Platelets: An Efficient Depot for Generation and Distribution of Lysosomal Idua in Enzyme-Deficient MPS I Mice.
Authors: Dai Mei , Cao Phuong , Pan Dao 
Published: October 2019

Abstract
Abstract 3157
Proviral integration into hematopoietic stem cells (HSC) by lentivirus vector (LV)-mediated gene transfer can provide the benefit of life-long therapeutic effect, yet it can also bring the risk of insertional oncogenesis. Platelets are terminally differentiated, enucleated cells full of secretory granules. Restricting transgene expression to platelets may reduce the risk for activating oncogenes in HSCs and most of their progeny, and take advantage of their rapid turnover and professional secretion function. In this study, we evaluated the feasibility of employing megakaryocyte-platelet as a depot for the over-expression and release of alpha-L-iduronidase (IDUA), the lysosomal enzyme deficient in patients with Mucopolysaccharidosis type I (MPS I, or Hurler Syndrome). Utilizing a human megakaryocytic DAMI cell line, we found that a hybrid human ankyrin-1 promoter (K) containing ALAS2 intron 8 enhancer and HS40 core element from human alpha LCR could introduce robust intracellular IDUA expression (over 500-fold of background) and significant enzyme release (120-fold). Upon introduction to megakaryocytic differentiation as verified by FACS analysis with PI staining and morphologic evaluation, the elevated intracellular IDUA levels remained unchanged, while released IDUA increased steadily by 30-fold. We then evaluated in vivo IDUA production/release from platelets in an enzyme-deficient MPS I mouse model. Lineage-negative bone marrow (BM) cells were isolated from MPS I mice using immunomagnetic cell sorting, and transduced twice with LV-K-IDUA-ires-GFP (KIiG) for a total multiplicity of infection at 30. Five-months after 1° transplantation, GFP+ platelets were detected by FACS analysis with CD41-PE staining and observed by Image X analysis. To generate animals with various transgene-dosages, 2° transplantation was performed into MPS I mice using a serial dilution of BM from 1° MPS/KIiG (51–65% transduction efficiency) with BM of MPS I. FACS analysis revealed that the percentage of GFP+ platelets was correlated directly to transduction efficiencies in 2° recipients as determined by real-time qPCR of BM 5 months post transplantation. Moreover, the intra-platelet IDUA enzyme activities were associated linearly with the changes of GFP+% platelets (r2=0.95) among MPS I mice with different transgene-dosages, suggesting that the lysosomal IDUA from transgene overexpression could be sorted and packaged into platelets with proper catalytic function. Noticeably 1% gene transfer efficiency as shown by 1% GFP+ platelets was sufficient to introduce the IDUA levels comparable to those found in wild-type platelets. To determine if platelet-derived IDUA could be released, we conducted Ca-induced platelet activation as verified by FACS analysis with Annexin V-APC and CD41-PE staining, resulting in linear correlation (r2=0.98) of released IDUA with intra-platelet IDUA. Interestingly, the plasma IDUA levels were also linearly correlated with GFP+% platelets (r2=0.96) among MPS I with different transgene-dosages. Competitive uptake assay using lymphoblastoid cells derived from a MPS I patient (LCLmps) demonstrated steady increase of intracellular IDUA with increasing amounts of extracellular enzyme levels, which was blocked by the presence of mannose-6-phosphate (M6P). This suggested that platelet-released IDUA retained its intercellular trafficking capability via M6P receptor. Finally normalization of aberrant lysosomal morphology was observed in LCLmps exposed to platelet-released IDUA as determined by in situ immunostaining with LysoTracker, indicating functional proficiency of platelet-derived IDUA to cross-correct deficit in patient's cells. These results demonstrate for the first time that megakaryocytes/platelets are capable of over-producing, packaging and storing a lysosomal enzyme which retains proper catalytic activity, lysosomal enzyme trafficking and endogenous M6PR-mediated uptake, as well as the ability of cross-correction in patient's cells. This data warrants further evaluation for the potential application of megakaryocytes/platelets in treating lysosomal storage diseases, especially for those, such as Gaucher disease, when the desirable enzyme is sensitive to neutral pH in serum and protection of enzyme in platelets may provide continuous, real-time enzyme release by low physiological levels of platelet activation.

Disclosures:
No relevant conflicts of interest to declare.