RUNX1 Regulates PLDN (Pallidin) Expression In Platelets: A Potential Mechanism For Dense Granule Deficiency In RUNX1 Haplodeficiency
Authors: Mao Guangfen , Fan Chengxiang , Jalagadugula Gauthami , Freishtat Robert , Rao A. Koneti 
Published: October 2019

Abstract
Transcription factor RUNX1 plays a major role in hematopoiesis. Haplodeficiency of RUNX1 is associated with familial thrombocytopenia, impaired platelet function and megakaryopoiesis, and predisposition to acute myeloid leukemia. Platelet abnormalities include impaired aggregation, secretion, protein phosphorylation, and activation of GPIIb-IIIa on platelet activation. Dense granule deficiency, either alone or in combination with alpha granule deficiency, has been reported in patients with RUNX1 haplodeficiency, but the mechanisms involved are unknown. We have reported a patient with inherited thrombocytopenia and abnormal platelet function (Gabbeta et al, Blood 87:1368-76, 1996) associated with a heterozygous nonsense mutation in transcription factor RUNX1 (Sun et al, Blood 103: 948-54, 2004). The platelets showed impaired aggregation, dense granule secretion, phosphorylation of pleckstrin and myosin light chain, diminished GPIIb-IIIa activation and decreased platelet protein kinase C-θ. Platelet mRNA expression profiling showed downregulation of several genes, including myosin light chain 9 (MYL9), platelet factor 4 (PF4), protein kinase C-θ (PRKB), and 12-lipoxygenase (ALOX12) in the patient compared to 6 normal subjects (Sun et al, J. Thromb Haemost. 5: 146-54, 2007). (We have shown that these genes are direct transcriptional targets of RUNX1.) In addition, PLDN, which encodes for protein pallidin, was four-fold down-regulated in platelets (fold change 0.239, p=0.029). In mouse model pallid, knock out of pldn leads to dense granule deficiency. Recent reports document mutations in PLDN in human subjects with platelet dense granule deficiency and the Hermansky-Pudlak syndrome. Pallidin is one of 8 subnunits that constitute the BLOC-1 (biogenesis of lysosome-related organelles complex-1), which plays a major role in granule/vesicle biogenesis. PLDN has two known human transcripts; PLDN1 is expressed ubiquitously, with exception of brain, while PLDN2 is expressed in brain, testes and leukocytes. We have addressed that PLDN is a direct transcriptional target of RUNX1 and hypothesize that its decreased expression constitutes a mechanism for the platelet dense granule deficiency in patients with RUNX1 haplodeficiency.
We validated the decreased expression of PLDN on platelet profiling by quantitative real-time PCR - PLDN1 mRNA expression was decreased by 16-fold in the patient relative to normal subjects. We studied the regulation of PLDN1 by RUNX1 in human erythroleukemia (HEL) cells treated with phorbol 12-myristate 13-acetate (PMA) for 24 h to induce megakaryocytic transformation. In silico analyses revealed the presence of 6 RUNX1 consensus binding sites in 2288bp of PLDN1 5’ upstream region from ATG. To determine endogenous interaction of RUNX1 with PLDN promoter, we performed chromatin immunoprecipitation (ChIP) assay using anti-RUNX1 antibody. There was RUNX1 binding to PLDN chromatin at regions encompassing the putative RUNX1 binding site 1 (-184 to -179 bp) and site 3 (-1370/-1365 bp). We performed electrophoretic mobility shift assay (EMSA) using probes with RUNX1 motifs and PMA-treated HEL cell nuclear extracts. With 30–34 bp probes encompassing site 1 (-184 to -179 bp) and site 3 (-1370 to -1365 bp), protein binding was observed that was competed by excess unlabelled probe and inhibited by anti-RUNX1 antibody indicating RUNX1 as the protein involved. To study the functional effect of the binding sites, the wild type PLDN1 promoter construct –2288/-2 bp containing the putative RUNX1 sites or mutant constructs with each site individually mutated were cloned into firefly luciferase reporter gene vector and transfected into HEL cells. Mutation of RUNX1 sites 1 and 3 individually caused 60-70% reduction in promoter activity indicating that these sites were functional. Studies on the other RUNX1 consensus sites in PLDN promoter are underway.

Conclusions
Our results provide the first evidence that PLDN gene is transcriptionally regulated by and is a direct target of RUNX1. These studies provide a cogent mechanism for the PLDN transcript downregulation observed in the patient platelets. More importantly, they provide a mechanism for the dense granule deficiency and impaired vesicle formation associated with RUNX1 haplodeficiency.

Disclosures:
No relevant conflicts of interest to declare.