Gene Therapy for Lysosomal Storage Disorders: Recent Advances and Limitations

Esteban Alberto Gonzalez Guilherme Baldo About the authors

Abstract

Lysosomal storage disorders (LSDs) are a group of diseases with multisystemic features. Current treatments have limitations and gene therapy arises as a promising treatment option. Here, we discuss some of the most recent studies for gene therapy in LSD, vectors used, and outcomes. In particular, the approaches used in animal models aiming to correct the central nervous system, the eye, and the bones are highlighted. Finally, we discuss the recent reports of clinical trials using this technology for these diseases. We conclude that gene therapy for LSD has gathered a substantial amount of evidence from animal models to know its potential and limitations. First evidences from clinical trials using both adeno-associated and lentiviral vectors show that this approach is safe and efficient and therefore could provide an effective treatment for several LSD in the near future.

Keywords
lysosomal storage disorders; gene therapy; vectors; Hurler; clinical trials

Biology of Lysosomal Storage Disorders

Lysosomal storage disorders (LSDs) are a group of about 60 heterogeneous diseases that result in lysosomal dysfunction, usually as a consequence of deficiency of an enzyme required for the metabolism of complex molecules. As a group, these diseases can reach an incidence of about 1:5000 to 1:10 000. Most of the LSDs are autosomal recessive disorders; however, a few are X-linked recessively inherited, such as Hunter syndrome (mucopolysaccharidoses [MPS] II) and Fabry disease.11 Alroy, J, Lyons, JA. Lysosomal storage diseases. J Inborn Errors Metab Screen. 2014;2:1–20. doi:10.1177/2326409813517663.
https://doi.org/10.1177/2326409813517663...

Storage of undegraded or partially degraded material occurs in the lysosome. The LSD can be grouped into categories, based on the class of the substrate that accumulates, including lipidoses (where storage of lipids occurs), MPS (storage of mucopolysaccharides or glycosaminoglycans), glycogenoses (storage of glycogen), and oligosaccharidoses (storage of small sugar chains). In a general view, these diseases are multisystemic. Clinical findings of several LSDs include organomegaly as well as central nervous system (CNS) dysfunction. These diseases are also known for their progressive nature, with high morbidity and mortality, although there are significant variations between different diseases and even within the same disease.11 Alroy, J, Lyons, JA. Lysosomal storage diseases. J Inborn Errors Metab Screen. 2014;2:1–20. doi:10.1177/2326409813517663.
https://doi.org/10.1177/2326409813517663...
,22 Pastores, GM, Hughes, DA. Non-neuronopathic lysosomal storage disorders: disease spectrum and treatments. Best Pract Res Clin Endocrinol Metab. 2015;29(2):173–182. doi:10.1016/j.beem.2014.08.005.
https://doi.org/10.1016/j.beem.2014.08.0...

The genes mutated in LSD produce proteins that are important for normal lysosomal and cell function, and therefore, they are ubiquitously expressed. When an enzyme is lacking, the substrate accumulates in all tissues, but mainly in those where the turnover for that particular undegraded substance is higher. Therefore, the storage of the undegraded substrate related to the primary gene defect is believed to be the main cause of the disease phenotype. However, there is more and more evidence that this storage triggers multiple cascades of events that lead to the storage of other compounds (such as secondary accumulation of gangliosides in MPS) and many of the disease manifestations as well.33 Bellettato, CM., Scarpa, M. Pathophysiology of neuropathic lysosomal storage disorders. J Inherit Metab Dis. 2010;33(4):347–362. doi:10.1007/s10545-010-9075-9.
https://doi.org/10.1007/s10545-010-9075-...

Principles of Gene Therapy for LSD

Treatment options until a few decades ago for most LSD were restricted to support measures. Specific treatments have been developed in the last 2 decades and are available for several LSDs nowadays. Each treatment has its limitations and some are still in development. The main characteristics of currently available treatments of LSD are summarized in Table 1.

Table 1
Specific Treatments Available for Lysosomal Storage Disorders.

