Lysosomal storage diseases (LSDs) are rare inherited metabolic disorders characterized by a dysfunction in lysosomes, leading to waste material accumulation and severe organ damage. Enzyme replacement therapy (ERT) and haematopoietic stem cell transplant (HSCT) have been exploited as potential treatments for LSDs but pre-clinical and clinical studies have shown in some cases limited efficacy. Intravenous ERT is able to control the damage of visceral organs but cannot prevent nervous impairment. Depending on the disease type, HSCT has important limitations when performed for early variants, unless treatment occurs before disease onset. In the attempt to overcome these issues, gene therapy has been proposed as a valuable therapeutic option, either ex vivo, with target cells genetically modified in vitro, or in vivo, by inserting the genetic material with systemic or intra-parenchymal, in situ administration. In particular, the use of autologous haematopoietic stem cells (HSC) transduced with a viral vector containing a healthy copy of the mutated gene would allow supra-normal production of the defective enzyme and cross correction of target cells in multiple tissues, including the central nervous system. This review will provide an overview of the most recent scientific advances in HSC-based gene therapy approaches for the treatment of LSDs with particular focus on metachromatic leukodystrophy (MLD) and mucopolysaccharidosis type I (MPS-I).

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In this study, we characterize the natural course of metachromatic leukodystrophy (MLD), explore intra/inter group differences, and identify biomarkers to monitor disease progression. This is a longitudinal observational study. Genotype and characteristics at disease onset were recorded. Time‐to‐event analyses were performed to assess time to major disease‐related milestones in different subgroups. Longitudinal trajectories of nerve conduction velocities (NCV), brain MRI score, and brainstem auditory evoked responses (BAERs) were described. We recruited 22 late‐infantile, 14 early‐juvenile, 5 late‐juvenile, and 4 adult MLD patients. Thirty‐four were prospectively evaluated (median FU time 43 months). In late‐infantile patients, the attainment of independent walking was associated with a later age at dysphagia. In early‐juvenile, the presence of isolated cognitive impairment at onset was not a favorable prognostic factor. Late‐infantile and early‐juvenile subjects showed similar rapid loss of ambulation and onset of seizures, but late‐infantile displayed earlier loss of trunk control, dysphagia, and death. We found significant differences in all major disease‐related milestones (except death) between early‐juvenile and late‐juvenile patients. Late‐juvenile and adult patients both presented with a predominant cognitive impairment, mild/no peripheral neuropathy, lower brain MRI score at plateau compared to LI/EJ, and later cerebellar involvement. NCV and BAER were consistently severely abnormal in late‐infantile but not in older subjects, in whom both NCV and BAER were variably affected, with no deterioration over time in some cases. This study clarifies intra/inter group differences between MLD subtypes and provides additional indications regarding reliable clinical and instrumental tools to monitor disease progression and to serve as areference to evaluate the efficacy of future therapeutic interventions inthe different MLD variants.

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faccanoni hospital, habilita care&research hospitals, sarnico, bergamo, italy; 2 department of clinicalsurgical, diagnostic and pediatric sciences, university of pavia, pavia, italy; 3 department of Mechanical and industrial Engineering, university of brescia, brescia, italy; 4 istituto di neuroriabilitazione ad alta complessità, habilita care&research hospitals, Zingonia di ciserano, bergamo, italy