Congenital Aspirin-like Defect As a Result of Autosomal Recessive Variants in PTGS1

Inherited defects of platelet function disorders are rare and difficult to diagnose due to lack of standardized platelet tests. An aspirin-like platelet defect is characterised by reduced thromboxane A2 (TXA2) signalling due to a defect in the arachidonic acid (AA) pathway in platelets. Patients with aspirin-like defect present with mild to moderate bleeding symptoms and impaired platelet aggregation responses to AA and ADP. This is similar to the irreversible effect of aspirin on platelets, which is mediated through inhibition of prostaglandin H synthase-1 also known as cyclooxygenase-1 (PTGS1/COX1). We for the first time report platelet function disorders due to autosomal recessive inheritance of variants in PTGS1.

In a total of 3563 cases with bleeding disorders, comprising 1169 whole genome sequenced probands of the BRIDGE-BPD study and 2394 panel sequenced index cases of the ThromboGenomics cohort, we identified 15 unrelated cases of each cohort with an aspirin-like platelet function defect. Two of these cases had rare a variants in PTGS1, the gene encoding COX-1, which catalyses the conversion of arachidonic acid to prostaglandin H2. The first case presented with epistaxis and peri-operative bleeding. She had reduced platelet aggregation responses to arachidonic acid, ADP, collagen and epinephrine. Incubation of control blood with collagen resulted in enhanced levels of thromboxane B2, PGD2, PGE2, 11-HETE and 15-HETE which was absent in the index case.We identified a homozygous missense variant in PTGS1, p.Trp322Ser with a Combined Annotation Dependant Depletion Score (CADD) of 31.0. This variant was absent from GnomAD. The variant co-segregated in an autosomal recessive inheritance mode, with aspirin-like defect phenotype in the seven family members who were investigated. PTGS1 was not expressed on the platelets by western blot. Immunophenotyping demonstrated absence on the platelet surface but presence on neutrophils. The second case of the presented with menorrhagia, nosebleeds, easy bleeding and bruising. She had reduced aggregation responses to arachidonic acid, ADP, collagen and epinephrine. We identified two variants in cis: a splice-donor variant (g. 125133553 T>A), CADD 24.3; and an upstream non-coding variant (g. 125132069 C>G), CADD 16.63. The frequency of these variants were respectively; 1.7 x 10^-5 and 8 x 10^-3 in GnomAD. Platelet RNA and protein expression studies in the propositus revealed alternative splicing with the generation of a smaller protein due the splice variant. In contrast, the non coding variant had no effect on promoter or enhancer activity and therefore, is likely benign. In this case, the mode of inheritance is autosomal dominant with a dominant negative effect, which has been reported previously. For the other 13 cases of the Bridge-BPD study we also interrogated the non-coding space and interactors in the arachidonic acid pathway, none of which had genetic variants explaining the phenotype. For the 15 ThromboGenomics cohort cases because they were sequenced on targeted platform similar investigations could not take place. These cases could have a non inherited cause for the platelet defect or it is also permissible that variation in a hitherto undefined pathways unique to individual cases might be causal.

In conclusion, we for the first time report autosomal recessive inheritance of variants in PTGS1 as cause for a rare inherited bleeding disorder. The effect of the mutation are selective loss of expression of PTGS1 within platelets and decreased enzyme function. Two previous reports demonstrated autosomal dominant inheritance. The first demonstrated autosomal dominant inheritance of variants in PTGS1 as modifier in a well characterized family with haemophilia A and platelet function disorder (Nance et al JTH 2016). The second reported rare heterozygous variants in PTGS1 in two cases with a bleeding tendency which was not further specified in the report (Bastida et al Haematologica 2018).