HLA frequencies and haplotypes in children with idiopathic thrombocytopenic purpura (ITP)

Genetic studies in pediatric ITP: outlook, feasibility, and requirements

The genomic revolution in medicine has not escaped attention of clinicians and scientists involved in medical management and research studies of immune thrombocytopenic purpura (ITP). In principle, ITP biology and care will benefit greatly from modern methods to understand the patterns of gene expression and genetic markers associated with fundamental parameters of the disease including predictors of remission, risk factors for severity, determinants of response to various therapies, and possibly biological sub-types. However, applying modern genetics to ITP carries severe challenges: (a) Achieving adequate sample sizes is a fundamental problem because ITP is rare (and in pediatric ITP, chronic cases constitute only about one fourth of the total); (b) familial transmission of childhood ITP is so rare that a convincing pedigree requires consideration of other immunologic or hematologic disorders; (iii) ITP is probably biologically heterogeneous, based on clinical observations, immunological studies, and animal models. Here we review the advantages and disadvantages of potential genetic approaches. Sufficient information is available to set reasonable bounds on which genetic analyses of ITP are feasible and how they are most likely to be accomplished. The highest priority is for accurate phenotypes to compare to genetic analyses. Several registries worldwide hold promise for accomplishing this goal.

Idiopathic thrombocytopenic purpura (ITP): is there a genetic predisposition?

Idiopathic thrombocytopenic purpura (ITP) is a diagnosis of exclusion. It is unknown, whether familial ITP exists. Familial cases would make a genetic susceptibility for ITP possible. Data of the Pediatric and Adult Registry on Chronic ITP (PARC-ITP) were reviewed and subsequently ITP patients from Basel investigated for cases with a positive family history. In 10 of 445 pediatric patients and in 2 of 21 patients from Basel the family history was positive. A surprisingly high number of ITP patients with a positive family history were identified, indicating the likely existence of a genetic susceptibility for ITP.

MHC Class I–Associated Peptides Identified From Normal Platelets and From Individuals With Idiopathic Thrombocytopenic Purpura

Major histocompatibility complex (MHC) class I molecules bind and display peptide antigens on the cell surface. CD8(+) T lymphocytes recognize peptides in association with class I proteins to initiate a cytotoxic immune response. To understand the specificity of such immune responses and to facilitate the development of therapies for disease, it is important to identify MHC-presented peptides. In this study, platelets, easily obtainable and often associated with immune-mediated disease, were selected to identify MHC class I-associated peptides. MHC-associated peptides presented on platelets of normal individuals and individuals with idiopathic thrombocytopenic purpura (ITP) were characterized. ITP is characterized by the premature immune destruction of platelets. It is associated with the production of antiplatelet autoantibodies, most often targeting platelet membrane GPIIb/IIIa or GPIb/IX. In addition to characterizing five fully and several partially sequenced peptides from platelets, the peptide GPRGA(L/I)S(L/I)(L/I) was identified from four of the five ITP patients. The anchor motif of this peptide correlates with the presence of the HLA-B7 allele. A BLAST search identified this peptide as GPIb (4-12). In conclusion, platelets from normal and ITP individuals can present peptides from general cellular proteins and platelet specific proteins, such as GPIb, to the immune system via MHC class I.

The HLA-system in immune thrombocytopenic purpura: its relation to the outcome of therapy

The distribution of HLA-A, -B, -C and -DR antigens was investigated in 154 patients with immune thrombocytopenia (99 adults and 55 children). An increased frequency of HLA-Bw56 (a split of HLA-Bw22) was found both in the adults (RR = 4.30) as well as in the children (RR = 7.91). Differences in HLA antigen distributions have also been analysed in patient subgroups with a good and a bad response to corticosteroids and splenectomy. The frequency of HLA-DR4 was low in those patients with a good response to prednisone as compared with those with a poor response. The opposite was found in splenectomized patients. In 22 patients who did not respond to splenectomy, LB4 (a local split of HLA-DR4) was completely absent. These results suggest that HLA-DR4 (LB4) may be a predictive marker for therapy in ITP patients, i.e. a bad response to corticosteroids and a favourable outcome of splenectomy. However, none of the differences were significant after correction was made for the number of antigens tested.

HLA antigens in idiopathic thrombocytopenic purpura

The lymphocytes of 52 patients with the clinical diagnosis of idiopathic thrombocytopenic purpura (ITP) were typed for the HLA-A, -B and -C antigens, and of 27 of those patients also for the DR antigens. ITP proved not to be significantly associated with any of the HLA-A, -C or -DR antigens tested for. On the platelets of 35 of these 52 patients autoantibodies were detected in the direct immunofluorescence test. In these 35 patients with autoimmune thrombocytopenia (AITP), an increased frequency of HLA-Bw38 was found (14.3% versus 2.6% in controls). The frequency of none of the HLA antigens was significantly increased in the group of 17 ITP patients without demonstrable autoantibodies. The difference in association with HLA-Bw38 between AITP and ITP without demonstrable autoantibodies was not significant.

Lack of association between HLA specificities and Wilms' tumour

HLA antigen distribution was determined in thirty patients with Wilms' tumour, and their frequencies compared with those of an ethnically matched control population. No statistically significant association was found between any single HLA antigen and Wilms' tumour disease. The value of prospective HLA typing studies, with special respect to genetic aspects, histopathological subgrouping and survival rate of Wilm's tumour patients is discussed.