The rationale for gene therapy in these diseases is the same of other enzyme-based approaches: the ability of a deficient cell to internalize a lysosomal enzyme from the extracellular media. This enzyme can be originated either from the bloodstream (in case of enzyme replacement) or from a corrected cell (in case of gene therapy and hematopoietic stem cell transplantation [HSCT]). This uptake is a saturable, receptor-mediated process, in most cases by the mannose-6-phosphate receptor.44 Sly, WS, Fischer, HD. The phosphomannosyl recognition system for intracellular and intercellular transport of lysosomal enzymes. J Cell Biochem. 1982;18(1):67–85. doi:10.1002/jcb.1982.240180107.
https://doi.org/10.1002/jcb.1982.2401801...

Based on this, LSDs are considered good targets for gene therapy, despite their multisystemic involvement. The correction of a small percentage of cells results in enzyme secretion into the circulation, and the enzyme can be then internalized by the deficient cells, resulting in biochemical correction even in cells whose DNA was not modified.55 Baldo, G., Giugliani, R., Matte, U. Gene delivery strategies for the treatment of mucopolysaccharidoses. Expert Opin Drug Deliv. 2014;11(3):449–459. doi:10.1517/17425247.2014.880689.
https://doi.org/10.1517/17425247.2014.88...

In LSD, long-term gene expression is desirable. Therefore, initially most clinical and preclinical trials used viral-based vectors. Nowadays, nonviral integrative approaches are being developed as well. The first studies on skin fibroblasts more than 2 decades ago used retroviruses and showed promising data.66 Anson, DS., Bielicki, J., Hopwood, JJ. Correction of mucopolysaccharidosis type I fibroblasts by retroviral-mediated transfer of the human alpha-L-iduronidase gene. Hum Gene Ther. 1992;3(4):371–379. doi:10.1089/hum.1992.3.4-371.
https://doi.org/10.1089/hum.1992.3.4-371...
However, tests in animal models evidenced that systemic injection of retroviruses would not correct some organs as the brain, as the enzymes cannot cross the blood–brain barrier (BBB).77 Ellinwood, NM., Vite, CH., Haskins, ME. Gene therapy for lysosomal storage diseases: the lessons and promise of animal models. J Gene Med. 2004;6(5):481–506. doi:10.1002/jgm.581.
https://doi.org/10.1002/jgm.581...
Also, several LSDs involve other organs that are not easily corrected, such as the bone, the joints, and the eye, which evidenced the need for specific approaches. We have recently published a review that focuses on general aspects of gene therapy for MPS.55 Baldo, G., Giugliani, R., Matte, U. Gene delivery strategies for the treatment of mucopolysaccharidoses. Expert Opin Drug Deliv. 2014;11(3):449–459. doi:10.1517/17425247.2014.880689.
https://doi.org/10.1517/17425247.2014.88...
Here, we focus on LSD, specifically the effects of gene therapy on hard-to-correct organs, and the advances in clinical trials recently published for these diseases.

Gene Therapy Approaches Aiming to Correct Visceral Organs

Improvement in visceral organs (liver, heart, lung, and kidneys) is of major importance in LSD. One of the most common findings in these diseases is organ enlargement, especially hepatosplenomegaly. The increase in the size of these organs can alter hematological parameters in patients as well as lead to compression of other organs, such as the lungs, causing respiratory complications as well.88 Linari, S., Castaman, G. Hematological manifestations and complications of Gaucher disease. Expert Rev Hematol. 2016;9(1):51–58. doi:10.1586/17474086.2016.1112732.
https://doi.org/10.1586/17474086.2016.11...

Most gene therapy approaches are able to correct visceral pathology in LSD. Systemic injection of different vectors has shown that most particles are internalized by liver cells and, to a lesser extent, other visceral organs. Even in cases where the liver is the only transduced organ, the enzyme produced in the liver is able to reach the circulation and cross correct other visceral tissues.99 Metcalf, JA, Linders, B, Wu, S. Upregulation of elastase activity in aorta in mucopolysaccharidosis I and VII dogs may be due to increased cytokine expression. Mol Genet Metab. 2010;99(4):396–407. doi:10.1016/j.ymgme.2009.12.003.
https://doi.org/10.1016/j.ymgme.2009.12....
,1010 Ponder, KP., O’Malley, TM., Wang, P. Neonatal gene therapy with a gamma retroviral vector in mucopolysaccharidosis VI cats. Mol Ther. 2012;20(5):898–907. doi:10.1038/mt.2012.9.
https://doi.org/10.1038/mt.2012.9...

