Assumption of heparin-induced thrombocytopenia in mucopolysaccharidoses

Background: Mucopolysaccharidoses (MPSs) are a group of lysosomal storage diseases, resulted from glycosaminoglycans’ breakdown failure (GAGs). The study aims to determine the presence of thrombocytopenia, its prevalence, clinical implication, and correlation with the types of MPSs and with the types of glycosaminoglycans storage. Methods: A retrospective cross-sectional study of complete blood count data was conducted among 108 children with a confirmed diagnosis of MPS in the National Medical Research Center for Children's Health, Moscow, Russian Federation. STATISTICA 10 (Stat Soft, Inc. 1984-2011) was used for statistical analysis. A p-value of ≤0.05 was considered significant. Results: The median age of children was 65 [IQR: 41; 102] months, range: 3 – 102 months. The male to female ratio was 3:1. The prevalence of fluctuating or persistent thrombocytopenia in all children with MPS was 19.0%. Thrombocytopenia occurred in patients with MPS I (14.0%), II (19.0%), and III (31.0%). Neither of the patients with MPS IV nor VI demonstrated low platelet count. There was a non-significant predominance (P=0.068) of thrombocytopenia frequency in patients with heparan sulfate storage (22.0%) compared with patients without heparan sulfate burden (0%). Conclusion: Routine checkups of patients with MPSs must include complete blood count with platelet measurement. Cases of thrombocytopenia that requiring treatment, immunomodulatory/ immunosuppressive therapy should be considered. Further research is needed to look for the laboratory confirmation of autoimmune variants of HIT in patients with MPS I, II, III.


Background
Mucopolysaccharidoses (MPSs) are a group of multisystem metabolic diseases resulting from deficiency of the Lysosomes enzymes involved in glycosaminoglycans' breakdown. The classical clinical picture of mucopolysaccharidoses does not comprise thrombocytopenia. Nevertheless, less increasing number of case reports describing thrombocytopenia in patients with Hunter and Sanfilippo syndromes [1][2][3][4][5][6][7][8][9] and even in a child with unspecified type of MPS [10] have appeared in recent years. Some of the articles described clinical manifestations of thrombocytopenia, including serious complications: intracranial hemorrhage, recurrent epistaxis, melena and anemia, extradural hemorrhage following ventriculoperitoneal shunt insertion, intraoperative bleeding during adenoidectomy, epistaxis, and skin hemorrhages [1,3,6,7,8,10]. Notably, most cases of low platelet count were encountered in patients with heparan sulfate breakdown failure (namely MPS II and III). Thus, the present study aims to analyze thrombocytopenia prevalence, its clinical implication, and correlation with the MPSs' and glycosaminoglycans' types in a rather large group of pediatric patients.

Design of study
A retrospective cross-sectional study was conducted from January 2007 to December 2016 at the National Medical Research Center for Children's Health to reveal thrombocytopenia in a group of children with MPSs (n=108).

Inclusion and exclusion criteria
All children with enzymatically and molecularly confirmed diagnoses of one of the MPSs' types were included in the study.

Procedures
Data on complete blood count were retrospectively searched through the medical charts, recorded in National Medical Research Center for Children's Health, and through medical records provided from regional medical institutions. Clinical implications of thrombocytopenia were thoroughly explored through all medical charts. One or more complete blood count measures were available for each of the 108 children. All samples independently of the presence of pathogenic therapy (regular enzyme-replacement therapy or exposure to hematopoietic stem cell transplantation) were used for analysis. However, we excluded the measures obtained during acute respiratory infections, after epileptic status or surgery, and those low platelet count measures suspicious of adverse drug events.

Definition of variables
Patients having one or more platelets count less than 150х109/L were considered thrombocytopenia cases. To evaluate the influence of the type of accumulated glycosaminoglycan on the development of thrombocytopenia, we analyzed the prevalence of thrombocytopenia in subgroups of patients with different types of MPSs. In respect to all published cases of thrombocytopenia included only patients with MPS II and III, we separately compared the prevalence of discussed disturbance in the subgroups of patients with and without heparan sulfate storage that is considered to have structural and functional similarity to heparin.

