Immune Thrombocytopenia (ITP) and Hβ-1 Tubulin: The Arg307His Substiution Is Over-Represented and Identifies Patients with More Severe Disease

Abstract 2525

Platelet production and function are dependent on the presence of the hematopoietic-specific β-tubulin isotype Hβ1 (Class VI), whose expression is restricted to platelets and megakaryocytes, constituting 90% of total platelet β-tubulin. Among the eight human β-tubulin isotypes, Hβ1 is the only isotype for which frequent non-synonymous single nucleotide polymorphisms (SNPs) have been described. Little is known regarding the role these SNPs play in platelet production and function. In ITP, It is accepted that both accelerated platelet destruction and decreased platelet production are central to the disease process but the exact pathophysiologic mechanisms underlying individual patient variation are unknown. Likewise, little is understood regarding the diverse clinical manifestations of ITP—which range from minor to life-threatening bleeding, and even thrombosis—among patients who otherwise appear to have similar disease. The central role of platelets in ITP together with the platelet-specific expression of Hβ1 tubulin prompted us to investigate the potential role of Hβ1 SNPs in the pathophysiology and disease manifestations of ITP. We sequenced the coding region of Hβ1 gene using genomic DNA extracted from whole blood of 98 mostly-Caucasian ITP patients and 360 Caucasian controls. Our results showed that one of the 6 reported SNPs, namely 27795494G>A leading to the substitution of arginine for histidine at amino acid 307 (Arg307His), was overrepresented in the ITP patient population as compared to the controls. Specifically, the A allele was overrepresented in the ITP population (A allele frequency of 19% versus 16%; p=0.04). Importantly, the frequency of the homozygous A/A genotype was also significantly higher in the ITP population compared to the control population (7.1% versus 3.9%; p=0.05), while we did not find any changes in the frequencies of the heterozygote and wild-type genotypes. We retrospectively examined the disease characteristics of the different genotype populations in the ITP group; namely the homozygous (A/A) Hβ1 SNP versus the heterozygote (A/G) and homozygote wild-type (G/G) groups, which were combined after all analyses showed them to be similar, and are herein referred to as A/G+G/G. Our analysis showed no significant differences in gender, age, age at initial ITP diagnosis and time since initial diagnosis, between the A/A and A/G+G/G genotypes; there was also no difference in the incidence of concurrent autoimmune conditions between the groups. However, we found that the percentage of patients with platelet counts less than 30K/μL at disease presentation was significantly higher in the A/A genotype compared to the combined A/G+G/G genotype group (100% versus 55%, p=0.035). Furthermore, the total number of different treatment types for ITP within each group was significantly different, with A/A patients requiring a mean of 7.6 treatments (range 4–14), while the A/G+G/G patients required a mean of 5.4 treatments (range, 0–14) (p=0.03) over the course of their diseases. There were no significant differences between the groups with regard to responses to individual treatments (intravenous immune globulin, anti-D, rituximab, eltrombopag, and romiplostim). Taken together, these results suggest that alterations in Hβ1 tubulin play a patholphysiologic role in the development of ITP and that patients with the homozygous A/A genotype have more severe disease at presentation and more difficulty with maintenance of long-term disease control.