Based on enzyme replacement therapy (ERT) studies, some potential exceptions to that rule are the heart valves and the podocytes in the kidney. In Fabry disease, the podocytes are especially affected and complete correction is usually not achieved by ERT.1111 Trimarchi, H, Canzonieri, R, Schiel, A. Podocyturia is significantly elevated in untreated vs treated Fabry adult patients. J Nephrol. 2016;29(6):791–797. doi:10.1007/s40620-016-0271-z.
https://doi.org/10.1007/s40620-016-0271-...
Thus, whereas these kidney cells appear to be somewhat refractory to treatment, this limitation is overcome, at least in part, by exposure to higher levels of the enzyme. A study using gene therapy with a pseudotyped rAAV2/8 vector in Fabry mice has shown the presence of high levels of the missing enzyme in renal cells 6 weeks after injection of 2 × 1012 particles and maintained normal levels of enzyme in the kidney even after 60 weeks.1212 Choi, JO., Lee, M., Park, HY., Jung, SC. Characterization of Fabry mice treated with recombinant adeno-associated virus 2/8-mediated gene transfer. J Biomed Sci. 2010;17(1):26. doi:10.1186/1423-0127-17-26.
https://doi.org/10.1186/1423-0127-17-26...

Higher levels of circulating enzyme are also needed to correct the heart valves. These structures are especially affected in different MPS, although other LSDs such as galactosialidosis can also show valve disease.1313 Bursi, F, Osranek, M, Seward, JB, O’Leary, PW. Mitral and aortic valve thickening associated with galactosialidosis: echocardiographic features of a lysosomal storage disease. Echocardiography. 2003;20(7):605–606. Mucopolysaccharidoses results from deficiency in one of the several enzymes involved in the degradation of glycosaminoglycans, and heart disease is characterized by mitral and aortic valve thickening and regurgitation and aortic dilatation.1414 Braunlin, EA., Harmatz, PR., Scarpa, M. Cardiac disease in patients with mucopolysaccharidosis: presentation, diagnosis and management. J Inherit Metab Dis. 2011;34(6):1183–1197. doi:10.1007/s10545-011-9359-8.
https://doi.org/10.1007/s10545-011-9359-...
Recent studies have shown that high levels of circulating enzyme are essential to correct heart valve disease in MPS VII dogs. Treating dogs from birth using a retroviral vector-transduced hepatocytes secreted β-glucuronidase into blood to an average of 3.6-fold normal levels for more than a decade. Retroviral vector (RV)-mediated gene therapy reduced cardiac valve disease in MPS VII dogs, although at later times in life, small abnormalities could be detected in some dogs.1515 Bigg, PW, Sleeper, MM, O’Donnell, PA. The effect of neonatal gene therapy with a gamma retroviral vector on cardiac valve disease in mucopolysaccharidosis VII dogs after a decade. Mol Genet Metab. 2013;110(3):311–318. doi:10.1016/j.ymgme.2013.06.015.
https://doi.org/10.1016/j.ymgme.2013.06....

Gene Therapy Approaches Aiming to Correct Brain Disease

Most LSDs have a CNS component, and frequently, it is the most affected system in these disorders. It is known that large molecules such as enzymes cannot freely move from the circulation to the brain cells, due to the BBB. This endothelial occlusive epithelium limits the number of molecules that can reach the brain. Therefore, gene therapy approaches to the CNS need to transduce directly neuron cells, to generate a modified molecule that uses one of the few receptors in the BBB to enter the brain, or to modify cells ex vivo that can then reach the brain after implant.

Several gene delivery systems have been evaluated and showed at least some evidence of CNS correction in animal models of multiple LSD. Both viral and nonviral vectors have been delivered either directly into the CNS or in the circulation. Transplantation of genetically modified stem cells ex vivo has also shown good response.1616 Wolf, DA, Banerjee, S, Hackett, PB, Whitley, CB, McIvor, RS, Low, WC. Gene therapy for neurologic manifestations of mucopolysaccharidoses. Expert Opin Drug Deliv. 2015;12(2):283–296. doi:10.1517/17425247.2015.966682.
https://doi.org/10.1517/17425247.2015.96...

