Thrombasthenias

Jeffrey W. Norris • Monica Aleman • Fern Tablin

Thrombasthenias, which are rare, often inherited, defects in platelet function, result in spontaneous or excessive bleeding. Unlike common clinical tests for specific coagulation factors, aPTT, PT, and platelet counts are unremarkable.

Glanzmann Thrombasthenia

Glanzmann thrombasthenia (GT) results from either quantita­tive or qualitative deficiencies of the platelet membrane integrin α∏_bβ₃ (glycoprotein IIb-IIIa). α∏_b and β₃ proteins, expressed by separate genes, form heterodimers that bind fibrinogen and mediate platelet aggregation.

Clinical signs are spontaneous bleeding episodes including chronic, intermittent epistaxis unrelated to exercise; petechial and ecchymotic hemorrhages; hematomas; and prolonged bleeding following surgical procedures or venipuncture. GT platelets have normal morphology, coagulation parameters, and fibrin degradation products.^1-5 GT platelets are unable to aggregate in response to collagen or ADP and blood from GT horses forms loose clots, with limited serum separation and decreased tensile strength.^1-5 Flow cytometry has documented marked decreases in α∏_bβ₃ integrin on the platelet surface.³

Genetic analyses of horses with GT have documented mutations in the gene encoding α∏_b (ITGA2B). Currently, there are three documented mutations in this gene associated with GT. The first mutation to be identified in a Thoroughbred-cross gelding was a homozygous mutation in exon 2.5 A Quarter Horse mare that was found to be heterozygous for this exon 2 missense mutation also had a 10-base pair deletion in the second ITGA2B allele associated with a splice site for exon 11.6 This deletion was subsequently found to occur with homozygosity in a Peruvian Paso horse as well.⁷ The third mutation in ITGA2B associated with GT was in an Oldenburg filly, which was homozygous for a distinct missense mutation in exon 2.8

Owners of GT horses should be alert for signs of bleed­ing requiring supportive care and regularly monitor for anemia.⁷

Atypical Equine Thrombasthenia

Another platelet defect was identified in a Thoroughbred filly with excessive prolonged bleeding following pin-firing.⁹ In addition to this index case, the Tablin laboratory identified five other Thoroughbreds with atypical equine thrombasthenia (AET).^9-11

AET platelets form normal clots, have normal expression of the ⅛_bβ₃ integrin, and aggregate normally in response to ADP.^9,10,12 However, AET platelets simulated with low-dose thrombin have significantly reduced fibrinogen binding.^10,11 AET platelets have limited prothrombinase activity and reduced factor V released from AET platelet alpha granules.^11,12

Treatment of bleeding has been managed with the antifi- brinolytic agent ε-aminocaproic acid. Thoroughbred horses with characteristics of AET should be considered at risk for excessive bleeding during surgery.

Simmental Hereditary Thrombopathy

Spontaneous bleeding in Simmental cattle, Simmental hereditary thrombopathy (SHT), was first described in animals from Canada and the United States.^13,14 Cattle present with spontane­ous hemorrhage, resultant anemia, and superficial hematomas. Prolonged bleeding secondary to tattooing or ear-tagging is commonly reported by owners.

Blood from SHT cattle has diminished clot retraction and reduced aggregation in response to ADP or collagen.^14-16 SHT platelets have normal morphology and normal calcium­dependent shape change in response to ADP.^14,17 However, SHT platelets fail to aggregate in response to the calcium ionophore A23187.^14,16,17

The genetic transmission of SHT was investigated in Cana­dian Simmental cattle, and a homozygous missense mutation in exon 7 of the gene encoding calcium diacylglycerol guanine nucleotide exchange factor I (CalDAG-GEFI), RASGRP2, was identified in a heifer. A number of clinically normal calves in this study were heterozygous for the mutation.^18,19 The mutation leads to a change in a conserved region of the catalytic domain of CalDAG-GEFI, a protein necessary for calcium-dependent activation of the integrin α_πbβ₃.¹⁹ SHT platelets have delayed calcium signaling resulting in delayed integrin activation.²⁰

Bovine Chediak-Higashi Syndrome

Chediak-Higashi syndrome (CHS), characterized by abnormal secretory granules in platelets, leukocytes, and melanocytes, has been identified in Holstein, Brangus, and Japanese Black cattle.^21-23 A marked reduction of CHS platelet dense granules leads to excessive bleeding.^25-27 Dense granules have mark­edly reduced ADP content and release disproportionately less ADP, yet platelets aggregate normally in response to ADP.^24,28,29 Importantly, CHS platelets have a slow and transient 2429

aggregation in response to collagen., A missense mutation was identified in the gene encoding the protein lysosomal trafficking regulator, LYST.ⁱ°^,i1 The mutation is autosomal recessive, and one study found that 8.8% of Japanese Black cattle were carriers.³²

Additional signs of CHS include hypopigmentation of skin, hair, and eyes; photophobia; and increased susceptibility to infection. Breed information combined with coat color dilution in cattle presenting with infections or inappropriate bleeding suggests CHS. However, coat color dilution in CHS Japanese Black cattle is mild and insufficient to discriminate affected from normal cattle.²⁷