Defective acid hydrolase secretion in RUNX1 haplodeficiency: Evidence for a global platelet secretory defect

Area of Science:

• Hematology
• Molecular Biology
• Genetics

Background:

• RUNX1 haplodeficiency is linked to thrombocytopenia, platelet dysfunction, and leukemia risk.
• Platelets contain dense granules (DG), alpha-granules (α-granules), and acid hydrolase (AH)-containing vesicles.
• While DG and α-granule deficiencies are known in RUNX1 mutations, AH vesicle secretion is poorly understood.

Purpose of the Study:

• To investigate platelet secretion defects in patients with RUNX1 mutations, focusing on AH-containing vesicles.
• To determine if RUNX1 mutations cause a global defect in platelet granule secretion.

Main Methods:

• Studied two related patients with RUNX1 mutation, mild thrombocytopenia, and easy bruising.
• Assessed platelet aggregation and dense granule (ATP, ADP), alpha-granule (β-thromboglobulin), and AH (β-glucuronidase, β-hexosaminidase, α-mannosidase) content.
• Measured dense granule, α-granule, and AH secretion upon stimulation with thrombin and A23187.
• Analyzed thromboxane A2 production and arachidonic acid incorporation.

Main Results:

• Platelet aggregation was impaired in response to various agonists.
• Granule contents were normal or minimally decreased.
• Dense granule secretion was diminished, and AH and α-granule secretion were impaired.
• Thromboxane production and arachidonic acid metabolism were normal, ruling out these pathways as the cause of defective secretion.

Conclusions:

• This study provides the first evidence of defective AH (lysosomal) secretion in patients with RUNX1 mutations.
• A global defect in secretion involving all three platelet granule types was identified, independent of granule content.
• RUNX1 mutations have pleiotropic effects, leading to multiple platelet defects.