Due to their characteristic of transducing only cells that are going through division, the use of gamma-retroviral vectors for CNS manifestations is limited. However, a few studies have shown that in animal models, a small percentage of the enzyme crosses the BBB through mechanisms still unknown when very high levels of enzyme are produced by liver cells transduced with retroviral vectors.1717 Baldo, G, Wozniak, DF, Ohlemiller, KK, Zhang, Y, Giugliani, R, Ponder, KP. Retroviral-vector-mediated gene therapy to mucopolysaccharidosis I mice improves sensorimotor impairments and other behavioral deficits. J Inherit Metab Dis. 2013;36(3):499–512. doi:10.1007/s10545-012-9530-x.
https://doi.org/10.1007/s10545-012-9530-...
That scenario is not likely to happen in patients, since upscaling from small mice to bigger mammals usually leads to a significant reduction in serum enzyme levels.1818 Ponder, KP, Haskins, ME. Gene therapy for mucopolysaccharidosis. Expert Opin Biol Ther. 2007;7(9):1333–1345. doi:10.1517/14712598.7.9.1333.
https://doi.org/10.1517/14712598.7.9.133...

Adeno-associated viral (AAV) vectors are emerging as the in vivo vector platform of choice for neurological diseases. Several studies using different AAV serotypes have been described. These viruses are capable of infecting cells that are not going through division and persist primarily as nonintegrative episomal units, although integration leading to hepatic tumors were reported in mice when neonatal treatment is performed.1919 Russell, DW . AAV vectors, insertional mutagenesis, and cancer. Mol Ther. 2007;15(10):1740–1743. doi:10.1038/sj.mt.6300299.
https://doi.org/10.1038/sj.mt.6300299...
,2020 Donsante, A, Vogler, C, Muzyczka, N. Observed incidence of tumorigenesis in long-term rodent studies of rAAV vectors. Gene Ther. 2001;8(17):1343–1346. doi:10.1038/sj.gt.3301541.
https://doi.org/10.1038/sj.gt.3301541...

Adeno-associated virus serotypes 1, 5, 9, and rh.10 are particularly efficient in transfecting neuronal mouse cells after in situ injections. Serotypes 9 and rh.10 can also cross the BBB and transduce CNS cells when applied intravenously in mice and dogs.2121 Fu, H., DiRosario, J., Killedar, S., Zaraspe, K., McCarty, DM. Correction of neurological disease of mucopolysaccharidosis IIIB in adult mice by rAAV9 trans-blood–brain barrier gene delivery. Mol Ther. 2011;19(6):1025–1033. doi:10.1038/mt.2011.34.
https://doi.org/10.1038/mt.2011.34...
,2222 Gurda, BL, De Guilhem De Lataillade, A, Bell, P. Evaluation of AAV-mediated gene therapy for central nervous system disease in canine mucopolysaccharidosis VII. Mol Ther. 2016;24(2):206–216. doi:10.1038/mt.2015.189.
https://doi.org/10.1038/mt.2015.189...

These vectors were recently used to treat animal models of GM1 gangliosidosis,2323 Weismann, CM, Ferreira, J, Keeler, AM. Systemic AAV9 gene transfer in adult GM1 gangliosidosis mice reduces lysosomal storage in CNS and extends lifespan. Hum Mol Genet. 2015;24(15):4353–4364. doi:10.1093/hmg/ddv168.
https://doi.org/10.1093/hmg/ddv168...
MPS IIIB,2424 Gilkes, JA, Bloom, MD, Heldermon, CD. Preferred transduction with AAV8 and AAV9 via thalamic administration in the MPS IIIB model: a comparison of four rAAV serotypes. Mol Genet Metab Reports. 2016;6:48–54. doi:10.1016/j.ymgmr.2015.11.006.
https://doi.org/10.1016/j.ymgmr.2015.11....
Sandhoff disease,2525 Gray-Edwards, HL, Brunson, BL, Holland, M. Mucopolysaccharidosis-like phenotype in feline Sandhoff disease and partial correction after AAV gene therapy. Mol Genet Metab. 2015;116(1-2):80–87. doi:10.1016/j.ymgme.2015.05.003.
https://doi.org/10.1016/j.ymgme.2015.05....
metachromatic leukodystrophy,2626 Miyake, N, Miyake, K, Asakawa, N, Yamamoto, M, Shimada, T. Long-term correction of biochemical and neurological abnormalities in MLD mice model by neonatal systemic injection of an AAV serotype 9 vector. Gene Ther. 2014;21(4):427–433. doi:10.1038/gt.2014.17.
https://doi.org/10.1038/gt.2014.17...
Krabbe disease,2727 Lin, DS, Hsiao, CD, Lee, AYL. Mitigation of cerebellar neuropathy in globoid cell leukodystrophy mice by AAV-mediated gene therapy. Gene. 2015;571(1):81–90. doi:10.1016/j.gene.2015.06.049.
https://doi.org/10.1016/j.gene.2015.06.0...
and MPS I.2828 Hinderer, C, Bell, P, Gurda, BL. Intrathecal gene therapy corrects CNS pathology in a feline model of mucopolysaccharidosis I. Mol Ther. 2014;22(12):2018–2027. doi:10.1038/mt.2014.135.
https://doi.org/10.1038/mt.2014.135...
Different doses, routes of delivery, and outcomes are summarized in Table 2. In general, these vectors were very effective in correcting disease phenotype, quickly becoming one of the most promising approaches for the treatment of LSD.

