Musculoskeletal Diseases


Muscle

There are dozens of monogenic disorders of skeletal muscle which cause progressive disability and frequently premature death. Most of these disorders should be amenable to gene therapy. Trials designed to test the safety and efficacy of gene therapy in primary muscle disorders have all employed adeno associate virus (AAV). While trials of systemic delivery of AAV for primary muscle disorders have recently been initiated, to date, completed trials have all utilized local intramuscular delivery.  

Duchenne muscular dystrophy (DMD): DMD is the most common muscular dystrophy. It is due to mutations in the gene for dystrophin which is on the X chromosome and therefore primarily affects boys. It is the most common muscular dystrophy and one of the most common inherited conditions of humankind affecting approximately 1:5000 boys. Boys lose their ability to ambulate in their early teens and die in early adulthood. 
Dystrophin is the largest gene in the human genome (11 kb coding region) and as such is too large to be packaged inside the AAV vector. Minidystrophins containing the most important regions of the gene while excluding redundant aspects have been developed for AAV gene therapy. A minidystrophin representing approximately 40% of the coding sequence of the human dystrophin gene was delivered using rAAV2.5 via intramuscular injection into the biceps of 6 boys. Minidystrophin was detectable in 2 of 4 subjects at 42 days but in neither of 2 subjects at 90 days post injection. Although an immune response to AAV capsid was not observed, concern was raised for T-cell response to dystrophin epitopes resulting in low efficacy.

Limb Girdle Muscular Dystrophy 2D (LGMD2D):  LGMD2D is a rare disorder due to mutations in the gene for α-sarcoglycan, inherited in an autosomal recessive pattern and thus affecting both boys and girls.  Similar to DMD, the disorder is marked by loss of ambulation and early death due to cardiopulmonary consequences. The full length of α-sarcoglycan is well within the packaging capacity of AAV.  In a clinical trial, an α-sarcoglycan transgene was delivered by AAV to a foot muscle in 6 patients.  5 of 6 patients demonstrated α-sarcoglycan expression with 2 of 3 patients demonstrating expression at 6 months.  The remaining subject who did not demonstrate expression exhibited an early rise in neutralizing antibodies and early T-cell response to AAV1 capsid.

Limb Girdle Muscular Dystrophy 2C (LGMD2C):  LGMD2C is a rare, autosomal recessive disorder affecting both boys and girls.  It is due to mutations in the gene for γ-sarcoglycan and results in loss of ambulation and early death.  AAV1 expressing γ-sarcoglycan was delivered locally to an arm muscle. At 30 days post treatment, biopsies from low dose groups did not express the transgene while 3 of 3 biopsies from the high dose cohort were positive for γ-sarcoglycan by immunohistochemistry but only 1 of 3 by western blot.

Pompe disease:  Pompe disease is a rare autosomal recessive disorder due to mutation in the gene for acid alpha glucosidase (GAA), the enzyme needed to degrade lysosomal glycogen.  The disorder can present in infancy for those individuals who produce no appreciable enzyme or later in life for those who produce up to 20% of normal activity.  AAV1 expressing acid alpha glucosidase (GAA) was delivered to the diaphragm muscle of 5 ventilator dependent children.  Measures of respiratory function improved with longer periods of unassisted breathing.  In a separate study, AAV1-CMV-GAA delivered to the diaphragms of Pompe children on partial/no mechanical ventilation and full time mechanical ventilation were compared.  Respiratory function increased in those with a higher baseline respiratory function and were maintained for at least 6-12 months.

Growth factors via gene therapy: Many of the factors governing muscle growth and regeneration are known. These include myostatin, a natural inhibitor of muscle growth.  Inhibition of myostatin causes muscle growth and regeneration and reduction in fat and fibrosis.  Follistatin is a natural inhibitor of myostatin and two clinical trials have assessed whether intramuscular delivery of follistatin via rAAV1 provides benefits to those with primary muscle diseases.  In Becker muscular dystrophy, a muscular dystrophy due to mutations in dystrophin but with a milder course than DMD, follistatin was delivered to the quadriceps muscle and muscle histology and walking distance were assessed.  Histological improvement and increased walking distance in 4 of 6 patients at 6 months were reported.  In sporadic inclusion body myositis, an acquired inflammatory myopathy, follistatin was similarly delivered intramuscularly via rAAV1 to quadriceps muscle of 6 individuals.  Histological features were reported to be improved compared to pretreatment biopsies and annualized walking distance was on average greater than that of untreated controls.  

