Thrombocytopenia and Platelet Ultra-Structural Abnormalities Associated with RUNX1 Haploinsufficiency in Monosomy 21 Mosaicism.
Authors: Kumar Riten , Pluthero Fred , Christensen Hilary , Kahr Walter H. 
Published: October 2019

Abstract
Abstract 2190
RUNX1, also known as core binding factor A2 (CBFA2), is a transcription factor that regulates the expression of several hematopoietic specific genes through a highly conserved DNA-binding region called the RUNT homology domain. The significant role of RUNX1 on megakaryopoiesis and leukemogenesis is supported by the finding that germline, heterozygous mutations in the RUNX1 gene on 21q22.12 forms the genetic basis of the autosomal dominant, familial platelet disorder with predisposition to acute myeloid leukemia (FDP/AML). Patients with FDP/AML present with mild to moderate thrombocytopenia, qualitative platelet defects and a propensity to develop AML. While several platelet abnormalities have been reported in this cohort including abnormalities in platelet aggregation and secretion, Mpl receptors, activation of GPIIb-IIIa, phosphorylation of myosin light chain and deficiency of platelet factor 4 (PF4); to the best of our knowledge, platelet electron microscopy findings have never been described. Furthermore, data on phenotypic manifestations of partial monosomy 21 are sparse and anecdotal. Published reports from literature describe patients with growth restriction, developmental delay, heart defects and dysmorphic facies. Thrombocytopenia, bleeding and even fatal hemorrhage have been noted, though detailed hematological manifestations are not consistently described. It is plausible that deletion of RUNX1 may be responsible for some of the characteristics observed in this cohort.
An 18 month old male with a history of unprovoked recurrent subdural hemorrhage was referred to our service for further investigation. Patient was born at 35 weeks of gestation with intra-uterine growth restriction (birth weight 1440 grams). Post-natal history was significant for dysmorphic facies, global developmental delay, seizure disorder, chronic thrombocytopenia (platelet count 70 – 140 × 109/L) and subaortic stenosis. Bleeding diathesis workup including INR, aPTT, von Willebrand antigen, ristocetin co-factor and multimer analysis were normal. PFA-100 showed consistently prolonged closure times with Col/Epi and Col/ADP cartridges and platelet aggregation showed dis-aggregation with ADP, but normal ATP release. Peripheral blood karyotype showed 46, XY, r(21) (p11.2q22.3)[22]/45, XY, −21[8]. Fluorescent-in-situ hybridization (FISH) analysis for the RUNX1 locus showed 2 copies of RUNX1 in 116/200 (58%) nuclei and one signal of RUNX1 (consistent with loss of the ring chromosome 21) in 84/200 (42%) nuclei analyzed. Transmission electron microscopy of patient platelets revealed frequent fused/large alpha granules, with approximately 20% of the platelet population characterized as agranular or having few granules. PF4 in patient total platelet lysates was not significantly decreased as assessed by immunoblotting.
To our knowledge this is the first demonstration of ultra-structural platelet abnormalities in a patient with RUNX1 haploinsufficiency. The molecular data from our patient helps elaborate the role of RUNX1 on both thrombocytopenia and platelet developmental abnormalities which also affect platelet function. Further use of electron microscopy in RUNX1 haplodeficient cohort may help better understand and characterize this rare platelet disorder.

Disclosures:
No relevant conflicts of interest to declare.