Based on the positive results from the animal models, a phase I/II clinical study in patients with MPS IIIA was conducted. Four patients were treated with intracerebral injections of an AAV rh.10 at a dose of 7.2 × 1011 viral genomes/patient simultaneously over a period of 2 hours. The use of this vector was safe and well tolerated. This study also showed moderate improvement in behavior, attention, and sleep, mainly in the patient treated earlier.2929 Tardieu, M, Zérah, M, Husson, B. Intracerebral administration of adeno-associated viral vector serotype rh.10 carrying human SGSH and SUMF1 cDNAs in children with mucopolysaccharidosis type IIIA disease: results of a phase I/II trial. Hum Gene Ther. 2014;25(6):506–516. doi:10.1089/hum.2013.238.
https://doi.org/10.1089/hum.2013.238...
An increase in the number of patients is still necessary, but AAV vectors surely arise as a viable and encouraging approach to treat LSD.

Table 2
AAV Gene Therapy in LSD With Brain Involvement: Preclinical Studies.

Other viruses demonstrated efficacy in correcting brain disease in LSD animal models. Notably, the use of lentiviruses to correct both stem cells (ex vivo gene therapy) and quiescent cells (in vivo gene therapy) was described in several diseases, including MPS IIIA, MPS IIIB, and cystinosis, to name a few.3030 Sergijenko, A, Langford-Smith, A, Liao, AY. Myeloid/microglial driven autologous hematopoietic stem cell gene therapy corrects a neuronopathic lysosomal disease. Mol Ther. 2013;21(10):1938–1949. doi:10.1038/mt.2013.141.
https://doi.org/10.1038/mt.2013.141...
3333 Kyosen, SO, Iizuka, S, Kobayashi, H. Neonatal gene transfer using lentiviral vector for murine Pompe disease: long-term expression and glycogen reduction. Gene Ther. 2010;17(4):521–530. doi:10.1038/gt.2009.160.
https://doi.org/10.1038/gt.2009.160...
These vectors can also be injected in situ, as shown for Krabbe mice.3434 Lattanzi, A, Salvagno, C, Maderna, C. Therapeutic benefit of lentiviral-mediated neonatal intracerebral gene therapy in a mouse model of globoid cell leukodystrophy. Hum Mol Genet. 2014;23(12):3250–3268. doi:10.1093/hmg/ddu034.
https://doi.org/10.1093/hmg/ddu034...

Lentiviruses (Table 3) can be also used to augment the efficacy of existing treatments as shown by studies in both MPS I and MPS II mice. Mucopolysaccharidoses I mice were treated with normal hematopoietic stem cell (HSC) modified by a lentivirus to overexpress alpha-L-iduronidase (IDUA), and it resulted in better correction in organs that are hard to treat by conventional HSCT, such as neurologic and skeletal disease.3636 Visigalli, I, Delai, S, Politi, LS. Gene therapy augments the efficacy of hematopoietic cell transplantation and fully corrects mucopolysaccharidosis type I phenotype in the mouse model. Blood. 2010;116(24):5130–5139. doi:10.1182/blood-2010-04-278234.
https://doi.org/10.1182/blood-2010-04-27...
A recent study demonstrated that although biochemical alterations in the brains of MPS II mice are not fully corrected by regular HSCT, curiously the HSCT modified with a lentivirus to overexpress iduronate sulfatase corrected neuronal manifestations in the brains of MPS II mice.3535 Wakabayashi, T, Shimada, Y, Akiyama, K. Hematopoietic stem cell gene therapy corrects neuropathic phenotype in murine model of mucopolysaccharidosis type II. Hum Gene Ther. 2015;26(6):357–366. doi:10.1089/hum.2014.158.
https://doi.org/10.1089/hum.2014.158...

