyser. Abnormalities of all of the various facets of platelet function testing were not always present, even in those patients with notably abnormal platelet nucleotide analysis. Furthermore, there did not appear to be any correlation between either particular abnormalities of platelet function tests or the total number of abnormal results and a bleeding tendency. These results also emphasise the need to comprehensively evaluate platelet function in any patient with a potential bleeding disorder as in the HPS patients grossly abnormal platelet nucleotides were detected in patients with otherwise normal results.
Our experience of HPS is that the bleeding tendency is generally very mild, despite the coinheritance of Fitzgerald factor deficiency and an unexplained prolongation of the activated partial thromboplastin time in one kindred. Fitzgerald factor is a high molecular weight kininogen thought to be implicated in the initiation of the intrinsic pathway of coagulation. Deficiency of Fitzgerald factor may be associated with a haemorrhagic tendency, but this is unusual. Only one child had a life threatening intracranial haemorrhage that occurred when probably insufficient vitamin K prophylaxis was given. A second child has relatively severe menorrhagia that requires ongoing treatment. The remaining cases have had trivial mucocutaneous bleeding occasionally requiring treatment with DDAVP and/or nasal cautery. However, this report does illustrate the importance of considering the diagnosis of HPS in any child with oculocutaneous albinism and ensuring that they receive appropriate therapy in situations identified as “high risk”. Our patients required only minimal interventional therapy. Importantly thus far, none of these patients have developed any features suggestive of multisystem disease (pulmonary fibrosis, granulomatous colitis) as has been reported in the Puerto Rican patients.
Further studies of these patients are underway to define the underlying genetic defect(s), possible phenotypic modifying factors, and the long term prognosis of these fascinating patients.
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