Bleeding Disorders Panel

This sequencing panel focuses on inherited bleeding disorders due to impaired platelet function, coagulation factor deficiencies or thrombocytopenia. With many factors contributing to clot formation, differential diagnosis of the various bleeding disorders can be time-intensive, labor-intensive, and difficult to interpret, especially in patients with milder symptoms. Genetic testing provides a means to examine multiple bleeding disorder genes simultaneously to quickly identify potential causes to disease. Differential diagnosis is especially helpful in employing appropriate therapies to mitigate bleeding episodes (Westbury et al. 2015; Simeoni et al. 2016; Lentaigne et al. 2016).

Platelet function disorders (PFDs) tested in this panel include defects in platelet adhesion/coagulation, receptor function, or secretion (Handin et al 2005). PFDs due to defects in platelet adhesion include Bernard-Soulier syndrome and Scott Syndrome; defects in receptor function include Glanzmann’s thrombasthenia, Thromboxane A2 receptor deficiency, GPVI collagen receptor deficiency, and P2Y12 ADP receptor deficiency. Defects in storage granules include Hermansky-Pudlak Syndrome.

Coagulation factor deficiency panel includes testing for a large group of inherited bleeding disorders including three types of hemophilia, von Willebrand disease and rare bleeding disorders (RBD). RBDs include inherited deficiencies in fibrinogen, factor (F) II, FV, FV +FVIII, FVII, FX, FXI, FXIII, plasminogen activator inhibitor, and alpha-s-plasmin inhibitor.

Inherited thrombocytopenias comprise a heterogeneous group of rare disorders characterized by low platelet counts. In adults, low platelet numbers are typically considered below 150,000/microL. Bleeding manifestations of thrombocytopenia include primarily excessive bruising (purpura), petechiae, prolonged bleeding from cuts or from surgical procedures, spontaneous nose bleeds, and in women, heavy menstrual flows. Thrombocytopenia and consequent bleeding diatheses range in severity from mild to severe. About half of the inherited thrombocytopenias are syndromic disorders characterized by physical and neurological anomalies, and immunodeficiencies (Balduini et al. 2013). Some inherited thrombocytopenias are associated with an increased risk of developing myelodysplastic syndrome (MDS) and acute leukemia (AL) (Churpek et al. 2013). It is important to distinguish inherited thrombocytopenias from immune / idiopathic thrombocytopenias (ITP) in order to inform clinical management and identify potential at risk family members.

Inherited platelet function disorders are inherited in an autosomal recessive manner due to pathogenic variants in the ITGB3, ITGA2B, AP3B1, BLOCK1S3, DTNBP1, HPS1, HPS3, HPS4, HPS5, HPS6, ANO6, GP1BA, GP9, GP1BB, P2RY12, CD36 and GP6 genes (Handin et al. 2005; Watson et al. 2013). Autosomal dominant forms include pathogenic variants in the TBXA2R, PLAU, PTGS1, and TBXAS1 genes.

Hemophilia A and B are inherited through an X-linked recessive manner through pathogenic variants in the F8 and F9 genes respectively and primarily affect males. VWD is inherited in both autosomal dominant and recessive manners through pathogenic variants in the VWF gene. RBDs are all inherited in an autosomal recessive manner with deficiencies in FVII, FXI, or FV accounting for ~80% of cases. RBD genes include FGA, FGB, FGG, F2, F5, F7, F10, F11, F12, F13A1, F13B, MCFD2, LMAN1, SERPINE1, SERPINF2, VKORC1, and GGCX. See individual test descriptions for additional information on the molecular biology of each gene.

Thrombocytopenia genes included in this panel have been associated with both syndromic and non-syndromic forms of inherited thrombocytopenia and represent the most well-documented forms of inherited thrombocytopenia reported in the literature. Thrombocytopenias are typically divided into three distinct groups based upon platelet size: large / macrothrombocytopenias, small / microthrombocytopenias, and thrombocytopenias with normal sized platelets (Westbury et al. 2015; Simeoni et al. 2016; Lentaigne et al. 2016).

This test is designed to identify variants in genes associated with a variety of bleeding phenotypes. In a recent study of patients with excessive bleeding, a next generation sequencing approach was used for differential diagnosis. Of the 61 patients with a suspected etiology, a corresponding pathogenic variant was identified in 91.8% of cases. In 76 patients with an unknown etiology, a causative pathogenic variant was identified in 10.5% of cases (Simeoni et al. 2016).

Large deletions in various inherited platelet disorders represent ~20% of causative variants found within AP3B1 and HPS3, ~10% in HPS6 and GP1BB, 2% in GP1BA, 5% in ITGA2B, 2% in ITGB3, and 100% in PLAU (Human Gene Mutation Database).

For coagulation deficiency genes, large deletions are represent 5% of causative variants in the F10, 2% in F11, 3% in F13A1, 2% in F5, 2% in F7, 6% in F8, 3% in F9, 15% in FGA, and 5% in VWF (Human Gene Mutation Database).

The majority of variants reported in the thrombocytopenia genes in this panel are missense and nonsense variants. Large deletions account for ~24% of the reported RUNX1 gene variants, but in general, large, multi-exon and whole gene deletions are rare among the thrombocytopenia panel genes. In addition to the RUNX1 gene, large deletions have also been reported in the GP1BB, MYH9, and WAS genes (Human Gene Mutation Database).

This test is performed using Next-Gen sequencing with additional Sanger sequencing as necessary.

This panel typically provides 92.9% coverage of all coding exons of the genes plus 10 bases of flanking noncoding DNA in all available transcripts along with other non-coding regions in which pathogenic variants have been identified at PreventionGenetics or reported elsewhere. We define coverage as ≥20X NGS reads or Sanger sequencing. PGnome panels typically provide slightly increased coverage over the PGxome equivalent. PGnome sequencing panels have the added benefit of additional analysis and reporting of deep intronic regions (where applicable).

Dependent on the sequencing backbone selected for this testing, discounted reflex testing to any other similar backbone-based test is available (i.e., PGxome panel to whole PGxome; PGnome panel to whole PGnome).