Table 3
Lentiviral Gene Therapy in LSD.

After preclinical studies, the use of these viruses finally reached the clinical stage. A lentiviral vector was used to transfer a functional Arylsulfatase A (ARSA) gene into HSCs from 3 presymptomatic patients with late infantile metachromatic leukodystrophy. After treatment, the patients showed extensive and stable ARSA gene replacement, no evidence of aberrant clonal behavior in the modified cells, and prevention of disease manifestation for up to 21 months beyond the predicted age of symptom onset.3737 Biffi, A, Montini, E, Lorioli, L. Lentiviral hematopoietic stem cell gene therapy benefits metachromatic leukodystrophy. Science. 2013;341(6148):12331 58-1233158. doi:10.1126/science.1233158.
https://doi.org/10.1126/science.1233158...
Recently, a report from a phase I trial with this approach was published. The results are quite promising since it showed no serious adverse events in 9 treated children. Furthermore, ARSA levels were restored and 8 patients had prevention of the disease symptoms.3838 Sessa, M, Lorioli, L, Fumagalli, F. Lentiviral haemopoietic stem-cell gene therapy in early-onset metachromatic leukodystrophy: an ad-hoc analysis of a non-randomised, open-label, phase 1/2 trial. Lancet. 2016;388(10043):476–487. doi:10.1016/S0140-6736(16)30374-9.
https://doi.org/10.1016/S0140-6736(16)30...
Along with AAVs, lentiviruses seem to be currently the 2 best options for gene therapy of LSD.

Gene Therapy Approaches Aiming to Correct Eye Disease

Along with the brain, the eye is also an organ isolated by an occlusive epithelium, the blood–retinal barrier. This structure prevents most substances to cross from the choriocapillaris to the retina.

One of the neuronal ceroid lipofuscinosis type 2 (CLN2) is characterized primarily by progressive loss of neurological functions and vision as well as generalized neurodegeneration and retinal degeneration. A recent study using an intra-cerebrospinal fluid (CSF) injection of an rAAV-2 in dogs has shown that the degeneration of the retina and loss of retinal function continued to progress, despite an improvement in the ganglion cell layer and in brain disease.3939 Whiting, REH, Jensen, CA, Pearce, JW, Gillespie, LE, Bristow, DE, Katz, ML. Intracerebroventricular gene therapy that delays neurological disease progression is associated with selective preservation of retinal ganglion cells in a canine model of CLN2 disease. Exp Eye Res. 2016;146:276–282. doi:10.1016/j.exer.2016.03.023.
https://doi.org/10.1016/j.exer.2016.03.0...

An interesting study using a retroviral vector in MPS I mice reached a somewhat similar conclusion. This vector only transduces liver cells that produce high levels of enzyme (up to 1000-fold normal levels in serum). The authors found that correction of eye disease, as happens to the brain, is only achieved in mice with the highest serum enzyme levels.4040 Chung, S, Ma, X, Liu, Y, Lee, D, Tittiger, M, Ponder, KP. Effect of neonatal administration of a retroviral vector expressing alpha-L-iduronidase upon lysosomal storage in brain and other organs in mucopolysaccharidosis I mice. Mol Genet Metab. 2007;90(2):181–192. doi:10.1016/j.ymgme.2006.08.001.
https://doi.org/10.1016/j.ymgme.2006.08....
These 2 studies suggest that intraocular delivery of a vector might be necessary for proper treatment of eye disease in LSD.

Intravitreal injection of AAV vectors is feasible and has been done even in clinical trials for Leber congenital amaurosis, for example. An intravitreal injection of an AAV vector was tested in MPS VII mice. Retinal function was improved, but the efficacy of the treatment depended heavily on parameters related to the injection procedure, such as the injection volume and vector dose. The authors concluded that intraocular AAV-mediated therapy may be efficacious for the retinal disease observed in LSD.4141 Hennig, AK, Ogilvie, JM, Ohlemiller, KK, Timmers, AM, Hauswirth, WW, Sands, MS. AAV-mediated intravitreal gene therapy reduces lysosomal storage in the retinal pigmented epithelium and improves retinal function in adult MPS VII mice. Mol Ther. 2004;10(1):106–116. doi:10.1016/j.ymthe.2004.03.018.
https://doi.org/10.1016/j.ymthe.2004.03....