There are several challenges remaining for gene therapy for muscle diseases.  In addition to widespread delivery and expression of transgene, these include how to modulate the immune system to dampen counterproductive immune responses to AAV capsid and transgene.  Trials of systemic delivery of newer generation AAV are in the planning and recruitment phases.  These trials should teach us important lessons for optimizing gene transfer to skeletal muscle.  

Cartilage

Disorders that affect the articular cartilage include traumatic injuries and progressive osteoarthritis (OA), both causing an irreversible but non-lethal disability of the articulating joints. If left untreated, traumatic injuries may further lead to OA. Thus far, none of the available pharmacological approaches (nonsteroidal anti-inflammatory drugs, opioid analgesics, intra-articular corticosteroid or hyaluronic acid injections) and surgical options (marrow-stimulating techniques like microfracture; autologous chondrocyte implantation, administration of mesenchymal stem cells; total joint replacement; osteotomy for OA) are unable to reliably restore the degraded cartilage that normally allows for a smooth gliding and weightbearing in diarthrodial joints.

Such disorders are also amenable to gene therapy using a variety of gene transfer vectors that have been tested in clinically relevant animal models by creation of experimental traumatic lesions or by induction of OA (surgical anterior cruciate ligament transection and medial meniscectomy). Most trials in these in vivo models were based on the administration of therapeutic candidate genes coding for antagonists of inflammatory cytokines to prevent cartilage degradation and/or for growth and transcription factors to promote neocartilage formation.

Traumatic lesions have been successfully treated in animals by direct injection of rAAV vectors coding for FGF-2 , IGF-I , and for the cartilage-specific transcription factor SOX9 and of adenoviral vectors combining the delivery of IGF-I and an IL-1 receptor antagonist (IL-1Ra) in rabbits and horses. Indirect administration of genetically modified cells has been also attempted to heal traumatic injuries including chondrocytes using nonviral FGF-2 and IGF-I  vectors and adenoviral IGF-I and BMP-7 vectors, and progenitor cells using nonviral TGF-β vectors and adenoviral BMP-2 and TGF- β vectors in rabbits, sheep, and horses. OA cartilage lesions have been managed by direct application of nonviral IL-1Ra vectors and of adenoviral IL-1Ra vectors or using IL-1Ra combined with FGF-2 and IGF-I in rabbits and horses while indirect administration of genetically modified synoviocytes has been performed via retroviral gene transfer of IL-1Ra in dogs.  Overall in these evaluations, the treatments could delay the degradation of the damaged cartilage and promote the formation of a new cartilage surface. The small rAAV vectors are particularly well suited as it can penetrate the dense extracellular cartilage matrix in-depth and transduce dividing progenitor cells or nondividing chondrocytes at very high efficiencies (up to 100%) and for sustained periods of time (several months).

Thus far, an initial phase I safety trial was reported by indirect gene transfer of TGF-β with retroviral vectors in patients with advanced knee OA using a cell line of human juvenile allogeneic chondrocytes established from a single new-born donor with polydactyly. These transduced cells were irradiated prior to intra-articular injection in vivo and combined (1:3) with untransduced, non-irradiated human juvenile allogeneic chondrocytes established from the same donor to amplify the paracrine effects of the secreted transgene product. As no serious adverse events were noted, a multicenter single-blinded phase IIa protocol was then conducted in patients with late-stage knee OA, showing again no adverse events and improvements in clinical outcomes of pain, function, and physical ability. Next, a placebo-controlled randomized trial was conducted in patients with moderate knee OA, revealing again improved clinical outcomes and leading to an ongoing phase III double-blinded randomized controlled protocol with about 1020 patients with knee OA. In parallel, an rAAV IL1-Ra vector is being examined by regulatory agencies for an intended clinical trial in OA patients.

2019
22nd Annual Meeting
April 29 – May 2 | Washington D.C.
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