Statistical analysis
We analyzed the frequency and values of thrombocytopenia in the whole group of children, in patients with different types of MPSs, and the groups with and without heparan sulfate storage. For descriptive statistic calculations, we used the minimal values of platelets' count (fixed for every patient during dynamic observation). Prevalence of thrombocytopenia in different types of MPSs was represented as absolute and relative frequency. Descriptive statistics of platelet count were expressed as medians, interquartile ranges, minimum and maximum. Fisher's two-tailed exact test was used to evaluate the difference between thrombocytopenia frequency in different types and groups of MPSs. We used STATISTICA 10 (Stat Soft, Inc. 1984-2011) for calculations. A P-value of ≤0,05 was considered statistically significant.

Results
One hundred eight children with enzymatically and molecularly confirmed diagnosis of one of the MPSs' types were evaluated.

Discussion
Despite a growing number of case reports describing thrombocytopenia in patients with MPSs, there is no single opinion about its pathogenic mechanism. Hypersplenism and substrate storage in bone marrow, as in Gaucher disease, are supposed to be the main causal factors by some researchers [4,[11][12][13]. To the best of your knowledge Us B. et al. [9] were the first and the only to consider an immune-mediated mechanism and/or possibly chronic antigenic stimulation due to GAG accumulation as the etiological factor of thrombocytopenia in MPS II. For the purpose of evolving our conception and summarizing the facts, a further discussion would be in this section considering the heparin-induced etiology of thrombocytopenia in MPSs. Heparin-induced thrombocytopenia (HIT) has not yet been discussed and explored in patients with these metabolic diseases. HIT is an immune prothrombotic complication of heparin therapy caused by antibodies to complexes of platelet factor 4 (PF4) and heparin. HIT antibodies may recognize PF4 bound to platelet chondroitin sulfate. Activation of platelets even in the absence of heparin, can explain spontaneous HIT syndrome (which resembles HIT clinically and serologically but occurs without proximate heparin exposure) [14]. Explanation of HIT possibility in MPSs can be structured as follows. Since the published case reports and our data have demonstrated the occurrence of thrombocytopenia exclusively in patients with heparan sulfate storage (MPS I, II, and III), heparan sulfate may play a significant role. As heparan sulfate is known to have structural and functional similarity to heparin [15][16][17], a well-known entity, called HIT, should be considered. Most research [1,6,7,9,10] reported the efficacy of immunomodulatory/immunosuppressive therapy (intravenous immunoglobulin and corticosteroids) for thrombocytopenia in MPS II and III patients and encountered in one of our patients, is in accordance with HIT. Furthermore, cases of spontaneous HIT after orthopedic surgery have been described and judged to be the result of glycosaminoglycans' release, triggering the generation of complexes with PF4 and subsequent antibody production [18][19][20][21][22]. In addition, the emergence of thrombocytopenia after cessation of enzyme replacement therapy in two patients with Hunter syndrome, presented by Jurecka A, et al. [23] point out to the link between hematologic disturbance and increased glycosaminoglycans' amount in the blood. Thus, the autoimmune variant of HIT is a probable manifestation of MPSs with heparan sulfate storage. This theoretical suggestion determines the need to investigate antibodies to PF4/heparin complexes, heparin-induced platelet aggregation, and serotonin release in patients with MPS I, II, and III. Determination of other predisposing factors, such as HLA alleles and killer-cell immunoglobulin-like receptors [23], would be of great interest.

Conclusion
In conclusion, it is reasonable to state that thrombocytopenia is not a rare manifestation in patients with MPS. The autoimmune variant of HIT is one of the probable explanations of the disturbance. Thus the practical approach to patients with MPS, especially I, II, and III, must include regular complete blood count with platelet measurement, and immunomodulatory/immunosuppressive therapy should be considered in cases of thrombocytopenia, requiring treatment. Furthermore, additional analysis of laboratory HIT markers in MPSs is a perspective of searching new markers of clinical manifestations and severity of these lysosomal storage diseases.