Gene Therapy Approaches Aiming to Correct the Bones

The bones are structures poorly cellularized and vascularized, with a rigid extracellular matrix that does not allow an adequate distribution of enzymes when they are applied intravenously or even in situ. This scenario makes treatment of bone tissue especially challenging, especially because they are responsible for a lot of burden that characterizes these diseases.

Increasing the affinity of the enzyme by modifying its structure was attempted for ERT in animal models, with a certain success.4242 Montaño, AM, Oikawa, H, Tomatsu, S. Acidic amino acid tag enhances response to enzyme replacement in mucopolysaccharidosis type VII mice. Mol Genet Metab. 2008;94(2):178–189. doi:10.1016/j.ymgme.2008.01.007.
https://doi.org/10.1016/j.ymgme.2008.01....
However, gene therapy approaches have not included such modifications so far. Instead of that both transduction of other organs and overexpression of the enzyme or modifications of stem cells with its subsequent reinfusion have been tried.

Although not the only group of LSD with bone involvement, the skeletal disease in the MPS disorders is one of the best studied. Patients with MPS usually present short stature and dysostosis, among other complications. Aiming to correct these characteristics, preclinical studies have shown some progress. Mucopolysaccharidoses VII mice treated either at birth or 7 weeks of age with a systemic lentiviral gene therapy resulted in improvement in parameters of bone mass and architecture as well as biochemical and enzymatic correction. Conversely, growth plate chondrocytes were not responsive to treatment, and vertebral and femoral bone length and growth plate height were not corrected.4343 Macsai, CE, Derrick-Roberts, ALK, Ding, X. Skeletal response to lentiviral mediated gene therapy in a mouse model of MPS VII. Mol Genet Metab. 2012;106(2):202–213. doi:10.1016/j.ymgme.2012.03.022.
https://doi.org/10.1016/j.ymgme.2012.03....
Neonatal treatment of MPS VII dogs with a retrovirus resulted in high levels of the enzyme in the serum, and despite a positive general clinical effect, treated MPS VII dogs still had hypoplastic ventral epiphyses with reduced calcification in the lumbar spine at 6 months, which evidence that bone and spine disease is not completely corrected in larger animals, even with high enzyme levels in serum.4444 Smith, LJ, Martin, JT, O’Donnell, P. Effect of neonatal gene therapy on lumbar spine disease in mucopolysaccharidosis VII dogs. Mol Genet Metab. 2012;107(1–2):145–152. doi:10.1016/j.ymgme.2012.03.013.
https://doi.org/10.1016/j.ymgme.2012.03....
Similar results were also found in another disease, MPS VI, when the feline model was treated with this vector.1010 Ponder, KP., O’Malley, TM., Wang, P. Neonatal gene therapy with a gamma retroviral vector in mucopolysaccharidosis VI cats. Mol Ther. 2012;20(5):898–907. doi:10.1038/mt.2012.9.
https://doi.org/10.1038/mt.2012.9...
Based on these results, it becomes evident that even if gene therapy is able to correct a vast number of cells, it will not be able to correct all disease symptoms in this group of disorders, and novel technologies should be considered to treat bone disease.

Concluding Remarks

Gene therapy for LSD has come a long way in the last 30 years. The animal models have been very useful to predict organs that are going to be corrected by each approach and which systems are still refractory to treatment. Central nervous system abnormalities seem to be corrected by specific vectors and injection routes, but new approaches aiming to correct bone and eye disease still need to be developed, because they are important aspects of these disorders. The first clinical studies suggest that AAV and lentiviral vectors have the potential to provide a single-dose, stable, safe, and effective treatment for LSD in the near future.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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Publication Dates

  • Publication in this collection
    16 May 2019
  • Date of issue
    2017

History

  • Received
    25 May 2016
  • Accepted
    25 May 2016
Latin American Society Inborn Errors and Neonatal Screening (SLEIMPN); Instituto Genética para Todos (IGPT) Rua Ramiro Barcelos, 2350, CEP: 90035-903, Porto Alegre, RS - Brasil, Tel.: 55-51-3359-6338, Fax: 55-51-3359-8010 - Porto Alegre - RS - Brazil
E-mail: rgiugliani@hcpa.edu.br